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The Relationship of Trade Wind Cumulus Distribution to Subcloud Layer Fluxes and Structure

Margaret A. LeMoneNational Center for Atmospheric Research, Boulder, Colorado 80303

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William T. PennellNational Center for Atmospheric Research, Boulder, Colorado 80303

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Abstract

A definite relationship between cloud distribution and sub-cloud layer structure and fluxes in fair weather is documented using measurements of wind, temperature, humidity and overhead cloud occurrence from the NCAR DeHasvilland aircraft. Three cases are used. These were extracted from data taken to the north of Puerto Rico on 14 and 15 December 1972 in mesoscale regions of reasonably uniform convection ranging from suppressed with very little shallow cloudiness to slightly enhanced with active (but non-precipitating) trade cumulus having tops to 2000 m. On both days synoptic conditions were suppressed and the surface winds were from the cast at 10 to 15 m s−1.

In the highly suppressed cases, there is evidence that cloud distribution was determined by subcloud layer circulations—roll vortices which persisted throughout the flight patterns. In the more enhanced case, the predominant coupling was by well-defined cloud scale updrafts which were traceable to at least 100 m below cloud base.

As a consequence of these interactions, the fluxes of moisture and momentum in the upper subcloud layer were found to be strongly coupled to cloud distribution. A comparison of direct measurements from the aircraft and the results of budget computations by other workers for several suppressed situations in the trades suggests that almost all of the fluxes out of the mixed layer are concentrated in mesoscale cloud patches and that a large function of the transport is due to motions on the scale of the individual cumulus.

Abstract

A definite relationship between cloud distribution and sub-cloud layer structure and fluxes in fair weather is documented using measurements of wind, temperature, humidity and overhead cloud occurrence from the NCAR DeHasvilland aircraft. Three cases are used. These were extracted from data taken to the north of Puerto Rico on 14 and 15 December 1972 in mesoscale regions of reasonably uniform convection ranging from suppressed with very little shallow cloudiness to slightly enhanced with active (but non-precipitating) trade cumulus having tops to 2000 m. On both days synoptic conditions were suppressed and the surface winds were from the cast at 10 to 15 m s−1.

In the highly suppressed cases, there is evidence that cloud distribution was determined by subcloud layer circulations—roll vortices which persisted throughout the flight patterns. In the more enhanced case, the predominant coupling was by well-defined cloud scale updrafts which were traceable to at least 100 m below cloud base.

As a consequence of these interactions, the fluxes of moisture and momentum in the upper subcloud layer were found to be strongly coupled to cloud distribution. A comparison of direct measurements from the aircraft and the results of budget computations by other workers for several suppressed situations in the trades suggests that almost all of the fluxes out of the mixed layer are concentrated in mesoscale cloud patches and that a large function of the transport is due to motions on the scale of the individual cumulus.

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