The marine boundary layer: impacts and modifications on a severe, deep convective system

The development and maintenance of severe deep convective storms depends critically on storm interactions with the boundary layer. In instances when convection crosses from land to a cooler water surface, the changing boundary layer properties can have an important effect on convective structure and intensity. When convective systems move over bounded water areas, such as lakes, their evolution over the lakes can be quite complicated. In these cases, interactions between the convective boundary layer (CBL) originating over land, marine boundary layer (MBL) and storm generated cold pools can occur.

When spring and summer convective systems move over the Great Lakes they commonly dissipate or weaken as their surface-based source of buoyant energy is depleted. However, numerous examples during which convective systems strengthen or maintain their intensity over the lakes have been observed. This presentation will focus on such an event on 26 July 2005. On this date, a line of convective storms developed on the upwind shore of Lake Erie and maintained intensity as it crossed the water surface, even sparking new convection. This study seeks to understand convective evolution over the lake in light of interactions between the CBL upwind of Lake Erie, the MBL associated with the lake surface, and the developing surface cold pool created by the convection.

Surface observations at the time of the convective line initiation show upwind environmental temperatures of around 32oC, with a cooler lake surface of 27oC. Buoy observations taken before development of the cold pool indicate that the near-water air temperature (29oC) was lower than those upwind over land. In addition, there is evidence of a lake breeze circulation during the event. Taken together, these suggest the presence of a relatively cool, stable MBL over Lake Erie relative to the CBL over land. The NAM40 model, NOAA rawinsonde and ACARS observations from commercial aircraft, as well as radar data from Cleveland, OH (KCLE) and Detroit, MI (KDTX) were used to attempt to quantify the structure and profiles of the CBL, MBL and resulting convective cold pool. These were then applied to squall line theory proposed by Rotunno, Weisman and Klemp (RKW, 1988) and Weisman and Klemp (2004) to offer an explanation for how lake interaction modified the original squall line system. Preliminary results suggest a complex interaction/modification of the three boundary layers; the CBL is modified (or perhaps elevated) by the cooler MBL as it is advected over Lake Erie by the synoptic wind, and the MBL is then displaced vertically by the cooler (25.5oC) cold pool. We propose that lifting due to interactions between these boundary layers contribute to further convective development as well as a change from linear to cellular convective structures along the squall line.