Mountain Volume Control on Deep-Convective Rain Amount during Episodes of Weak Synoptic Forcing

Abstract

Thermally driven upslope flows in mountainous areas provide favorable conditions for diurnal deep moist convection especially during episodes of weak synoptic forcing. The present study investigates the response of deep convection to axisymmetric orography as a function of orographic width and height by running ensembles of idealized convection-resolving simulations with a horizontal grid spacing of ?x = 1 km, full-physics parameterizations, and an interactive land surface. Deep convection is explicitly resolved and not parameterized. To cover a wide range of orographic scales, simulations are conducted with heights between 250 and 4000 m and widths between 5 and 30 km. The mountain slope strongly affects upslope wind speed characteristics, the timing and intensity of local updrafts, and local rain intensity. Although the day-to-day variability is substantial, the statistical-mean rain amount extracted by the mountain scales almost linearly with the mountain volume. Simulations with alternative mountain geometries, multiple peaks, and large-scale flow suggest that the linear scaling is valid for a surprisingly large portion of the parameter space. The scaling breaks down in the limit of relatively strong large-scale flows, sufficiently tall mountains, or elongated mountains. The existence of the simple linear scaling over such a wide range of configurations suggests that the response of thermally driven orographic deep convection over many mountainous areas is strongly affected by mountain volume. As a consequence, the rain amount is disproportionally dominated by the large horizontal scales of orography, as they contribute mostly to the mountain volume.