Low-Level Updrafts in Stable Layers Forced by Convection

Abstract

An investigation is made of the stability of a convectively unstable atmosphere in the presence of a stably stratified layer beneath, which is moving with a constant velocity relative to the upper air. This work is an extension of the linear model presented as part of the recent study of Orlanski and Ross in which they sought to explain the structure of their simulated squall line. A stability analysis shows that two modes are possible: 1) the gravitational or convective mode due to the unstable stratification in the upper layer which modifies the stable region below and 2) the classical Kelvin-Helmholtz mode due to shear across the interface. The Kelvin?Helmholtz mode is of limited physical interest in this case. On the other hand, the gravitational mode produces an updraft structure similar to updrafts in the stable lower layer of a convective system. Analysis of the horizontal displacement of the surface convergence for this mode relative to the convergence in the convective zone shows this displacement to depend primarily on the wind, stratification, and depth of the stable lower layer. The resulting relationship provides a method for determining whether a dual or single updraft will occur in a convective system.