Initiation of Deep Convection along Boundary Layer Convergence Lines in a Semitropical Environment

Abstract

The initiation of deep convection through forcing along boundary layer convergence lines is examined using observations from the Convection and Precipitation/Electrification Experiment conducted in east-central Florida during the summer of 1991. The study is concerned with the evolution and interaction of two converging air masses that were initially separated by an intervening boundary layer characterized by neutral stability and horizontal convective rolls. As anticipated, major thunderstorm erupt when the east coast breeze eventually collides with thunderstorm outflows from the west, but unexpected convection takes place prior to their merger along a well-defined confluence zone associated with a persistent quasi-stationary roll vortex signature. Analyses using wavelet transforms confirm that linear boundary layer reflectivity features are strongly correlated with radial convergence associated with roll vortices. In this study, complementary interactions between roll vortex convergence lines and the sea-breeze front are not sufficient to trigger deep convection. However, organized convergence along the eastward-spreading thunderstorm outflows did interact periodically with roll vortex convergence maxima to initiate a series of new storms. Results from two-dimensional numerical model simulations replicate many of the observed boundary layer features. Surface heating produces circulations similar to sea-breeze frontal zones that appear near the coastlines and progress steadily toward each other as the interior boundary layer deepens. Vertical velocity maxima develop over the associated convergence zones, but weaker periodic maxima also occur within the interior air mass at intervals similar to the spacing of observed horizontal roll vortices. As the boundary layer deepens, a layer immediately above it cools, confirming organized large-scale ascent within the interior air mass. When surface heating is removed, circulation associated with this large-scale ascent collapses to a near-steady state where the width of the remaining prominent updraft is similar to its depth. This results from a balance between momentum advected by large-scale circulations and excess pressure developed at low levels near the center of the interior domain.