| Summary: | 碩士 === 國立臺灣大學 === 大氣科學研究所 === 107 === In this study, the sustenance of the nocturnal CIs within mesoscale convective systems (MCSs) developed on 15 July 2015 during the Plains Elevated Convection at Night (PECAN) field campaign were investigated with a combination of in-situ observations and a set of Weather Research and Forecasting (WRF) experiments.
Observational analyses revealed that systems with a greater percentage of CIs near the system edge had greater maintainability than system where CIs tended to cluster in system rear. Two hypotheses were proposed to explain this phenomenon: (a) environmental instability near the system edge CIs were greater due to enhanced moisture above the boundary layer and (b) the kinematic-microphysical structures of systems with system edge CIs evolved in a manner that was favorable for system maintenance. Specifically, dual-polarimetric observations indicate stronger, more extended rear-inflow jet (RIJ) and increased riming growth within the convective updrafts for these systems.
A set of microphysical sensitivity experiments were performed to evaluate the two hypotheses. Since the ambient environmental instabilities were similar between the experiments, internal processes would play a dominate role if significant inter-model differences in updraft strength were found.
Statistical analyses suggest that simulated systems were stronger when rimed particles can sediment at different terminal velocities with regard to their sizes. RIJs in these systems tended to the stronger and more horizontally expanded, allowing more system edge CIs. In these experiments, preferential sedimentation of melting graupel increased the buoyancy gradient near system edge and created stronger negative buoyancy pressure perturbation, which enhanced the system RIJs. Stronger and more horizontally extended RIJs could subsequently strengthen the system by extra riming and deposition when the RIJs transported the graupel back to the updrafts.
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