梅雨锋暴雨个例的中尺度数值模拟研究(Ι)——中α尺度双雨带

利用ＰＳＵ／ＮＣＡＲ的中尺度数值模式ＭＭ５,取１９８１年６月２７日梅雨锋暴雨过程作为个例进行了数值模拟,模拟的降水结果同华东中尺度天气试验实测取得的该暴雨过程的稠密降水资料分析结果相比较,发现直到中β尺度程度上,两者的分布结构和演变过程等基本特征都是近于一致的．不论在观测分析中还是在模拟结果中,几乎与梅雨锋相平行,均出现了中α尺度（南、北）双雨带,它们均由数个中β尺度雨团组织和近乎东移所成,双雨带之间均具有些此强彼弱的演变特征．所有这些构成了梅雨锋雨区或暴雨主要的中尺度时空特征．     

基于强降水分布特征的湖北省梅雨分类及讨论

利用1960～2014年湖北省50个气象站逐日降水资料,研究了本地区梅雨期气候特征,将梅雨年聚类为4类并探讨偏强年份2类梅雨的合理性.结果表明：湖北省梅雨入梅推迟,出梅提前,梅雨期长度缩短,雨强下降,强度减弱;梅雨期降水日数显著减少,强降水过程增加但区域性强降水过程减少.K聚类分析后发现Ⅱ类梅雨强度大于Ⅰ类梅雨,Ⅱ类梅雨期内暴雨、大暴雨及区域性暴雨多于Ⅰ类;Ⅱ类梅雨期雨带稳定少动,以区域性强降水为主,Ⅰ类梅雨期内6月雨带较为散乱,单点强降水为主,7月雨带与Ⅱ类相似.从两类梅雨期南海季风爆发时间、副高面积、强度及脊线环流因子结合梅雨期内灾损验证梅雨聚类合理,为湖北省梅雨诊断分析提供参考.     

一次梅雨锋强降水过程中云和降雨的结构特征

本文利用地面雨量计、天气雷达、FY-2C、CloudSat云廓线雷达等观测资料,对2007年7月7-8日发生在淮河流域的一次典型梅雨锋强降水过程中云和降雨的结构特征进行了综合分析。结果表明:此过程主要受准东西向低空切变线的影响,雨带略呈西北-东南向的带状分布形式,梅雨锋锋前强的上升运动有利于触发低层对流不稳定能量的释放,加强锋区的垂直环流。切变线上不断新生的对流单体,最终发展成为东西向贯通的梅雨锋云系,成熟期的梅雨锋云系由南向北依次平行分布着强对流性降水带、过渡带、层状降水区和宽广的非降水性云砧。梅雨锋云系发展的各个阶段,云体的垂直结构及水凝物的回波特征都有很大区别。     

一次梅雨锋强降水过程中云和降雨的结构特征

本文利用地面雨量计、天气雷达、FY-2C、CloudSat云廓线雷达等观测资料,对2007年7月7-8日发生在淮河流域的一次典型梅雨锋强降水过程中云和降雨的结构特征进行了综合分析。结果表明：此过程主要受准东西向低空切变线的影响,雨带略呈西北-东南向的带状分布形式,梅雨锋锋前强的上升运动有利于触发低层对流不稳定能量的释放,加强锋区的垂直环流。切变线上不断新生的对流单体,最终发展成为东西向贯通的梅雨锋云系,成熟期的梅雨锋云系由南向北依次平行分布着强对流性降水带、过渡带、层状降水区和宽广的非降水性云砧。梅雨锋云系发展的各个阶段,云体的垂直结构及水凝物的回波特征都有很大区别。     

1998年二度梅雨锋的演变及结构特征

应用气象常规资料对1998-07-21-24日暴雨时段的二度梅雨锋的演变和其不同阶段的水平、垂直结构进行了诊断分析,确认了二度梅雨锋结构的一些普遍特征,如在对流层低层表现为θe锋而不是温度的强烈对比;同时揭示了1998-07的二度梅雨锋结构不同于一般梅雨锋的典型特征:700 hPa以下的梅雨锋向南倾斜或近乎垂直,切变线位于梅雨锋南侧,梅雨锋具有相当正压性.结果还表明:1998年二度梅雨锋结构不同于温带锋面结构,而是更倾向于赤道辐合带的性质,并且在各个阶段其结构并不完全相同.     

2020年海洋气象研究热点回眸

 回顾了2020年影响东亚气候的梅雨锋、季风、台风、海气通量、风暴潮等天气过程与机制研究取得的重要进展,探讨了海洋气象学的未来发展方向。     

梅雨锋暴雨个例的中尺度数值模拟研究（I）——中α尺度双雨带

利用ＰＳＵ／ＮＣＡＲ的中尺度数值模式ＭＭ５,取１９８１年６月２７日梅雨锋暴雨过程作为个例进行了数值模拟,模拟的降水结果同华东中尺度天气试验实测取得的该暴雨过程的稠密降水资料分析结果相比较,发现直到中β尺度程度上,两者的分布结构和演变过程等基本特征都是近于一致的,不论在观测分析中还是模拟结果中,几乎与梅雨锋都平行,均出现了α尺度（南,北）双雨带,它们均由整个中β尺度雨团组织和近乎东移所成,双雨带之间     

2003年江淮连续梅雨暴雨过程的中尺度数值试验

为了分析天气研究及预报WRF(weather research and forecasting)模式对我国天气现象的适用性,利用WRF模式对2003年淮河汛期的3次梅雨锋暴雨过程进行了一组数值模拟试验,对3次天气过程模拟试验采用相同的参数设置,模拟区域根据实况降水落区作了相应设置.模拟结果分析表明,WRF模式能有效模拟我国梅雨锋暴雨的环流背景和天气形势,较好地反映影响暴雨的中尺度系统的发生和发展等特征,还可以较好地模拟出雨带的范围、位置和走向,对降水中心的模拟基本可用.     

98.7特大暴雨的动力学及能量诊断分析

利用非静力中尺度模式MM5的客观分析资料对引起这次特大暴雨的可能机制进行了讨论。指出，在“98．7”暴雨发生发展期间，梅雨锋系的冷（暖）锋后（前）部分别有与冷（暖）锋平行的θse密集（稀疏）带，这表明θse能很好地反映气团与梅雨锋扰动在地面附近的热力结构特征。对该过程的三维空间动力学锋生函数进行的诊断结果发现，925hPa的锋生带与武汉及其周边地区的暴雨带相当接近，揭示了暴雨带与锋生强迫有一定的动力学关系。而850hPa的锋生分布为实际未来48h的暴积降水量分布提供了可能预报依据。从能量的观点来看，在暖锋区为高温、高湿和高能区，并且，不稳定的大气层结是产生此次强暴雨过程的主要物理条件之一。     

一次梅雨锋暴雨在不同物理过程下的模拟结果比较

本文研究对象为2005年7月江淮流域的一次梅雨锋暴雨,通过选择WRF模式中五组不同的微物理方案和积云参数化方案,进行两重网格嵌套模拟,并对模拟结果进行诊断分析。通过比较得出如下结论:Betts-Miller-Janjic方案与Ferrier(new Eta)微物理方案的组合最能模拟出降水雨带的方向、位置和雨量,其次是New-Kain-Fritsch和Ferrier(new Eta)方案的组合,而且细网格的模拟效果更好;积云参数化方案的选择比微物理方案的选择对降水模拟的影响大;当微物理方案相同时,Betts-Miller-Janjic方案比New-Kain-Fritsch方案在细网格模拟下更准确反映梅雨锋暴雨的热力结构和垂直运动的特征,对暴雨的落区有一定指示作用。     

Meridional seesaw-like distribution of the Meiyu rainfall over the Changjiang-Huaihe River Valley and characteristics in the anomalous climate years

Although Meiyu rainfall has its in-phase spatial variability over the Changjiang-Huaihe River Valley (CHRV) in most years, it is distributed in some years like a seesaw to the north and south of the Changjiang River, when the precipitation tends to be nearly normal throughout the valley, which would inevitably increase difficulties of making short-term prediction of the rainfall. For this reason, EOF analysis is made on 15 related stations’ precipitation from June to July during 1951─2004, revealing that the EOF2 mode shows largely a north-south seesaw-like pattern, and thereby classifying Meiyu patterns into two types: "northern drought and southern flood (NDSF)" and "northern flood and southern drought (NFSD)". Afterwards, the authors investigated ocean-atmospheric characteristics when these two anomalous types occured using the NCEP reanalysis (version 1) and the extended reconstructed SSTs (version 2). The results show that in the NDSF years, the low-level frontal area and moisture convergence center lie more southward, accompanied by weaker subtropical summer mon- soon over East Asia, with the western Pacific subtropical high and 200 hPa South Asia High being more southward. Both the Northern and Southern Hemisphere Annular Modes are stronger than normal in preceding February; SST is higher off China during boreal winter and spring and the opposite happens in the NFSD years. Also, this seesaw-form Meiyu rainfall distribution might be affected to some degree by the previous ENSO event.     

Regional integrated environmental model system and its simulation of East Asia summer monsoon

A continuous 22-year simulation in Asia for the period of 1 January 1979 to 31 December 2000 was conducted using the Regional Integrated Environmental Model System （RIEMS 2.0） with NCEP Reanalysis II data as the driving fields. The model processes include surface physics state package （BATS le）, a Grell cumulus parameterization, and a modified radiation package （CCM3） with the focus on the ability of the model to simulate the summer monsoon over East Asia. The analysis results show that （1） RIEMS reproduces well the spatial pattern and the seasonal cycle of surface temperature. When regionally averaged, the summer mean temperature biases are within 1-2℃（2） For precipitation, the model reproduces well the spatial pattern, and temporal evolution of the East Asia summer monsoon rain belt, with steady phases separated by more rapid transitions, is reproduced. The rain belt simulated by RIEMS 2.0 is closer to observation than by RIEMS 1.0. （3） RIEMS 2.0 can reasonably reproduce the large-scale circulation.     

Decadal northward shift of the Meiyu belt and the possible cause

The feature of the decadal movement of the Meiyu belt and their association with East Asia atmospheric circulation for the period of 1979―2007 is examined in this study. It is shown that the Meiyu belt of China has a decadal shift in the late 1990s. There is a remarkable difference between the periods before and after 1999, with the Meiyu belt mainly located to the south of the Yangtze River valley before 1999, but afterward mainly located in the Huaihe River valley. At the same time, a distinctive tropospheric warming and stratospheric cooling trend is found in the mid-latitudes of East Asia, which causes the upper tropospheric pressure surfaces upward bulging and the tropoause elevated in the subtropics and associated widening of the subtropical area over East Asia. Accompanying the subtropics widening, the subtropical westerly jet over East Asia shifts northward and the East Asia tropical Hadley circulation expands poleward, which results in the northward shift of the Meiyu belt.     

Meiyu in the middle and lower reaches of the Yangtze River since 1736

＂Yu Xue Fen Cun＂ records during the Qing Dynasty are used to identify the starting and ending dates of Meiyu at the period of 1736-1911. These results, along with the instrumental meteorological records, are used to reconstruct the series of length and precipitation of Meiyu during 1736-2000 over the middle and lower reaches of the Yangtze River. The characteristics of Meiyu are analyzed since 1736. Moreover, the strength of East Asian Summer Monsoon and locations of rainband are discussed, based on the relationship between the length of Meiyu and the Index of East Asian Summer Monsoon. It is found that the starting and ending dates and the length of Meiyu have significant interannual and interdecadal variations. Apart from 7-8 years, 20-30 years and 40 years cycles for the lengths of Meiyu, the centennial oscillation is also presented. The length of Meiyu, monsoon rainband movement over eastern China, and the strength of East Asian Summer Monsoon （EASM） have a very good correlation, which can be expressed in the following： during the periods of 1736-1770, 1821-1870 and 1921-1970, the EASM was stronger, and the monsoon rainband was located in North China and South China easily, corresponding to the decreased length of Meiyu. Whereas during the periods of 1771-1820, 1871-1920 and 1971-2000, the EASM was weaker and monsoon rainband usually stopped at the middle and lower reaches of the Yangtze River, corresponding to the increased length of Meiyu.     

“Climate effect” of the northeast cold vortex and its influences on Meiyu

The Northeast Cold Vortex (NECV) is an important weather system in the middle and high latitudes in East Asia. Its time scale is synoptic, yet the frequent activities of NECV have significant "climate effect" which influences not only the monthly temperature in the lower troposphere in Northeast China but also the Meiyu rainfall in East Asia. On the basis of ERA-40 reanalysis data provided by ECMWF, the "climate effect" of NECV and its relationship with Meiyu in East Asia are studied. It is shown that there is significant correlation between NECV during the Meiyu period and rainfall amount: strong NECV corresponds to more Meiyu rainfall and weak NECV corresponds to less rainfall. In strong NECV years, the dry and cold air from the north is led to the south by NECV, converges with the lower-level warm and wet southwesterly on the north verge of Meiyu region, thus forms an unstable stratification of "upper dryness and lower wetness" . Triggered by ascending motion, the Meiyu rainfall amount is more than usual. It is on the contrary in weak NECV years. The anomalous SST in north Pacific in the previ-ous year may be a factor that results in the anomalous NECV at Meiyu period. The land-sea thermal contrast in summer facilitates NECV, while that in winter inhibits NECV. All of the above provide a meaningful result for the short-term climate prediction of NECV and Meiyu.     

基于EOF和REOF分析江淮梅雨量的时空分布

基于江淮梅雨区域50个气象台站1960-2002年的梅雨量资料,利用EOF、REOF展开方法,分析了江淮梅雨降水的空间分布及时间演变特征.EOF展开方法表明江淮梅雨前三个特征向量累积方差贡献比例为88.9%,其主要特征是三个特征向量场呈现纬向带状分布;第一特征向量场空间分布均为正值,说明江淮梅雨量的干湿变化具有一致性,但是各特征向量场之间的特点相差明显.REOF展开方法表明可以把江淮梅雨划分为6个典型的梅雨量场;后一个时间系数序列的变化幅度比前一个时间序列要小;主成分旋转后载荷要比旋转前分布均匀得多;第一时间系数的变化等同于各站平均梅雨量变化.     

中国大地形对冻雨天气云微物理过程的影响

云贵高原的冻雨是中国南方冬季最严重的气象灾害。利用天气研究和预报模式(weather research and forecasting model, WRF),对云贵高原大地形进行了敏感性实验,模拟了不同地形高度对大气环流、云微物理参量及冻雨分布的影响,结果显示：随着云贵高原大地形的抬升,准静止锋的锋线一直维持在24°N附近,但锋面越来越陡,表明高原大地形是形成云贵准静止锋的主要因素,准静止锋长时间滞留在贵州中东部和湖南西部,是该区冻雨高发的原因。随着高原的抬升,从全境的云滴碰并增长逐渐在高原东部出现冰晶融化过程,高原主体上的冻雨以过冷暖雨机制为主,而低海拔的东部地区以融化机制为主。随着高原抬升,高空逆温层及冻雨区均从贵州西北向贵州中部及东部地区移动,逆温层的存在及其冷-暖-冷的垂直结构是冻雨形成非常重要的因子。当云贵高原降至原地形的一半时,冻雨量最大,其后随着高原抬升,冻雨量开始减少。该研究有助于更深入了解云贵高原大地形对冻雨形成、范围和强度的影响机制。