Numerically Simulation Of Complex Charge Structure In Severe Thunderstorms

The purpose of this study was to understand the process of complex charge in severe thunderstorms. A three-dimensional dynamics-electrification coupled model was used to simulate a severe thunderstorm observed on Jun 29,2000 during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) to permit analysis of the conditions that might easily cause+CG flashes and analyze evolution characteristic of charge structure. The results showed:(1) Strong updrafts play an important role in the occurrence of intracloud flashes. However, frequent+CG flashes require not only strong updrafts but also strong downdrafts in the lower cloud region, conditions that correspond to the later phase of the mature stage and the period of the heaviest solid precipitation of a thunderstorm.(2) The charge distributed in updraft regions with normal tripole charge structure when stronger updraft and without obvious downdraft existed in severe thunderstorms. Once storms intensity strengthen further with obvious downdraft and strong precipitation, it will result in positive and negative charge regions emerged in updraft and downdraft regions with more complex charge distribution. During this stage, strong updrafts elevated each charge area in the updraft region to a higher level, which resulted in an inverted tripole charge structure.(3) A wide mid-level region of strong positive charge caused largely by positively charged graupel at relative lower temperature, presented in the middle of the updraft region because of a non-inductive ice-ice collisional charging mechanism with ample effective liquid water content in mixed phase region. However, the charge structure revealed five vertically stacked charge regions in downdraft region, alternating in polarity. Since inductive charging scheme function made much of the graupel and hail with negative charge in lower region.(4) Much of the graupel/hail outside the updrafts was lowered to cloud-base by strong downdrafts. In this area, the graupel/hail was charged negatively because of the transportation of negatively charged graupel/hail from higher regions of negative charge in the updrafts, and via the inductive charging mechanism of collisions between graupel/hail and cloud droplets at the bottom of the cloud. Consequently, a large region of negative charge formed near the ground. This meant that+CG flashes were initiated more easily in the lower inverted dipole, i.e., the middle region of positive charge and lower region of negative charge. Frequent+CG flashes began almost synchronously with dramatic increases in the storm updrafts, hail volume, and total flash rate. Therefore, the occurrence of+CG flashes appears a good indicator of storm intensification and it could have some use as a predictor of severe weather in the form of hail. The occurrence of inverted tripole charge structure betokened thunderstorm with stronger intensity and damage.