弱天气系统强迫下黄土高原强对流云的初生及演变

Initiation and Evolution of Storm over Loess Plateau for Weak Synoptic Forcing Situations

基于NCAR的风暴识别追踪系统(Thunderstorm Identification Tracking Analysis and Nowcasting,TITAN)及其对流云识别算法,在个例分析的基础上,归纳总结弱天气系统强迫下陕北黄土高原对流云的初生、演变机制。结果表明:(1)夏季弱天气系统强迫下,对流云的触发可分为地形、干线、冷池、暖堆、辐合线等5种机制,其中地形强迫作用最为显著,其次为干线、辐合线、暖堆、冷池;(2)合适的湿度场是触发对流云的关键条件,如果有干线存在,即使动力或热力条件较弱,也会触发较多对流云生成,但当抬升作用较弱时,对流云通常不会进一步发展;(3)地形、暖堆、边界层辐合线的作用在于加速对流云触发并促使其强度加强,但这些因子在不同时段所起的作用是不同的,清晨至正午地形抬升占主导作用,正午前后地面暖堆作用加强,午后地面辐合线的作用更为显著。

Using Thunderstorm Identification Tracking Analysis and Nowcasting System( TITAN) that developed by NCAR,the storm initiation and evolution over the loess plateau were analyzed. The results showthat:( 1) Forced by weak synoptic system in summer,the most common initiation mechanism of convective cloud over the loess plateau was terrain forcing,the remaining triggering mechanism followed by dry line,convergence line,warm area and clod pool.( 2) Appropriate humidity is a key condition for triggering convective. If there is the humidity gradient values,even though dynamic force and heating power are weak,much convective cloud may form. However,when the uplifting power is weak,the convective cloud can' t develop further.( 3) The effect of topography,warm area and boundary layer convergence line is to speed up the occurring of the convective cloud and to strengthen it. However,all these factors affect it differently in different time of a day,topographic lifting having strong influence in the morning,the force of warm areas increasing around 12: 00( local time),boundary layer convergence line functioning mainly in the afternoon. By analyzing many similar cases,a conceptual model of storm initiation and evolution is also presented.