A Modeling Study of the Impacts of 3-Dimensional Topography on Shallow-Convective Clouds

Shallow-convective clouds (SCCs) play important roles in the Earth’s atmospheric system by affecting radiative balance, large-scale circulation, and transport of pollutants. It is common sense that topography exerts substantial impacts on SCCs. However, the underlying mechanisms are not well understood. Here, we performed large-eddy simulations (LESs) to investigate how three-dimensional (3D) topography affected SCCs. The 3D topography was constructed using two widely used two-dimensional (2D) topographies, a bell-shaped ridge varying in the x-direction and a series of sinusoidal ridges varying in the y-direction. The bell-shaped ridge was the major ridge. The upper parts and lower parts of the sinusoidal ridges were the minor ridges and minor valleys, respectively. The wavelength of the sinusoidal ridges was systematically varied. LESs were also performed separately using the 2D topographies. In the simulations with 3D topography, the upslope winds were mainly over the minor ridges and the return flows were mainly over the minor valleys, which was different from those in the simulations using 2D topographies. The upslope winds promoted the development of SCCs over the major ridges by producing large thermals and high humidity, similar to in the simulations using 2D topographies. Increasing the wavelength of minor ridges enlarged the region with convergence, and thereby increased the size of SCCs. Our results suggest that it is necessary to consider the 3D topography instead of the more conventional 2D topographies when investigating the topographic impacts on SCCs.