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

David W. Martin Space Science and Engineering Center, University of Wisconsin, Madison 53706

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Dhirendra N. Sikdar Geological Sciences Department, University of Wisconsin, Milwaukee 53201

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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 (<1 to ∼3 days). Downdrafts in the first cluster were weak. The second cluster was without significant downdrafts as it formed, but gradually assumed the appearance of tropical squall lines. Massive squall-line downdrafts were observed in the third cluster. Conditions favoring deep convection were high absolute moisture content of the subcloud layer, relatively high moisture content in the middle troposphere, a weak trade inversion, large-scale 950 mb convergence, and cyclonic or weakly anticyclonic 950 mb relative vorticity. Deep convection tended to parallel centers of cyclonic vorticity at 950 mb. Downdrafts were stronger where there was a distinct wind speed maximum in the middle troposphere.

The clusters occurred with a persistent westward moving cloud wave: one toward the apex, and two at the base close to the Intertropical Cloud Band (ITCB). Surface θe 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.

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 (<1 to ∼3 days). Downdrafts in the first cluster were weak. The second cluster was without significant downdrafts as it formed, but gradually assumed the appearance of tropical squall lines. Massive squall-line downdrafts were observed in the third cluster. Conditions favoring deep convection were high absolute moisture content of the subcloud layer, relatively high moisture content in the middle troposphere, a weak trade inversion, large-scale 950 mb convergence, and cyclonic or weakly anticyclonic 950 mb relative vorticity. Deep convection tended to parallel centers of cyclonic vorticity at 950 mb. Downdrafts were stronger where there was a distinct wind speed maximum in the middle troposphere.

The clusters occurred with a persistent westward moving cloud wave: one toward the apex, and two at the base close to the Intertropical Cloud Band (ITCB). Surface θe 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.

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