The Prelimiary Application of Radar Data to the Lightning Warning of Isolated Storm Cells

The radar data, the sounding data and the lightning location data of 20 isolated cells in Beijing from July to August in 2005 are analyzed. Considering the analysis and the overseas studies, it is concluded that 40 dBz is an appropriate predictor for the regional lightning warning, because the difference of 40 dBz echo top between thunderstorms and non-thunderstorms is maximal among various echoes. The application of this predictor is not as simple as other research results. According to the analysis on the evolution of various echoes, it is considered that the threshold of 40 dBz echo top height should be located at the 0 ℃ stratification level. In the meantime, whether 40 dBz echo top height reaches the -10 ℃ stratification level, the proportion of some kinds of echo should be taken into account. The forecast accuracy will be improved markedly when the factors mentioned before are taken into consideration. Based on the above studies, a forecast process of lightning is given. First, it is checked that whether the 0 ℃ stratification level is reached by the 40 dBz echo top height or not. If it is affirmative, the cell will have the great potential of becoming a thunderstorm. If not, the cell will be concluded to be a non-thunderstorm. Then, checking whether the -10 ℃ stratification level is reached by the 40 dBz echo top height will be necessary to judge the thunderstorm further. If that height is reached, the first lighting in the cell will occur in 15 minutes. If not, the P value should be used for subsidiary discrimination. If the P value exceeds 5% and keeps above this level for a period of time, the first lightning will also occur in 15 minutes. A prediction test for this method is carried out using the data of the other 22 isolated cells in Beijing in the same period. The test result shows that the method is effective in judging whether a normal cell will become a non-thunderstorm cell or not and predicting the time of the first lightning in a thunderstorm. In addition, a linear relationship is found between two time intervals. One interval is between the occurring time of the first intercloud flash (IC) and the first cloud-to-ground (CG) in a cell. Another is between the appearing of the first 25 dBz in the cell and the V value (change rate of 35 dBz echo thickness in the cell) reaching its maximum. A reference is given for the CG prediction.