渤海西岸一次极端大风的特征和成因分析

Characteristics and Causal Analysis of an Extreme Gale on West Coast of Bohai Sea

利用NCEP-FNL资料、FY-4卫星红外云图资料、雷达资料和自动站资料,对2021年4月15日渤海西岸由大尺度冷空气和中尺度对流系统共同作用形成的极端大风进行特征和成因分析。结果表明:①FY-4卫星红外云图云顶亮温表现出指状特征,雷达反射率因子表现为两个弱回波带在渤海西岸合并加强为一条带状回波,随着系统东移由带状回波演变为弓状回波。②上冷下暖的不稳定层结为锋面触发对流提供有利环境条件,在径向速度图上出现较大范围速度模糊和中层径向辐合。③此次大风过程具有雷暴大风和冷空气大风混合的大风特征,大风成因是由大尺度冷空气产生的动量下传、大尺度变压风、梯度风以及中尺度雷暴冷池出流共同导致的。

The observed process constitutes a combination of systemic cold air gale and thunderstorm gale.The convective system is induced and moves along with the front,resulting in sudden and extreme gale conditions akin to thunderstorm gales.Upon transitioning,the convective system mirrors the persistent traits of a systemic cold air gale.This study uses NCEP-FNL data,FY-4 satellite infrared cloud data,radar data,wind profile data,and automatic weather station data to investigate the characteristics and causes of the extreme gales.The principal findings are as follows:(1)The eastward progression of high trough at 500 hPa and low vortex at 850 hPa established a widened warm region at 850 hPa,as well as an unstable stratified complex along the west coast of the Bohai Sea.The transition of the front triggered convection,leading to extreme windy weather along the west coast.The reflectivity factor revealed that two bands of weak echoes along the west coast of the Bohai Sea were converging and intensifying into a single band,which then developed from a band to a bow echo with a gap in the front,as the system moved eastward.Notably,substantial velocity blurring and mid-level radial convergence emerged from the velocity diagram.(2)The gale event occurred in early spring when short-term forecasts did not accommodate convective weather,accounting only for the manifestation of frontal precipitation and cold air gales.Prior to the emergence of convection,cold advection at 5oo hPa and pronounced warm advection at 700-925 hPa were present over the west coast of the Bohai Sea.The unstable stratum between the upperlevel cold and lower-level warm layers furnished favourable environmental conditions for the frontal transition to induce convection.In this context,the CAPE and DCAPE values derived from the NCEP forecast model served as a reference for thunderstorm and gale forecasting.(3)The gale progression was influenced by the downward momentum transfer of large-scale cold air,variable pressure winds,gradient winds induced by large-scale cold air,and the effects of mesoscale thunderstorm cold pool outflow invoked by convective systems.The systemic cold air gales,propagated due to the downward momentum transfer of large-scale cold air,variable pressure winds and gradient winds,exerted an augmenting impact on the mesoscale thunderstorm outflow.