Characteristics of large-scale orographic precipitation: Evaluation of linear model in idealized problems

This paper investigates mechanisms underlying large-scale orographic precipitation and introduces global measures for assessing the relative importance of physical processes involved; including moisture advection, condensation, precipitation evolution and fallout. The measures proposed quantify the hydrological cycle in mountainous terrain. The discriminating character of the measures is demonstrated in numerical simulations of idealized moist, warm air-flow over a mountain ridge. A series of experiments, with constant ambient wind and moist static stability uniform in the lower troposphere, is conducted for a range of thermodynamic parameters, such as surface temperature and relative humidity, and a range of heights and widths of the ridge. These calculations corroborate the classical picture of large-scale precipitation falling on the windward side, for mountain ridges wider than about 15 km, given the observed prevailing wind. However, as the ridge width decreases below this threshold, the model results show that cloud water and hydrometeors are advected into the lee, where they evaporate in the descending flow. Overall, this reduces the total surface precipitation expected from the wide-ridge scenario. Furthermore, increasing the ridge height amplifies the precipitation due to enhanced condensation. Finally, raising the surface temperature tends to reduce the fraction of the incoming water vapor that turns into precipitation, primarily due to enhanced evaporation of hydrometeors. The measures derived discriminate the physical processes involved in many orographic precipitation scenarios. In particular, they were applied to test how well an elementary linear model predicts orographic precipitation compared to a fully nonlinear (viz. reference) simulation with an elaborate numerical code. The test results are encouraging, as they suggest that the linear model provides a valuable integral prediction, in spite of heavily abbreviated physics.