Suppression of Severe Winds in Simulations of Nocturnal Quasi-Linear Convective Systems

Severe winds at the surface are frequently produced by quasi-linear convective systems (QLCSs) during the day, but are often suppressed by the presence of a stable boundary layer (SBL) during the night, the dynamics of which are not well known. Previous hypotheses have held that the reduction in magnitude of the negative buoyancy within convective downdrafts by the SBL prevents the downdrafts from reaching the surface.

This presentation summarizes a set of idealized (CM1) simulations of QLCSs in the absence of a SBL and in the presence of a SBL of varying depth and stability. These simulations are used to examine the dynamics underlying the production and suppression of severe surface winds within QLCSs. In agreement with previous studies, it is found that the QLCS must be producing a surface-based cold pool in order for severe winds to occur. However, it is found that even in relatively deep and strongly stable boundary layers in which severe surface winds do not occur, downdraft parcels still routinely reach the ground.  Rather than a lack of air spreading out from downdrafts hitting the ground, the horizontal wind speeds are reduced by the development of a buoyancy-induced high pressure near the leading edge of the cold pool. This buoyancy-induced high pressure results from the buoyancy minimum being above the surface within significant SBLs rather than at the surface, as it is in QLCSs with no SBL.