THE SIMULATION OF A CONVECTIVE CLOUD IN A 3D MODEL WITH EXPLICIT MICROPHYSICS .2. DYNAMICAL AND MICROPHYSICAL ASPECTS OF CLOUD MERGER

The development and merger of pairs of convective clouds in a shear-fr ee environment were simulated in an explicit microphysical cloud model . The occurrence or nonoccurrence of updraft merger and the timing of merger depended critically on the initial spacing of the thermal pertu rbations imposed in the model's initialization. In the unmerged cases the presence of a neighbor cloud was detrimental to cloud development at all times. In the merged cases this negative interaction was still operating but only until the onset of updraft merger. Based on the vis ual form of the updraft merger, it was hypothesized that low-level mer ger was a consequence of mutual advection, that is, that each cloud ca ught its neighbor in its radial inflow and advected it inward. This lo w-level advection hypothesis was quantified by considering a potential how induced by two line sinks whose strengths were set equal to the l ow-level mass flux into the numerically simulated clouds. The merger t imes obtained from the advection hypothesis were in good agreement wit h the merger times observed in the simulations. Moreover, if merger di d not occur, the advection hypothesis suggested that merger should not have occurred. The merger process was accompanied by the presence of trimodal drop spectra at the upper levels of the cloud. It was shown t hat the drop size distribution depends nor only on the autoconversion and accretion rates, but also on the nonlinear interaction between var ious source and sink terms affecting rain formation, particularly on t he rates of condensation-evaporation, sedimentation, and breakup proce sses. The analysis of raindrop trajectories showed the details of rain formation in different cloud regions and the effect of dynamical cond itions on the growth of rain particles.