Understanding the effects of multi-scale flow interactions on convective boundary layer depth

Granite Peak, located in the Dugway Proving Ground in western Utah, is an isolated mountain rising ~800 m above the surrounding terrain. It has an approximately ellipsoidal shape with main axes respectively ~10- and ~6-km long. A flat dry lake (playa) lies west and northwest of the peak, while a NW-sloping plain covered by herbaceous vegetation extends to the east.

Because of these topography and land-use features, different flow phenomena occur in the area. In particular, fair weather days with strong solar forcing feature the onset of upslope winds along the sidewalls of Granite Peak and of lake breezes between the playa and the adjacent plain. The presence of such wind systems may have a non-negligible impact on the structure of the convective boundary layer (CBL). For instance, it is known that upslope flows can cause a local reduction of CBL depth at the foot of mountain slopes, by advecting potentially warm and stable air downwards within their elevated return branch.

This study aims to understand how slope winds and lake breezes affect the atmospheric boundary layer around Granite Peak, with a particular focus on the CBL depth. Several sets of semi-idealized very-large eddy simulations are presented. A factor separation approach is employed to assess the pure contributions to the CBL depth of topographic effects and of the spatial heterogeneity of sensible heat fluxes. The nonlinear interaction between these two factors is shown to locally reduce the CBL thickness compared to a case where neither is active.

Different prototypical flows, with large-scale winds impinging on Granite Peak from different quadrants, are analysed. Finally, an attempt to characterize the coherent turbulent structures embedded in the CBL is made.

Modelled scenarios are compared with estimates of the CBL depth obtained from airborne backscatter lidar scans, performed during a recent field campaign held during the MATERHORN project.