贺兰山东麓极端暴雨的中尺度特征

利用近10年宁夏逐时自动气象站降水、银川CD雷达、FY-2、探空和ECMWF再分析0.125°x 0.125°等高分辨率多源气象资料,在中尺度系统分型基础上,对比分析贺兰山东麓6次极端暴雨的中尺度特征.结果表明:(1)低空偏(东)南急流夜间增强并配合贺兰山地形,在东坡山前触发或增强了暴雨中小尺度系统,造成地形处降水增幅...

The Mesoscale Characteristics of Extreme Rainstorm in the Eastern Region of Helan Mountain

Using high-resolution multi-source data such as the hourly precipitation of automatic weather stations in the past 10 years,Yinchuan CD radar,Temperature of Black Body of FY-2 satellite,radiosonde data and ECMWF reanalysis data(0. 125°×0. 125°),the mesoscale characteristics of six extreme rainstorms in the Eastern Region of Helan Mountain were analyzed based on the classification of mesoscale systems. The rainstorm dominated by linear convection system is deep convection,while the rainstorm dominated by nonlinear convection system is mixed convection. The results show that:(1)The low-level southeast jet enhances and triggers mesomicro scale rainstorm systems in the eastern slope of Helan Mountain at night,leading to the extreme rainstorms are convective rainstorms with short-time heavy rainfall. The rainstorm is mainly concentrated in the eastern slope of Helan Mountain especially centralizes in the mouth of shanhong ditch with remarkable characteristics of night rain.(2)All the extreme rainstorm environment fields meet the three basic conditions of convective rainstorm. Low-level southeast jet at 700 hPa transports warm and moist flow to heavy rainfall areas,which promotes the atmospheric static instability,dynamic,thermodynamic and topography lifting trigger mechanism.There is no obvious low-level jet at 850 hPa in the rainstorm dominated by deep convection system. Vapor source is the Bay of Bengal. Limited water vapor transport condition but lower atmospheric stability is more favorable to convective rainstorm. However,the path of low-level jet at 850 hPa coincides with low-level southeast jet at 700 hPa in the rainstorm dominated by mixed convection system. Moisture transport from the Bay of Bengal,the South China Sea,the Yellow Sea and the Bohai Sea are the source of the extreme rainstorm. The more moisture transport contributes to sustained rainstorm.(3)Extreme rainstorm is mainly divided into convective precipitation in the warm zone of front,frontal convective precipitation and stratiform precipitation of frontal zone. Convective precipitation in the warm zone of front mainly occurs in the mountain areas,and its trigger mechanism is topographic uplifting. Frontal convective is triggered by warm and moisture flow lifted on the cold surface,and it could occur in plains and mountains. The movement of convective system is consistent with lower wind field. It propagates along mountain in the mountain areas but along the low-level jet in plains. Usually the movement of convective system is parallel to the direction of propagation. However,when the lower level dominated by east wind in the mountain areas,it moves approximately perpendicular to the direction of propagation with obvious train effect.(4)There is a weak cold air in linear convective rainstorm,and the types of rainstorm are mainly convective precipitation in the warm zone of front and frontal convective precipitation. Convective system propagates along the mountain and form the linear zonal echo with high degree of organization,its movement could be parallel or perpendicular to the direction of propagation. It develops strongly and maintains a long time in front of the mountain affected by topographic uplift. So the rainstorm has features such as the short duration,the small range,the great rainfall intensity and the intermittent precipitation. Cumulative rainfall of 3~4 h of rainstorm accounted for about 85% of the total amount of the process,and the area mean rainfall is much smaller than heavy rain magnitude,which reflects the feature of topographic strong convective rainstorm.(5)Cold air is active in nonlinear convective precipitation. It is dominated by frontal convective precipitation and stratiform precipitation of frontal zone. Convective system propagates along the mountain in the mountain areas and along the jet stream axis in plains,forming the nonlinear echo. It moves parallel to the direction of propagation in plains while its movement could be parallel or perpendicular to the direction of propagation in the mountain areas. Convective system moves quickly with weak intensity and low degree of organization. Rainstorm has features such as the long duration,the large range,the small rainfall intensity,the continuous precipitation and the large cumulative rainfall. The area mean rainfall is close to or equal to the heavy rain magnitude. It has the remarkable characteristic of mixed precipitation.(6)The precipitation intensity is positively correlated with CAPE amplification,echo intensity,duration of strong echo,echo tops and vertically integrated liquid,but negatively correlated with TBB. Correlations are clearer in rainstorm dominated by linear deep convection. The positive correlation between precipitation intensity R and echo intensity Z is the most significant when the echo intensity is more than 40 dBZ(R=3. 67×10-8×Z5. 222+4. 835).