Aircraft Observations of Meteorological Conditions along Africa's West Coast between 30°–35° South

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  • 1 Oceanography Department, University of Cape Town, Rondebosch 7700, South Africa
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Abstract

Meteorological conditions along the west coast of Africa between 30° and 35°S were monitored by means of an instrumented aircraft flown at a height of 150 m over the continental shelf and coastal margin. Case studies, selected from 600 h of aerial survey data encompassing a variety of weather conditions, are used to illustrate mesoscale gradients in the wind, air and sea temperature fields forced by local topographic features. These results focus on the summer season, when the periodic eastward ridging of the South Atlantic high pressure cell gives rise to pulses in SE trade wind flow of variable depth. A composite of four grids describes many of the summer mesoclimatic features found along the west coast, including: divergence of southerly wind flow over the continental shelf in response to friction and seabreezes over the coast; low-level wind jets off the mountainous capes at 33° and 34°S; and wind shadows leeward of the capes where subsidence is enhanced. Topographic channeling is contrasted under inversion heights of 1800 and 600 m using two aerial surveys at the beginning and end of a SE wind event. In the deep flow case, weak meteorological gradients were recorded, while the shallow wind field exhibited an alongshore variability which perturbed the air/sea temperature fronts over the continental shelf. Analyses of wind vorticity, divergence and dewpoint temperature provide evidence of topographically anchored circulations during shallow SE trades. Over a smaller domain encompassing the Cape Peninsula (34°S, 18°E), aerial survey results define mesoscale meteorological conditions during four discrete phases in the subtropical/midlatitude weather cycle. Influences of the synoptic-scale climatology on local topographic and thermal circulations are described.

Abstract

Meteorological conditions along the west coast of Africa between 30° and 35°S were monitored by means of an instrumented aircraft flown at a height of 150 m over the continental shelf and coastal margin. Case studies, selected from 600 h of aerial survey data encompassing a variety of weather conditions, are used to illustrate mesoscale gradients in the wind, air and sea temperature fields forced by local topographic features. These results focus on the summer season, when the periodic eastward ridging of the South Atlantic high pressure cell gives rise to pulses in SE trade wind flow of variable depth. A composite of four grids describes many of the summer mesoclimatic features found along the west coast, including: divergence of southerly wind flow over the continental shelf in response to friction and seabreezes over the coast; low-level wind jets off the mountainous capes at 33° and 34°S; and wind shadows leeward of the capes where subsidence is enhanced. Topographic channeling is contrasted under inversion heights of 1800 and 600 m using two aerial surveys at the beginning and end of a SE wind event. In the deep flow case, weak meteorological gradients were recorded, while the shallow wind field exhibited an alongshore variability which perturbed the air/sea temperature fronts over the continental shelf. Analyses of wind vorticity, divergence and dewpoint temperature provide evidence of topographically anchored circulations during shallow SE trades. Over a smaller domain encompassing the Cape Peninsula (34°S, 18°E), aerial survey results define mesoscale meteorological conditions during four discrete phases in the subtropical/midlatitude weather cycle. Influences of the synoptic-scale climatology on local topographic and thermal circulations are described.

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