Research On Recognition Technology Of Tornado Echo Characteristics In The Doppler Radar

Tornado is a strong,small-scale air vortex.It is caused by strong convective air movement under strong and unstable weather conditions.It is usually a strong cyclone generated by a funnel-shaped cloud that extends from a thunderstorm cloud to the ground.Jiangsu Province is in the high incidence area of tornado disasters in China.Statistics of China Weather Network also show that Jiangsu ranks first among the 11 provinces and cities with the most strong tornadoes from 1961 to 2015.Therefore,it is of great significance to carry out in-depth research on the tornado in this area.In this paper,using the 44 tornado processes in Jiangsu Province that occurred between 2008 and 2017 were analyzed and discussed by sounding data,NCEP reanalysis data and the " Yearbook of China Meteorological disasters" and so on,concluded as follow:(1)Tornadoes in Jiangsu Province mainly occur in more than 75% of the total in July and August each year,and they have a good correspondence with the time of the Meiyu period in Jianghuai.More than 70% of the tornadoes occur in the afternoon in the day,indicating that the thermal conditions are Tornado generation plays a vital role.The high-risk areas of tornadoes are mainly concentrated in Yancheng,Huai’an,Xuzhou and Suqian.(2)Tornadoes in Jiangsu Province mostly occur under the unstable environmental conditions of the warm air flow from the ocean to the inland,and the elevation of the condensate before and after the tornado process is usually low,and the vertical wind shear in the low layer is large.(3)Through synthetic analysis,it can be seen that the A-index of Jiangsu Province is above 15 during the tornado period,and most areas of central Jiangsu have high-value areas with an A-index greater than 20.It can also be seen that the K index of Jiangsu province is greater than 35 during the tornado period.Comprehensive analysis of the A index and K index can be seen that the atmosphere presents a significant instability during the tornado period,which is conducive to strong convective weather such as tornadoes.generate.(4)Comparative analysis of the environmental parameters of the tornado period and the precipitation period in Jiangsu Province shows that the K index and the SI index are not significantly different in both cases.The convective effective potential energy CAPE value is most important during the tornado period Above the precipitation period.Low-level vertical wind shear and uplift condensation height LCL show a clear difference: the uplift condensation height during the tornado occurs mainly at 940-960 h Pa,and the tornado will not occur when it is lower than 900 h Pa;The time period is obviously higher than that of the precipitation period,and its value is mainly set above 10 × 10-3 / s.Therefore,the convective effective potential energy,uplift condensation height and low-level vertical wind shear can be used as important indicators to distinguish precipitation and tornado.This paper uses Jiangsu Doppler radar data to analyze several typical tornado processes in northern Jiangsu and draws the following conclusions:(1)In order to adapt to the default value of the tornado warning algorithm in Jiangsu Province,the false alarm rate can be reduced by increasing the reflectance factor threshold.The NSSL TDA algorithm can be used as the basis of tornado warning,but more importantly,it should be Considered as a supplementary guidance tool for the tornado warning decision-making process,it should be used in conjunction with the forecaster’s knowledge of measured conditions and atmospheric forecast conditions.(2)Most of the tornadoes in Jiangsu Province occur at the hook-shaped echoes,and the middle cyclone circulation is formed by the combined effect of the inflow from the lower layer and the outflow caused by the downstream subsidence airflow.When the maximum shear height of the middle cyclone has a significant downward trend and its shear value increases,the middle cyclone will gradually strengthen and develop to the lower layer;when the radar observes a significant cyclonic rotation at the elevation angle of the lower layer and the next The timing of positive and negative speeds indicates that the convective clouds that may cause tornadoes have developed to the lower layers.No matter whether there is a TVS feature,it should consider issuing a tornado warning in time to provide valuable time for disaster prevention and mitigation.