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    Role of Extratropical Cyclones in the Recently Observed Increase in Poleward Moisture Transport into the Arctic Ocean
    Gian A. VILLAMIL-OTERO, Jing ZHANG, Juanxiong HE, Xiangdong ZHANG
    DOI: 10.1007/s00376-017-7116-0
    Abstract   ( 57 ) PDF (1989KB) (30)
    Poleward atmospheric moisture transport (AMT) into the Arctic Ocean can change atmospheric moisture or water vapor content and cause cloud formation and redistribution, which may change downward longwave radiation and, in turn, surface energy budgets, air temperatures, and sea-ice production and melt. In this study, we found a consistently enhanced poleward AMT across 60°N since 1959 based on the NCAR--NCEP reanalysis. Regional analysis demonstrates that the poleward AMT predominantly occurs over the North Atlantic and North Pacific regions, contributing about 57% and 32%, respectively, to the total transport. To improve our understanding of the driving force for this enhanced poleward AMT, we explored the role that extratropical cyclone activity may play. Climatologically, about 207 extratropical cyclones move across 60°N into the Arctic Ocean each year, among which about 66 (32% of the total) and 47 (23%) originate from the North Atlantic and North Pacific Ocean, respectively. When analyzing the linear trends of the time series constructed by using a 20-year running window, we found a positive correlation of 0.70 between poleward yearly AMT and the integrated cyclone activity index (measurement of cyclone intensity, number, and duration). This shows the consistent multidecadal changes between these two parameters and may suggest cyclone activity plays a driving role in the enhanced poleward AMT. Furthermore, a composite analysis indicates that intensification and poleward extension of the Icelandic low and accompanying strengthened cyclone activity play an important role in enhancing poleward AMT over the North Atlantic region.
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    Climatology of Cloud-base Height from Long-term Radiosonde Measurements in China
    Yong ZHANG, Lejian ZHANG, Jianping GUO, Jinming FENG, Lijuan CAO, Yang WANG, Qing ZHOU, Liangxu LI, Bai LI, Hui XU, Lin LIU, Ning AN, Huan LIU
    DOI: 10.1007/s00376-017-7096-0
    Abstract   ( 75 ) PDF (3162KB) (28)
    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. 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 (< 2.4 km above ground level). In terms of seasonality, the CBH reaches its maximum in summer and minimum in winter. A low cloud base tends to occur frequently (> 70%) over the TP, PRD and SCB. In contrast, at most sites over the Yangtze River Delta (YRD) and the NCP, about half the cloud belongs to the high-cloud category. The CBH does not exhibit marked diurnal variation in summer, throughout all CRN sites, probably because the 1400 LST soundings in summertime are launched for improving the forecasting of high-impact weather. To the best of our knowledge, this is the first CBH climatology produced from sounding measurements in China, and provides a useful reference for obtaining observational cloud base information.
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    Aerosol Optical Properties and Radiative Impacts in the Pearl River Delta Region of China during the Dry Season
    Boru MAI, Xuejiao DENG, Zhanqing LI, Jianjun LIU, Xiang’ao XIA, Huizheng CHE, Xia LIU, Fei LI, Yu ZOU, Maureen CRIBB
    DOI: 10.1007/s00376-017-7092-4
    Abstract   ( 9 ) PDF (2937KB) (8)
    Aerosol optical properties and direct radiative effects on surface irradiance were examined using seven years (2006--2012) of Cimel sunphotometer data collected at Panyu—the main atmospheric composition monitoring station in the Pearl River Delta (PRD) region of China. During the dry season (October to February), mean values of the aerosol optical depth at 550 nm, the Ångström exponent, and the single scattering albedo at 440 nm (SSA) were 0.54, 1.33 and 0.87, respectively. About 90% of aerosols were dominated by fine-mode strongly absorbing particles. The size distribution was bimodal, with fine-mode particles dominating. The fine mode showed a peak at a radius of 0.12 μm in February and October (~0.10 μm3 μm−2). The mean diurnal shortwave direct radiative forcing at the surface, inside the atmosphere (FATM), and at the top of the atmosphere, was −33.4 ± 7.0, 26.1 ± 5.6 and −7.3 ± 2.7 W m−2, respectively. The corresponding mean values of aerosol direct shortwave radiative forcing efficiency were −60.0 ± 7.8, 47.3 ± 8.3 and −12.8 ± 3.1 W m−2 τ−1, respectively. Moreover, during the study period, FATM showed a significant decreasing trend (p < 0.01) and SSA increased from 0.87 in 2006 to 0.91 in 2012, suggesting a decreasing trend of absorbing particles being released into the atmosphere. Optical properties and radiative impacts of the absorbing particles can be used to improve the accuracy of inversion algorithms for satellite-based aerosol retrievals in the PRD region and to better constrain the climate effect of aerosols in climate models.
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    Remarkable Link between Projected Uncertainties of Arctic Sea-Ice Decline and Winter Eurasian Climate
    Hoffman H. N. CHEUNG, Noel KEENLYSIDE, Nour-Eddine OMRANI, Wen ZHOU
    DOI: 10.1007/s00376-017-7156-5
    Abstract   ( 79 ) PDF (3136KB) (34)
    We identify that the projected uncertainty of the pan-Arctic sea-ice concentration (SIC) is strongly coupled with the Eurasian circulation in the boreal winter (December--March; DJFM), based on a singular value decomposition (SVD) analysis of the forced response of 11 CMIP5 models. In the models showing a stronger sea-ice decline, the Polar cell becomes weaker and there is an anomalous increase in the sea level pressure (SLP) along 60°N, including the Urals--Siberia region and the Iceland low region. There is an accompanying weakening of both the midlatitude westerly winds and the Ferrell cell, where the SVD signals are also related to anomalous sea surface temperature warming in the midlatitude North Atlantic. In the Mediterranean region, the anomalous circulation response shows a decreasing SLP and increasing precipitation. The anomalous SLP responses over the Euro-Atlantic region project on to the negative North Atlantic Oscillation--like pattern. Altogether, pan-Arctic SIC decline could strongly impact the winter Eurasian climate, but we should be cautious about the causality of their linkage.
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    First Surface-based Estimation of the Aerosol Indirect Effect over a Site in Southeastern China
    Jianjun LIU, Zhanqing LI
    DOI: 10.1007/s00376-017-7106-2
    Abstract   ( 25 ) PDF (2872KB) (16)
    The deployment of the U.S. Atmospheric Radiation Measurement mobile facility in Shouxian from May to December 2008 amassed the most comprehensive set of measurements of atmospheric, surface, aerosol, and cloud variables in China. This deployment provided a unique opportunity to investigate the aerosol--cloud interactions, which are most challenging and, to date, have not been examined to any great degree in China. The relationship between cloud droplet effective radius (CER) and aerosol index (AI) is very weak in summer because the cloud droplet growth is least affected by the competition for water vapor. Mean cloud liquid water path (LWP) and cloud optical depth (COD) significantly increase with increasing AI in fall. The sensitivities of CER and LWP to aerosol loading increases are not significantly different under different air mass conditions. There is a significant correlation between the changes in hourly mean AI and the changes in hourly mean CER, LWP, and COD. The aerosol first indirect effect (FIE) is estimated in terms of relative changes in both CER (FIECER) and COD (FIECOD) with changes in AI for different seasons and air masses. FIECOD and FIECER are similar in magnitude and close to the typical FIE value of ~0.23, and do not change much between summer and fall or between the two different air mass conditions. Similar analyses were done using spaceborne Moderate Resolution Imaging Spectroradiometer data. The satellite-derived FIE is contrary to the FIE estimated from surface retrievals and may have large uncertainties due to some inherent limitations.
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    The 30--60-day Intraseasonal Variability of Sea Surface Temperature in the South China Sea during May--September
    Jiangyu MAO, Ming WANG
    DOI: 10.1007/s00376-017-7127-x
    Abstract   ( 13 ) PDF (3741KB) (5)
    This study investigates the structure and propagation of intraseasonal sea surface temperature (SST) variability in the South China Sea (SCS) on the 30--60-day timescale during boreal summer (May--September). TRMM-based SST, GODAS oceanic reanalysis and ERA-Interim atmospheric reanalysis datasets from 1998 to 2013 are used to examine quantitatively the atmospheric thermodynamic and oceanic dynamic mechanisms responsible for its formation. Power spectra show that the 30--60-day SST variability is predominant, accounting for 60% of the variance of the 10--90-day variability over most of the SCS. Composite analyses demonstrate that the 30--60-day SST variability is characterized by the alternate occurrence of basin-wide positive and negative SST anomalies in the SCS, with positive (negative) SST anomalies accompanied by anomalous northeasterlies (southwesterlies). The transition and expansion of SST anomalies are driven by the monsoonal trough--ridge seesaw pattern that migrates northward from the equator to the northern SCS. Quantitative diagnosis of the composite mixed-layer heat budgets shows that, within a strong 30--60-day cycle, the atmospheric thermal forcing is indeed a dominant factor, with the mixed-layer net heat flux (MNHF) contributing around 60% of the total SST tendency, while vertical entrainment contributes more than 30%. However, the entrainment-induced SST tendency is sometimes as large as the MNHF-induced component, implying that ocean processes are sometimes as important as surface fluxes in generating the 30--60-day SST variability in the SCS.
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    Organizational Modes of Severe Wind-producing Convective Systems over North China
    Xinlin YANG , Jianhua SUN
    DOI: 10.1007/s00376-017-7114-2
    Abstract   ( 22 ) PDF (2140KB) (29)
    Severe weather reports and composite radar reflectivity data from 2010--14 over North China were used to analyze the distribution of severe convective wind (SCW) events and their organizational modes of radar reflectivity. The six organizational modes for SCW events (and their proportions) were cluster cells (35.4%), squall lines (18.4%), nonlinear-shaped systems (17.8%), broken lines (11.6%), individual cells (1.2%), and bow echoes (0.5%). The peak month for both squall lines and broken lines was June, whereas it was July for the other four modes. The highest numbers of SCW events were over the mountains, which were generally associated with disorganized systems of cluster cells. In contrast, SCW associated with linear systems occurred mainly over the plains, where stations recorded an average of less than one SCW event per year. Regions with a high frequency of SCW associated with nonlinear-shaped systems also experienced many SCW events associated with squall lines. Values of convective available potential energy, precipitable water, 0--3-km shear, and 0--6-km shear, were demonstrably larger over the plains than over the mountains, which had an evident effect on the organizational modes of SCW events. Therefore, topography may be an important factor in the organizational modes for SCW events over North China.
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    Asymmetric Relationship between the Meridional Displacement of the Asian Westerly Jet and the Silk Road Pattern
    Xiaowei HONG, Riyu LU, Shuanglin LI
    DOI: 10.1007/s00376-017-6320-2
    Abstract   ( 63 ) PDF (2131KB) (32)
    In previous work, a significant relationship was identified between the meridional displacement of the Asian westerly jet (JMD) and the Silk Road Pattern (SRP) in summer. The present study reveals that this relationship is robust in northward JMD years but absent in southward JMD years. In other words, the amplitude of the SRP increases with northward displacement of the jet but shows little change with southward displacement. Further analysis indicates that, in northward JMD years, the Rossby wave source (RWS) anomalies, which are primarily contributed by the planetary vortex stretching, are significantly stronger around the entrance of the Asian jet, i.e., the Mediterranean Sea--Caspian Sea area, with the spatial distribution being consistent with that related to the SRP. By contrast, in southward JMD years, the RWS anomalies are much weaker. Therefore, this study suggests that the RWS plays a crucial role in inducing the asymmetry of the JMD--SRP relationship. The results imply that climate anomalies may be stronger in strongly northward-displaced JMD years due to the concurrence of the JMD and SRP, and thus more attention should be paid to these years.
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    Projected Changes in Temperature and Precipitation Extremes over China as Measured by 50-yr Return Values and Periods Based on a CMIP5 Ensemble
    Ying Xu, Xuejie Gao, Filippo Giorgi, Botao Zhou, Ying Shi, Jie Wu, Yongxiang Zhang
    DOI: 10.1007/s00376-017-6269-1
    Abstract   ( 2 ) PDF (1534KB) (0)
    Future changes in the 50-yr return level for temperature and precipitation extremes over mainland China are investigated based on a CMIP5 multi-model ensemble for RCP2.6, RCP4.5 and RCP8.5 scenarios. The following indices are analyzed: TXx and TNn (the annual maximum and minimum of daily maximum and minimum surface temperature), RX5day (the annual maximum consecutive 5-day precipitation) and CDD (maximum annual number of consecutive dry days). After first validating the model performance, future changes in the 50-yr return values and return periods for these indices are investigated along with the inter-model spread. Multi-model median changes show an increase in the 50-yr return values of TXx and a decrease for TNn, more specifically, by the end of the 21st century under RCP8.5, the present day 50-yr return period of warm events is reduced to 1.2 yr, while extreme cold events over the country are projected to essentially disappear. A general increase in RX5day 50-yr return values is found in the future. By the end of the 21st century under RCP8.5, events of the present RX5day 50-yr return period are projected to reduce to < 10 yr over most of China. Changes in CDD-50 show a dipole pattern over China, with a decrease in the values and longer return periods in the north, and vice versa in the south. Our study also highlights the need for further improvements in the representation of extreme events in climate models to assess the future risks and engineering design related to large-scale infrastructure in China.
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    Study of Aerosol Direct and Indirect Effects and Auto-conversion Processes over the West African Monsoon Region Using a Regional Climate Model
    Zeinab SALAH, Ahmed SHALABY, Allison L. STEINER, Ashraf S. ZAKEY, Ritesh GAUTAM, Mohamed M. ABDEL WAHAB
    DOI: 10.1007/s00376-017-7077-3
    Abstract   ( 51 ) PDF (1832KB) (21)
    This study assesses the direct and indirect effects of natural and anthropogenic aerosols (e.g., black carbon and sulfate) over West and Central Africa during the West African monsoon (WAM) period (June--July--August). We investigate the impacts of aerosols on the amount of cloudiness, the influences on the precipitation efficiency of clouds, and the associated radiative forcing (direct and indirect). Our study includes the implementation of three new formulations of auto-conversion parameterization [namely, the Beheng (BH), Tripoli and Cotton (TC) and Liu and Daum (R6) schemes] in RegCM4.4.1, besides the default model’s auto-conversion scheme (Kessler). Among the new schemes, BH reduces the precipitation wet bias by more than 50% over West Africa and achieves a bias reduction of around 25% over Central Africa. Results from detailed sensitivity experiments suggest a significant path forward in terms of addressing the long-standing issue of the characteristic wet bias in RegCM. In terms of aerosol-induced radiative forcing, the impact of the various schemes is found to vary considerably (ranging from −5 to −25 W m−2).
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