The effect of shear on heat budgets in a simulated Mesoscale Convective System

Abstract

The evolution and structure of simulated Mesoscale Convective Systems (MCS) were examined using the Collaborative Model for Multiscale Atmospheric Simulations. Three numerical simulations were performed, with the amount of vertical wind shear in the lowest 2.5 km varying from 10-25 m s⁻¹. A fourth simulation was conducted using the weak wind shear profile and incorporating ice microphysics. An analysis domain was produced every two hours for the northern and southern portions of the convective line in order to analyze the temporal and spatial variability of each system. Horizontal cross-sections across each MCS revealed that increasing amounts of shear produced larger, stronger and more symmetric convective systems. The structure of the weak shear simulation was very similar to conceptual models of asymmetric squall lines. More intense convection was located along the center and southwest flanks of the line. In contrast, the northern section of the line was characterized by more isolated cells that weakened over time. A broad, cyclonic mesovortex was found in the mid-levels of the stratiform region behind the northern portion of the convective line. As the vertical wind shear increased, there was no preferred location for the most intense convection and the size of the mid-level cyclonic circulation decreased. However, a strong rear-inflow jet was produced in the moderate and strong shear cases. This jet remained elevated to the back of the convective line and may have aided in the maintenance of the strength of the convection. Profiles of area-averaged vertical velocity in the convective region showed that as shear increased the temporal variability in the vertical motion decreased. The strength and number of strong updrafts declined after a maximum vertical velocity was achieved. Also, the maximum vertical velocity occurred later in the simulation as the amount of vertical wind shear increased. As the area-averaged vertical velocity decreased, the height at which the peak velocity occurs decreased as well. Vertical profiles of heating followed a similar temporal pattern as the profiles of vertical velocity. Little spatial variability in the heat budgets was observed. The height at which the peak heating occurred was considerably lower than the peak of vertical velocity. Also, a secondary peak of heating was observed at upper-levels.