Feedback effect of electric field force on electrification and charge structure in thunderstorm

Abstract

The electrification within the thunderstorm, caused mainly by inductive and noninductive charging mechanism, can produce strong local electric field inside the thundercloud. Due to the resulting electric field force, the vertical velocity of the graupel and hail particles which are the main in-cloud charge carriers, would change. As a feedback, this variation could affect the original electrification and charge structure of the thunderstorm. In order to investigate such a feedback effect, a weather research forecasting (WRF) model coupled with explicit lightning physics including charging and discharge lightning scheme (hereafter WRF-Elec) is employed and modified in this study. We derive the formulas for calculating the mass-weighted mean terminal velocities of graupel and hail under the balance among gravity, resistance and electric field force. Then, the National Sever Storm Laboratory (NSSL) two-moment bulk microphysics scheme is modified by adding the calculating code with consideration of electric field force (EFF) acting on the fall speed of graupel and hail particles. Eventually, the two-coupled WRF-Elec is developed successfully.#br#Based on this modified WRF-Elec, sensitivity tests are conducted to quantitatively investigate the influences of EFF on the thunderstorm electrification and the corresponding charge structure in an idealized supercell case. The results show that during the rapid enhancement of the thunderstorm, the grid-scale mass-weighted mean fall speed of graupel and hail vary significantly in consideration of EFF, with the maximum values both exceeding 4 m/s, although this situation occurs within a local area and lasts a short time. The action of EFF tends to enhance the falling of graupel and weaken the falling of hail. The influences of EFF on those graupel and hail particles with smaller-size and lower number concentration are stronger, as determined by composite factors of the strength and polarity of electric field, the diameter and number concentration of graupel and hail, and their charge density and polarity as well. The adjustment of the terminal velocity of the graupel and hail in consideration of EFF, eventually results in increasing the rate of both inductive and noninductive charge separation, where the inductive charging is the much more significant one. This leads to a grid-scale total charge density variation of -0.6-1.2 nC/m3 and a redistribution of the charge structure in the thunderstorm, and correspondingly, an increase of the local vertical electric field by 5 kV/m, thus producing stronger lightning eventually. In addition, due to the effect of electric field force, the mass mixing ratio of four precipitation particles including graupel, hail, ice crystal and snow is changed in the ranges of -0.09-0.24, -0.16-0.04, -0.04-0.05, and -0.01-0.006 g/kg, respectively. Therefore, the electric field force in thunderstorm affects not only the electrification and charge structure, but also the microphysical process. Generally, the overall influence of EFF on electrification tends to be positive, and the feedback effect of EFF on the charge structure should not be neglected.