Rainfall redistribution over low hills due to flow perturbation

A simple potential flow model is developed for airflow over low hills. The model readily allows incorporation of simple rain microphysics so that drop trajectories can be traced in the region of a hill complex. Rainfall intensity variations are found in the horizontal and in the vertical, and a straightforward modification to the model allows for time dependence to be studied. The model is shown to serve as a useful diagnostic tool to describe rain drop redistribution due to perturbed airflow. In particular, increased rainfall is predicted in the lee of hill peaks and depleted rainfall is predicted on ridges. The redistribution effects dominate at low levels, in contrast to predictions based on seeder–feeder enhancement. Comparison with field measurements shows good qualitative agreement over low hills and, in some cases, close quantitative agreement. Redistribution of rain appears to be a significant process that should be considered in rainfall interpolation and prediction schemes involving hilly terrain.