How mountain geometry affects aerosol-cloud-precipitation interactions: part II. Deep convective clouds

Abstract 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 simulations with a symmetric mountain shape, 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 shapes, 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 produces less precipitation because its drop growth is slower than in the clean case and its latent heating is weaker than in the polluted case. As a result, cloud interaction is suppressed. In the gentle upslope cases, the control case shows the most precipitation due to sufficient droplet supply and latent heating which promote vertical growth and cloud interaction. The clean case lacks enough droplets, while the polluted case suffers from weak convection despite strong aerosol-induced heating. Consequently, both cases exhibit suppressed cloud interaction and mixed-phase processes.