热带大气对流垂直结构与降水模拟偏差的关系:基于GAMIL3模式的分析

针对LASG/IAP发展的大气环流模式GAMIL(Grid-point Atmospheric Model of IAP LASG)的两个版本GAMIL2(G2)和GAMIL3(G3),评估了其对热带降水气候态以及对流垂直结构的模拟能力,在此基础上探究了新版本模式降水模拟改进的原因以及热带对流垂直结构与降水模拟偏差的关系。两个版本的GAMIL模式都较好地捕捉到了热带降水的主要特征,且G3的模拟结果整体优于G2。新版本的主要改进在于显著减小了热带西北太平洋正降水偏差。水汽收支诊断显示,模式降水偏差主要来源于蒸发项和水汽垂直平流动力项,而后者的偏差则来自于对流强度和对流垂直结构的共同作用。对流垂直结构偏差主要存在于赤道印度洋与赤道大西洋区域,表现为大气低层辐合分量偏小,对流卷出层高度偏高;在热带西北太平洋与赤道东太平洋区域,模式较好地还原了典型的“头重型”和“脚重型”对流垂直结构,但依然存在有整体性的对流偏深。湿静力能(MSE)收支显示,热带西北太平洋区域过量的净能量通量是模式垂直运动偏差的主要来源。而对流垂直结构偏深造成的总湿稳定度(Gross Moist Stability,简称GMS)偏大,在一定程度上抵消了模式中的净能量通量偏差,抑制了模拟的对流强度。诊断结果显示,G3中热带西北太平洋区域的降水改善主要源于对流强度正偏差的减小。G3中对流阈值和层云阈值的下调,使得对流发生频率增加,从而抑制了过大的对流强度。热带对流垂直结构与降水偏差有着紧密且多样的联系,在未来模式发展中应当予以重视。

短时强降水临近预报的思路与方法

对短时强降水主观临近预报的主要思路和方法进行综述。(1)短时强降水(flash heavy rain)是指1 h雨量在20 mm或3 h雨量在50 mm以上的降水事件。短时强降水事件的识别主要由雨强和降水持续时间两个要素确定。(2)雨强临近估计的主要根据是天气雷达反射率因子和雨强之间的经验关系,即Z-R关系。对流性雨强的估计,最简单易行的方法是将对流性降水分为大陆强对流型和热带海洋型两种类型,分别采用不同的Z-R关系。雨强估计的主要误差来源包括不适当的Z-R关系、地形对雷达波束阻挡、冰雹"污染"、强降水和冰雹对雷达波束的衰减、硬件定标偏差、被大雨淋湿的天线罩导致的衰减等。(3)判断是否出现强降水的另一要素是降水的持续时间。沿着回波移动方向高降水率的区域尺度越大,降水系统移动越慢,则持续时间越长。对于导致强降水的β中尺度对流系统,其雷达回波的移动矢量是平流矢量和传播矢量的合成。如果平均风方向(平流方向)与回波传播方向交角大于90°,称为后向传播,此时回波移速小于平均风速,移动较慢,易导致强降水。在有利于强降水的环境条件下,含有中气旋或更大尺度涡旋的β中尺度对流系统会明显增大强降水的可能。

CONVECTIVE ANOMALIES IN TROPICAL OCEAN AREAS AND LONG-LEAD FORECAST OF SUMMER RAINFALL IN SHANDONG

This study focuses on deep convection anomalies in tropical regions in winter-spring period and their possible influence on the following summer rainfall in Shandong province. On the basis of monthly precipitation wet and dry summers in Shandong are defined according to a precipitation index. Then monthly OLR data, observed by NOAA satellites, are used to diagnose the features of deep convection for both wet and dry summers. It is found that negative anomalies seem dominant prior to wet summers, while large areas of positive anomalies appear prior to dry summers in tropical oceans. The differences are remarkable especially in the western, middle and eastern tropical Pacific as well as in the tropical Indian Ocean. Correlative analysis confirms the relations between OLR and precipitation. Subtropical High, which plays an essential role in summer rainfall, is also connected with the deep conviction. Altogether eight EOF-CCA forecast models are established on the basis of the above study. The assessment of the models relies on the gauge observing precipitation in 1997 and 1998. The results show that models using spring OLR data appear to be more practicable than those using winter OLR data, and the models established with OLR in western Pacific and the Indian Ocean perform better than the others.

Intraseasonal oscillation features of the South China Sea summer monsoon and its response to abnormal Madden and Julian Oscillation in the tropical Indian Ocean

By applying the OLR and wind data, rainfall data and the Madden and Julian Oscillation （MJO） index, the paper deals with in traseasonal oscillation features and interannual differences of the South China Sea （SCS） summer monsoon, distribution of its LF circulation and convection fields and rainfall, and path of summer monsoon ISO spreading, as well as impact of tropical IndoMJO on SCS summer monsoon ISO during 19792008. It is found that （1） there are three intraseasonal oscillations of the SCS summer monsoon Intraseasonal Oscillation （ISO） in summer （from May to August） in the climate normal. The SCS summer monsoon ISO goes through six phases （exclusive of weak phase） at every complete fluctuation： developing, the strongest, weakening, restraining, the weakest, and recovering. Due to tropical LC convection spreading to the east and north, the LR convection and circulation fields in the lst3rd and 4th6th phases present the antiphase in the Arabian SeaWest Pa cific latitudinal band. Its corresponding rain bands in the lst3rd and 4th6th phases als present antiphase roughly. The rain band, mainly in tropical regions in the south of 20N, moves eastward with LR convection shifting eastward, while the rain band moves northward with LR convection shifting northward in East Asia （EA） subtropical regions in the north of 20N. （2） The SCS summer monsoon ISO presents significant interannual variations in intensity. There are three stronger monsoon in traseasonal oscillations in summer in the strong SCS monsoon ISO year. The first two oscillations from the tropical Indian Ocean ISO spread northward to the Bay of Bengal first, and then to the South China Sea （SCS） along the 10-20N latitudinal band. They are strengthened there and stimulate the ISO moving to the north to form the tropical IndoISO. Finally they spread to South China （SC） by relay way in the longitudelatitude direction. Moreover, in the weaker SCS summer monsoon ISO, the oscillation weakens greatly and irregularly in intensity with the weaker ISO spreading in the longitudelatitude direction. In average conditions, the tropical Indian ISO spreads to the SCS by about 20 days （one half ISO periods）. （3） MJO1 （the first modal of MJO index provided by the CPC） averaged value in the lst2nd pentads of April has the negative correlation with the SCS monsoon ISO intensity. The tropical IndoMJO is slightly stronger in the subsequent May to August when it is more ac tive in the lst2nd pentads of April, and the ISO also spreads strongly to the SCS, so that the SCS summer monsoon ISO strengthens. Conversely, the SCS summer monsoon ISO weakens. The abnormal MJO in the lst2nd pentads of April contrib utes to a certain theory basis for us to predict the subsequent SCS summer monsoon ISO intensity and analyze the related re gions＇ abnormal rainfall.