The simulation of a convective cloud in a 3D model with explicit microphysics. Part II: Dynamical and microphysical aspects of cloud merger

Abstract

The development and merger of pairs of convective clouds in a shear-free 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 perturbations 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 visual form of the updraft merger, it was hypothesized that low-level merger was a consequence of mutual advection, that is, that each cloud caught its neighbor in its radial inflow and advected it inward. This low-level advection hypothesis was quantified by considering a potential flow induced by two line sinks whose strengths were set equal to the low-level mass flux into the numerically simulated clouds. The merger times obtained from the advection hypothesis were in good agreement with the merger times observed in the simulations. Moreover, if merger did 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 that the drop size distribution depends not only on the autoconversion and accretion rates, but also on the nonlinear interaction between various source and sink terms affecting rain formation, particularly on the rates of condensation-evaporation, sedimentation, and breakup processes. The analysis of raindrop trajectories showed the details of rain formation in different cloud regions and the effect of dynamical conditions on the growth of rain particles.