Possible contributions of lid conditions during explosive cyclogenesis.

Abstract

Maritime soundings acquired during the Genesis of Atlantic Lows Experiment (GALE) Intensive Observing Periods OP's) 6 and 11 are evaluated to deterrnine the presence of mid-tropospheric, dry continental air that forms a capping in- ;rsion over cool, moist marine air, which is defined to be a lid. The strength of these lid conditions is evaluated using the id Strength Index (LSI) developed by Carlson et al. (1980) for continental thunderstorms. The environmental factors confuting to lid conditions and the lid's possible effects on explosive cyclogenesis are analyzed. During IOP 6. a predominantly >nal flow advects warm, dry continental air over the region upstream of the convective heat release. Consequently, the air- •a fluxes and horizontal advection of moist air into the central region of the low are trapped below the lid. With the superosition of upper-level forcing that lifts and weakens the lid, the IOP 6 low develops explosively. During IOP 11. the tid-tropospheric air flow is more meridional and no-lid conditions occur upstream from the cyclone region. Consequently, ie air-sea fluxes and horizontal advection of moisture is not confined to the lower troposphere. Although cyclogenesis occurs t this environment, it is not explosive. The presence of lid conditions during IOP 6 and the absence of lid conditions during DP 1 1 are suggested to be one of the factors that differentiated explosive and non-explosive maritime cyclogenesis.