Saturation limiters for water vapour advection schemes: impact on orographic precipitation

Abstract

A common fault of atmospheric general circulation models (AGCMs) is an overestimation oforographic precipitation. One basic reason is that water vapour advection schemes do not useinformation about the local temperature. When water vapour is advected from a warm gridpoint to a colder one, supersaturation may occur on the way, and the water vapour advectedmay partly precipitate before reaching the latter. This process is particularly important whenmoisture is advected upward mountain slopes along terrain-following coordinates. Not takingit in account, i.e., letting all the moisture reach the colder point, leads to artificial drying of thewindward valleys and foothills, and to overestimation of rainfall over summits and plateaux.This spurious behaviour is amplified by the resulting biases in the circulation, due to misplacementof the moisture convergence. It is a general bias, although its magnitude may be reduced, for instance when σ-coordinates are replaced by hybrid coordinates, or increased by highlydiffusive schemes such as the upstream finite differencing. A simple way of correcting this biasis to test the advected water vapour with respect to saturation values, and redistribute itaccordingly over the grid points found along the advecting path. This method is tested on afinite difference model using σ-coordinates and an upstream advection scheme. The effect onthe distribution of moisture and rainfall is dramatic: precipitation is displaced from summitsand plateaux to slopes and foothills, leading to much more realistic patterns, in particular forthe Indian and Amazonian monsoons.