Aspects of the Evolution Of the Marine Boundary layer during Cold-Air Outbreaks off the Southeast Coast of the United States

Abstract

Remote sensing and in situ measurements that were obtained in February 1986 were combined with the results from a boundary-layer model to study aspects of the evolution of the marine boundary layer (MBL) during two cold-air outbreaks (CAO) off the southeast coast of the United States. Maximum total heat fluxes occurred over the Gulf Stream northeast of Cape Hatteras where very cold, dry air encountered the Gulf Stream and where the wind speeds were very large. Maximum 6-h values for the total heat flux were as high as 2015 W m?2. The magnitude of the area-average heat flux in the MBL was influenced not only by the surface air and dewpoint temperatures and the surface wind speeds off the coast but also by the area covered by Gulf Stream water (sea surface temperature greater than 22°C). The heat flux distribution off the coast rapidly produced large horizontal temperature (?10°C) and humidity (4 g kg?1) differences over distances on the order of 100 km. The large influx of moisture at the surface in response to the large latent heat fluxes caused the lifting condensation level relative to the surface to lower, influencing the production of offshore cloudiness and subsequently lowering the cloud-base height. Destabilization of the (or increased instability in the) boundary layer also occurred over short distances offshore. The boundary-layer depth increased rapidly, the rate of increase being largest between the coast and the Gulf Stream.