Mountain Shape and Aerosol Loading Effects on the Orographic Precipitation from Shallow-Warm Convective Clouds

In idealized simulations of orographic precipitation from shallow-warm convective clouds over a bell-shaped mountain, the sensitivity of aerosol effects to the upslope steepness is studied by considering different upslope angles and aerosol concentration levels. Based on three categories of aerosol number concentrations and three different upslope angles of the bell-shaped mountain, a total of nine cases are considered. For a detailed representation of drop size distributions, a WRF-bin model that includes a bin microphysics scheme is used. A higher aerosol number concentration leads to production of more cloud droplets, inhibiting the growth of cloud droplets into raindrops. As a result, the total and maximum precipitation amounts decrease, and the location of maximum precipitation shifts downstream. Furthermore, the stronger condensational heat release associated with the increased production of cloud droplets means that stronger convection is generated, resulting in more liquid drops of small sizes being distributed over a deeper layer. Aerosol effects are more clearly seen in the case with narrower windward-width compared to the symmetric mountain case. If the windward-width of the mountain is narrower, the steeper upslope generates stronger convection with a shorter advection time scale, resulting in strong precipitation being concentrated over a narrow area. On the other hand, if the windward-width of the mountain is wider, the gentler upslope generates weaker convection with a sufficiently long advection time scale so that a large portion of liquid drops precipitate over the wide upslope before reaching the peak. The orographic precipitation amount and the location of its maximum are, therefore, more sensitive to the aerosol number concentration when the mountain upslope angle is steeper.