A Modelling Study of the Factors Governing the Convective Boundary Layer Height over Isolated Mountain Ridges

Convective boundary layer (CBL) processes in mountainous regions have a demonstrated importance in initiating moist convection and in venting pollutants to the free atmosphere. Compared to the CBL over plains, the daytime boundary layer near mountains is affected not only by convective turbulence, but also by organized airmotions such as thermally-induced breezes. The latter encompass a wide spectrum of scales, going from slope winds to mountain-plain circulations.

The phenomena affecting the structure of the CBL in valleys (e.g., valley inversions, cross-valley flows, in-valley subsidence) have been extensively investigated in the past, with both field experiments and modelling studies. In contrast, the CBL over isolated mountain ridges has received much less attention and is not well understood yet. One of the most interesting quantities in this context is the CBL height, that is, the altitude up to which the atmosphere is subject to convective mixing. The spatial and temporal variability of the CBL height over complex terrain has relevant implications to air quality and to the transport of trace gases.

This study presents the results of idealized large-eddy simulations of flow over an elongated ridge. The simulation set is designed to understand the role of topographic characteristics, surface heating, free-atmospheric stability and advection on the CBL height over mountain ridges. Scale analysis and heat engine theory are used to explain the numerical evidence and to link the modelled CBL height to the governing external parameters. Results about the spatial distribution of first- and second-order turbulence moments in the vicinity of the ridge are also presented.