A numerical simulation of slantwise convection: its structure and evolution

Abstract

Weather forecasters have had difficulty forecasting mesoscale precipitation bands occurring in the regions of frontal zones. Within the last twenty years, slantwise convection (SC) through the release of conditional symmetric instability (CSI) has gained more acceptance as the cause of the rainbands under certain conditions. Bennetts and Hoskins (1979) predict that under saturated conditions, CSI would be released, forming slantwise roll circulations with sloping updrafts and downdrafts (slantwise convection), and could be a possible explanation for frontal rainbands. This study uses the Penn State/NCAR Mesoscale Model (MM5) as a diagnostic tool to simulate a slantwise convective case from the Genesis of Atlantic Lows Experiment (GALE, Dirks et al. 1988) intensive observing period 1 (IOP1, 18 January 1986 to 20 January 1986). The model results show that SC formed in an 'upscale' development as suggested by Xu (1986) with SC dominating over upright convection. The results showed that in SC's development stage, the beginning of a roll circulation with a slanted updraft and a broad downdraft with a rear inflow jet. SC in its mature stage exhibits a well developed roll circulation, a 3-dimensional structure with along-line variability, stages of re-development suggesting a quasi-steady state, and the non-conservation of negative equivalent potential vorticity (-EPV). Eventually, SC decays as it can no longer generate CSI for release. The SC compared well with other SC bands in the event and showed similar structure as observed in radar data.