Cloud Interactions and Merging on Day 261 of GATE

Cloud interactions and merging processes in pairs of moderate-sized convective cells are studied by using radar data with 5-min resolution from day 261 of GATE, in relation to a three-dimensional cloud model. The radar data indicate that most of the clouds are parallel cells, in that the two clouds lie along a line parallel to the wind shear vector. This vector, obtained by subtracting the lower level wind from the upper level wind, is from the north in the layer to 3 km. The two elements of the parallel pairs of cells appear simultaneously in half of the cases. Within the simultaneous cells, the northern echo, called the upshear cell, tends to be stronger than the southern echo, the downshear cell. If, however, the two cells do not appear simultaneously, the downshear cell usually develops earlier and also becomes stronger than the upshear cell. A number of numerical simulations initiated with a pair of impulses with varying spacing, intensity, and timing are performed. The numerical results are in fair agreement with the radar observations. Except for the parallel upshear cell, all the cells are suppressed. The suppression can be attributed to the circulation of the adjacent cell forcing the inner downdrafts to develop inside the lateral boundaries of the clouds. In contrast, the upshear cell behaves like an isolated one, because of the increased moisture flux from the direction of the downshear cell. The sensitivity tests on the varying timing and intensity of the two impulses show that neither the use of nonsimultaneous nor nonidentical impulses promotes merging. On the contrary, the minimum edge-to-edge separation between the echoes is larger than that in the simulations with simultaneous and identical impulses. Merging is found to have a considerable influence on cloud development. Both the radar observations and numerical simulations show a substantial increase in the maximum area, maximum echo top, and maximum reflectivity factor of the echoes as a result of the merging process. The numerical experiments indicate that the perturbation pressure structure caused by precipitation, downdrafts, and the formation of a cloud bridge, a vertically thin cloudy area connecting the neighboring clouds, is crucial to trigger echo merging.