Mesoscale and Convective–Scale Downdrafts as Distinct Components of Squall-Line Structure

Abstract

This paper describes the two different kinds of downdraft air frequently observed to the rear of some squall lines at low levels. The primary data source is measurements taken during aircraft penetrations of certain low-latitude squall lines; they are supplemented by satellite data, radar data, surface meteorological data, and soundings ahead of and behind the squall lines. A shallow layer of cool, near-saturated air occupies the lowest few hundred meters and is separated by a marked stable layer from a deep layer of highly unsaturated air. The lowest layer is hypothesized to be the product of convective-scale saturated downdrafts, and the drier air is shown to be the result of mesoscale unsaturated downdrafts as described by Zipser (1969). Over a warm ocean, there is a large latent and sensible heat flux from the surface into the lowest layer, which rapidly becomes a new mixed layer and incorporates the drier air from above by entrainment. Mesoscale sinking in the post-squall region is shown to slow the deepening of the shallow mixed layer. The surface dew point drops during squall passage, but is observed to recover more slowly than the temperature toward ambient values. Frequently, tile dew point reaches its absolute minimum value several hours after squall passage, clearly indicating that enhanced evaporation from the surface can be less than the moisture flux through the top of the mixed layer. An idealized model describing a class of squall lines is presented and discussed. The thermodynamic transformations that take place in each layer of air are identified hypothetically, and they can account for the observed properties of each airstream both before and after passage through the system. The proposed structure permits the coexistence of convective-scale saturated downdrafts and mesoscale unsaturated downdrafts, the former in the active convective clouds of the squall line, the latter farther to the rear.