Far- and near-field influence of a mesoscale mountain on the diurnal cycle of summertime moist convection

Rainfall climatologies over mountains and their vicinity are largely controlled by thermally-driven flows during summer. In this study, the control of a single mesoscale mountain over the spatial distribution and the diurnal cycle of moist convection is investigated using an idealized numerical framework. Cloud-resolving simulations of flow over a Gaussian-shaped mountain are performed for a conditionally unstable atmosphere under diurnal radiative forcing. The simulations are run for 35 days and the equilibrium diurnal cycle during the last 20 days is studied.

A reference simulation shows considerable spatial variability in terms of timing and amount of convective rainfall. This heterogeneity is found to relate to different physical mechanisms responsible for convective initiation. While the thermally-driven slope-flow favors strong convective precipitation over the mountain's lee slope during the morning, instabilities in the boundary layer are swept out by the emerging cold-pool in the vicinity of the mountain. As a consequence, the mountain's near-field receives less rainfall than its far-field where an unperturbed boundary-layer growth allows for sporadic convective initiation. Detailed analysis of this reference simulation and additional sensitivity experiments provide further explanation to this frequently observed rainfall pattern over mountains and adjacent plains.