The Wake Low in a Midlatitude Mesoscale Convective System Having Complex Convective Organization

Abstract

An analysis has been carried out of the surface pressure field in a highly complex mesoscale convective system that occurred on 3-4 June 1985 during the Oklahoma-Kansas Preliminary Regional Expeximent for STORM-Central (OK PRE-STORM). During its mature stage the storm consisted of two primary intersecting convective bands approximately 200 km in length, one oriented NIE-SW (to the north) and the other N-S (to the south), with a stratiform precipitation region extending to the northwest of the bands. Stratifonn precipitation was weak to nonexistent in the southernmost portion of the storm. Although the organization of the storm was complex, the surface pressure field resembled those associated with simpler, quasi-linear squall systems containing trading stratifom regions: a mesohigh existed neat the convective line and a wake low was observed to the rear of the stratiform region. A strong system-relative, descending rear inflow jet was observed in the northern part of the storm near the wake low. Significantly, only the northern portion of the storm had a trailing stratiform region and it was only in that region that a wake low and a descending mu inflow jet occurred. An analysis of dual-Doppler radar data taken in the northern part of the storm indicates remarkably strong, localized subsidence at low levels within the rear inflow jet, up to 6 m s?1 on a 10-km scale at the back edge of the trailing stratiform region. The maximum sinking occurred (a) to the rear of the highest reflectivity portion of the trailing stratiform region, (b) within the region of the strongest low-level reflectivity gradient, and (c) was coincident with the strongest surface pressure gradient [up to 2 mb (5 km)?1] ahead of the wake low center. These findings indicate that the trailing stratiform precipitation regions of mesoscale convective systems can be dynamically significant phenomena, generating rapidly descending inflow jets at their back edges and, con-sequently, producing pronounced lower-tropospheric warming, intense surface pressure gradients and strong low-level winds.