Abstract Cloud interactions and merging process in pairs of moderate-sized convective cells am studied using radar data with 5 min resolution from day 261 of GATE, in connection with a three-dimensional cloud model. The radar data indicate that most of the clouds are so-called 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 up 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 carried out. The numerical...