IDEALIZED SIMULATION OF OROGRAPHIC RAINFALL WITH A MESOSCALE ATMOSPHERIC MODEL

Abstract

The impact of different atmospheric and topographic conditions on the rainfall distribution around a single, infinitely long ridge is investigated. The main objective of the study was to understand the conditions that are responsible for commonly observed rainfall patterns like high rainfall yields on the side of the ridge facing the wind and rain-shadows on the lee side. A modified version of MM5, a non-hydrostaticmodel that is widely used for limited area atmospheric simulations for research and operational purposes, was used for this study. The effects of different mountain ridge dimensions, large-scale wind conditions and large-scale moisture profiles on the amount and distribution of orographic precipitation, were clarified. Ridge height has a positive correlation with the amount of rainfall generated while the ridge width did notshow such a clear relationship. Slowing of wind speed resulted in the spreading of rainfall outwards from the mountain peak. Reduction of moisture caused a reduced rainfall yield. Most of the simulations produced highest rainfall amounts on the windward slopes. However, changing the topography, wind speed or moisture, a rain shadow condition could not be attained. The introduction of a reversal of the winds in the upper atmosphere as observed in Asian summer monsoon and many other large-scale atmospheric flows onthe globe, created a rain shadow by confining the rainfall completely to the upstream of the ridge top.