A Composite Analysis of the Boundary Layer Accompanying a Tropical Squall Line

Abstract

Rawinsonde data from the GARP Atlantic Tropical Experiment (GATE) have been used to prepare a composite of the boundary layer associated with a tropical squall line on 12 September 1974. Soundings from fifteen GATE ships have been composited with respect to the center of the squall radar echo, as determined by Gamache and Houze (1982), for a 9-hour period during which the squall system was approximately in steady-state moving to the southwest at 13.5 m s?1 Particular emphasis is given to the modified boundary layer or wake that trails the leading edge of the squall system. Immediately behind the ?200 km wide squall line to a distance of ?100 km exists a very cool, stable layer of air several hundred meters deep just above the ocean surface. This region is followed by a Shallow mixed layer ?100 m deep which gradually increases to ?500 m (a depth characteristic of undisturbed regions) at distances behind the line ranging from 500 km on the northwest side to ?300 km on the southeast side. Surface temperatures and specific humidities within the squall wake are as low as 4°C and 3 g kg?1 below the surrounding environmental values. Composites of wind, surface pressure, temperature, specific humidity, moist static energy, stability atop the mixed layer inversion, and fluxes of sensible and latent heat have been computed and are displayed relative to the squall center. The shallowest mixed layer depths coincide with the surface wind diffluence center located beneath the northwest portion of the mesoscale precipitating anvil cloud trailing the squall line. The composite observations provide a coherent, three-dimensional view of a tropical squall boundary layer which should prove valuable for models of squall wake recovery.