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.