A Modeling Study of the Topographic Effects on Shallow Convective Clouds

Shallow convective clouds (SCCs) play important roles in the Earth system. Previous studies mostly focus on SCCs over the oceans or plains. It is unclear how topography affects SCCs. In this study, the impacts of isolated ridges on the development of SCCs are investigated using large-eddy simulations, where the maximum height and the half-width of the ridge are systematically varied. In all simulations, the potential temperature over the ridge top is higher than over the plain, and the difference increases with the volume of the ridge. Upslope winds are only produced in simulations where the maximum slope angle is >0.5°. The vapor transport by upslope winds tends to increase the humidity over the ridge top. On the contrary, the dry air entrained from above the convective boundary layer tends to decrease the humidity over the ridge top. The upslope winds from the two sides of the ridge collide near the ridge top. This produces wide updrafts, and thereby facilitates the development of SCCs. As the ridge geometry varies, the variation of the depth of SCCs is collectively determined by the variations of the temperature, humidity, and updrafts. The depth of the SCCs increases with the maximum height of the ridge. It also increases as the half-width increases from 2 to 8 km, but only slightly changes as the half-width further increases to 16 km. The results of this study can potentially be used to implement the topographic effects in the parameterizations of SCCs.