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    Differences between Convective and Stratiform Precipitation Budgets in a Torrential Rainfall Event
    Yongjie HUANG, Yaping WANG, and Xiaopeng CUI
    DOI: 10.1007/s00376-019-8159-1
    Abstract   ( 22 ) PDF (3047KB) (4)
    Differences in rainfall budgets between convective and stratiform regions of a torrential rainfall event were investigated using high-resolution simulation data produced by the Weather Research and Forecasting (WRF) model. The convective and stratiform regions were reasonably separated by the radar-based convective--stratiform partitioning method, and the three-dimensional WRF-based precipitation equation combining water vapor and hydrometeor budgets was further used to analyze the rainfall budgets. The results showed that the magnitude of precipitation budget processes in the convective region was one order larger than that in the stratiform region. In convective/stratiform updraft regions, precipitation was mainly from the contribution of moisture-related processes, with a small negative contribution from cloud-related processes. In convective/stratiform downdraft regions, cloud-related processes played positive roles in precipitation, while moisture-related processes made a negative contribution. Moisture flux convergence played a dominant role in the moisture-related processes in convective or stratiform updraft regions, which was closely related to large-scale dynamics. Differences in cloud-related processes between convective and stratiform regions were more complex compared with those in moisture-related processes. Both liquid- and ice-phase processes were strong in convective/stratiform updraft regions, and ice-phase processes were dominant in convective/stratiform downdraft regions. There was strong net latent heating within almost the whole troposphere in updraft regions, especially in the convective updraft region, while the net latent heating (cooling) mainly existed above (below) the zero-layer in convective/stratiform downdraft regions.
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    Microphysical Properties of Rainwater in Typhoon Usagi (2013): A Numerical Modeling Study
    Lin DENG, Wenhua GAO , Yihong DUAN, and Yuqing WANG
    DOI: 10.1007/s00376-019-8170-6
    Abstract   ( 19 ) PDF (5418KB) (5)
    A 2-km resolution simulation using the Weather Research and Forecasting model with Morrison microphysics was employed to investigate the rainwater microphysical properties during different stages of Typhoon Usagi (2013) in the inner-core and outer region. The model reproduced the track, intensity, and overall structure of Usagi (2013) reasonably. The simulated raindrop size distribution showed a rapid increase in small-size raindrop concentration but an oscillated decrease in large-size ones in the inner-core region, corresponding well with the upward motion. It was found that there existed two levels (1.25 and 5.25 km) of maximum number concentration of raindrops. The ice-related microphysics at high levels was stronger than the warm-rain processes at low levels. The larger raindrops formed by self-collection in the inner-core suffered from significant breakup, but the raindrops outside the eyewall did not experience evident breakup. Model results indicated that the dominant terms in the water vapor budget were the horizontal moisture flux convergence (HFC) and local condensation and deposition. The evaporation from the ocean surface (PBL) was ~10% of the HFC in the inner core, but up to 40% in the outer region as the air therein was far from saturation. Furthermore, water vapor in the outer region was obtained equally through evaporation from the cloud and inward transportation from the environment. An earlier start of cloud microphysical processes in the inner-core region was evident during the intensification stage, and the continuous decreasing of condensation in both the inner-core and outer regions might imply the beginning of the storm weakening.
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    Sub-seasonal to Seasonal Hindcasts of Stratospheric Sudden Warming by BCC_CSM1.1(m): A Comparison with ECMWF
    Jian RAO, Rongcai REN, Haishan CHEN, Xiangwen LIU, Yueyue YU, and Yang YANG
    DOI: 10.1007/s00376-018-8165-8
    Abstract   ( 27 ) PDF (5448KB) (5)
    This study focuses on model predictive skill with respect to stratospheric sudden warming (SSW) events by comparing the hindcast results of BCC_CSM1.1(m) with those of the ECMWF’s model under the sub-seasonal to seasonal prediction project of the World Weather Research Program and World Climate Research Program. When the hindcasts are initiated less than two weeks before SSW onset, BCC_CSM and ECMWF show comparable predictive skill in terms of the temporal evolution of the stratospheric circumpolar westerlies and polar temperature up to 30 days after SSW onset. However, with earlier hindcast initialization, the predictive skill of BCC_CSM gradually decreases, and the reproduced maximum circulation anomalies in the hindcasts initiated four weeks before SSW onset replicate only 10% of the circulation anomaly intensities in observations. The earliest successful prediction of the breakdown of the stratospheric polar vortex accompanying SSW onset for BCC_CSM (ECMWF) is the hindcast initiated two (three) weeks earlier. The predictive skills of both models during SSW winters are always higher than that during non-SSW winters, in relation to the successfully captured tropospheric precursors and the associated upward propagation of planetary waves by the model initializations. To narrow the gap in SSW predictive skill between BCC_CSM and ECMWF, ensemble forecasts and error corrections are performed with BCC_CSM. The SSW predictive skill in the ensemble hindcasts and the error corrections are improved compared with the previous control forecasts.
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    Progress in Semi-arid Climate Change Studies in China
    Jianping HUANG , Jieru MA, Xiaodan GUAN, Yue LI, and Yongli HE
    DOI: 10.1007/s00376-018-8200-9
    Abstract   ( 6 ) PDF (2747KB) (0)
    This article reviews recent progress in semi-arid climate change research in China. Results indicate that the areas of semi-arid regions have increased rapidly during recent years in China, with an increase of 33% during 1994--2008 compared to 1948--1962. Studies have found that the expansion rate of semi-arid areas over China is nearly 10 times higher than that of arid and sub-humid areas, and are mainly transformed from sub-humid/humid regions. Meanwhile, the greatest warming during the past 100 years has been observed over semi-arid regions in China, and mainly induced by radiatively forced processes. The intensity of the regional temperature response over semi-arid regions has been amplified by land--atmosphere interactions and human activities. The decadal climate variation in semi-arid regions is modulated by oceanic oscillations, which induce land--sea and north--south thermal contrasts and affect the intensities of westerlies, planetary waves and blocking frequencies. In addition, the drier climates in semi-arid regions across China are also associated with the weakened East Asian summer monsoon in recent years. Moreover, dust aerosols in semi-arid regions may have altered precipitation by affecting the local energy and hydrological cycles. Finally, semi-arid regions in China are projected to continuously expand in the 21st century, which will increase the risk of desertification in the near future.
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    Causes of the Extreme Hot Midsummer in Central and South China during 2017: Role of the Western Tropical Pacific Warming
    Ruidan CHEN, Zhiping WEN, Riyu LU, and Chunzai WANG
    DOI: 10.1007/s00376-018-8177-4
    Abstract   ( 18 ) PDF (2431KB) (1)
    This study investigates why an extreme hot midsummer occurred in Central and South China (CSC) during 2017. It is shown that the western North Pacific subtropical high (WNPSH) was abnormally intensified and westward-extending, resulting in anomalous high pressure and consequent extreme heat over CSC. The abnormal WNPSH was favored by the warming of the western tropical Pacific (WTP), which was unrelated to ENSO and manifested its own individual effect. The WTP warming enhanced the convection in-situ and led to anomalous high pressure over CSC via a local meridional circulation. The influence of the WTP was confirmed by CAM4 model experiments. A comparison between the 2017 midsummer and 2010 midsummer (with a stronger WNPSH but weaker extreme heat) indicated that the influence of the WNPSH on extreme heat can be modulated by the associated precipitation in the northwestern flank.
    The role of the WTP was verified by regression analyses on the interannual variation of the WTP sea surface temperature anomaly (SSTA). On the other hand, the WTP has undergone prominent warming during the past few decades, resulting from decadal to long-term changes and favoring extreme warm conditions. Through a mechanism similar to the interannual variation, the decadal to long-term changes have reinforced the influence of WTP warming on the temperature over CSC, contributing to the more frequent hot midsummers recently. It is estimated that more than 50% of the temperature anomaly over CSC in the 2017 midsummer was due to the WTP warming, and 40% was related to the decadal to long-term changes of the WTP SSTA.
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    Investigating the Initial Errors that Cause Predictability Barriers for Indian Ocean Dipole Events Using CMIP5 Model Outputs
    Rong FENG, Wansuo DUAN
    DOI: 10.1007/s00376-018-7214-7
    Abstract   ( 69 ) PDF (3450KB) (11)
    By analyzing the outputs of the pre-industrial control runs of four models within phase 5 of the Coupled Model Intercomparison Project, the effects of initial sea temperature errors on the predictability of Indian Ocean Dipole events were identified. The initial errors cause a significant winter predictability barrier (WPB) or summer predictability barrier (SPB). The WPB is closely related with the initial errors in the tropical Indian Ocean, where two types of WPB-related initial errors display opposite patterns and a west–east dipole. In contrast, the occurrence of the SPB is mainly caused by initial errors in the tropical Pacific Ocean, where two types of SPB-related initial errors exhibit opposite patterns, with one pole in the subsurface western Pacific Ocean and the other in the upper eastern Pacific Ocean. Both of the WPB-related initial errors grow the fastest in winter, because the coupled system is at its weakest, and finally cause a significant WPB. The SPB-related initial errors develop into a La Niña–like mode in the Pacific Ocean. The negative SST errors in the Pacific Ocean induce westerly wind anomalies in the Indian Ocean by modulating the Walker circulation in the tropical oceans. The westerly wind anomalies first cool the sea surface water in the eastern Indian Ocean. When the climatological wind direction reverses in summer, the wind anomalies in turn warm the sea surface water, finally causing a significant SPB. Therefore, in addition to the spatial patterns of the initial errors, the climatological conditions also play an important role in causing a significant predictability barrier.
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