Application of a Three-Dimensional Cloud Model to the Study of Gate Showers.

An analysis of 5 minute 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 procedure generates a unique data set which is used to compare with the results of a three-dimensional cloud model and to study cloud interactions and merging processes between convective echoes. 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 km('2) and living shorter than 60 min. The modes of the maximum echo top and maximum reflectivity factor distributions are around 2.5 km and 30 dBz, respectively. A fully three-dimensional cloud model including precipitation processes is used to simulate convective clouds on this day. A number of single cloud experiments are carried out to verify the numerical results against the radar observations. In addition, several two-cloud developments are simulated to study cloud interactions and merging processes. Comparison of the modelled echo parameters against the observed ones 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. The two-cloud simulations suggest that both the alignment of the clouds in relation to the wind shear vector and the spacing between the cells are important factors in determining the type of cloud interactions. Merging takes place when the spacing between the two elements becomes small enough. The numerical simulations indicate that the perturbation pressure structure is crucial to trigger echo merging.