Latent Heat Flux over the North Atlantic Ocean—A Case Study

Abstract

Nimbus-7 SMMR data and ship observations are combined to compute the latent heat flux using the bulk aerodynamic method. Sea surface temperature (SST) and the surface humidity are determined with the microwave data. The surface wind field is derived from an analysis of ship observations of wind speed and surface pressure by means of a boundary-layer model by Bumke and Hasse. The microwave-derived SSTs are calibrated against those calculated from Advanced Very High-Resolution Radiometer (AVHRR) data. To get reliable results in the northern parts of the North Atlantic, only ascending (daytime) orbits of Nimbus-7 were used. Daytime data show a larger bias due to solar heating of the instrument but lack the complicating effects of differential cooling when the satellite enters the earth's shadow at the beginning of the descending orbits. The evaporation fields are derived over the North Atlantic for individual overpasses of the satellite during July 1983, with a spatial resolution of 1° × 1°. High temporal and spatial gradients are observed, which are consistent with the prevailing synoptic situations. In the area south of Greenland and east of Canada, where the Labrador Current is located, latent heat flux (LE) is negative even in the monthly mean. The reliability of the negative values is demonstrated by a case study. They coincide well with ship observations of fog events. The flux of latent heat can be determined with an acceptable accuracy of 25–40 W m−2 for individual values if the bias of the SMMR data can be reliably removed.