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Observational Studies of Mesoscale Cellular Convection

E. M. AgeeDept. of Geosciences, Purdue University, West Lafayelle, Ind. 47907

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K. E. DowellDept. of Geosciences, Purdue University, West Lafayelle, Ind. 47907

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Abstract

An observational study of mesoscale cellular convection occurring over vast regions of the North Atlantic and North Pacific has been done for the period 1 January 1969 through 30 June 1970. Satellite cloud photography from the ESSA 7, ESSA 9 and ATS 3 satellites and conventional rawinsonde data have been analyzed for a total of 38 cases, consisting of 25 open and 13 closed convective patterns. Computations have shown that: 1) the average diameter for open cells is 30 km and for closed cells 32 km; 2) the average convective depth for open cells is 2.3 km, greater than the 1.3 km average for closed cells; 3) the average aspect (diameter-to-depth) ratio for open cells, 15:1, is less than that for closed cells, 28:1; 4) the aspect ratio is inversely proportional to increasing convective depth; 5) sea surface temperature exceeds the air temperature on the average by 2.1C in open cells but is 0.4C less in closed cells; 6) directional and magnitude shear (in the vertical) of the horizontal wind is small, less than 7° km−1 and 2 m sec−1 km−1, respectively, but indicative of backing or cold air advection in open cells and veering or warm air advection in closed cells; 7) a characteristic lapse for the convecting layer of 8.2C km−1; and 8) a strong total heat flux of 218 1y(4 hr) −1 from the sea to the atmosphere in regions of open cell and a weaker total heat flux of 65 1y(4 hr) −1 from the air to the sea in regions of closed cells.

Open cellular patterns which preferably occur in cyclonic synoptic-scale flow portray the oceans as a major energy source for driving the atmosphere's circulation. Closed cellular patterns, on the other hand, usually occurring under conditions of anticyclonic synoptic-scale flow, portray the oceans as a weak sink for the atmosphere's energy.

Abstract

An observational study of mesoscale cellular convection occurring over vast regions of the North Atlantic and North Pacific has been done for the period 1 January 1969 through 30 June 1970. Satellite cloud photography from the ESSA 7, ESSA 9 and ATS 3 satellites and conventional rawinsonde data have been analyzed for a total of 38 cases, consisting of 25 open and 13 closed convective patterns. Computations have shown that: 1) the average diameter for open cells is 30 km and for closed cells 32 km; 2) the average convective depth for open cells is 2.3 km, greater than the 1.3 km average for closed cells; 3) the average aspect (diameter-to-depth) ratio for open cells, 15:1, is less than that for closed cells, 28:1; 4) the aspect ratio is inversely proportional to increasing convective depth; 5) sea surface temperature exceeds the air temperature on the average by 2.1C in open cells but is 0.4C less in closed cells; 6) directional and magnitude shear (in the vertical) of the horizontal wind is small, less than 7° km−1 and 2 m sec−1 km−1, respectively, but indicative of backing or cold air advection in open cells and veering or warm air advection in closed cells; 7) a characteristic lapse for the convecting layer of 8.2C km−1; and 8) a strong total heat flux of 218 1y(4 hr) −1 from the sea to the atmosphere in regions of open cell and a weaker total heat flux of 65 1y(4 hr) −1 from the air to the sea in regions of closed cells.

Open cellular patterns which preferably occur in cyclonic synoptic-scale flow portray the oceans as a major energy source for driving the atmosphere's circulation. Closed cellular patterns, on the other hand, usually occurring under conditions of anticyclonic synoptic-scale flow, portray the oceans as a weak sink for the atmosphere's energy.

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