A Case Study of West Atlantic Cloud Clusters: Part 2. Dynamic Structure and Environment

Abstract

We describe the behavior, dynamics and setting of three cloud clusters. The clusters occurred in the western Atlantic Ocean between 21 and 24 July 1969. Wind and cloud analyses made from intensive ship and satellite observations of the Barbados Oceanographic and Meteorological Experiment (BOMEX) were supplemented by analyses of thermodynamic structure (temperature, moisture and stability) from Martin and Sikdar (1975). Each of the clusters moved west to northwest. In each cluster high-energy near-surface air was ingested on the front or right flank and vented in the upper troposphere toward the right flank or rear. The clusters were distinguished by a range of speeds (6–15 m s⁻¹), maximum areas (30 000–200 000 km²) and lifetimes (_e was high within the cloud wave, and 950 mb relative vorticity was mostly cyclonic. The trailing edge of the cloud wave marked a surge in the northeast trades. The cloud wave was linked with a layer of warm, dry Saharan air between 650 and 850 mb. Baroclinicity across the front of the Saharan air supported a 20 m s⁻¹ east-southeasterly jet at 650 mb. There was a ridge over the trailing edge of the cloud wave, and a trough over the cloud wave, 200–500 km downstream. Air advanced relative to the wave, sinking as it approached the ridge and rising in passing from ridge to trough. In this case the strongest controls on deep convection were exercised from the middle troposphere.