晴空回波在强对流天气临近预报中的应用

该文介绍了利用多普勒天气雷达资料判断环境风场辐合、辐散的两种方法:径向速度图像特征定性识别法和EVAD技术定量计算法,并结合2009年8月两次强对流天气过程详细介绍了两种方法的具体应用。统计分析了2005-2008年5-9月晴空回波特征以及不同高度散度和对流出现时间的关系,晴空回波出现在距离雷达中心50km范围之内,反射率因子多在10～20dBZ,径向速度大部分为-5～5m/s;强对流天气出现之前,对流层低层均会出现辐合,可以利用低层连续5个体扫出现辐合作为环境场具有辐合条件的指标。根据统计结果建立了强对流天气临近预警系统,该系统在2009年6-8月试运行,预警命中率为88.9%,虚警率为29.8%,临界成功指数为64.5%,辐合提前对流天气出现时间平均为7.1h,对降低强对流天气漏报率,提高强对流天气的临近预报水平有重要意义。

The Application of Clear air Echo to Early Warning of Severe Convective Weather

Using Doppler weather radar data, two methods are implemented to estimate the divergence of environmental wind field. The first method is radial velocity image qualitative identification, through which the wind divergence is qualitatively judged by calculating the difference between positive and negative radial velocity area, and the wind speed given by comparing the value of positive and negative radial velocity in the same range rings. The other is EVAD (Extended Velocity Azimuth Display) quantitative analysis, which estimates the divergence of each level in 10—50 km around radar. They are applied in the analysis of two strong convective weather processes occurred in August 2009. The relationships between clear air echo characteristics, divergence in different levels and the time of convection appearance are statistically investigated by analyzing 50 processes from May to September during 2005 to 2008. It's found that clear air echo appears within 50 km from radar center, the reflectivity are 10—20 dBZ, and the radial velocity is about±5 m/s. There are always convergences in low levels before convective weather, so it can be used as indicator of convergent environment field when consecutive five convergences appear on low level. As the altitude lowers, convergence happens ahead of convective weather much more, and on the level of 0.5 km, the forecast time of simple rainstorm can bring forward longer than that of simultaneous multiform convective weather.
Based on the results, a severe convective weather automatic warning system (SWEAWAR) is established and run on trial from June to August of 2009 except for 5 days without radar records. Among the 57 warnings, the hit ratio is 88.9% and false warnings ratio is 29.8%, the critical success index for the early warning is 64.5% and the convergence occurs 7.1 hours ahead of the convective weather on average. The SWEAWAR system seldom misses but generate quite a few false warnings too. Overall, it can help to reduce the missing report rate and improve forecast of severe convective weather.