Cloud Formations Leeward of India during the Northeast Monsoon

Abstract

TIROS VII photographs revealed that lines and groupings of clouds off the southwestern coast of India recurred frequently in preferred locations during the northeast monsoon season. Observations made on a meteorological flight of a C-54Q aircraft through this area 3 hr after a TIROS passage allowed a detailed study of the atmospheric conditions associated with this phenomenon. From aircraft observations of clouds, winds, temperatures, humidifies, and turbulence, and from ship and land observations, a description of the phenomenon is given and possible causes are discussed. The offshore air was generally clear except for several lines of small low cumulus clouds which divided the area into lobes of clear air. These lines were located off-shore and to the north of the valleys. The potential virtual temperature of the air in the cloud lines was 1.5C lower than the surrounding air at the flight level and about the same as the air near the sea surface. Either this air had been mixed previously by some mechanism and the clouds are produced by buoyancy forces, or air parcels were lifted by an external force. Convergence, the most likely process which can lift air, was produced by mesoscale eddies leeward of the mountains or by the merging of small jets flowing from the cast through the valleys with the air flowing from the north over the Arabian Sea. Other possible mechanisms capable of producing cloud lines are discussed.