On the Moisture Extraction Efficiency of Mountains via Deep Convection

The elevated heating by mountains provides favorable conditions for the initiation of vigorous deep moist convection, in particular in the warm season during episodes of weak synoptic forcing. Complex orography contains features at a wide range of length scales that strongly affect precipitation patterns. In this study we aim to systematically quantify the deep-convective response to a single, three-dimensional mountain as a function of its extent and height using ensembles of idealized convection-resolving simulations with full physics parameterizations. To this end we conduct numerical experiments with a simplified but interactive land surface and a single Gaussian mountain and systematically evaluate the impact of heights ranging from 250 m to 1500 m and widths ranging from 5 km to 30 km. Additional experiments with locally perturbed soil moisture are conducted to test the role of soil-moisture heterogeneity for mountain precipitation. Despite the strong impact of the mountain height and mountain width on the diurnal cycle of precipitation and precipitation statistics the total rain amount scales approximately linearly with the volume of the mountain.The volume scaling is also found in test simulations with multiple Gaussian peaks or sheared background flow. The mean rain intensity over the mountain on the other hand reaches a maximum at a width of ~ 20 km for a fixed mountain height. Although the details of the thermal forcing strongly differ, the characteristics of the rain response to the mountain width are qualitatively similar to the corresponding response of rain to the horizontal scale of tropical islands. In contrast, surface evaporation over the mountain monotonously decreases with mountain volume and can therefore be dismissed as significant contributor to the rain amount. This extends recent findings that mountain evaporation is of minor importance for precipitation for mountains that exceed a certain height (Imamovic et al., 2017). Finally, we explore mountain flow regimes in search for a departure from the volume scaling.
References:
Imamovic, A., Schlemmer, L., and Schär, C. (2017). Collective impacts of orography and soil moisture on the soil moisture-precipitation feedback. Geophysical Research Letters, 44, 11,682–11,691. https://doi.org/10.1002/2017GL075657