Satellite and human visual observation are two of the most important observation approaches for cloud cover. In this study, the total cloud cover (TCC) observed by MODIS onboard the Terra and Aqua satellites was compared with Synop meteorological station observations over the North China Plain and its surrounding regions for 11 years during daytime and 7 years during nighttime. The Synop data were recorded eight times a day at 3-h intervals. Linear interpolation was used to interpolate the Synop data to the MODIS overpass time in order to reduce the temporal deviation between the satellite and Synop observations. Results showed that MODIS-derived TCC had good consistency with the Synop observations; the correlation coefficients ranged from 0.56 in winter to 0.73 in summer for Terra MODIS, and from 0.55 in winter to 0.71 in summer for Aqua MODIS. However, they also had certain differences. On average, the MODIS-derived TCC was 15.16% higher than the Synop data, and this value was higher at nighttime (15.58%-16.64%) than daytime (12.74%-14.14%). The deviation between the MODIS and Synop TCC had large seasonal variation, being largest in winter (29.53%-31.07%) and smallest in summer (4.46%-6.07%). Analysis indicated that cloud with low cloud-top height and small cloud optical thickness was more likely to cause observation bias. Besides, an increase in the satellite view zenith angle, aerosol optical depth, or snow cover could lead to positively biased MODIS results, and this affect differed among different cloud types.
Clouds are an important element in climate dynamics, atmospheric radiation, as well as atmospheric physics (Warren et al., 2007). Clouds can strongly affect the radiation balance of Earth, as they have a cooling effect due to the enhancement of planetary albedo and a heating effect resulting from the greenhouse effect of clouds (Ramanathan et al., 1989). The cloud fraction reflects the cloud's spatial domain and is a crucial factor in energy exchange in the climate system (Lu et al., 2015). Thus, it is essential to detect the spatial distribution and temporal variation of total cloud cover (TCC).
Ground-based observation and satellite remote sensing are the two most commonly used cloud observation methods and have high spatial coverage and long time series (Lu et al., 2015). Ground-based observation includes human visual observations and ground-based automatic cloud detection (Kazantzidis et al., 2012). Since ground-based automatic cloud detection is restricted by short time series and low spatial coverage, human visual observation is still the most important source of cloud information (Kotarba, 2009; Feister et al., 2010; Huo and Lu, 2012; Lu et al., 2015). Visual observation is conducted at meteorological stations, which are also called Synop stations. This is the most traditional observation approach to obtain long-term cloud fraction data, and offers a relatively dense spatial coverage.
Satellite remote sensing is another important observation approach to obtain cloud fraction data. Compared with Synop observations, satellite data are not influenced by subjective factors. This observation method also provides the opportunity to obtain continuous and spatially uniform observations of cloud conditions (Kästner et al., 2004; Fontana et al., 2013). Nevertheless, the data quality varies with the different characteristics of satellites, such as the spectral, spatial and temporal resolution of the sensors (Fontana et al., 2013). In recent decades, satellite remote sensing has been developing rapidly and is considered to be the most important method of remote sensing in cloud detection. The MODIS instrument, onboard the Aqua and Terra satellites, is a passive imager with 36 spectral channels and a spatial resolution of 250 to 1000 m. Previous studies have shown that MODIS has higher cloud recognition capabilities, as well as better calibration and geometry, compared with other operational sensors (Platnick et al., 2003; Lu et al., 2015). Comparisons between MODIS and other satellites have indicated that the observational quality of MODIS represents an improvement over ISCCP and AVHRR (Heidinger et al., 2002; Kotarba, 2015).
Satellite-derived TCC has been compared with visual surface observations (Meerkötter et al., 2004; Kotarba, 2009; Fontana et al., 2013; Ma et al., 2014; Lu et al., 2015) and ground-based instruments (Key et al., 2004; An and Wang, 2015) in different regions of the world. The results show good consistency between satellite and surface observations in some regions (Kästner et al., 2004; Meerkötter et al., 2004), but also that MODIS tends to overestimate the cloud cover when compared with the surface observations in other regions (Kotarba, 2009; Fontana et al., 2013). The satellite-observed TCC is generally higher in winter and lower in summer, as determined from the observations of ISCCP, AVHRR and MODIS (Rossow et al., 1993; Kästner et al., 2004; Kotarba, 2009). (Meerkötter et al., 2004) pointed out that, in areas with serious haze pollution in the Mediterranean, the satellite-observed cloud cover is much higher than in clean areas. Research in China has shown that the consistency between satellite and visual-surface-observed TCC is probably affected by air pollution and snow cover (Lu et al., 2015). Also, the cloud cover from satellite and surface observations has been reported to show greater deviation over the North China Plain (NCP) compared with other regions (Ma et al., 2014).
The NCP is an area with serious air pollution. Rapid economic growth over the past three decades has resulted in severe atmospheric pollution and frequent haze events (Che et al., 2014; Chen and Wang, 2015; Li, 2016). The aggravated pollution is accompanied by high aerosol loading levels (Qiu and Yang, 2000; Luo et al., 2001; Li et al., 2013; Zhang et al., 2013) and reductions in visibility (Che et al., 2007) and solar radiation (Che et al., 2005; Liang and Xia, 2005; Xia, 2010). In regions with high aerosol optical depth(AOD), the so-called shadowing effect caused by aerosols will lead to a smaller Synop-detected value of cloud fraction compared with the true value (Lu et al., 2015). Another important affect caused by high AOD is that MODIS tends to misjudge aerosol plumes as cloud in regions with heavy aerosol concentrations (Shang et al., 2014; Mao et al., 2015). However, comparisons between satellite and visual surface observations are still rare over areas with high atmospheric pollution like the NCP, particularly over the long term and in recent high-haze years.
In this paper, we present a detailed comparison of MODIS cloud cover data with Synop observations over the NCP and its surrounding regions during the period from December 2002 to November 2013 in daytime, and December 2002 to November 2009 at nighttime. We assess the discrepancies between the two datasets over high haze pollution regions and analyze these discrepancies with respect to cloud with different cloud-top heights (CTHs) and cloud optical thicknesses (COTs). The possible factors (particularly in terms of aerosol) related to the discrepancies between MODIS and Synop data are discussed for different cloud types.
Eleven years (December 2002 to November 2013) of MODIS-derived TCC and Synop TCC data during daytime and seven years (December 2002 to November 2009) during nighttime were used to analyze the observational consistency of the two datasets over the NCP and its surrounding regions. Five provinces (Liaoning, Hebei, Shandong, Shanxi and Henan) and two municipalities (Beijing and Tianjin) were chosen as the research area, as shown in Fig. 1a, because there is a high AOD center and frequent haze pollution during winter over these regions (Wang et al., 2015a, 2015b, 2015c, 2015d).
The satellite-observed TCC was derived from MODIS onboard the Terra and Aqua satellites, which passes over each region of the world twice a day in daytime and at nighttime. For Terra, the overpass time is around 1130 LST (Local Standard Time, UTC+8) during the daytime and 2330 LST at nighttime. For Aqua, meanwhile, the overpass time is around 1330 LST during daytime and 0130 LST at nighttime. The MODIS collection 6 MYD06/MOD06 and MYDATML2/MODATML2 cloud and aerosol products were used, downloaded from the Level 1 and Atmospheric Archive and Distribution System (http://ladsweb.nascom.nasa.gov/). The cloud detection results were recorded into 1-km (at nadir) spatial resolution MODIS cloud mask. According to the cloudiness likelihood of a given pixel, it was labeled as "cloudy", "uncertain——probably cloudy", "probably clear" or "confidently clear". The first two conditions were regarded as cloudy and the latter two as clear when calculating the cloud fraction (Platnick et al., 2003). The cloud mask product was generated into cloud fractions at 5-km resolution by calculating the proportion of cloudy pixels from every 25-pixel cloud mask group (Menzel et al., 2008).
For comparison of satellite and surface observations, the usual approach is to average the satellite-derived cloud fraction or cloud mask data within the field of view (FOV) of the surface observation. Previous studies have found that a FOV with a radius of 30 or 35 km agrees better with the observers' FOV at each Synop station (Minnis et al., 2003; Meerkötter et al., 2004; Dybbroe et al., 2005; Fontana et al., 2013). In China, studies have found that satellite and surface observations correlate best when using a FOV with a 35-km radius (Lu et al., 2015). At each Synop station, we calculated the average MODIS cloud fraction within the surrounding 35-km radius to obtain the MODIS-observed TCC from Terra and Aqua, separately.
The surface TCC data are visual estimations of cloud cover and cloud type produced by observers at meteorological observation stations, which are sited in open areas away from buildings and trees in order to ensure the FOV is unaffected. The data were provided by the China Meteorological Sharing Service System (CMDSSS, 2016). In total, 121 Synop stations were chosen in the research area. Synop observations were performed at eight times a day at 3-h intervals——at 0200, 0500, 0800, 1100, 1400, 1700, 2000 and 2300 LST. To minimize the effect of the time differences between Synop and MODIS observations, possible approaches include choosing the Synop TCC nearest to the MODIS overpass time (Lu et al., 2015), calculating the average of two time points adjacent to the MODIS overpass time (Fontana et al., 2013), and interpolating the Synop TCC to the MODIS overpass time (Kotarba, 2009). In this study, the Synop TCC at three times nearest the overpass time (0800, 1100 and 1400 LST during daytime and 2000, 2300 and 0200 LST at nighttime for Terra; 1100, 1400 and 1700 LST during daytime and 2300, 0200 and 0500 at nighttime for Aqua) were interpolated to the satellites' overpass times with linear interpolation in order to reduce the errors caused by observational time deviation.
In terms of the dark conditions at nighttime seriously influencing the accuracy of visual surface observations (Minnis et al., 2003), the main existing method is to choose observations made at illuminations greater than that from a half-moon at zenith. The illumination of the moonlight from the lunar altitude and phase can be determined by the ephemeris and date (Hahn et al., 1992). The Extended Edited Cloud Report Archive (EECRA) is a dataset compiled based on global surface observation datasets. EECRA offers the relative lunar illuminance and flags denoting sufficient illumination from moonlight, twilight, or sunlight during the period 1971 to 2009 for land-based stations. In this study, 81 stations in or near the research area were chosen. For each Synop station to be compared, the nearest EECRA station was identified and their illuminations considered to be approximately equal.
Synop observations of cloud types divide the cloud at three levels into 10 types, separately. For the sake of analysis of cloud with different forms, we redivided clouds into 10 categories following the classification method defined by the International Meteorological Organization. The 10 cloud types were: cumulus cloud (Cu), cumulonimbus cloud (Cb), stratocumulus cloud (Sc) stratocumulus cloud (St), nimbostratus cloud (Ns), altostratus cloud (As), altocumulus cloud (Ac), cirrus cloud (Ci), cirrostratus cloud (Cs), and cirrocumulus cloud (Cc).
For the analysis of the factors influencing observations, five auxiliary datasets of CTH, COT, AOD at 550nm, satellite view zenith angle (VZA), and snow cover, were used. All were derived from Terra and Aqua MODIS Collection 6 data products. The AOD data were derived from the Deep Blue (DB) and Dark Target (DT) combined algorithm, and only the highest quality flag (QF=3) AOD data were used. The DT algorithm was developed to detect AOD over dark surfaces such as vegetation and ocean (Remer et al., 2005; Levy et al., 2007a, 2007b). In contrast, the DB algorithm can retrieve AOD over bright surfaces such as desert and snow (Hsu et al., 2004; Bilal and Nichol, 2015). The DT/DB algorithm is a "best of" AOD product with a wide coverage and high precision (Green et al., 2009; Levy et al., 2013; Bilal and Nichol, 2015). The snow cover data were derived from the MODIS snow and sea ice products MOD10/MYD10, which provide the snow cover and ice cap at a 0.05
In order to realize the overall distribution of MODIS- and Synop-observed TCC, we first calculated the climatic field as well as the temporal variation of the TCC. As shown in Fig. 1, the cloud fraction showed distinct seasonal changes. The TCC observed by MODIS was generally greater than that from the Synop observations. The latter showed the lowest TCC in winter and highest in summer, yet the MODIS value was high both in summer and winter, and relatively low in spring and winter. The TCC observed by the two methods showed best consistency in summer and greatest deviation in winter. Analysis of the TCC climatic field is shown in Fig. 2. In general, the TCC of the southern part was higher than the northern part, which was roughly the same for MODIS and Synop observations. Meanwhile, it is notable that in winter the MODIS-observed TCC in the northern part was much larger than the Synop observation during daytime, while at nighttime both the MODIS- and Synop-derived TCC showed low values. In the southern part, the MODIS-observed TCC was high both in daytime and at nighttime, while the Synop observation was relatively low.
We conducted a detailed 11-year (December 2002-November 2013) comparison between the MODIS-derived TCC from the Terra and Aqua satellites. Figure 3 compares the monthly averaged TCC observed by Aqua and Terra for all stations. The correlation coefficient
A comparison between the MODIS and Synop TCC was conducted daily during the period December 2002-November 2013, and the statistical results are shown in
Table 1 explicitly shows that the deviation at nighttime was greater than that during daytime. The
The difference between MODIS and Synop TCC also varied with season (Table 2). As shown in Table 2, the deviation between the two datasets was greatest in winter. In winter, the
In contrast, the difference between the MODIS and Synop TCC was smallest and most consistent in summer. Both the
The deviation between MODIS and Synop TCC in spring and autumn was between that of summer and winter, and did not show any great difference. The high
Considering different cloud types may influence both MODIS and Synop observations and further influence the
Figure 5a shows the average CTH under different
Figure 5b facilitates further discussion on the
The other characteristic, COT, is discussed based on the results in Fig. 6. Figure 6a shows that the average COT with
It can be inferred from the analysis above that MODIS tends to detect cloud with low CTH and small COT that is otherwise undetected by Synop observations, meaning there may be cases that Cu and Sc clouds are detected by MODIS but undetected or underestimated by Synop observations. Given that previous research has proven that MODIS tends to judge the layer of aerosols at low altitude as cloud (Shang et al., 2014; Mao et al., 2015), it is possible that MODIS in the present study judged the aerosol layer as cloud, leading to the high
To explore the possible factors influencing the consistency and deviation between MODIS and Synop TCC observations, we analyzed the relationship between the deviation with AOD, VZA, and snow cover.
Analysis of the relationships between cloud types and VZA (Figs. 7b-d) showed that high VZA would lead to the
Figure 8 shows the spatial distribution of Aqua MODIS AOD and the averaged
Note that in the Liaoning area the
To further investigate the impact of AOD on the difference between satellite and Synop TCC observations for each cloud type, the
Analysis of different cloud types (Figs. 9b-d) showed that the
To further investigate the influence of snow cover, the relationship between snow cover and the
This study compared MODIS (Terra and Aqua) and Synop surface observed cloud fraction over the NCP and its surrounding regions during the period from December 2002 to November 2013 during daytime, and from December 2002 to November 2009 at nighttime. The comparison showed that certain differences existed between MODIS- and Synop-observed TCC. MODIS observed a significantly higher value, and this phenomenon was more obvious at nighttime. At nighttime, the mean difference between Synop and Terra/ Aqua MODIS was 15.58% and 16.64%, respectively; and this was greater than during daytime, being 12.74% and 14.14% for Terra and Aqua, respectively. The regression correlation coefficient between Synop and Terra/Aqua MODIS at nighttime was 0.65 and 0.64, respectively, which was smaller than during daytime (0.69 and 0.67 for Terra and Aqua, respectively). The comparison also revealed considerable changes in different seasons. The mean differences for Terra MODIS and Aqua MODIS in winter (29.53% and 31.07%, respectively) were much higher than in the other three seasons (ranging from 4.46% to 13.64%), and the correlation coefficients in winter (0.56 and 0.55 for Terra and Aqua, respectively) were less than in the other three seasons (ranging from 0.71 to 0.73).
Analysis of the effect of cloud characteristics on the observational deviation found that CTH and COT had an obvious influence on
Analysis showed that a large VZA would lead to a larger MODIS-observed TCC, and this effect was more obvious for clouds occurring in clumps than cloud covering the whole sky. Besides, thin clouds like Cs would lead to a negative
Acknowledgements. This work was supported by the National Natural Science Foundation of China (Grant Nos. 41590874 and 41590875) and the Ministry of Science and Technology of China (Grant No. 2014CB953703). The MODIS cloud and aerosol properties were provided by the Level 1 and Atmosphere Archive and Distribution System of the NASA Goddard Space Flight Center. We are grateful to the China Meteorological Administration for providing the visual surface cloud cover data.
Clouds determine the amount of solar radiation incident to the surface. Accurately quantifying cloud fraction is of great importance but is difficult to accomplish. Satellite and surface cloud observations have different fields of view (FOVs); the lack of conformity of different FOVs may cause large discrepancies when comparing satellite- and surface-derived cloud fractions. From the viewpoint of surface-incident solar radiation, this paper compares Moderate Resolution Imaging Spectroradiometer (MODIS) level-2 cloud-fraction data with three surface cloud-fraction datasets at five Surface Radiation Network (SURFRAD) sites. The correlation coefficients between MODIS and the surface cloud fractions are in the 0.80-0.91 range and vary at different SURFRAD sites. In a number of cases, MODIS observations show a large cloud-fraction bias when compared with surface data. The variances between MODIS and the surface cloud-fraction datasets are more apparent when small convective or broken clouds exist in the FOVs. The magnitude of the discrepancy between MODIS and surface-derived cloud fractions depends on the satellite's view zenith angle (VZA). On average, relative to surface cloud-fraction data, MODIS observes a larger cloud fraction at VZA > 40 degrees and a smaller cloud fraction at VZA < 20 degrees. When comparing long-term MODIS averages with surface datasets, Aqua MODIS observes a higher annual mean cloud fraction, likely because convective clouds are better developed in the afternoon when Aqua is observing.
Abstract This study evaluates the performance of different MODerate resolution Imaging Spectroradiometer (MODIS) aerosol algorithms during fine particle pollution events over the Beijing-Tianjin-Hebei region using Aerosol Robotic Network aerosol optical depth (AOD). These algorithms include the Deep Blue (DB) Collection 5.1 (C5) and Collection 6 (C6) algorithms at 1065km resolution, the Dark Target (DT) C5 and C6 algorithms at 1065km, the DT C6 algorithm at 365km, and the Simplified Aerosol Retrieval Algorithm (SARA) at 50065m, 365km, and 1065km resolutions. The DB C6 retrievals have 34–39% less uncertainties, 2–3 times smaller root-mean-square error (RMSE), and 3–4 times smaller mean absolute error (MAE) than DB C5 retrievals. The DT C6 has 4–8% lower bias, 4–12% less overestimation, and smaller RMSE and MAE errors than DT C5. Due to underestimation of surface reflectance and the use of inappropriate aerosol schemes, 87–89% of the collocations of the DT C6 at 365km fall above the expected error (EE), with overestimation of 64–79% which is 15–27% higher than that for the DT C6 at 1065km. The results suggest that the DT C6 at 365km resolution is less reliable than that at 1065km. The SARA AOD has small RMSE and MAE errors with 90–96% of the collocations falling within the EE. Overall, the SARA showed 15–16% less uncertainty than the DB C6 (1065km), 69–72% less than the DT C6 (1065km), and 79–83% less than the DT C6 (365km) retrievals.
In January 2013, North China Plain experienced several serious haze events.Cimel sunphotometer measurements at seven sites over rural, suburban andurban regions of North China Plain from 1 to 30 January 2013 wereused to further our understanding of spatial-temporal variation of aerosoloptical parameters and aerosol radiative forcing (ARF). It was found thatAerosol Optical Depth at 500 nm (AODsub500 nm/sub) during non-pollution periodsat all stations was lower than 0.30 and increased significantly to greaterthan 1.00 as pollution events developed. The Angstrom exponent (Alpha) waslarger than 0.80 for all stations most of the time. AODsub500 nm/sub averagesincreased from north to south during both polluted and non-polluted periodson the three urban sites in Beijing. The fine mode AOD during pollutionperiods is about a factor of 2.5 times larger than that during thenon-pollution period at urban sites but a factor of 5.0 at suburban andrural sites. The fine mode fraction of AODsub675 nm/sub was higher than80% for all sites during January 2013. The absorption AODsub675 nm/sub atrural sites was only about 0.01 during pollution periods, while~0.03–0.07 and 0.01–0.03 during pollution and non-pollutionperiods at other sites, respectively. Single scattering albedo variedbetween 0.87 and 0.95 during January 2013 over North China Plain. The sizedistribution showed an obvious tri-peak pattern during the most seriousperiod. The fine mode effective radius in the pollution period was about0.01–0.08 μm larger than during non-pollution periods, while thecoarse mode radius in pollution periods was about 0.06–0.38 μm lessthan that during non-pollution periods. The total, fine and coarse modeparticle volumes varied by about 0.06–0.34 μmsup3/sup, 0.03–0.23 μmsup3/sup, and 0.03–0.10 μmsup3/sup, respectively, throughout January2013. During the most intense period (1–16 January), ARF atthe surface exceeded 6150 W msup612/sup, 61180 W msup612/sup, and 61200 W msup612/sup at rural, suburban, and urban sites, respectively. The ARFreadings at the top of the atmosphere were approximately 6130 W msup612/sup inrural and 6140–60 W msup612/sup in urban areas. Positive ARF at the top of theatmosphere at the Huimin suburban site was found to be different from othersas a result of the high surface albedo due to snow cover.
Trends in Chinese global radiation, direct horizontal radiation, diffuse radiation, clearness index, diffuse fraction and percentage of possible sunshine duration for the period 1961-2000 were evaluated based on data for daily surface solar radiation and monthly sunshine duration. Annual means for all six variables were calculated for each station and for China as a whole. Linear regression analysis was used to characterize long-term annual trends in these variables. Over the latter half of the 20th century, there have been significant decreases in global radiation (-4.5 W/mper decade), direct radiation (-6.6 W/mper decade), clearness index (-1.1% per decade), and the percentage of possible sunshine duration (-1.28% per decade), but diffuse fraction has increased (1.73% per decade). Although there is some evidence that conditions have improved in the last decade, the consistent spatial and temporal variations of these variables support the theory that increased aerosol loadings were at least partially responsible for the observed decreases in global radiation and direct radiation, the clearness index, and the monthly percentage of possible sunshine duration over much of China.
Trends in Chinese horizontal visibility, the frequency of visibility >19 km, and haziness for the period between 1981 and 2005 were evaluated based on data for daily horizontal visibility. Annual means were calculated for each station and for China as a whole. Linear regression analysis was used to characterize long-term annual trends in these variables. Over the past 25 years, there has been a significant decrease in horizontal visibility (-2.1 km per decade from 1990 to 2005) and the frequency of visibility >19 km (-3.5% per decade) but a significant increase in the 75th percentile annual extinction coefficients (25% per 25 year). According to rapid increase of total energy consumption, the consistent spatial and temporal variations of visibility and haze support the speculation that increased aerosol loadings were responsible for the observed decreases in horizontal visibility over much of East China.
Abstract Haze is a severe hazard that greatly influences traffic and daily life with great economic losses and threats to human health. To enhance understanding of the haze occurrences, this study examined the haze variations over North China and their associated atmospheric circulations for the period of 1960–2012 using daily visibility data. Results indicate that the haze events over this region primarily occur in boreal winter of year and mainly in the morning of day. The results of the analysis of the long-term variations indicate that the annual haze days were relatively few in the 1960s but increased steeply in the 1970s and have remained stable to the present. Some differences are obvious among seasons. A stably increasing trend is discernable in summer and autumn, relatively low in the 1960s and the 1990s–2000s and relatively high in the 1970s–1980s in spring and winter. Haze variations in urban regions are quite similar to haze variations in rural regions but with more haze days in urban regions because of the high aerosol emissions. Further analyses indicate that the occurrences of severe haze events in boreal winter generally correlate with the weakened northerly winds and the development of inversion anomalies in the lower troposphere, the weakened East Asian trough in the midtroposphere, and the northward East Asian jet in the high troposphere. All of these factors provide a favorable atmospheric background for the maintenance and development of haze events in this region.
New methods and software for cloud detection and classification at high and midlatitudes using Advanced Very High Resolution Radiometer (AVHRR) data are developed for use in a wide range of meteorological, climatological, land surface, and oceanic applications within the Satellite Application Facilities (SAFs) of the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), including the SAF for Nowcasting and Very Short Range Forecasting Applications (NWCSAF) project. The cloud mask employs smoothly varying (dynamic) thresholds that separate fully cloudy or cloud-contaminated fields of view from cloud-free conditions. Thresholds are adapted to the actual state of the atmosphere and surface and the sun-satellite viewing geometry using cloud-free radiative transfer model simulations. Both the cloud masking and the cloud-type classification are done using sequences of grouped threshold tests that employ both spectral and textural features. The cloud-type classification divides the cloudy pixels into 10 different categories: 5 opaque cloud types, 4 semitransparent clouds, and 1 subpixel cloud category. The threshold method is fuzzy in the sense that the distances in feature space to the thresholds are stored and are used to determine whether to stop or to continue testing. They are also used as a quality indicator of the final output. The atmospheric state should preferably be taken from a short-range NWP model, but the algorithms can also run with climatological fields as input.
A comparison between different types of ground-based sensors has been carried out to derive macroscopic cloud data such as cloud cover and cloud-base heights. The instruments compared in the campaign at the Meteorological Observatory Lindenberg in the period May to September 2006 include an infrared (IR) sky scanner called Nubiscope, a Daylight VIS/NIR Whole Sky Imager (WSI), a ceilometer LD-40 measuring in the near infrared region (NIR) and a Ka band cloud radar measuring in the micro wave band (extremely high frequency or EHF) region. In addition, our data analysis included regular hourly cloud observations by weather observers, and vertical profiles of temperature, humidity and winds taken from six-hourly radio soundings at the site. The comparison has been focused on performance and features of the Nubiscope as a prototype instrument for automatic cloud observations. Cloud cover (CC) derived from the Nubiscope cloud algorithm compares quite well with CC derived from both WSI and from observations. CC differences are within 2 Okta in 67% of cases between Nubiscope and observations, and in 90% of cases between Nubiscope and WSI. The cloud detection capability as derived from the zenith signals of Nubiscope and WSI shows coincidence in about 90% of cases. For cloud-base heights (CBHs) from Nubiscope data and ceilometer as well as from radar reflectivity, the comparison showed a general good correspondence in the lower and middle troposphere up to heights of about 6 km with some systematic difference due to the different detection methods. For the upper troposphere above 6 km the differences become widespread and more random. Cloud detection capabilities of the instruments are also illustrated by a case study of moving clouds with patterns similar to contrails that were erroneously classified as such by the weather observer mainly due to lack of height information that the ceilometer did not provide. By combined information from WSI, radio sonde humidity and radar, they were shown not to be contrails, but most likely low-level water clouds either of natural origin or built from aircraft at their ascent or descent flight close to the airport.
78 MODIS mid-morning cloud fraction data set derived from the Terra/MODIS MOD35 cloud mask product between (03/2000–02/2012) 78 MODIS early-afternoon cloud fraction data set derived from the Aqua/MODIS MYD35 cloud mask product (07/2000–02/2012) 78 High spatial resolution (0.05°) cloud cover climatology over the Swiss Alps 78 Validation with Synop observations over the full time period 78 Close agreement between satellite- and ground-based observations of cloud cover
Collocated Interagency Monitoring of Protected Visual Environments (IMPROVE) particulate matter (PM) less than 2.5 microm in aerodynamic diameter (PM2.5) chemically speciated data, mass of PM less than 10 microm in aerodynamic diameter (PM10), and Aerosol Robotic Network (AERONET) aerosol optical depth (AOD) and size distribution at Bondville, IL, were compared with satellite-derived AOD. This was done to evaluate the quality of the Geostationary Operational Environmental Satellite (GOES) and Moderate Resolution Imaging Spectroradiometer (MODIS) AOD data and their potential to predict surface PM2.5 concentrations. MODIS AOD correlated better to AERONET AOD (r = 0.835) than did GOES AOD (r = 0.523). MODIS and GOES AOD compared better to AERONET AOD when the particle size distribution was dominated by fine mode. For all three AOD methods, correlation between AOD and PM2.5 concentration was highest in autumn and lowest in winter. The AERONET AOD-PM2.5 relationship was strongest with moderate relative humidity (RH). At low RH, AOD attributable to coarse mass degrades the relationship; at high RH, added AOD from water growth appears to mask the relationship. For locations such as many in the central and western United States with substantial coarse mass, coarse mass contributions to AOD may make predictions of PM2.5 from AOD data problematic. Seasonal and diurnal variations in particle size distributions, RH, and seasonal changes in boundary layer height need to be accounted for to use satellite AOD to predict surface PM2.5.
Visual observations of cloud cover are hindered at night due to inadequate illumination of the clouds. This usually leads to an underestimation of the average cloud cover at night, especially for the amounts of middle and high clouds, in climatologies based on surface observations. The diurnal cycles of cloud amounts, if based on all the surface observations, are therefore in error, but they can be obtained more accurately if the nighttime observations are screened to select those made under sufficient moonlight. Ten years of nighttime weather observations from the Northern Hemisphere in December were classified according to the illuminance of moonlight or twilight on the cloud tops, and a threshold level of illuminance was determined, above which the clouds are apparently detected adequately. This threshold corresponds to light from a full moon at an elevation angle of 6℃, light from a partial moon at higher elevation, or twilight from the sun less than 9℃ below the horizon. It permits the use of about 38 % of the observations made with the sun below the horizon. The computed diurnal cycles of total cloud cover are altered considerably when this moonlight criterion is imposed. Maximum cloud cover over much of the ocean is now found to be at night or in the morning, whereas computations obtained without benefit ofthe moonlight criterion, as in our published atlases, showed the time of maximum to be noon or early afternoon in many regions. Cloud cover is greater at night than during the day over the open oceans far from the continents, particularly in summer. However, near-noon maxima are still evident in the coastal regions, so that the global annual average oceanic cloud cover is still slightly greater during the day than at night by 0.3 %. Over land, where daytime maxima are still obtained but with reduced amplitude, average cloud cover is 3.3 % greater during the daytime. The diurnal cycles of total cloud cover we obtain are compared with those of ISCCP for a few regions; they are generally in better agreement if the moonlight criterion is imposed on the surface observations. Using the moonlight criterion, we have analyzed 10 years (1982-91) of surface weather observations over land and ocean, worldwide, for total cloud cover and for the frequency of occurrence of clear sky, fog, and precipitation. The global average cloud cover (average of day and night) is about 2 % higher if the moonlight criterion is imposed than if all observations are used. The difference is greater in winter than in summer, because of the fewer hours of darkness in summer. The amplitude of the annual cycle of total cloud cover over the Arctic Ocean and at the South Pole is diminished by a few percent when the moonlight criterion is imposed. The average cloud cover for 1982-91 is found to be 55 % for Northern Hemisphere land, 53 % for Southern Hemisphere land, 66 % for Northern Hemisphere ocean, and 70 % for Southern Hemisphere ocean, giving a global average of 64 %. The global average for daytime is 64.6 %; for nighttime 63.3 %
Focuses on a study concerning the improvement in a cloud-masking capability of data from a Moderate Resolution Imaging Spectroradiometer (MODIS) relative to data from an Advanced Very High Resolution Radiometer (AVHRR). Comparison of cloud masks; Analysis of the cloud-mask results between AVHRR and MODIS; Conclusions.
 A global 2-month comparison is presented between the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and the Moderate Resolution Imaging Spectroradiometer (MODIS) for both cloud detection and cloud top height (CTH) retrievals. Both CALIOP and MODIS are part of the NASA A-Train constellation of satellites and provide continuous near-coincident measurements that result in over 28 million cloud detection comparisons and over 5 million CTH comparisons for the months of August 2006 and February 2007. To facilitate the comparison, a computationally efficient and accurate collocation methodology is developed. With the collocated MODIS and CALIOP retrievals, nearly instantaneous comparisons are compiled regionally and globally. Globally, it is found that the MODIS 1-km cloud mask and the CALIOP 1-km averaged layer product agreement is 87% for cloudy conditions for both August 2006 and February 2007. For clear-sky conditions the agreement is 85% (86%) for August (February). The best agreement is found for nonpolar daytime and the poorest agreement in the polar regions. Differences in cloud top heights depend strongly on cloud type. Globally, MODIS underestimates the CTH relative to CALIOP by 1.4 脗 2.9 km for both August 2006 and February 2007. This value of 1.4 km is obtained using the CALIOP 1 km layer products. When compared to the CALIOP 5-km products, the differences increase to 2.6 脗 3.9 km as a result of CALIOP's increased sensitivity to optically thin cirrus. When only high clouds above 5 km are considered, the differences are found to be greater than 4 km with individual comparisons having differences larger than 10 km. These large differences (>10 km) represent approximately 16% of the nonpolar high cloud retrievals (>5 km). For high clouds it is found that MODIS retrieves a cloud top height for 90% of the clouds detected by the CALIOP 5-km layer products. The large MODIS underestimates for optically thin cirrus occur for cases when MODIS reverts to a window brightness temperature retrieval instead of CO2 slicing. A systematic bias is found for marine low-level stratus clouds, with MODIS overestimating the CTH by over 1 km in dense marine stratocumulus regions. The cause of the bias was identified in the MODIS Collection 5 algorithm; an application of a modified algorithm reduced this bias.
Retrieving aerosol properties from satellite remote sensing over a bright surface is a challenging problem in the research of atmospheric and land applications. In this paper we propose a new approach to retrieve aerosol properties over surfaces such...
ABSTRACT Naked-eye observation of cloud cover has widely resisted automation. Replacement of human observation by instruments is an inexorable trend for the development of ground-based macroscopic cloud observation. In this paper, cloud covers from an all-sky imager (AST) are compared with those from a meteorological observer (MO) through field experiments performed at three sites in China. The correlation coefficient between ASI and MO is 0.77 for all cases. The ASI cloud fractions have great agreement with MO for clear sky, overcast sky, and sky loaded with low- and middle-level clouds. About 78% of the ASI cases had deviations between +/- 1 tenth compared to MO cloud cover. High-level cloud (or aerosol) is the main reason causing this difference. It is partially due to MO, who takes aerosol as high, thin cloud. Another reason might be that ASI made a wrong estimation for high-level cloud (or aerosol) because of its detector and the cloud-determination algorithm. Distinguishing high, thin cloud from aerosol is a challenge, and is the main problem that needs to be resolved for future developments of ASI. A new, improved method is discussed at the end of this paper.
A five-year cloud climatology (1992 to 1996) of the Alpine region in a 15-km resolution has been evaluated by means of the APOLLO cloud detection algorithm applied to daytime AVHRR data of several NOAA satellites. The study area comprises three different climatic regions, the moderate climate north of the Alps, the Alpine climate and the Mediterranean climate in the Po-valley. Synoptic observations of the total cloud cover at 40 stations have been compared to the satellite based monthly mean data. Hourly ground observations allowed to estimate the variance in the monthly mean diurnal cycle of total cloud cover due to the fact that the satellite overpass time shifts from noon to afternoon for the NOAA-11 platform and for different NOAA satellites as well. This time shift of satellite observation effects the cloud climatology only slightly, because the changes of the cloud cover between 11 and 16 UTC are in most cases considerably smaller than the yearto-year variability. Furthermore, these cloud cover variations due to the time of the day are in monthly means below the validation accuracy. The comparison of monthly means reveals an overestimation of the satellite cloud cover of about 10% mainly due to additional detection of thin cirrus. A good agreement is found in the Alpine and rural moderate climates (corr. coeff. r > 0.75), whereas the cloud detection in the satellite data is too high in the Mediterranean zone due to urban and aerosol haze effects. In both data sets a rather small amplitude of the annual cycle of cloud cover results in the mountains compared to the lowlands. The high spatial variability of cloud cover in mountainous terrain is obvious with the satellite data and is substantiated by the sparse synoptic stations within the Alps. Eine 5-j01hrige (1992–1996) Wolkenklimatologie der Alpenregion in einer 15-km-Aufl02sung wurde mit dem APOLLO Wolkenerkennungsalgorithmus erzeugt. Sie basiert auf t01glichen AVHRR-Daten (Mittags überflüge) von verschiedenen NOAA-Satelliten. Das Auswertegebiet umfasst drei verschiedene Klimaregionen: das gem0108igte Klima n02rdlich der Alpen, das Alpenklima und das mediterrane Klima in der Po-Ebene. Synoptische Beobachtungen der Gesamtwolkenbedeckung an 40 SYNOP-Stationen wurden mit den satellitengestützten, monatlich gemittelten Daten verglichen. Die stündlichen Stationsbeobachtungen erm02glichen eine Absch01tzung der 02nderung der Gesamtbew02lkung im Tagesgang und damit den Einfluss aus unterschiedlichen 05berflugzeiten der Satelliten. Dies ist insbesondere für den NOAA-11-Satelliten wichtig, dessen 05berflugzeit sich im Laufe der Jahre von Mittag auf Nachmittag verschob, bei den anderen NOAA-Satelliten ist es 01hnlich. Es zeigte sich, dass diese Zeitverschiebung der Satellitenbeobachtung die Ergebnisse der Wolkenklimatologie nur wenig beeinflusst, weil die Ver01nderungen der Wolkenbedeckung zwischen 11 und 16 UTC im Monatsmittel in den meisten F01llen betr01chtlich kleiner als die Jahr-zu-Jahr Variabilit01t sind. Ferner liegen diese tageszeitlichen Ver01nderungen unterhalb der Validierungsgenauigkeit. Der Vergleich der Monatsmittelwerte zeigt eine 05bersch01tzung der Satelliten-Wolkenbedeckung von ca. 10% zu den Bodenstationen, haupts01chlich bedingt durch die zus01tzliche Erkennung von dünnem Zirrus in den Satellitendaten. Insgesamt besteht eine recht gute 05bereinstimmung für das alpine und das gem0108igte Klima in l01ndlichen Gebieten (Korr. koeff. r0,75), w01hrend in der Mittelmeerzone und in Gro08st01dten die Satellitendaten die Bew02lkung übersch01tzen, was auf Aerosol- und Dunsteffekte zurückzuführen ist. Beide Datens01tze weisen eine geringe Amplitude des Jahresgangs der Bew02lkung über Bergen im Vergleich zum Flachland auf. Die hohe r01umliche Variabilit01t des Bedeckungsgrades im gebirgigen Gel01nde wird besonders deutlich in den Satellitendaten und gleichfalls best01tigt durch die r01umlich weniger gut aufgel02sten Stationsbeobachtungen im Alpengebiet.
A simple whole sky imaging system, based on a commercial digital camera with a fish-eye lens and a hemispheric dome, is used for the automatic estimation of total cloud coverage and classification. For the first time, a multi color criterion is applied on sky images, in order to improve the accuracy in detection of broken and overcast clouds under large solar zenith angles. The performance of the cloud detection algorithm is successfully compared with ground based weather observations. A simple method is presented for the detection of raindrops standing on the perimeter of hemispheric dome. Based on previous works on cloud classification, an improved k-Nearest-Neighbor algorithm is presented, based not only on statistical color and textural features, but taking also into account the solar zenith angle, the cloud coverage, the visible fraction of solar disk and the existence of raindrops in sky images. The successful detection percentage of the classifier ranges between 78 and 95% for seven cloud types.
All-weather Arctic cloud analyses primarily derived from a surface-based hemispheric all-sky imager are compared against ISCCP D-1 cloud amount, type, and phase during the sunlit polar season. Increasing surface temperatures and decreasing ice cover over the past decade have altered heat and moisture fluxes around the Arctic, providing conditions more conducive for cloud generation. Shipboard and ice camp measurements from field experiments conducted over an 8-year period show cloudy skies in 7095% of the record. Most of these occurrences are stratiform or multi-level, multi-form cloud, increasing in amount with time through the season. Collocated ISCCP retrievals underestimate cloud amount at small solar zenith angles and overestimate at large angles, sometimes by as much as 50%. Satellite assessments of cloud form classify 95% of scenes as having multiple cloud types, the majority of which are mid-level ice cloud and low-level liquid cloud. Despite large discrepancies in diurnal cloud amount, regional averages of ISCCP pixel cloudiness over the length of the experiments agree within 5% of surface observations.
Two main sources for global cloud climatologies are visual surface observations and observations made by spaceborne sensors. Satellite observations compared with surface data show in most cases differences ranging from 610215% up to 61021%, depending on sensor and observation conditions. These differences are partially controlled by sensors' cloud detection capabilities — a higher number of spectral bands and higher spatial resolution are believed to allow discrimination of clouds from land/ocean/snow background. A Moderate-Resolution Imaging Spectroradiometer (MODIS) produces images of the atmosphere in 36 spectral bands with a spatial resolution of 250–1000m, thus having a capacity for cloud detection far more advanced than other operating sensors. In this study, instantaneous MODIS cloud observations were compared with surface data for Poland for January (winter) and July (summer) 2004. It was found that MODIS observed 4.38% greater cloud amount in summer conditions and 7.28% in winter conditions. Differences were greater at night (7–8%) than in daytime (0.5–7%) and correlations ranged between 0.577 (winter night) and 0.843 (winter day, summer day and night).
The goal of the International Satellite Cloud Climatology Project (ISCCP) is to provide global cloud amount statistics for atmospheric radiation flux modeling, which is a key element of climate change studies. However, ISCCP estimates rely on two spectral channels only, while the most advanced satellite sensors offer over 20 spectral bands, and thus a higher probability of correct cloud detection. We validated the accuracy of ISCCP mean monthly cloud amount statistics using the state-of-the-art, 36-spectral channel Moderate-resolution Imaging Spectroradiometer (MODIS) instrument aboard the Terra and Aqua satellites. Based on the MODIS Level 2 Cloud Mask we developed a dedicated Level 3 product for Central Europe (2004–2009). For the first time, MODIS swath data were projected onto an ISCCP equal-area grid, which guaranteed an exact geometrical agreement between both climatologies. Results showed that there was a close correlation between ISCCP and MODIS data (ρ02=020.872, α02=020.99), especially at warmer part of the year (ρ02≥020.940, α02>020.99). However, ISCCP estimations were found to be unreliable in wintertime when surface was covered with snow. The presence of snow resulted in a significant underestimate of cloud amount by 0.45 for individual ISCCP grid boxes. Our results suggest that MODIS cloud climatology is more reliable when estimates of mean monthly cloud amount are required.
 Since first light in early 2000, operational global quantitative retrievals of aerosol properties over land have been made from Moderate Resolution Imaging Spectroradiometer (MODIS) observed spectral reflectance. These products have been continuously evaluated and validated, and opportunities for improvements have been noted. We have replaced the surface reflectance assumptions, the set of aerosol model optical properties, and the aerosol lookup table (LUT). This second-generation operational algorithm performs a simultaneous inversion of two visible (0.47 and 0.66 μ m) and one shortwave-IR (2.12 μ m) channel, making use of the coarse aerosol information content contained in the 2.12 μ m channel. Inversion of the three channels yields three nearly independent parameters, the aerosol optical depth ( τ ) at 0.55 μ m, the nondust or fine weighting ( η ), and the surface reflectance at 2.12 μ m. Retrievals of small-magnitude negative τ values (down to 610.05) are considered valid, thus balancing the statistics of τ in near zero τ conditions. Preliminary validation of this algorithm shows much improved retrievals of τ , where the MODIS/Aerosol Robotic Network τ (at 0.55 μ m) regression has an equation of: y = 1.01x + 0.03, R = 0.90. Global mean τ for the test bed is reduced from 650.28 to 650.21.
The twin Moderate resolution Imaging Spectroradiometer (MODIS) sensors have been flying on Terra since 2000 and Aqua since 2002, creating an extensive data set of global Earth observations. Here, we introduce the Collection 6 (C6) algorithm to retrieve aerosol optical depth (AOD) and aerosol size parameters from MODIS-observed spectral reflectance. While not a major overhaul from the previous Collection 5 (C5) version, there are enough changes that there are significant impacts to the products and their interpretation. The C6 aerosol data set will be created from three separate retrieval algorithms that operate over different surface types. These are the two "Dark Target"(DT) algorithms for retrieving (1) over ocean (dark in visible and longer wavelengths) and (2) over vegetated/dark-soiled land (dark in the visible), plus the "Deep Blue"(DB) algorithm developed originally for retrieving (3) over desert/arid land (bright in the visible). Here, we focus on DT-ocean and DT-land (# 1 and # 2). We have updated assumptions for central wavelengths, Rayleigh optical depths and gas (H2O, O-3, CO2, etc.) absorption corrections, while relaxing the solar zenith angle limit (up to <= 84 degrees) to increase poleward coverage. For DT-land, we have updated the cloud mask to allow heavy smoke retrievals, fine-tuned the assignments for aerosol type as function of season/location, corrected bugs in the Quality Assurance (QA) logic, and added diagnostic parameters such topographic altitude. For DT-ocean, improvements include a revised cloud mask for thin-cirrus detection, inclusion of wind speed dependence on the surface reflectance, updates to logic of QA Confidence flag (QAC) assignment, and additions of important diagnostic information. At the same time, we quantified how "upstream"changes to instrument calibration, land/sea masking and cloud masking will also impact the statistics of global AOD, and affect Terra and Aqua differently. For Aqua, all changes will result in reduced global AOD (by 0.02) over ocean and increased AOD (by 0.02) over land, along with changes in spatial coverage. We compared preliminary data to surface-based sun photometer data, and show that C6 should improve upon C5. C6 will include a merged DT/DB product over semi-arid land surfaces for reduced-gap coverage and better visualization, and new information about clouds in the aerosol field. Responding to the needs of the air quality community, in addition to the standard 10 km product, C6 will include a global (DT-land and DT-ocean) aerosol product at 3 km resolution.
With the development of economy in the past thirty years, many large cities in the eastern and southwestern China are experiencing increased haze events and atmospheric pollution, causing significant impacts on the regional environment and even climate. However, knowledge on the aerosol physical and chemical properties in heavy haze conditions is still insufficient. In this study, two winter heavy haze events in Beijing occurred in 2011 and 2012 were selected and investigated by using the ground-based remote sensing measurements. We used CIMEL CE318 sun-sky radiometer to derive haze aerosol optical, physical and chemical properties, including aerosol optical depth (AOD), size distribution, complex refractive indices and fractions of chemical components like black carbon (BC), brown carbon (BrC), mineral dust (DU), ammonium sulfate-like (AS) components and aerosol water content (AW). The retrieval results from a total of five haze days showed that the aerosol loading and properties during the two winter haze events were relatively stable. Therefore, a parameterized heavy haze characterization was drawn to present a research case for future studies. The averaged AOD is 3.2 at 440 nm and ngstrm exponent is 1.3 from 440870 nm. The coarse particles occupied a considerable fraction of the bimodal size distribution in winter haze events, with the mean particle radius of 0.21 and 2.9 m for the fine and coarse mode respectively. The real part of the refractive indices exhibited a relatively flat spectral behavior with an average value of 1.48 from 440 to 1020 nm. The imaginary part showed obviously spectral variation with the value at 440 nm (about 0.013) higher than other three wavelengths (e.g. about 0.008 at 675 nm). The chemical composition retrieval results showed that BC, BrC, DU, AS and AW occupied 1%, 2%, 49%, 15% and 33% respectively on average for the investigated haze events. The comparison of these remote sensing results with in situ BC and PMlt;subgt;2.5lt;/subgt; measurements were also presented in the paper.
Long-term trends in downwelling solar irradiance and associated climatic factors over China are studied in the paper. Decreasing trends in global and direct radiation are observed over much of China. The largest decrease occurs in South and East China (east of about 100℃ E and south of about 40℃ N). The spatial pattern of observed trends in diffuse irradiance is complex and inhomogeneous. An intriguing aspect of trends in global and direct irradiance is the rather abrupt decrease in annual and seasonal mean values from 1978 onward. The decreasing trends in solar radiation in China did not persist into the 1990s. The spatial and temporal patterns of trends in sunshine duration are consistent with that of global and direct irradiance. A decreasing trend in rainy days is observed over much of China, which is in agreement with the secular trend in cloud amount. The fact that trends in cloud amount and solar radiation are quite similar suggests that the cloud amount is not the primary cause for the decrease in solar radiation. Visibility in the eastern part of China has deteriorated heavily as a result of the rapid increase in aerosol loading. The statistical analysis showed that atmospheric transmission under clear conditions decreased rapidly. These facts suggest that the rapid increase in aerosol loading should be one of the principle causes for the decrease in solar radiation. The observed diurnal temperature range decreases remarkably in China, which is closely related to the increase in aerosols. The effects of anthropogenic air pollutants on climate should be further studied and included in the simulation of climate and projection of climate scenario.Keywords. Atmospheric composition and structure (Aerosol and particles; General or miscellaneous) Meteorology and atmospheric dynamics (Radiative processes)
In this study, we conducted a comparison between surface-observed total cloud cover (TCCs) and Moderate Resolution Imaging Spectroradiometer (MODIS)-derived total cloud cover (TCCm) over China. A statistical method was applied to estimate the average field of view (FOV) of surface observers, and the radius range of FOV was 20–25, 25–35, 35–50, and 25–4502km for spring, summer, autumn, and winter, respectively. More differences would be added in the comparison when the satellite’s FOV was smaller or larger than the average FOV. Monthly mean TCCs was 74.78%, 74.41%, 66.5%, and 74.06% for each season and the corresponding TCCm was 75.27%, 78.34%, 73.82%, and 82.12%. The correlation between two data sets was stronger in spring (0.727) and summer (0.736) than in autumn (0.710) and winter (0.667). Over 60% of the differences were within the 6110% to 10% range, and more differences occurred for smaller TCCs. As a special feature, we found that the dust, haze, and snow cover over specific regions in China were the possible causes of the significant differences. Generally, these two data sets were in good agreement over China, and can complement each other especially in those significant difference cases to provide more accurate TCC data sets.
Abstract Earth observation satellites can provide systematic and continuous monitoring of clouds. High spatial resolution (565km) cloud fraction data are retrieved from the Moderate Resolution Imaging Spectroradiometer cloud mask products of Terra (1265years) and Aqua (965years) over China. Long-term trends of cloud fraction for morning and afternoon observations clearly reflected seasonal variations. We found more clouds in the afternoon than in the morning. There is a strong correlation between satellite and surface observations of the daily cloud fraction for the period of 2012, with correlation coefficients of 0.678 and 0.7 for morning and afternoon, respectively. However, the analyses of the monthly mean cloud fraction between satellite and surface observations showed a larger discrepancy in the winter. In order to investigate the differences between satellite and ground-based cloud fraction over different underlying surfaces, a statistical test was carried out for six areas. The results indicated statistically significant increased correlations ( p 65<650.0001) between satellite and ground-based cloud fraction in northern China after removing winter data, especially in Northeastern Forest and Taklimakan Desert, while the correlation coefficients in southern China did not show significant changes.
Characterizing the earth's global cloud field is important for the proper assessment of the global radiation budget and hydrologic cycle. This characterization can only be achieved with satellite measurements. For complete daily coverage across the globe, polar-orbiting satellites must take observations over a wide range of sensor zenith angles. This paper uses Moderate Resolution Imaging Spectroradiometer (MODIS) Level-3 data to determine the effect that sensor zenith angle has on global cloud properties including the cloud fraction, cloud-top pressure, effective radii, and optical thickness. For example, the MODIS cloud amount increases from 57% to 71% between nadir and edge-of-scan (0903046700°) observations, for clouds observed between 3500°N and 3500°S latitude. These increases are due to a combination of factors, including larger pixel size and longer observation pathlength at more oblique sensor zenith angles. The differences caused by sensor zenith angle bias in cloud properties are not readily apparent in monthly mean regional or global maps because the averaging of multiple satellite overpasses together 'washes out' the zenith angle artifact. Furthermore, these differences are not constant globally and are dependent on the cloud type being observed.
Abstract The cloud detection algorithm for passive sensors is usually based on a fuzzy logic system with thresholds determined from previous observations. In recent years, haze and high aerosol concentrations with high aerosol optical depth (AOD) occur frequently in China and may critically impact the accuracy of the Moderate Resolution Imaging Spectroradiometer (MODIS) cloud detection. Thus, we comprehensively explore this impact by comparing the results from MODIS/Aqua (passive sensor), Cloud-Aerosol Lidar with Orthogonal Polarization/CALIPSO (lidar sensor), and Cloud Profiling Radar/CloudSat (microwave sensor) of the A-Train suite of instruments using an averaged AOD as an index for an aerosol concentration value. Case studies concerning the comparison of the three sensors indicate that MODIS cloud detection is reduced during haze events. In addition, statistical studies show that an increase in AOD creates an increase in the percentage of uncertain flags and a decrease in hit rate, a consistency index between consecutive sets of cloud retrievals. On average, AOD values lower than 0.1 give hit rate values up to 80.0% and uncertainty values lower than 16.8%, while AOD values greater than 1.0 reduce the hit rate below to 66.6% and increase the percentage of uncertain flags up to 46.6%. Therefore, we can conclude that the ability of MODIS cloud detection is weakened by large concentrations of aerosols. This suggests that use of the MODIS cloud mask, and derived higher-level products, in situations with haze requires caution. Further improvement of this retrieval algorithm is desired as haze studies based on MODIS products are of great interest in a number of related fields.
A 14-year (1990-2003) high resolution European Cloud Climatology has been generated by use of NOAA/AVHRR data. For selected areas we present spatially averaged monthly means of total cloud cover derived from noon overpasses and compare them with surface SYNOP observations. The climatologies do not reveal a significant trend of cloud cover over the 14-year period. However, both data sets show a clear latitudinal variability and a seasonal dependence which is more pronounced in the satellite than in the SYNOP observations. Mean differences between satellite and SYNOP data range from about -2% to -10% in all seasons except summer when the mean difference is as large as -15.3%. As a special feature we notice the broad minimum of cloud cover during the extreme dry and hot summer in 2003 in Central Europe.
The Moderate Resolution Imaging Spectroradiometer (MODIS) on the NASA Earth Observing System (EOS) Terra and Aqua platforms provides unique measurements for deriving global and regional cloud properties. MODIS has spectral coverage combined with spatial resolution in key atmospheric bands, which is not available on previous imagers and sounders. This increased spectral coverage/spatial resolution, along with improved onboard calibration, enhances the capability for global cloud property retrievals. MODIS operational cloud products are derived globally at spatial resolutions of 5 km (referred to as level-2 products) and are aggregated to a 1℃ equal-angle grid (referred to as level-3 product), available for daily, 8-day, and monthly time periods. The MODIS cloud algorithm produces cloud-top pressures that are found to be within 50 hPa of lidar determinations in single-layer cloud situations. In multilayer clouds, where the upper-layer cloud is semitransparent, the MODIS cloud pressure is representative of the radiative mean between the two cloud layers. In atmospheres prone to temperature inversions, the MODIS cloud algorithm places the cloud above the inversion and hence is as much as 200 hPa off its true location. The wealth of new information available from the MODIS operational cloud products offers the promise of improved cloud climatologies. This paper 1) describes the cloud-top pressure and amount algorithm that has evolved through collection 5 as experience has been gained with in-flight data from NASA Terra and Aqua platforms; 2) compares the MODIS cloud-top pressures, converted to cloud-top heights, with similar measurements from airborne and space-based lidars; and 3) introduces global maps of MODIS and High Resolution Infrared Sounder (HIRS) cloud-top products.
The Moderate Resolution Imaging Spectroradiometer (MODIS) is one of five instruments aboard the Terra Earth Observing System (EOS) platform launched in December 1999. After achieving final orbit, MODIS began Earth observations in late February 2000 and has been acquiring data since that time. The instrument is also being flown on the Aqua spacecraft, launched in May 2002. A comprehensive set of remote sensing algorithms for cloud detection and the retrieval of cloud physical and optical properties have been developed by members of the MODIS atmosphere science team. The archived products from these algorithms have applications in climate change studies, climate modeling, numerical weather prediction, as well as fundamental atmospheric research. In addition to an extensive cloud mask, products include cloud-top properties (temperature, pressure, effective emissivity), cloud thermodynamic phase, cloud optical and microphysical parameters (optical thickness, effective particle radius, water path), as well as derived statistics. We will describe the various algorithms being used for the remote sensing of cloud properties from MODIS data with an emphasis on the pixel-level retrievals (referred to as Level-2 products), with 1-km or 5-km spatial resolution at nadir. An example of each Level-2 cloud product from a common data granule (5 min of data) off the coast of South America will be discussed. Future efforts will also be mentioned. Relevant points related to the global gridded statistics products (Level-3) are highlighted though additional details are given in an accompanying paper in this issue.
Using a method developed by Qiu (Qiu, J., 1998. A method to determine atmospheric aerosol optical depth using total direct solar radiation. J. Atmos. Sci. 55, 734–758), 0.75 μm aerosol optical depths at five meteorological observatories in north China during 1980–1994 are retrieved from global direct solar radiation, and variation characteristics of the depths and visibility are analyzed. These observatories are located in the cities of Wulumuqi, Geermu, Harbin, Beijing and Zhengzhou. It is found that during 1980–1994 the aerosol optical depths show an increasing trend at all five sites. During winter the trend is stronger. In winter at Beijing and Wulumuqi, the depth increased by a factor of about two in 15 years. Pollution caused due to the burning of fossil fuel is the main cause of the change. In spring at Geermu the depth is larger and its increase is the quickest among the four seasons, mainly due to desert dust events. The Pinatubo volcanic eruption in 1991 had a significant influence on the aerosol optical depth. The yearly averaged depths over five sites in 1992 after the eruption increased by 0.068 to 0.212, compared to those in 1990, while from 1992 to 1994 they generally show a decreasing trend. In some cities such as Zhengzhou and Geermu, both visibility and optical depth show an increasing trend during 1980–1994, a possible reason for this is that the aerosol particle vertical distribution shifts up in the troposphere. At Geermu, Harbin, Beijing and Zhengzhou, optical depths in summer are larger, which may be because of the growth of aerosol particles growing in the moist summer. Apart from Geermu, at the other four sites visibility in winter is smaller, especially at Wulumuqi and Harbin. At Harbin, visibility in summer is about twice larger than that in winter, but the difference between depths is small, implying the turbid lower troposphere in winter and the larger extinction coefficient in the upper troposphere during summer.
The study of climate and climate change is hindered by a lack of information on the effect of clouds on the radiation balance of the earth, referred to as the cloud-radiative forcing. Quantitative estimates of the global distributions of cloud-radiative forcing have been obtained from the spaceborne Earth Radiation Budget Experiment (ERBE) launched in 1984. For the April 1985 period, the global shortwave cloud forcing [-44.5 watts per square meter (W/m(2))] due to the enhancement of planetary albedo, exceeded in magnitude the longwave cloud forcing (31.3 W/m(2)) resulting from the greenhouse effect of clouds. Thus, clouds had a net cooling effect on the earth. This cooling effect is large over the mid-and high-latitude oceans, with values reaching -100 W/m(2). The monthly averaged longwave cloud forcing reached maximum values of 50 to 100 W/m(2) over the convectively disturbed regions of the tropics. However, this heating effect is nearly canceled by a correspondingly large negative shortwave cloud forcing, which indicates the delicately balanced state of the tropics. The size of the observed net cloud forcing is about four times as large as the expected value of radiative forcing from a doubling of CO(2). The shortwave and longwave components of cloud forcing are about ten times as large as those for a CO(2) doubling. Hence, small changes in the cloud-radiative forcing fields can play a significant role as a climate feedback mechanism. For example, during past glaciations a migration toward the equator of the field of strong, negative cloud-radiative forcing, in response to a similar migration of cooler waters, could have significantly amplified oceanic cooling and continental glaciation.
The Moderate Resolution Imaging Spectroradiometer (MODIS) aboard both NASA0964s Terra and Aqua satellites is making near-global daily observations of the earth in a wide spectral range (0.4109“15 0204m). These measurements are used to derive spectral aerosol optical thickness and aerosol size parameters over both land and ocean. The aerosol products available over land include aerosol optical thickness at three visible wavelengths, a measure of the fraction of aerosol optical thickness attributed to the fine mode, and several derived parameters including reflected spectral solar flux at the top of the atmosphere. Over the ocean, the aerosol optical thickness is provided in seven wavelengths from 0.47 to 2.13 0204m. In addition, quantitative aerosol size information includes effective radius of the aerosol and quantitative fraction of optical thickness attributed to the fine mode. Spectral irradiance contributed by the aerosol, mass concentration, and number of cloud condensation nuclei round out the list of available aerosol products over the ocean. The spectral optical thickness and effective radius of the aerosol over the ocean are validated by comparison with two years of Aerosol Robotic Network (AERONET) data gleaned from 132 AERONET stations. Eight thousand MODIS aerosol retrievals collocated with AERONET measurements confirm that one standard deviation of MODIS optical thickness retrievals fall within the predicted uncertainty of 02”0367 = 00±0.03 00±0.050367 over ocean and 02”0367 = 00±0.05 00± 0.150367 over land. Two hundred and seventy-one MODIS aerosol retrievals collocated with AERONET inversions at island and coastal sites suggest that one standard deviation of MODIS effective radius retrievals falls within 02”reff = 00±0.11 0204m. The accuracy of the MODIS retrievals suggests that the product can be used to help narrow the uncertainties associated with aerosol radiative forcing of global climate.
A new 8-year global cloud climatology has been produced by the International Satellite Cloud Climatology Project (ISCCP) that provides information every 3 h at 280-km spatial resolution covering the period from July 1983 through June 1991. If cloud detection errors and differences in area sampling are neglected, individual ISCCP cloud amounts agree with individual surface observations to within 15% rms with biases of only a few percent. When measurements of small-scale, broken clouds are isolated in the comparison, the rms differences between satellite and surface cloud amounts are about 25%, similar to the rms difference between ISCCP and Landsat determinations of cloud amount. For broken clouds, the average ISCCP cloud amounts are about 5% smaller than estimated by surface observers (difference between earth cover and sky cover), but about 5% larger than estimated from very high spatial resolution satellite observations (overestimate due to low spatial resolution offset by underestimate due to finite radiance thresholds). Detection errors, caused by errors in the ISCCP clear-sky radiances or incorrect radiance threshold magnitude are the dominant source of error in monthly average cloud amounts. The ISCCP cloud amounts appear to he too low over land by about 10%, somewhat less in summer and somewhat more in winter, and about right (maybe slightly low) over oceans. In polar regions, ISCCP cloud amounts are probably too low by about 15%–25% in summer and 5%–10% in winter. Comparison of the ISCCP climatology to three other cloud climatologies shows excellent agreement in the geographic distribution and seasonal variation of cloud amounts; there is little agreement about day/night contrasts in cloud amount. Notable results from ISCCP are that the global annual mean cloud amount is about 63%, being about 23% higher over oceans than over land, that it varies by <1% rms from month to month, and that it has varied by about 4% on a time wale ≈2–4 years. The magnitude of interannual variations of local (280-km scale) monthly mean cloud amounts is about 7%–9%. Longitudinal contrasts in cloud amount are just as large as latitudinal contrasts. The largest seasonal variation of cloud amount occurs in the tropics, being larger in summer than in winter; the seasonal variation in middle latitudes has the opposite phase. Polar regions may have little seasonal variability in cloud amount. The ISCCP results show slightly more nighttime than daytime cloud amount over oceans and more daytime than nighttime cloud amount over land.
The Moderate Resolution Imaging Spectroradiometer (MODIS) standard cloud product is prone to misidentifying areas that are heavily polluted with aerosols as cloudy regions over the North China Plain (NCP) and to retrieving aerosol characteristics as cloud parameters. Based on the differences in physical and optical properties between aerosols and clouds, we propose a new approach to distinguish aerosol-laden areas from cloudy regions using MODIS level 2 cloud properties (e.g., cloud fraction, cloud phase, and cloud top pressure products). The approach was applied to 22 haze-fog cases that occurred in the 2011 and 2012 winters over the NCP. The aerosol identification results were then compared with MODIS-flagged aerosol areas, which were inferred from the noncloud obstruction flag and the suspended dust flag in the MODIS cloud mask product. The results indicated that approximately 60% of the MODIS-flagged aerosol areas were correctly identified using our approach. Among the analyzed cases, two cases exhibited substantial differences; the aerosol areas detected using the newly proposed method were approximately 2.5 times larger than that of the MODIS-flagged area. Further comparisons with aerosol distributions along the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) orbit for the two cases demonstrated that approximately 60%-80% of the CALIOP observed aerosols were identified using our method, while less than 10% of the CALIOP observed aerosols were consistent with the MODIS flagging.
Two model experiments, namely a control (CTL) experiment without aerosol–radiation feedbacks and a experiment with online aerosol–radiation (RAD) interactions, were designed to study the radiative feedback on regional radiation budgets, planetary boundary layer (PBL) meteorology and haze formation due to aerosols during haze episodes over Jing–Jin–Ji, China, and its near surroundings (3JNS region of China: Beijing, Tianjin, Hebei, East Shanxi, West Shandong and North Henan) with a two-way atmospheric chemical transport model. The impact of aerosols on solar radiation reaching Earth's surface, outgoing long-wave emission at the top of the atmosphere, air temperature, PBL turbulence diffusion, PBL height, wind speeds, air pressure pattern and PM2.5 has been studied focusing on a haze episode during the period from 7 to 11 July 2008. The results show that the mean solar radiation flux that reaches the ground decreases by about 15% in 3JNS and 20 to 25%in the region with the highest aerosol optical depth during the haze episode. The fact that aerosol cools the PBL atmosphere but warms the atmosphere above it leads to a more stable atmospheric stratification over the region, which causes a decrease in turbulence diffusion of about 52% and a decrease in the PBL height of about 33%. This consequently forms a positive feedback on the particle concentration within the PBL and the surface as well as the haze formation. Additionally, aerosol direct radiative forcing (DRF) increases PBL wind speed by about 9% and weakens the subtropical high by about 14 hPa, which aids the collapse of haze pollution and results in a negative feedback to the haze episode. The synthetic impacts from the two opposite feedbacks result in about a 14% increase in surface PM2.5. However, the persistence time of both high PM2.5 and haze pollution is not affected by the aerosol DRF. On the contrary over offshore China, aerosols heat the PBL atmosphere and cause unstable atmospheric stratification, but the impact and its feedback on the planetary boundary layer height, turbulence diffusion and wind is weak, with the exception of the evident impacts on the subtropical high.
The ultimate goal of climate research is to produce climate predictions on various time scales. In China, efforts to predict the climate started in the 1930 s. Experimental operational climate forecasts have been performed since the late 1950 s,based on historical analog circulation patterns. However, due to the inherent complexity of climate variability, the forecasts produced at that time were fairly inaccurate. Only from the late 1980 s has seasonal climate prediction experienced substantial progress, when the Tropical Ocean and Global Atmosphere project of the World Climate Research program(WCRP) was launched. This paper, following a brief description of the history of seasonal climate prediction research, provides an overview of these studies in China. Processes and factors associated with the climate variability and predictability are discussed based on the literature published by Chinese scientists. These studies in China mirror aspects of the climate research effort made in other parts of the world over the past several decades, and are particularly associated with monsoon research in East Asia. As the climate warms, climate extremes, their frequency, and intensity are projected to change, with a large possibility that they will increase. Thus, seasonal climate prediction is even more important for China in order to effectively mitigate disasters produced by climate extremes, such as frequent floods, droughts, and the heavy frozen rain events of South China.
In the middle latitudes of both hemispheres, seasonal anomalies of cloud cover are positively correlated with surface temperature in winter and negatively correlated in summer, as expected if the direction of causality is from clouds to temperature.
This study investigates dimming and brightening of surface solar radiation (SSR) during 1961–2005 in China as well as its relationships to total cloud cover (TCC). This is inferred from daily ground-based observational records at 45 pyranometer stations. A statistical method is introduced to study contributions of changes in the frequency of TCC categories and their atmospheric transparency to the secular SSR trend. The surface records suggest a renewed dimming beyond 2000 in North China after the stabilization in the 1990s; however, a slight brightening appears beyond 2000 in South China. Inter-annual variability of SSR is negatively correlated with that of TCC, but there is a positive correlation between decadal variability of SSR and TCC in most cases. The dimming during 1961–1990 is exclusively attributable to decreased atmospheric transparency, a portion of which is offset by TCC frequency changes in Northeast and Southwest China. The dimming during 1961–1990 in Northwest and Southeast China primarily results from decreased atmospheric transparency under all sky conditions and the percentage of dimming stemming from TCC frequency changes is 11% in Northwest and 2% in Southeast China. Decreased atmospheric transparencies during 1991–2005 in North China in most cases lead to the dimming. TCC frequency changes also contribute to the dimming during this period in North China. This feature is more pronounced in summer and winter when TCC frequency changes can account for more than 80% of dimming. In South China, increased atmospheric transparencies lead to the brightening during 1991–2005. A substantial contribution by TCC frequency changes to the brightening is also evident in spring and autumn.