How Mountain Geometry Affects Aerosol-Cloud-Precipitation Interactions: Part II. Deep Convective Clouds

The sensitivity of aerosol effects on orographic precipitation from deep convective clouds to mountain upslope steepness is examined using the Weather Research and Forecasting (WRF) model coupled with a bin microphysics scheme. During the early stage, the sensitivity resembles that of warm, shallow orographic convection as discussed in Part I. As time progresses, interactions between vigorously developed lower-layer clouds and upstream-extending upper-layer clouds become crucial for enhancing surface precipitation via melting and direct sedimentation of ice-phase particles such as graupel and hail. In the symmetric mountain cases, higher aerosol number concentration enhances surface precipitation through stronger condensational latent heating and more active mixed-phase processes (freezing, Wegener-Bergeron-Findeisen process, and riming). Under asymmetric mountain configurations, however, the sensitivities are non-monotonic. In the steep upslope cases, fast liquid drop growth in the clean case and strong latent heating in the polluted case both support cloud development and enhance precipitation. In contrast, the control case exhibits weaker precipitation due to slower drop growth than the clean case and weaker heating than the polluted case, suppressing the cloud interaction. In the gentle upslope cases, the control case shows enhanced precipitation due to sufficient droplet supply and latent heating which help clouds grow through the freezing level. In contrast, the clean case lacks sufficient cloud droplets, while the polluted case suffers from weak convection despite strong aerosol-induced heating. In both cases, the cloud interaction and mixed-phase processes are suppressed.