Thermally-driven orographic convection initiation is sensitive to terrain steepness

Abstract

Diurnal mountain winds favor the onset of deep moist convection, both because the related horizontal and vertical transports of heat and moisture make the environment more supportive, and because they may lift air parcels above their level of free convection. We study the effect of cross-valley circulations on convection initiation under synoptically undisturbed and convectively inhibited conditions, using idealized large-eddy simulations with the Weather Research and Forecasting (WRF) model. We consider quasi-2D mountain ranges of different heights and widths and we contrast convection initiation over relatively steep mountains (20 % average slope) and less steep ones (10 %). Under identical environmental conditions, steeper mountains cause stronger thermal updrafts at ridgetops, but lead to a delayed onset and lower intensity of deep moist convection. Analysis of the ridgetop moisture budget reveals the competing effects of moisture advection by the mean thermally driven circulation and of turbulent moisture transport. At mountaintops, the divergence of the turbulent moisture flux offsets the convergence of the advective moisture flux almost entirely. Considering in-cloud vertical profiles of equivalent potential temperature, we demonstrate that buoyant updrafts over steeper mountains are more strongly affected by the turbulent entrainment of environmental air. This depletes their moisture and cloud water content and makes them less effective at initiating deep convection. The weaker convection over steeper mountains is a robust finding, valid over a range of background environmental stability and mountain sizes.