A diagnostic case study of the 9-11 August period during the GARP Atlantic Tropical Experiment

Abstract

Several analysis techniques are used to determine the structure of a disturbance which traversed the GATE A/B ship array from 9 to 11 August 1974. Emphasis is placed on the identification of an easterly wave and the tropospheric structure associated with it. The disturbance is also viewed as a pre-tropical cyclone cloud cluster. The vorticity, divergence and vertical motions fields are compared to fields derived from western Pacific cloud clusters. The disturbance proves to be associated with an easterly wave which is best defined at 600 mb and is cold core in the mid-troposphere. The convection within the disturbance is closely associated with the position of the 600 mb trough axis and is also aligned with the surface flow. Two low level vortices developed within the disturbance but in regions of minimum convection. The northern vortex persisted and strong convection developed around it. A mid-tropospheric jet (~20 m/s) is evident after the wave passage. Associated with the jet is warm, dry Saharan air which significantly changes the temperature and moisture fields over the array. Vertical wind shears are evident throughout the period and are associated with an upper-tropospheric jet (rv40 m/s) and a mid-tropospheric jet (~20 m/s). Despite the vertical wind shear, the horizontal wind field appears very organized. Streamline analysis for the 600 mb and 150 mb levels show evidence of compensating motion between the two levels (i.e. a trough at 600 mb is compensated by a ridge at 150 mb). The vorticity, divergence and vertical motion fields appear different across the array. The intensity and shape of the profile are highly dependent on the area over which the computations are made. Qualitatively there appears to be little difference between profiles from the Western Pacific and the GATE A/B-scale ship array.