Possible roles of fall speed parameters of different graupel densities on microphysics and electrification in an idealized thunderstorm

Abstract

AbstractGraupel is often parametrized as “medium‐density” ice particles with a bulk density of 400 kg m−3 and corresponding fixed fall speed parameters in numerical models. In natural clouds, however, graupel has an extensive range of densities, and its fall speed is closely related to its density ρg. In this study, the possible responses of the microphysical and electrical structures of a simulated thunderstorm to varying ρg and its corresponding fall speed parameters were examined using the Advanced Research Weather and Forecasting Model (ARW‐WRF) with an explicit charging and discharge lightning scheme. Six sensitivity tests were performed with different ρg and corresponding fall speed parameters. ρg ranges from 350–850 kg m−3, to represent relatively low‐ (350, 450 kg m−3), medium‐ (550, 650 kg m−3), and high‐ (750, 850 kg m−3) density assumptions. In low‐density cases, the ice water path (IWP) could be comparable with the liquid water path (LWP), while the LWP exceeds the contribution of the IWP to the total water path (TWP) in high‐density cases. The results show that melting rates and precipitation were enhanced when ρg was increased from low to high values, resulting in a smaller size and lighter mean mass due to a shorter residence time and faster fall speed.Different assumptions about graupel density also resulted in different electrical structures in the simulated clouds. The clouds produced in the low‐ and medium‐density cases are mainly charged with conventional tripole or positive dipole structures, while the ones formed in the high‐density cases present “bottom‐heavy” tripole structures. The upper positive regions become weaker, with reduced negative noninductive charging rates, as graupel falls faster and less graupel is negatively charged at higher altitude. It is also found that a faster fall speed of graupel does not necessarily mean stronger flash density, although lightning activities are correlated with higher fall speed.