A Case Study of Atlantic Cloud Clusters: Part 1. Morphology and Thermodynamic Structure

Abstract

Ground based data from the Barbados Oceanographic and Meteorological Experiment (BOMEX) have been combined with satellite data in a study of the time changing cloud and thermodynamic structure of cloud clusters and their environment. Digital images from the third Applications Technology Satellite (ATS-3) were analyzed as ?movie loop? sequences on a computer controlled image storage, display, and processing device called McIDAS. Three-to six-hourly soundings from the five BOMEX ships were corrected for radiation and lag errors in moisture, then analyzed in time sections of temperature anomaly, relative humidity, and equivalent potential temperature. The integration of satellite and surface sounding data was accomplished through plexiglas models in which time section strips were treated as space sections. Extended meridional and quasi-zonal space sections of temperature anomaly, relative humidity, and equivalent potential temperature then were constructed from the models. The time sections reveal a complex stratification of the tropical troposphere, with multiple interwoven layers superimposed on a basic pattern of surface mixed and cloud layers, a trade inversion layer, and an upper tropospheric layer. These layers weakened and sometimes vanished in the vicinity of cloud clusters and the Intertropical Cloud Band (ITCB). Shallower convective systems and patterns were also reflected in this laminar structure, but mainly at lower levels. ATS picture-sequences covered three clusters: two along the ITCB and one to the north of the ITCB. The first of the ITCB clusters contained moderate convection with little organization as it traversed the ship array; the second had distinct squall characteristics. The northern cluster formed in two stages just east of the array: an arc of congestus cloud gradually increasing in area, then explosive development of cumulonimbi. This cluster and the squall cluster formed along the leading edge of a middle tropospheric layer of Saharan origin. Cluster formation occurred close to but not within the areas of greatest parcel instability, where deep convection apparently was inhibited by a strong trade inversion and a very dry mid-troposphere.