Clouds are critical to the global radiation budget and hydrological cycle, but knowledge is still poor concerning the observed climatology of cloud-base height (CBH) in China. Based on fine-resolution sounding observations from the China Radiosonde Network (CRN), the method used to estimate CBH was modified, and uncertainty analyses indicated that the CBH is good enough. The accuracy of CBH estimation is verified by the comparison between the sounding-derived CBHs and those estimated from the micro-pulse lidar and millimeter-wave cloud radar. As such, the CBH climatology was compiled for the period 2006-16. Overall, the CBH exhibits large geographic variability across China, at both 0800 Local Standard Time (LST) and 2000 LST, irrespective of season. In addition, the summertime cloud base tends to be elevated to higher altitudes in dry regions [i.e., Inner Mongolia and the North China Plain (NCP)]. By comparison, the Tibetan Plateau (TP), Pearl River Delta (PRD) and Sichuan Basin (SCB) have relatively low CBHs (
Cloud plays a significant role in the Earth's energy budget by reflecting and absorbing incoming solar radiation and reducing outgoing thermal radiation (Stephens et al., 2012; Zhou et al., 2016). The impacts of cloud on the radiation balance of the Earth-atmosphere system depend not only on the vertical structure and distribution of the cloud but also on its base and top heights and optical properties (Wild, 2012; Zhang et al., 2017a). Cloud radiative forcings, especially those induced by aerosol-cloud interaction (Wang et al., 2015; Guo et al., 2016a, 2017), further contribute to uncertainties in weather forecasting and climate prediction (Li et al., 2016). Cloud properties have changed and will continue to change in a warming climate, including changes in cloud height, cloud cover, and morphology (Eastman and Warren, 2013). For decades, both observations and models have been extensively used to elucidate the characteristics of cloud and their corresponding changes on the daily, seasonal and yearly time scales (Klein and Hartmann, 1993; Baker and Peter, 2008; Clement et al., 2009; Davies and Molloy, 2012; Shonk et al., 2012; Zelinka et al., 2013; Myers and Norris, 2015; Miao et al., 2017).
The properties of cloud are generally associated with cloud types, and the climate effects caused by various cloud types differ greatly; in some circumstances they are even totally opposite. For instance, high cloud has a warming effect on the surface, as opposed to the cooling effect of low-level clouds (IPCC, 2013). Unfortunately, cloud profiles are poorly understood at present and remain a primary source of uncertainty in global weather and climate studies (Stephens, 2005).
The retrieval of cloud profiles relies primarily on satellite- (e.g., Minnis and Harrison, 1984; Morel and Senesi, 2002) and surface-based observations (e.g., Dong et al., 2006). The advent of spaceborne active cloud radar (e.g., CloudSat) has allowed for a better portrayal of cloud vertical structure on both regional and global scales, making it one of the most popular data sources for cloud studies (Stephens et al., 2002; Sassen and Wang, 2008; Naud et al., 2010; Evan and Norris, 2012; Chen et al., 2016).
Cloud-base height (CBH) is an important cloud macrophysical parameter, strongly affecting the energy exchanges between the cloud layer and the ground surface (Wild, 2012). Using the cloud profiles and CBH data determined from the active-sounding measurements of CloudSat and CALIPSO has helped resolve the long-standing debate over the underestimation of thermal back-radiation, which has been updated from the previous estimate of 324 W m
In comparison to satellite measurements, ground-based cloud observations based on instruments such as cloud radars, lidars (e.g., Borg et al., 2011) and ceilometers (e.g., Martucci et al., 2010; Costa-Surós et al., 2009), can provide CBH measurements with higher accuracy and continuous temporal coverage (Sharma et al., 2016). In North America, an automated observation system employs ceilometers to observe cloud. However, these ceilometers observe cloud at a cutoff range, generally lower than 4 km (Dai et al., 2006). In other parts of the world, these instruments are generally deployed at very few locations. Fortunately, radiosondes can also penetrate cloud layers to provide in-situ measurements such as temperature, relative humidity and pressure, which are fundamental to estimating the profiles of atmospheric and cloud properties (Poore et al., 1995; Zhang et al., 2010). More importantly, operational radiosonde networks make it possible to retrieve the CBH over a large scale. For example, (Wang and Rossow, 1995) successfully obtained the cloud vertical structure based on relative humidity (RH) sounding data alone, and (Chernykh et al., 2001) analyzed worldwide long-term cloud-base and -top height trends based on RH and temperature sounding data.
Warren et al. (1986, 1988) produced a database from surface land stations and ship measurements and compiled a climatology of CBH over land and ocean by focusing on different cloud types (Warren et al., 2007; Eastman et al., 2011; Eastman and Warren, 2014). (Sun et al., 2007) comprehensively analyzed the temporal trend of cloud-top heights and frequencies over the United States since the early 1950s, based mainly on human observations at weather stations. (An et al., 2017) interpreted the diurnal, seasonal and interannual variability of the CBH over the contiguous United States based on an automated surface observing system.
However, notwithstanding a few CBH studies (e.g., Zhang et al., 2010) that were limited to a specific region of interest in China, no attempt (to the best of our knowledge) has been made to examine the climatology of CBH across China from long-term sounding observations from the China Radiosonde Network (CRN; Guo et al., 2016b). Accordingly, the main goals of the present study were to estimate the CBH with a robust method, and then elucidate the spatial and temporal characteristics of the CBH across China based on long-term soundings from the CRN. The remainder of the paper is organized as follows: Section 2 describes the data and the methods employed in this study. The uncertainty of the CBH estimation method is analyzed and discussed in section 3, followed in the same section by an analysis of the climatology, diurnal variation and seasonal differences of the CBH across China. The main conclusions are summarized in section 4.
The operational CRN consists of 120 sites equipped with L-band sounding systems, including GFE (L) 1 secondary wind radar and GTS1 digital electronic radiosondes (Guo et al., 2016b). The new-generation GTS1 digital radiosonde takes measurements of temperature, pressure, RH, wind speed, and wind direction twice a day [at 0800 Local Standard Time (LST, UTC+8) and 2000 LST], with a sampling frequency of once every 1.2 seconds. The vertical resolution varies from site to site, and from sounding to sounding at the same radiosonde site. A previous intercomparison of GTS1 against Vaisala RS80 indicated that they are in good agreement in terms of their profile measurements in the troposphere, despite a large bias in the upper atmospheric levels (Bian et al., 2011).
The setup of CRN dates back to 2002 when the China Meteorological Administration decided to upgrade existing radiosonde instruments; however, the GTS1 radiosonde at 14 sites only officially came into operation in 2006, as illustrated in Fig. 1. The L-band sounding systems with GTS1 radiosondes began to be widely deployed in 2007 across China, and had gradually expanded to 120 operational radiosonde stations by 2011. As such, each site of the CRN with GTS1 radiosonde data differs in their period of coverage (Fig. 1b). Overall, roughly 60% (97%) of radiosonde sites have more than 10 (6) years of L-band sounding data, which provides an even distribution across China and sufficient samples to characterize the climatology of the CBH from sounding data.
Therefore, the sounding data collected from the CRN during the period 1 December 2006 to 31 December 2016 were used to estimate the CBH across China. In summer (June-July-August), two additional soundings were launched (at 0200 LST and 1400 LST) to improve the predictability of high-impact weather at selected sites of the CRN, depending on the locations of large-scale synoptic weather systems. In this way, we in total made use of 421 729 profiles across China, including 1979 profiles at 0200 LST, 208 445 profiles at 0800 LST, 9726 profiles at 1400 LST, and 201 579 profiles at 2000 LST.
Cloud generally forms at an RH of about 100%. It is, however, rarely observed in radiosonde observations for the reasons listed in (Wang and Rossow, 1995). In particular, the thermal lag tends to result in lower-than-normal RH values by about 3%, which is mainly caused by the temperature in the hygristor element being roughly 1
In our analysis, the method proposed by (Wang and Rossow, 1995) was modified to better detect the cloud base, given the high temporal vertical resolution (1.2 s). First, the soundings under rainy conditions (rainfall amount
To understand the frequency distribution of cloud with various CBH, the sounding-derived CBH retrievals were grouped into three height intervals: 0.6-2 km; 2-3.6 km; and above 3.6 km, with the 0.6-2-km category representing low cloud (Sun et al., 2007). Due to the spatial inhomogeneity, all sites were interpolated onto a regular
The estimated CBH depends on the RH threshold values, which inevitably induces uncertainty in the CBH climatology. Uncertainty analysis, therefore, is imperative, and this is achieved by investigating the magnitude of changes in the CBH due to the selection of various critical RH thresholds.
Accordingly, scatterplots were produced that compare the CBH calculated using 84% and 83% as the critical RH versus those calculated using 84% and 85%. As expected, Figs. 2a and b show that the CBH based on RH = 85% (83%) is typically higher (lower) than that based on RH = 84%. However, the CBHs calculated using various RH thresholds are significantly correlated (
But what about the accuracy of the CBH estimated from the chosen RH threshold? Based on a field campaign carried out by our group last summer in Xingtai (37.05
Figure 3 presents the geographic distribution of CBH derived from the 0800 LST radiosonde observations for the whole period from 2006 to 2016. Clearly, the CBH at 0800 LST does not exhibit strong latitudinal dependence. However, some strong spatial and seasonal variation stands out. On average, the CBH reaches its peak (
Spatially, most sites in southwestern China, including the Tibetan Plateau (TP), Pearl River Delta (PRD) and Sichuan Basin (SCB), have a relatively low CBH (
Similar to 0800 LST, the spatial variation of CBH at 2000 LST also does not exhibit distinct latitudinal dependence. Moreover, the spatial distributions of the 2000 LST CBH bear resemblance to those of the 0800 LST CBH, and the magnitude of the mean CBH in the four seasons changes little (Fig. 4). In terms of CBH seasonality, the springtime mean CBH is 2.86 km, which is slightly lower than that in summertime (2.92 km), and most radiosonde sites in winter have the lowest CBH. More specifically, the PRD has the lowest CBH in winter, and the CBH in NCP is highest in spring. The CBH diurnal and seasonal features in China are generally consistent with those obtained in the contiguous United States (An et al., 2017), which means that the geographic distribution of CBH in different seasons and at different times of the day have similar patterns.
Figure 5 shows the spatial distributions of CBH frequency for cloud bases within various altitudes at 0800 LST and 2000 LST during the period 2006-16. One striking feature is that the TP, PRD, and SCB are characterized by a high frequency (
To improve the forecasting accuracy of high-impact stormy weather in summer, atmospheric profile measurements at 0200 LST and 1400 LST are required at most sites of the CRN, in addition to the usual operational soundings at 0800 LST and 2000 LST. Figure 6 presents frequency histograms for the CBHs at 0200 LST, 0800 LST, 1400 LST, and 2000 LST during the period 2006-16 for all radiosonde sites in China. As also can be seen, 50% of valid CBH retrievals are less than 2.4 km AGL, which is slightly higher than 2.0 km for the CBHs at 2000 LST. On average, the frequency distribution of CBHs at 0200 LST is quite similar to that at 1400 LST. That is, about half of the CBHs are less than 1.6 km AGL.
The additional one or two soundings (i.e., 0200 LST and 1400 LST) at some sites of the CRN provide us with a unique opportunity to investigate the diurnal cycle of CBH across China from a radiosonde perspective. Figure 7 shows the spatial distributions of average CBH at 0200 LST, 0800 LST, 1400 LST and 2000 LST for the period 2006-16. Overall, the average CBH at 0200 LST and 1400 LST is slightly lower that that at 0800 LST and 2000 LST. Our understanding is that the soundings at 0200 LST and 1400 LST are mainly launched under atmospheric conditions favorable for the initiation and development of precipitation, which will inevitably lead to the more frequent occurrence of low precipitating cloud. This at least accounts for the lower CBH observed at 1400 LST despite the stronger solar-radiation induced convection in summer.
Interestingly, we can still see a distinct spatial discrepancy in the CBH at 1400 LST, which is the same as that obtained at 0800 LST and 2000 LST. For instance, most of the sites on the NCP, and in Xinjiang, are characterized by high CBH, which is generally greater than 3.0 km AGL. The lowest CBH at 1400 LST tends to occur in the PRD, which is lower than 1.2 km AGL.
As for the annual average CBH at 0800 LST, the maximum CBH tends to occur on the NCP and in the YRD, as opposed to the minimum CBH seen on the TP. In terms of the geographic distribution of the annual average CBH at 2000 LST, the contrast does not change too much.
Accounting for the large longitudinal difference (63
Taking advantage of extensive high-resolution radiosonde measurements acquired from 120 sites of the CRN during 2006-16, we produced a climatology of CBH in China by applying an RH threshold method to 421 729 individual radiosonde atmospheric profiles. The statistics regarding the CBH at diurnal and seasonal time scales across China have been presented. To the best of our knowledge, the climatology of CBH produced in this study is the first to elucidate the spatial and temporal distribution of CBH in China from a large-scale sounding perspective.
Overall, CBH does not show any distinct latitudinal dependence; rather, it exhibits large spatial variability across China, at both 0800 LST and 2000 LST, for all seasons. In particular, the highest cloud tends to occur in relatively dry regions, such as Inner Mongolia, Xinjiang, and the NCP. In contrast, the TP, YRD and SCB have relatively low CBH (
This study is just the first step in an attempt to fully understand the spatial and temporal distribution of CBH in China, which will provide a viable approach to obtain observational cloud-base information. Such information will in turn assist in better quantifying the cloud response to the increased aerosol pollution in recent years (Guo et al., 2011). However, the factors accounting for such a large spatial discrepancy have not been fully analyzed, which merits further detailed investigation in the future. In addition, more insight should be gained into the true diurnal cycle of CBH if more routine 1400 LST sounding observations become available.
Acknowledgements. This work was carried out under the auspices of the Megacities Experiment on Integrated Meteorological Observations in China initiated by the China Meteorological Administration (CMA), the Ministry of Science and Technology of China (Grant Nos. 2017YFC1501701, 2017YFC1501401, 2017YFA0603501 and 2016YFA0600403), the National Natural Science Foundation of China (Grant Nos. 91544217, 41771399 and 41471301), the Chinese Academy of Meteorological Sciences (Grant Nos. 2017Z005 and 2017R001), and the Fundamental Research Funds for the Central Universities (Grant No. 2017STUD17). The authors would like to acknowledge the CMA for providing the radiosonde data used in this study. Last but not least, we thank the anonymous reviewers for their constructive suggestions and comments, which helped improve this manuscript.
Electronic supplementary material: Supplementary material is available in the online version of this article at https://doi.org/10.1007/s00376-017-7096-0.
Clouds constitute the largest single source of uncertainty in climate prediction. A better understanding of small-scale cloud processes could shed light on the role of clouds in the climate system.
GTS1 digital radiosonde, developed by the Shanghai Changwang Meteorological Science and Technology Company in 1998, is now widely used in operational radiosonde stations in China. A preliminary comparison of simultaneous humidity measurements by the GTS1 radiosonde, the Vaisala RS80 radiosonde, and the Cryogenic Frostpoint Hygrometer (CFH), launched at Kunming in August 2009, reveals a large dry bias produced by the GTS1 humidity sensor. The average relative dry bias is in the order of 10% below 500 hPa, increasing rapidly to 30% above 500 hPa, and up to 55% at 310 hPa. A much larger dry bias is observed in the daytime, and this daytime effect increases with altitude. The GTS1 radiosonde fails to respond to humidity changes in the upper troposphere, and sometimes even in the middle troposphere. The failure of GTS1 in the middle and upper troposphere will result in significant artificial humidity shifts in radiosonde climate records at stations in China where a transition from mechanical to digital radiosondes has occurred. A comparison of simultaneous temperature observations by the GTS1 radiosonde and the Vaisala RS80 radiosonde suggests that these two radiosondes provide highly reproducible temperature measurements in the troposphere, but produce opposite biases for daytime and nighttime measurements in the stratosphere. In the stratosphere, the GTS1 shows a warm bias (0.5 K) in the daytime and a relatively large cool bias (-0.2 K to -1.6 K) at nighttime.
The sensitivity of the millimeter cloud radar (MMCR) to optically thin single-layer cirrus at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site is investigated using collocated Raman lidar observations. The sensitivity is characterized in terms of cloud optical depth (OD) and infrared (IR) radiative flux using over three years of coincident Raman lidar and MMCR observations. For cases when the Raman lidar is not fully attenuated (OD < 2.0) the MMCR detects approximately 70% of the total cloud OD with the majority of missed cloud OD occurring near cloud top. If only MMCR observations are used for computing cloudy top-of-the-atmosphere (TOA) IR flux, the missed cloud OD results in TOA flux biases from 0 to over 100 W/m; however, the most frequently occurring bias is approximately 16 W/m. This result highlights the importance of combining Raman lidar, or other sensitive cloud lidars that are able to measure cloud extinction directly, with the MMCR in order to accurately characterize the cloud radiative forcing for thin cirrus cases.
Abstract Many efforts have been taken to investigate aerosol-cloud interactions from space, but only a few studies have examined the response of vertical cloud structure to aerosol perturbations. Three-dimensional cloud climatologies of eight different cloud types identified from the CloudSat level-2 cloud product during the warm season (May-September) in 2008-10 over eastern China were first generated and analyzed. Using visibility as a proxy for cloud condensation nuclei, in combination with satellite-observed radar reflectivity, normalized contoured frequency by altitude diagrams of the differences in cloud radar reflectivity Z profiles under polluted and clean conditions were constructed. For shallow cumulus clouds (shallow Cu) Z tends to be inhibited, and it is enhanced in the upper layers for deep cumulus (deep Cu), nimbostratus (Ns), and deep convective clouds (DCC) under polluted conditions. Overall, analyses of the modified center of gravity (MCOG) and cloud-top height (CTH) also point to a similar aerosol effect, except for the nonsignificant changes in MCOGs and CTHs in deep Cu. The impacts of environmental factors such as lower-tropospheric stability and vertical velocity are also discussed for these types of clouds. Although consistent aerosol-induced elevations in MCOGs and CTHs for Ns and DCC clouds are observed, the effect of meteorology cannot be completely ruled out, which merits further analysis.
Abstract Clouds are important to climate and climate trends. To determine trends in cloud–base heights and cloud–top heights, the Comprehensive Aerological Reference Data Set (CARDS) and the method of Chernykh and Eskridge are used to diagnose cloud base, top, and amount. Trends in time series of cloud bases and tops at 795 radiosonde stations from 1964 to 1998 are presented. It was found that trends in cloud–base height and cloud–top height are seasonally dependent and a function of cloud cover amount. There was a small increase in multilayer cloudiness in all seasons. Geographical distributions of decadal changes of cloud bases and tops were spatially nonuniform and depended upon the season. To estimate the errors made in calculating the heights of cloud boundaries, an analysis was made of the response of the thermistors and hygristors. Thermistors and hygristors are linear sensors of the first order. From this it is shown that the distance between calculated inflection points (cloud boundaries) of obse...
Abstract Feedbacks involving low-level clouds remain a primary cause of uncertainty in global climate model projections. This issue was addressed by examining changes in low-level clouds over the Northeast Pacific in observations and climate models. Decadal fluctuations were identified in multiple, independent cloud data sets, and changes in cloud cover appeared to be linked to changes in both local temperature structure and large-scale circulation. This observational analysis further indicated that clouds act as a positive feedback in this region on decadal time scales. The observed relationships between cloud cover and regional meteorological conditions provide a more complete way of testing the realism of the cloud simulation in current-generation climate models. The only model that passed this test simulated a reduction in cloud cover over much of the Pacific when greenhouse gases were increased, providing modeling evidence for a positive low-level cloud feedback.
61Ceilometers allow retrieving long-term cloud base height distributions.61Cloud base height has a remarkable seasonal evolution at the analyzed site.61Multilevel or multilayered cloud systems are hardly detected by ceilometers.
Self-consistent stereo measurements by the Multiangle Imaging SpectroRadiometer (MISR) on the Terra satellite yield a decrease in global effective cloud height over the decade from March 2000 to February 2010. The linear trend is -44 ± 22 m/decade and the interannual annual difference is -31 ± 11 m between the first and last years of the decade. The annual mean height is measured with a sampling error of 8 m, which is less than the observed interannual fluctuation in global cloud height for most years. A maximum departure from the 10-year mean, of -80 ± 8 m, is observed towards the end of 2007. These height anomalies correlate well with the changes in the Southern Oscillation Index, with the effective height increasing over Indonesia and decreasing over the Central Pacific during the La Ni09a phase of the oscillation. After examining the net influence of Central Pacific/Indonesia heights on the global mean anomaly, we conclude that the integrated effects from outside these regions dominate the global mean height anomalies, confirming the existence of significant teleconnections.
Data collected at the Department of Energy Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) Central Facility (SCF) are analyzed to determine the monthly and hourly variations of cloud fraction and radiative forcing between January 1997 and December 2002. Cloud fractions are estimated for total cloud cover and for single-layered low (009“3 km), middle (309“6 km), and high clouds (>6 km) using ARM SCF ground-based paired lidar09“radar measurements. Shortwave (SW) and longwave (LW) fluxes are derived from up- and down-looking standard precision spectral pyranometers and precision infrared radiometer measurements with uncertainties of 09030410 W m-2. The annual averages of total and single-layered low-, middle-, and high-cloud fractions are 0.49, 0.11, 0.03, and 0.17, respectively. Both total- and low-cloud amounts peak during January and February and reach a minimum during July and August; high clouds occur more frequently than other types of clouds with a peak in summer. The average annual downwelling surface SW fluxes for total and low clouds (151 and 138 W m-2, respectively) are less than those under middle and high clouds (188 and 201 W m-2, respectively), but the downwelling LW fluxes (349 and 356 W m-2) underneath total and low clouds are greater than those from middle and high clouds (337 and 333 W m-2). Low clouds produce the largest LW warming (55 W m-2) and SW cooling (-91 W m-2) effects with maximum and minimum absolute values in spring and summer, respectively. High clouds have the smallest LW warming (17 W m-2) and SW cooling (-37 W m-2) effects at the surface. All-sky SW cloud radiative forcing (CRF) decreases and LW CRF increases with increasing cloud fraction with mean slopes of -0.984 and 0.616 W m-2 %-1, respectively. Over the entire diurnal cycle, clouds deplete the amount of surface insolation more than they add to the downwelling LW flux. The calculated CRFs do not appear to be significantly affected by uncertainties in data sampling and clear-sky screening. Traditionally, cloud radiative forcing includes not only the radiative impact of the hydrometeors, but also the changes in the environment. Taken together over the ARM SCF, changes in humidity and surface albedo between clear and cloudy conditions offset 09030420% of the NET radiative forcing caused by the cloud hydrometeors alone. Variations in water vapor, on average, account for 10% and 83% of the SW and LW CRFs, respectively, in total cloud cover conditions. The error analysis further reveals that the cloud hydrometeors dominate the SW CRF, while water vapor changes are most important for LW flux changes in cloudy skies. Similar studies over other locales are encouraged where water and surface albedo changes from clear to cloudy conditions may be much different than observed over the ARM SCF.
Abstract A worldwide climatology of the diurnal cycles of low clouds is obtained from surface observations made eight or four times daily at 3- or 6-h intervals from weather stations and ships. Harmonic fits to the daily cycle are made for 5388 weather stations with long periods of record, and for gridded data on a 5 degrees x 5 degrees or 10 degrees x 10 degrees latitude-longitude grid over land and ocean areas separately. For all cloud types, the diurnal cycle has larger amplitude over land than over ocean, on average by a factor of 2. Diurnal cycles of cloud amount appear to be proprietary to each low cloud type, showing the same cycle regardless of whether that type dominates the diurnal cycle of cloud cover. Stratiform cloud amounts tend to peak near sunrise, while cumuliform amounts peak in the afternoon; however, cumulonimbus amounts peak in the early morning over the ocean. Small latitudinal and seasonal variation is apparent in the phase and amplitude of the diurnal cycles of each type. Land areas show more seasonality compared to ocean areas with respect to which type dominates the diurnal cycle. Multidecadal trends in low cloud cover are small and agree between day and night regardless of the local climate. Diurnal cycles of base height are much larger over land than over the ocean. For most cloud types, the bases are highest in the midafternoon or early evening.
Synoptic weather observations from ships throughout the World Ocean have been analyzed to produce a climatology of total cloud cover and the amounts of nine cloud types. About 54 million observations contributed to the climatology, which now covers 55 years from 1954 to 2008. In this work, interannual variations of seasonal cloud amounts are analyzed in 10脗℃ grid boxes. Long-term variations O(5-10 yr), coherent across multiple latitude bands, remain present in the updated cloud data. A comparison to coincident data on islands indicates that the coherent variations are probably spurious. An exact cause for this behavior remains elusive. The globally coherent variations are removed from the gridbox time series using a Butterworth filter before further analysis. Before removing the spurious variation, the global average time series of total cloud cover over the ocean shows low-amplitude, long-term variations O(2%%) over the 55-yr span. High-frequency, year-to-year variation is seen O(1%%-2%%). Among the cloud types, the most widespread and consistent relationship is found for the extensive marine stratus and stratocumulus clouds (MSC) over the eastern parts of the subtropical oceans. Substantiating and expanding upon previous work, strong negative correlation is found between MSC and sea surface temperature (SST) in the eastern North Pacific, eastern South Pacific, eastern South Atlantic, eastern North Atlantic, and the Indian Ocean west of Australia. By contrast, a positive correlation between cloud cover and SST is seen in the central Pacific. High clouds show a consistent low-magnitude positive correlation with SST over the equatorial ocean. In regions of persistent MSC, time series show decreasing MSC amount. This decrease could be due to further spurious variation within the data. However, the decrease combined with observed increases in SST and the negative correlation between marine stratus and sea surface temperature suggests a positive cloud feedback to the warming sea surface. The observed decrease of MSC has been partly but not completely offset by increasing cumuliform clouds in these regions; a similar decrease in stratiform and increase in cumuliform clouds had previously been seen over land. Interannual variations of cloud cover in the tropics show strong correlation with an ENSO index.
Measurements by the Multiangle Imaging SpectroRadiometer (MISR) instrument exhibit a decreasing trend in global mean effective cloud top height (2000-2011). Here we show that this trend is likely related to an artifact in the data present during the early years of the MISR mission that caused a systematic reduction in the number of retrievals of clouds at lower elevations relative to clouds at higher elevations. After the application of a post-hoc method for removing the bias associated with missing retrievals the MISR effective cloud height anomalies exhibit a positive trend over time.
This paper analyzes TOMS AOD at 50002nm (1980–2001), along with MODIS data (2000–2008) at 55002nm to investigate variations at one-degree grid over eight typical regions in China and the trends in AODs, temporally and spatially. In contrast to recently reported global decrease in AOD over global ocean beginning around 1990, we find there virtually exists no apparent AOD transition in China for that: firstly no notable upward tendencies in AOD during 1980–1992 for the relative low value (+0.001/decade), then during 1996–2001 a discernible ascending tendency with larger magnitude at 0.01/decade, and finally, since 2000, a weak upward trend with02+0.004/decade. The large increases during 1996–2001 are presumably consequences of large increases in industrial activities and bear a strong resemblance to the long-term decreasing observations of incident solar radiation and cloud cover in China. Specifically, in late 1990’s, only in Taklimakan Desert a negative trend with a maximum magnitude of02610.04/decade is detected. However, over regions such as Jingjinji and Pearl River Delta influenced by industrial activities, positive tendencies at02+0.01/decade are observed. Seasonal patterns in the AOD regional long-term trend are evident. AODs exhibit generally similar seasonality and the summer dominates higher AOD value than the autumn. In particular, during the period 1980–2001, all the eight regions except Taklimakan Desert witness the maximum aerosols in winter while there is not such seasonality during the period 2000–2008. Geographically, we also document spatial patterns of AOD variations over China. Results reveal that no apparent upward trends in AOD (about 15% per decade) are observed in 1980’s, while beginning 1990 till 2008, both data (TOMS and MODIS) are indicative of a significant AOD increase across China, especially in 1990’s it is indeed the case, roughly in accordance with the overall trends at regional scale.
The radiative and microphysical effects of aerosols can affect the development of convective clouds. The objective of this study is to reveal if the overall aerosol effects have any discernible impact on the diurnal variations in precipitation and lightning by means of both observational analysis and modeling. As the first part of two companion studies, this paper is concerned with analyzing hourly PM, precipitation, and lightning data collected during the summers of 2008-2012 in the Pearl River Delta region. Daily PMdata were categorized as clean, medium, or polluted so that any differences in the diurnal variations in precipitation and lightning could be examined. Heavy precipitation and lightning were found to occur more frequently later in the day under polluted conditions than under clean conditions. Analyses of the diurnal variations in several meteorological factors such as air temperature, vertical velocity, and wind speed were also performed. They suggest that the influence of aerosol radiative and microphysical effects serve to suppress and enhance convective activities, respectively. Under heavy pollution conditions, the reduction in solar radiation reaching the surface delays the occurrence of strong convection and postpones heavy precipitation to late in the day when the aerosol invigoration effect more likely comes into play. Although the effect of aerosol particles can be discernible on the heavy precipitation through the daytime, the influence of concurrent atmospheric dynamics and thermodynamics cannot be ruled out.
Abstract The important roles of planetary boundary layer (PBL) in climate, weather and air quality have long been recognized, but little has been known about the PBL climatology in China. Using the fine-resolution sounding observations made across China and a reanalysis data, we conducted a comprehensive investigation of the PBL in China from January 2011 to July 2015. The boundary layer height (BLH) is found to be generally higher in spring and summer than that in fall and winter. The comparison of seasonally averaged BLH derived from observations and reanalysis shows good agreement. The BLH derived from three- or four-times-daily soundings in summer tends to peak in the early afternoon, and the diurnal amplitude of BLH is higher in the northern and western sub-regions of China than other sub-regions. The meteorological influence on the annual cycle of BLH are investigated as well, showing that BLH at most sounding sites is negatively associated with the surface pressure and lower tropospheric stability, but positively associated with the near-surface wind speed and temperature. This indicates that meteorology plays a significant role in the PBL processes. Overall, the key findings obtained from this study lay a solid foundation for us to gain a deep insight into the fundamentals of PBL in China, which helps understand the roles of PBL playing in the air pollution, weather and climate of China.
Abstract Summer precipitation plays critical roles in the energy balance and the availability of fresh water over eastern China. However, little is known regarding the trend in local-scale precipitation (LSP). Here, we developed a novel method to determine LSP events in the summer afternoon throughout eastern China from 1970 to 2010 based on hourly gauge measurements. The LSP occurrence hours decrease at an annual rate of 0.25%, which varies considerably by region, ranging from 0.14% over the Yangtze River Delta to 0.56% over the Pearl River Delta. This declining frequency of LSP is generally accompanied by an increase in rain rate of LSP but a decrease in visibility, whose linkage to LSP events was investigated. In particular, more LSP events tended to form when the atmosphere was slightly polluted. Afterwards, LSP was suppressed. These findings have important implications for improving our understanding of the climatology of daytime precipitation at local scales.
CALIPSO CloudSat increases surface downward LW to 345Wm-2 Multi-layer clouds provide better agreement with CERES Surface radiation and other surface flux uncertainties overlap One year of instantaneous top-of-atmosphere (TOA) and surface shortwave and longwave irradiances are computed using cloud and aerosol properties derived from instruments on the A-Train Constellation: the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite, the CloudSat Cloud Profiling Radar (CPR), and the Aqua Moderate Resolution Imaging Spectrometer (MODIS). When modeled irradiances are compared with those computed with cloud properties derived from MODIS radiances by a Clouds and the Earth's Radiant Energy System
Abstract The seasonal cycle of low stratiform clouds is studied using data from surface-based cloud climatologies. The impact of low clouds on the radiation budget is illustrated by comparison of data from the Earth Radiation Budget Experiment with the cloud climatologies. Ten regions of active stratocumulus convection are identified. These regions fall into four categories: subtropical marine, midlatitude marine, Arctic stratus, and Chinese stratus. With the exception of the Chinese region, all the regions with high amounts of stratus clouds are over the oceans. In all regions except the Arctic, the season of maximum stratus corresponds to the season of greatest lower-troposphere static stability. Interannual variations in stratus cloud amount also are related to changes in static stability. A linear analysis indicates that a 6% increase in stratus fractional area coverage is associated with each 1℃C increase in static stability. Over midlatitude oceans, sky-obscuring fog is a large component of the summ...
The increasing severity of droughts/floods and worsening air quality from increasing aerosols in Asia monsoon regions are the two gravest threats facing over 60% of the world population living in Asian monsoon regions. These dual threats have fueled a large body of research in the last decade on the roles of aerosols in impacting Asian monsoon weather and climate. This paper provides a comprehensive review of studies on Asian aerosols, monsoons, and their interactions. The Asian monsoon region is a primary source of emissions of diverse species of aerosols from both anthropogenic and natural origins. The distributions of aerosol loading are strongly influenced by distinct weather and climatic regimes, which are, in turn, modulated by aerosol effects. On a continental scale, aerosols reduce surface insolation and weaken the land-ocean thermal contrast, thus inhibiting the development of monsoons. Locally, aerosol radiative effects alter the thermodynamic stability and convective potential of the lower atmosphere leading to reduced temperatures, increased atmospheric stability, and weakened wind and atmospheric circulations. The atmospheric thermodynamic state, which determines the formation of clouds, convection, and precipitation, may also be altered by aerosols serving as cloud condensation nuclei or ice nuclei. Absorbing aerosols such as black carbon and desert dust in Asian monsoon regions may also induce dynamical feedback processes, leading to a strengthening of the early monsoon and affecting the subsequent evolution of the monsoon. Many mechanisms have been put forth regarding how aerosols modulate the amplitude, frequency, intensity, and phase of different monsoon climate variables. A wide range of theoretical, observational, and modeling findings on the Asian monsoon, aerosols, and their interactions are synthesized. A new paradigm is proposed on investigating aerosol-monsoon interactions, in which natural aerosols such as desert dust, black carbon from biomass burning, and biogenic aerosols from vegetation are considered integral components of an intrinsic aerosol-monsoon climate system, subject to external forcing of global warming, anthropogenic aerosols, and land use and change. Future research on aerosol-monsoon interactions calls for an integrated approach and international collaborations based on long-term sustained observations, process measurements, and improved models, as well as using observations to constrain model simulations and projections.
ABSTRACT Twelve case studies of multilayer cloud-base height (CBH) retrievals from two collocated ceilometers (Vaisala CL31 and Jenoptik CHM15K) have been analyzed. The studies were performed during the period from September to December 2008 at the Mace Head Atmospheric Research Station in Ireland. During the period of measurement, the two instruments provided vertical profiles of backscattered laser signal as well as the manufacturer's operational cloud-base product. The cases selected covered a diverse range of cloud-cover conditions, ranging from single to multiple cloud layers and from cloud-base heights varying from only a few hundreds meters per day up to 3-5 km in a few hours. The results show significant offsets between the two manufacturer-derived CBHs along with a considerable degree of scatter. Using a newly developed temporal height-tracking (THT) algorithm applied to both ceilometers, significant improvement in the correlation between CBH derived from both instruments results in a correlation coefficient increasing to R 2 = 0.997 (with a slope of 0.998) from R 2 = 0.788 (with an associated slope of 0.925). Also, the regression intercept (offset) is reduced from 160 m to effectively zero (-3 m). For the worst individual case study, using the THT algorithm resulted in the correlation coefficient improving from R 2 = 0.52, using the manufacturer's output, to R 2 = 0.97 with a reduction in the offset reducing from 569 to 32 m. Applying the THT algorithm to the backscatter profiles of both instruments led to retrieved cloud bases that are statistically consistent with each other and ensured reliable detection of CBH, particularly when inhomogeneous cloud fields were present and changing rapidly in time. The THT algorithm also overcomes multiple false cloud-base detections associated with the manufacturer's output of the two instruments.
Meteorological conditions within the planetary boundary layer (PBL) are closely governed by large-scale synoptic patterns and play important roles in air quality by directly and indirectly affecting the emission, transport, formation, and deposition of air pollutants. Partly due to the lack of long-term fine-resolution observations of the PBL, the relationships between synoptic patterns, PBL structure, and aerosol pollution in Beijing have not been well understood. This study applied the obliquely rotated principal component analysis in T-mode to classify the summertime synoptic conditions over Beijing using the National Centers for Environmental Prediction reanalysis from 2011 to 2014, and investigated their relationships with PBL structure and aerosol pollution by combining numerical simulations, measurements of surface meteorological variables, fine-resolution soundings, the concentration of particles with diameters less than or equal to 2.5 m, total cloud cover (CLD), and reanalysis data. Among the seven identified synoptic patterns, three types accounted for 67 % of the total number of cases studied and were associated with heavy aerosol pollution events. These particular synoptic patterns were characterized by high-pressure systems located to the east or southeast of Beijing at the 925 hPa level, which blocked the air flow seaward, and southerly PBL winds that brought in polluted air from the southern industrial zone. The horizontal transport of pollutants induced by the synoptic forcings may be the most important factor affecting the air quality of Beijing in summer. In the vertical dimension, these three synoptic patterns featured a relatively low boundary layer height (BLH) in the afternoon, accompanied by high CLD and southerly cold advection from the seas within the PBL. The high CLD reduced the solar radiation reaching the surface, and suppressed the thermal turbulence, leading to lower BLH. Besides, the numerical sensitive experiments show that cold advection induced by the large-scale synoptic forcing may have cooled the PBL, leading to an increase in near-surface stability and a decrease in the BLH in the afternoon. Moreover, when warm advection appeared simultaneously above the top level of the PBL, the thermal inversion layer capping the PBL may have been strengthened, resulting in the further suppression of PBL and thus the deterioration of aerosol pollution levels. This study has important implications for understanding the crucial roles that meteorological factors (at both synoptic and local scales) play in modulating and forecasting aerosol pollution in Beijing and its surrounding area.
A hybrid bispectral threshold method (HBTM) is developed for hourly regional cloud and radiative parameters from geostationary satellite visible and infrared radiance data. The quantities derived with the HBTM include equivalent blackbody temperatures for clear skies, for the total cloud cover and for the cloud cover at three levels in the atmosphere; the total fractional cloud cover and the fractional cloud amounts at three altitudes; and the clear-sky and total cloud reflectance characteristics. Geostationary satellite data taken during November 1978 are analyzed. A minimum reflectance technique is used to determine clear-sky brightness. A visible bidirectional reflectance model is derived for clear ocean areas. Clear-sky radiative temperature is found with a bispectral clear radiance technique during daylight hours. An empirical model is derived to predict clear-sky temperature at night. A combination of previously published infrared threshold and bispectral techniques is used to determine the remaining parameters. Sources of uncertainty are discussed and means to minimize them are proposed. Monthly mean, regional fractional cloudiness determined with this method agrees well with more conventional subjective techniques. On the average, the present results are approximately 0.05 less than corresponding surface observations; this is consistent with previous comparisons of satellite- and surface-based nephanalyses. Comparisons between subjective analyses of satellite photographs and the HBTM yielded average differences in mean regional cloudiness, mean hourly cloudiness and instantaneous cloud amounts of 0.04, 0.05 and 0.11 respectively. Root-mean-square differences in these same quantities derived by two satellite data analysts were 0.03, 0.04 and 0.08 respectively.
Abstract Climate models' simulation of clouds over the eastern subtropical oceans contributes to large uncertainties in projected cloud feedback to global warming. Here, interannual relationships of cloud radiative effect and cloud fraction to meteorological variables are examined in observations and in models participating in phases 3 and 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5, respectively). In observations, cooler sea surface temperature, a stronger estimated temperature inversion, and colder horizontal surface temperature advection are each associated with larger low-level cloud fraction and increased reflected shortwave radiation. A moister free troposphere and weaker subsidence are each associated with larger mid- and high-level cloud fraction and offsetting components of shortwave and longwave cloud radiative effect. It is found that a larger percentage of CMIP5 than CMIP3 models simulate the wrong sign or magnitude of the relationship of shortwave cloud radiative effect to sea surface temperature and estimated inversion strength. Furthermore, most models fail to produce the sign of the relationship between shortwave cloud radiative effect and temperature advection. These deficiencies are mostly, but not exclusively, attributable to errors in the relationship between low-level cloud fraction and meteorology. Poor model performance also arises due to errors in the response of mid-and high-level cloud fraction to variations in meteorology. Models exhibiting relationships closest to observations tend to project less solar reflection by clouds in the late twenty-first century and have higher climate sensitivities than poorer-performing models. Nevertheless, the intermodel spread of climate sensitivity is large even among these realistic models.
Cloud vertical distributions across extratropical warm and cold fronts are obtained using two consecutive winters of –Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) observations and National Centers for Environmental Prediction reanalysis atmospheric state parameters over the Northern and Southern Hemisphere oceans (30°–70°N/S) between November 2006 and September 2008. These distributions generally resemble those from the original model introduced by the Bergen School in the 1920s, with the following exceptions: 1) substantial low cloudiness, which is present behind and ahead of the warm and cold fronts; 2) ubiquitous high cloudiness, some of it very thin, throughout the warm-frontal region; and 3) upright convective cloudiness near and behind some warm fronts. One winter of GISS general circulation model simulations of Northern and Southern Hemisphere warm and cold fronts at 2° × 2.5° × 32 levels resolution gives similar cloud distributions but with much lower cloud fraction, a shallower depth of cloudiness, and a shorter extent of tilted warm-frontal cloud cover on the cold air side of the surface frontal position. A close examination of the relationship between the cloudiness and relative humidity fields indicates that water vapor is not lifted enough in modeled midlatitude cyclones and this is related to weak vertical velocities in the model. The model also produces too little cloudiness for a given value of vertical velocity or relative humidity. For global climate models run at scales coarser than tens of kilometers, the authors suggest that the current underestimate of modeled cloud cover in the storm track regions, and in particular the 50°–60°S band of the Southern Oceans, could be reduced with the implementation of a slantwise convection parameterization.
Cloud layer thicknesses are derived from base and top altitudes by combining 14 years (19751988) of surface and upper-air observations at 63 sites in the Northern Hemisphere. Rawinsonde observations are employed to determine the locations of cloud-layer top and base by testing for dewpoint temperature depressions below some threshold value. Surface observations serve as quality cheeks on the rawinsonde-determined cloud properties and provide cloud amount and cloud-type information. The dataset provides layer-cloud amount, cloud type, high, middle, or low height classes, cloud-top heights, base heights and layer thicknesses, covering a range of latitudes from 0℃ to 80℃N. All data comes from land sites: 34 are located in continental interiors, 14 are near coasts, and 15 are on islands. The uncertainties in the derived cloud properties are discussed. For clouds classified by low-, mid-, and high-top altitudes, there are strong latitudinal and seasonal variations in the layer thickness only for high clouds. High-cloud layer thickness increases with latitude and exhibits different seasonal variations in different latitude zones: in summer, high-cloud layer thickness is a maximum in the Tropics but a minimum at high latitudes. For clouds classified into three types by base altitude or into six standard morphological types, latitudinal and seasonal variations in layer thickness are very small. The thickness of the clear surface layer decreases with latitude and reaches a summer minimum in the Tropics and summer maximum at higher latitudes over land, but does not vary much over the ocean. Tropical clouds occur in three base-altitude groups and the layer thickness of each group increases linearly with top altitude. Extratropical clouds exhibit two groups, one with layer thickness proportional to their cloud-top altitude and one with small (鈮1000 m) layer thickness independent of cloud-top altitude.
CloudSat supports a 94 GHz cloud profiling radar as part of the innovative A-train formation of satellites studying the Earths clouds and atmosphere. Using the vertical profiles of clouds and precipitation, an algorithm has been developed to determine the type of clouds present. Because cloud type corresponds to specific cloud physical properties, this step is needed to apply other algorithms to derive quantitative cloud content and radiative data. This cloud type algorithm is applied to the initial 1-year of radar data to obtain the global distribution of various cloud types over the land and ocean. These initial results appear consistent with previous global cloud type distributions, but with some differences that provide insights into the limitations of CloudSat measurements.
Clouds play a tangible role in the Earth's atmosphere and in particular, the cloud base height (CBH) which is linked to cloud type is one of the important characteristic to describe the influence of clouds on the environment. In present study, CBH observations from ceilometer CL31 have been extensively studied during May 2013 to January 2015 over Ahmedabad (23.03℃ N, 72.54℃ E), India. A detail comparison has been performed with the use of ground-based CBH measurements from ceilometer CL31 and CBH retrieved from MODIS (Moderate Resolution Imaging Spectroradiometer) onboard Aqua and Terra satellite. Some interesting features of cloud dynamics viz. strong downdraft and updraft have been observed over Ahmedabad which revealed different cloud characteristics during monsoon and post-monsoon periods. CBH shows seasonal variation during Indian summer monsoon and post-monsoon period. Results indicate that ceilometer is one of the excellent instruments to precisely detect low and mid-level clouds and MODIS satellite provides accurate retrieval of high-level clouds over this region. The CBH algorithm used for MODIS satellite is also able to capture the low-level clouds.
Many studies have investigated the effects that misrepresentation of sub-grid cloud structure can have on the radiation budget. In this study, we perform 20-year simulations of the current climate using an atmosphere-only version of the Met Office Unified Model to investigate the effects of cloud approximation on model climate. We apply the “Tripleclouds” scheme for representing horizontal cloud inhomogeneity and “exponential-random” overlap, both separately and in combination, in place of a traditional plane-parallel representation with maximum-random overlap, to the clouds within the radiation scheme. The resulting changes to both the radiation budget and other meteorological variables, averaged over the 2002years, are compared. The combined global effect of the parameterizations on top-of-atmosphere short-wave and long-wave radiation budget is less than 102W02m 612 , but changes of up to 1002W02m 612 are identified in marine stratocumulus regions. A cooling near the surface over the winter polar regions of up to 3°C is also identified when horizontal cloud inhomogeneity is represented, and a warming of similar magnitude is found when exponential-random overlap is implemented. Corresponding changes of the same sign are also found in zonally averaged temperature, with maximum changes in the upper tropical troposphere of up to 0.5°C. Changes in zonally averaged cloud fraction in this location were of opposite sign and up to 0.02. The individual effects on tropospheric temperature of improving the two components of cloud structure are of similar magnitudes to about 2% of the warming created by a quadrupling of carbon dioxide.
CloudSat is a satellite experiment designed to measure the vertical structure of clouds from space. The expected launch of CloudSat is planned for 2004, and once launched, CloudSat will orbit in formation as part of a constellation of satellites (the A-Train) that includes NASA's Aqua and Aura satellites, a NASA-CNES lidar satellite (CALIPSO), and a CNES satellite carrying a polarimeter (PARASOL). A unique feature that CloudSat brings to this constellation is the ability to fly a precise orbit enabling the fields of view of the CloudSat radar to be overlapped with the CALIPSO lidar footprint and the other measurements of the constellation. The precision and near simultaneity of this overlap creates a unique multisatellite observing system for studying the atmospheric processes essential to the hydrological cycle.The vertical profiles of cloud properties provided by CloudSat on the global scale fill a critical gap in the investigation of feedback mechanisms linking clouds to climate. Measuring these profiles requires a combination of active and passive instruments, and this will be achieved by combining the radar data of CloudSat with data from other active and passive sensors of the constellation. This paper describes the underpinning science and general overview of the mission, provides some idea of the expected products and anticipated application of these products, and the potential capability of the A-Train for cloud observations. Notably, the CloudSat mission is expected to stimulate new areas of research on clouds. The mission also provides an important opportunity to demonstrate active sensor technology for future scientific and tactical applications. The CloudSat mission is a partnership between NASA's JPL, the Canadian Space Agency, Colorado State University, the U.S. Air Force, and the U.S. Department of Energy.
Climate change is governed by changes to the global energy balance. A synthesis of the latest observations suggests that more longwave radiation is received at the Earth's surface than previously thought, and that more precipitation is generated.
Abstract U.S. weather stations operated by NOAA's National Weather Service (NWS) have undergone significant changes in reporting and measuring cloud ceilings. Stations operated by the Department of Defense have maintained more consistent reporting practices. By comparing cloud-ceiling data from 223 NWS first-order stations with those from 117 military stations, and by further comparison with changes in physically related parameters, inhomogeneous records, including all NWS records based only on automated observing systems and the military records prior to the early 1960s, were identified and discarded. Data from the two networks were then used to determine changes in daytime ceiling height (the above-ground height of the lowest sky-cover layer that is more than half opaque) and ceiling occurrence frequency (percentage of total observations that have ceilings) over the contiguous United States since the 1950s. Cloud-ceiling height in the surface-3.6-km layer generally increased during 1951-2003, with more significant changes in the period after the early 1970s and in the surface-2-km layer. These increases were mostly over the western United States and in the coastal regions. No significant change was found in surface-3.6-km ceiling occurrence during 1951-2003, but during the period since the early 1970s, there is a tendency for a decrease in frequency of ceilings with height below 3.6 km. Cloud-ceiling heights above 3.6 km have shown no significant changes in the past 30 yr, but there has been an increase in frequency, consistent with the increase in ceiling height below 3.6 km. For the surface-3.6-km layer, physically consistent changes were identified as related to changes in ceiling height and frequency of occurrence. This included reductions in precipitation frequency related to low ceiling frequency, and surface warming and decreasing relative humidity accompanying increasing ceiling heights during the past 30 yr.
Aerosol-cloud (AC) interactions remain uncharacterized due to difficulties in obtaining accurate aerosol and cloud observations. In this study, we quantified the aerosol indirect effects (AIE) on warm clouds over Eastern China based on near-simultaneous retrievals from MODIS/AQUA, CALIOP/CALIPSO, and CPR/CLOUDSAT between June 2006 and December 2010. The seasonality of aerosols from ground-based PM 10 (aerosol particles with diameter of 10m or less) significantly differed from that estimated using MODIS aerosol optical depth (AOD). This result was supported by the lower level frequency profile of aerosol occurrence from CALIOP, indicative of the significant role of CALIOP in the AC interaction. To focus on warm clouds, cloud layers with base (top) altitudes above 7 (10) km were excluded. The combination of CALIOP and CPR was applied to determine the exact position of warm clouds relative to aerosols out of the following six scenarios in terms of AC mixing states: 1) aerosol only (AO); 2) cloud only (CO); 3) single aerosol layer-single cloud layer (SASC); 4) single aerosol layer-double cloud layers (SADC); 5) double aerosol layers - single cloud layer (DASC); and 6) others. The cases with vertical distance between aerosol and cloud layer less (more) than 100m (700m) were marked mixed (separated), and the rest as uncertain. Results showed that only 8.95% (7.53%) belonged to the mixed (separated and uncertain) state among all of the collocated AC overlapping cases, including SASC, SADC, and DASC. Under mixed conditions, the cloud droplet effective radius (CDR) decreased with increasing AOD at moderate aerosol loading (AOD<0.4), and then became saturated at an AOD of around 0.5, followed by an increase in CDR with increasing AOD, known as boomerang shape. Under separated conditions, no apparent changes in CDR with AOD were observed. We categorized the AC dataset into summer- and winter-season subsets to determine how the boomerang shape varied with season. The response of CDR to AOD in summer exhibited similar but much more deepened boomerang shape, as compared with the all year round case. In contrast, CDR in winter did not follow the boomerang shape for its continued decreasing with increasing AOD, even after the saturation zone (AOD around 0.5) of a cloud droplet.
A method is described to use rawinsonde data to estimate cloud vertical structure, including cloud-top and cloud-base heights, cloud-layer thickness, and the characteristics of multilayered clouds. Cloud-layer base and top locations are identified based on three criteria: maximum relative humidity in a cloud of at least 87%, minimum relative humidity of at least 84%, and relative humidity jumps exceeding 3% at cloud-layer top and base, where relative humidity is with respect to liquid water at temperatures greater than or equal to 0°C and with respect to ice at temperatures less than 0°C. The analysis method is tested at 30 ocean sites by comparing with cloud properties derived from other independent data sources. Comparison of layer-cloud frequencies of occurrence with surface observations shows that rawinsonde observations (RAOBS) usually detect the same number of cloud layers for low and middle clouds as the surface observers, but disagree more for high-level clouds. There is good agreement between the seasonal variations of RAOBS-determined top pressure of the highest cloud and that from the International Satellite Cloud Climate Project (ISCCP) data. RAOBS-determined top pressures of low and middle clouds agree better with ISCCP, but RAOBS often fail to detect very high and thin clouds. The frequency of multilayered clouds is qualitatively consistent with that estimated from surface observations. In cloudy soundings at these ocean sites, multilayered clouds occur 56% of the time and are predominately two layered. Multilayered clouds are most frequent (≈70%) in the Tropics (10°S0210°N) and least frequent at subtropical eastern Pacific stations. The frequency of multilayered clouds is higher in summer than in winter at low-latitude stations (30°S0230°N), but the opposite variation appears at the two subtropical stations. The frequency distributions of cloud top, cloud base, and cloud-layer thickness and cloud occurrence as a function of height are also presented. The lowest layer of multilayered cloud systems is usually located in the atmospheric boundary layer.
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 is the fourth of a series of atlases to result from a study of the global cloud distribution from ground-based observations. The first two atlases (NCAR/TN-201+STR and NCAR/TN-241+STR) described the frequency of occurrence of each cloud type and the co-occurrence of different types, but included no information about cloud amounts. The third atlas (NCAR/TN-273+STR) described, for the land areas of the earth, the average total cloud cover and the amounts of each cloud type, and their geographical, diurnal, seasonal, and interannual variations, as well as the average base heights of the low clouds. The present atlas does the same for the ocean areas of the earth.
Using five climate model simulations of the response to an abrupt quadrupling of CO2, the authors perform the first simultaneous model intercomparison of cloud feedbacks and rapid radiative adjustments with cloud masking effects removed, partitioned among changes in cloud types and gross cloud properties. Upon CO2 quadrupling, clouds exhibit a rapid reduction in fractional coverage, cloud-top pressure, and optical depth, with each contributing equally to a 1.1 W m(-2) net cloud radiative adjustment, primarily from shortwave radiation. Rapid reductions in midlevel clouds and optically thick clouds are important in reducing planetary albedo in every model. As the planet warms, clouds become fewer, higher, and thicker, and global mean net cloud feedback is positive in all but one model and results primarily from increased trapping of longwave radiation. As was true for earlier models, high cloud changes are the largest contributor to intermodel spread in longwave and shortwave cloud feedbacks, but low cloud changes are the largest contributor to the mean and spread in net cloud feedback. The importance of the negative optical depth feedback relative to the amount feedback at high latitudes is even more marked than in earlier models. The authors show that the negative longwave cloud adjustment inferred in previous studies is primarily caused by a 1.3 W m(-2) cloud masking of CO2 forcing. Properly accounting for cloud masking increases net cloud feedback by 0.3 W m(-2) K-1, whereas accounting for rapid adjustments reduces by 0.14 W m(-2) K-1 the ensemble mean net cloud feedback through a combination of smaller positive cloud amount and altitude feedbacks and larger negative optical depth feedbacks.
 The Atmospheric Radiation Measurement Mobile Facility (AMF) was deployed in Shouxian, Anhui Province, China from 14 May to 28 December 2008. Radiosonde data obtained during the AMF campaign are used to analyze cloud vertical structure over this area by taking advantage of the first direct measurements of cloud vertical layers from the 95 GHz radar. Single-layer, two-layer, and three-layer clouds account for 28.0%, 25.8%, and 13.9% of all cloud configurations, respectively. Low, middle, high and deep convective clouds account for 20.1%, 19.3%, 59.5%, and 1.1% of all clouds observed at the site, respectively. The average cloud base height, cloud top height, and cloud thickness for all clouds are 5912, 7639, and 1727 m, respectively. Maximum cloud top height and cloud thickness occurred at 1330 local standard time (LST) for single-layer clouds and the uppermost layer of multiple layers of cloud. For lower layer clouds in multiple-layer cloud systems, maximum cloud top height and cloud thickness occurred at 1930 LST. Diurnal variations in the thickness of upper level clouds are larger than those of lower level clouds. Multilayer clouds occurred more frequently in the summer. The absolute differences in cloud base heights from radiosonde and micropulse lidar/ceilometer comparisons are less than 500 m for 77.1%/68.4% of the cases analyzed.
Using the data collected over the Southern Great Plains ARM site from 2006 to 2010, the surface Active Remote Sensing of Cloud (ARSCL) and CloudSat-CALIPSO satellite (CC) retrievals of total cloud and six specified cloud types [low, mid-low (ML), high-mid-low (HML), mid, high-mid (HM) and high] were compared in terms of cloud fraction (CF), cloud-base height (CBH), cloud-top height (CTH) and cloud thickness (CT), on different temporal scales, to identify their respective advantages and limitations. Good agreement between the two methods was exhibited in the total CF. However, large discrepancies were found between the cloud distributions of the two methods at a high (240-m) vertical grid spacing. Compared to the satellites, ARSCL retrievals detected more boundary layer clouds, while they underestimated high clouds. In terms of the six specific cloud types, more low- and mid-level clouds but less HML- and high-level clouds were detected by ARSCL than by CC. In contrast, the ARSCL retrievals of ML- and HM-level clouds agreed more closely with the estimations from the CC product. Lower CBHs tended to be reported by the surface data for low-, ML- and HML-level clouds; however, higher CTHs were often recorded by the satellite product for HML-, HM- and high-level clouds. The mean CTs for low- and ML-level cloud were similar between the two products; however, the mean CTs for HML-, mid-, HM- and high-level clouds from ARSCL were smaller than those from CC.
The planetary boundary layer turbulence and moist convection parameterizations have been modified recently in the NASA Goddard Institute for Space Studies (GISS) Model E2 atmospheric general circulation model (GCM; post-CMIP5, hereafter P5). In this study, single column model (SCM P5) simulated cloud fractions (CFs), cloud liquid water paths (LWPs) and precipitation were compared with Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) groundbased observations made during the period 200208. CMIP5 SCM simulations and GCM outputs over the ARM SGP region were also used in the comparison to identify whether the causes of cloud and precipitation biases resulted from either the physical parameterization or the dynamic scheme. The comparison showed that the CMIP5 SCM has difficulties in simulating the vertical structure and seasonal variation of low-level clouds. The new scheme implemented in the turbulence parameterization led to significantly improved cloud simulations in P5. It was found that the SCM is sensitive to the relaxation time scale. When the relaxation time increased from 3 to 24 h, SCM P5-simulated CFs and LWPs showed a moderate increase (10%20%) but precipitation increased significantly (56%), which agreed better with observations despite the less accurate atmospheric state. Annual averages among the GCM and SCM simulations were almost the same, but their respective seasonal variations were out of phase. This suggests that the same physical cloud parameterization can generate similar statistical results over a long time period, but different dynamics drive the differences in seasonal variations. This study can potentially provide guidance for the further development of the GISS model.
Cloud feedbacks strongly influence the magnitude of global warming. Climate model simulations show that these feedbacks vary strongly as the spatial patterns of sea surface temperatures change over time.