How Mountain Geometry Affects Aerosol-Cloud-Precipitation Interactions: Part I. Shallow Convective Clouds

This study examines how upslope geometry controls aerosol effects on orographic precipitation through two-dimensional idealized simulations of orographic precipitation from shallow warm convective clouds over a bell-shaped mountain with 1-km height. A total of nine cases are simulated by considering three different prescribed aerosol number concentrations and three different windward-widths of the mountain. For a detailed representation of drop size distributions, the Weather Research and Forecasting (WRF) model that includes a bin microphysics scheme is used with a horizontal grid size of 250 m and 40] terrain-following vertical levels. A higher aerosol number concentration leads to the production of more cloud droplets, inhibiting the growth of cloud droplets into raindrops in the cases with the symmetric mountain (the windward-side half-width a(1) = 10 km). As a result, the total and maximum surface precipitation amounts decrease and the location of the maximum surface precipitation amount shifts downstream. The aerosol effects on orographic precipitation are more clearly seen in the cases with the narrow windward-width (a(1) = 5 km) compared to the cases with the symmetric mountain and the wide windward-width (a(1) = 20 km). In the cases with the narrow windward-width, the steep upslope generates strong convection with a short advection timescale (similar to 600 s), resulting in more precipitation being concentrated over a narrow area of the mountain downslope compared to the cases with the symmetric mountain and the wide windward-width. On the other hand, in the cases with the wide windward-width, the gentle upslope generates weak convection with a sufficiently long advection timescale (similar to 2400 s) so that a large portion of liquid drops precipitates out on the wide mountain upslope before reaching the peak.