Effects of aerosols on the lifecycle of a mesoscale cloud cluster over the Indian peninsula: A numerical study with a bin-based cloud microphysics scheme

The effects of aerosols (i.e., CCN) on the lifecycle of a well-documented mesoscale cloud cluster (CC) over the Indian peninsula are investigated in this study. The WRF model coupled with a sophisticated spectral bin microphysics scheme is employed to simulate the observed cloud system under various CCN scenarios. The CCN sensitivity experiments were carried out using three different background CCN concentrations (250, 1000, and 3000 cm⁻³). The prevailing environment featured a relatively dry mid-level. The introduction of more aerosols weakened the convection and moistened the upper troposphere in the initiation and mature phases. The major impacts of the enhanced aerosols include the dissipation of shallow clouds, a decrease in the number of convective cells and their overall coverage, enhanced convective organization in the early phase(s) of the lifecycle, enhancement of the cloud-free area, etc. The key microphysical changes due to enhanced aerosols are the increase in cloud liquid water, presence of numerous smaller cloud droplets, enhancement of condensation and evaporation, formation of smaller ice crystals, reduced snow mass and reduction in the aggregation process, high graupel mass and number and a reduction in graupel size, fewer raindrops with slight enhancement in raindrop size, etc. Cloud growth is significantly limited in the high aerosol scenarios due to large evaporation favored by a relatively dry environment and no invigoration effect is noted. A significant reduction in the rainfall (and associated rainfall-type) from isolated convective cores is noted due to high aerosols, especially in the initiation and mature phases. The eventual impact on the surface precipitation is a decrease in overall rainfall in the enhanced aerosol scenarios, with suppression of heavy rain. The study indicates that in a dry environment, the microphysical changes in various CCN scenarios cumulatively lead to macrophysical changes, which are found to be the primary controller of the overall surface rainfall associated with the CC.