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Comparisons of Results from a Three-Dimensional Cloud Model with Statistics of Radar Echoes on Day 261 of GATE

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  • 1 Department of Meteorology, McGill University, Montreal, Canada H3A 2K6
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Abstract

An analysis of 5 min resolution Quadra data on day 261 of GATE, (0953–1451 GMT) is made to yield statistics of maximum area, echo top, lifetime and maximum reflectivity factor in medium-sized convective cells. The results, obtained by tracking 140 echoes throughout their lifetime, indicate that the maximum area is log-normally distributed, 90% of the echoes being smaller than 40 km2 and existing less than 60 min. The modes of the maximum echo top and maximum reflectivity factor distributions are around 2.5 km and 30 dBZ, respectively. Further stratification of the data according to minimum edge to edge separation (d) reveals that merging cells (d = 0) have an average lifetime three times longer and a maximum area five times larger than isolated ones (d > 7 km). For a fixed maximum area, however, echo parameters generally decrease with decreasing d.

A fully three-dimensional cloud model including precipitation processes is used to simulate the development of an isolated and two adjacent cells. Comparison of modeled and observed echo parameters indicates a fair degree of realism in the simulations. The computed maximum reflectivity factor, however, is considerably higher than that of the observations because of the unrealistic drop-size distribution assumed in the model. Results of two cloud simulations suggest that the alignment of the clouds in relation to the wind-shear vector is an important factor in addition to d in determining the intensity of cloud development. The upshear cell of the parallel clouds, even with a small d value, behaves similarly to an isolated one. The suppression experienced by adjacent cells is attributed to the reduced low-level moisture convergence.

Abstract

An analysis of 5 min resolution Quadra data on day 261 of GATE, (0953–1451 GMT) is made to yield statistics of maximum area, echo top, lifetime and maximum reflectivity factor in medium-sized convective cells. The results, obtained by tracking 140 echoes throughout their lifetime, indicate that the maximum area is log-normally distributed, 90% of the echoes being smaller than 40 km2 and existing less than 60 min. The modes of the maximum echo top and maximum reflectivity factor distributions are around 2.5 km and 30 dBZ, respectively. Further stratification of the data according to minimum edge to edge separation (d) reveals that merging cells (d = 0) have an average lifetime three times longer and a maximum area five times larger than isolated ones (d > 7 km). For a fixed maximum area, however, echo parameters generally decrease with decreasing d.

A fully three-dimensional cloud model including precipitation processes is used to simulate the development of an isolated and two adjacent cells. Comparison of modeled and observed echo parameters indicates a fair degree of realism in the simulations. The computed maximum reflectivity factor, however, is considerably higher than that of the observations because of the unrealistic drop-size distribution assumed in the model. Results of two cloud simulations suggest that the alignment of the clouds in relation to the wind-shear vector is an important factor in addition to d in determining the intensity of cloud development. The upshear cell of the parallel clouds, even with a small d value, behaves similarly to an isolated one. The suppression experienced by adjacent cells is attributed to the reduced low-level moisture convergence.

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