Idealized simulations of orographically-induced thermal circulations triggering deep moist convection

Processes on the meso-gamma scale play an essential role in triggering deep moist convection. In mountainous terrain, thermally-driven circulations–such as slope, valley, and plain-to-mountain winds–can provide the necessary trigger mechanism to lift air parcels above the level of free convection. Despite its relevance, the impact of thermally-driven circulations on convection initiation has so far not been systematically quantified.

We study the effect of the cross-valley circulation on convection initiation with idealized large-eddy simulations with the WRF model, considering quasi-2D mountain ranges of different heights and widths and different instability profiles. Despite being idealized by using simplified initial profiles, an idealized mountain geometry, and periodic lateral boundary conditions, the simulations employ a complete suite of physics parametrizations to achieve an adequate representation of the essential processes.

The vertical and horizontal redistribution of heat and moisture is quantified using the newly developed budget analysis tool WRFlux, and an attempt is made to determine how the initial stratification and terrain characteristics affect the time scale of convective destabilization. One distinctive finding is that steeper mountain ranges feature a later onset and lower intensity of deep moist convection due to a weaker thermal circulation.

We present different scaling arguments for the vertical profiles of vertical velocity and horizontal convergence above the mountain ridge. Surprisingly, classical velocity scales for the boundary layer over flat and horizontally homogeneous terrain work well also in our idealized mountainous terrain. In contrast, a framework that was specifically designed for estimating the strength of thermal circulations, the heat-engine framework, shows the poorest performance.