Evolutions and Cloud-to-ground Lightning Features of Two Mesoscale Convective Thunder Storm Systems in Yunnan

The circulation background of mesoscale convective thunderstorm system is diagnostically analyzed from 21 September to 23 September in 2010 by NCEP/NCAR data, and the evolutions and cloud-to-ground lightning activity features of two mesoscale convective thunderstorm systems are analyzed by synchronous stack of lightning detection system observations and FY-2E satellite images. Results show that the advantageous circulation background conditions of high energy, high humidity, and lifting dynamism are also supplied for the mesoscale convective thunderstorm systems by the weakened thermal depression. Mesoscale arc convective cloud belts appears first, on which partial convective cloud clusters gradually develop to mesoscale storm cloud clusters and then run into mesoscale convective storm complexes moving to the west along the same route, leading to strong thunderstorm weather and frequent CG lightning activities on the way.The negative cloud-to-ground (negative CG) lightning is predominant compared to the positive cloud-to-ground (positive CG) lightning during the whole lifetime of a thunder cloud cluster. But not only the storm cloud cluster contracture and the CG lightning activity feature change with time, but also positive CG and negative CG lightning frequency is well related to the cloud top temperature, which is related to the three-negative-polar structure of thunder cloud over the lower latitude plateau of China. When the cloud top temperature TBB descends and TBB isoline density increases, the thunder cloud cluster develops gradually, the low TBB center locates to its foreside, and the negative CG lightning frequency leaps. When the TBB descends to the minimum, the thunder cloud cluster develops to the maturation, negative CG lightning frequency gets to an apex, and positive CG lightning begin to take place. When the TBB ascends and TBB isoline density decreases, the thunder cloud cluster weakens gradually, the low TBB center closes up its center, the negative CG lightning frequency decreases rapidly, and positive CG lightning frequency increases to the apex gradually. In the meantime when TBB of a thunder cloud cluster is lower, convective development is stronger and CG lightning activity is more furious.The storm cloud cluster contracture and the CG lightning spatial distribution are asymmetric. In its foreside TBB is lower and the TBB isoline density and grads are bigger than those in its rearward. Negative CG lightning mainly cluster in its foreside with big TBB grads and within the low center where TBB is no higher than-56℃, while sparse positive CG lightning usually disperse within the low center when TBB is no higher than-56℃, namely taking place in the rearward of dense negative CG lightning. The activity of positive CG and negative CG lightning are negatively correlated. Positive CG lightning hardly takes place during the negative CG rapid incremental phase, it usually begins during the negative CG lightning mild phase, and it increases when negative CG lightning weaken. So negative CG lightning is the result of storm cloud cluster development and positive CG lightning is the result of storm cloud cluster developing to mature.