我国中东部地区夏季中尺度对流系统形成前物理量诊断分析

利用2008—2010年夏季(6—8月)FY-2地球静止卫星红外云图资料识别出我国中东部地区(110°E—124°E,27°N—40°N)共208个中尺度对流系统(mesoscale convective system,MCS)和 174个不能增长发展成为MCS的普通雷暴群(widespread convections,WCS).提取MCS形成前约6 h和WCS成熟时(个数最多)的NCEP再分析资料(时间间隔6 h,空间分辨率1°×1°),通过对表征水汽、动力和热力等条件的基本物理量和一些常用衍生物理量采用平均值、标准差等常用统计方法、动态合成和评估方法逐步筛选和分析诊断两种系统环境物理量场,最终从众多物理量中挑选出了能显著区别两种系统的物理量(即MCS形成的关键物理量),分别为强天气威胁指数、修正的K指数、地面抬升指数、2 m比湿和0~3 km垂直风切变,希望对预报我国中东部地区MCS发生与否提供一定的科学依据.

Diagnostic analysis of physical quantities for the precursor environment of mesoscale convective system during summer in central-eastern China

Mesoscale convective systems (MCSs) can devastate property and possessions by generating heavy rainfall and severe weather during summer over central-eastern China. If the critical parameters in determining whether concentrated convection would undergo upscale growth into an MCS are found, then the damages produced by severe weather can be prevented and reduced in central-eastern China. The NCEP data were used to compare the environments prior to MCS development with the environments of widespread convections (WCS) that did not undergo upscale growth and organization into MCS across central-eastern China (110°E—124°E,27°N—40°N) during summer (June—August) of 2008—2010.This analysis is based on the recognition result of 208 MCSs and 174 WCS. One method used in this analysis involved taking a single value of a given field for each case at a specific point. For MCSs the NCEP analysis data (time resolution every 6 h, spatial resolution 1°×1°) were extracted at 6 h prior to development from the location of centroid of the MCS at initiation, which were taken at the time of the maximum number of convective cells from the approximate center of the group of thunderstorms for WCS. A statistical analysis of mean and standard deviations of the data was made once all of the point-value data were compared for the different conditions. A 14°×10° movable grid centered on each case for the precursor environments of MCS and WCS was used to create composites of numerous parameters. An objective method of evaluation (POD, FAR, HSS, TS and BS) including making a dichotomous forecast was also implemented to test the skill and accuracy of the various parameters in forecasting MCSs.The precursor environments of hundreds of MCSs and instances of WCS were examined to gain insights into environmental conditions that support a concentrated group of thunderstorms into an MCS. Significant differences were found during these physical quantities between MCSs and WCS: (1) A statistical analysis of point-value data was performed to compare the precursor environments of MCSs and WCS through analyzing some basic physical and commonly derived quantities by using the methods of statistics of mean and standard deviations. The different quantities were obtained to identify MCSs and WCS: 2 m specific humidity, vapor flux divergence at 1000 and 850 hPa, relative humidity, specific humidity, temperature-dewpoint spread and temperature advection of 850 and 700 hPa, index of conditional convective stability (ILC), Showalter index (SI), lifted index on surface (LFTXS), convective available potential energy on surface (CAPES), total index (TTI), severe weather threat index(SWEAT), modified K index(MK), 0~1 km vertical wind shear and 0~3 km vertical wind shear. (2) Obvious characterization were found between MCSs and WCS by using storm-relative composites: 2 m specific humidity, relative humidity and temperature-dewpoint spread of 850, temperature advection of 850 and 700 hPa, SI, LFTXS, CAPES, TTI, SWEAT, MK, 0~1 km vertical wind shear and 0~3 km vertical wind shear. (3) Based on the previous results, five most important parameters in determining whether concentrated convective would undergo upscale growth into an MCS are obtained by using evaluation methods: LFTXS, SWEAT, MK, 2 m specific humidity and 0~3 km vertical wind shear.Statistically significant differences were found between these conditions through analyzing the environments of MCSs and WCS. The 2 m specific humidity stands for the water vapor in the atmosphere which is an essential factor for severe precipitation weather. MK and LFTXS represent a physical mechanism of thermal and instability factors that act to make thunderstorms occur. SWEAT represents the dynamic and thermal conditions that enhance and sustain the development of thunderstorms. The vertical wind shear further enhances upward motion at the leading edge of storm-generated cold pools, which promotes storm interaction and organization. The combination of these mechanisms appears to be the most favorable setting that leads to MCS formation and development.