A 3-DIMENSIONAL MODEL OF AIR-FLOW AND OROGRAPHIC RAINFALL ENHANCEMENT

A three-dimensional model of airflow over hills has been used in conjunction with a two-dimensional model of orographic rainfall enhancement. This development has allowed the accurate production of rainfall contour maps in areas of complex terrain. A comparison has been made, using a variety of atmospheric stability profiles, between the wind fields and rainfall distributions over a bell-shaped hill and those over a two dimensional ridge of similar height. The results show that, in conditions of either stably stratified air or with an inversion layer aloft, three-dimensional airflow effects can lead to much reduced rainfall over a bell-shaped hill. It is shown that in such situations the use of a two-dimensional approximation can introduce significant errors. An example is presented of the rainfall distribution in complex hill terrain over south Kintyre on the western coast of Scotland.