STRUCTURE OF THE MARINE ATMOSPHERIC BOUNDARY-LAYER OVER AN OCEANIC THERMAL FRONT - SEMAPHORE EXPERIMENT

The Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneit es Oceaniques: Recherche Experimentale (SEMAPHORE) experiment, the thi rd phase of which took place between October 4 and November 17, 1993, was conducted over the oceanic Azores Current located in the Azores ba sin and mainly marked at the surface by a thermal front due to the gra dient of the sea surface temperature (SST) of about 1 degrees to 2 deg rees C per 100 km. The evolution of the marine atmospheric boundary la yer (MABL) over the SST front was studied with two aircraft and a ship in different meteorological conditions. For each case, the influence of the incoming air direction with respect to the orientation of the o ceanic front was taken into account. During the campaign, advanced ver y high resolution radiometer pictures did not show any relation betwee n the SST field and the cloud cover. The MABL was systematically thick er on the warm side than on the cold side. The mean MABL structure des cribed from aircraft data collected in a vertical plane crossing the o ceanic front was characterized by (1) an atmospheric horizontal gradie nt of 1 degrees to 2 degrees C per 100 km in the whole depth of the mi xed layer and (2) an increase of the wind intensity from the cold to t he warm side when the synoptic wind blew from the cold side. The surfa ce sensible heat(latent heat) flux always increased from the cold to t he warm sector owing to the increase of the wind and of the temperatur e (specific humidity) difference between the surface and the air. Turb ulence increased from the cold to the warm side in conjunction with th e MABL thickening, but the normalized profiles presented the same stru cture, regardless of the position over the SST front. In agreement wit h the Action de Recherche Programme te Petite Echelle and Grande Echel le model, the mean temperature and momentum budgets were highly influe nced by the horizontal temperature gradient. In particular, the strong ageostrophic influence in the MABL above the SST front seems linked w ith the secondary circulation due to the SST front.