Influence of Terrain Smoothing on Simulated Convective Boundary Layer Depths in Mountainous Terrain

Many applications in climate and weather research rely on the correct estimation of the planetary boundary layer (PBL) depth, the height above the surface up to which turbulent mixing takes place. During the day, the PBL depth can show large spatial and temporal variability, especially in mountainous terrain. Many applications use output from numerical models run at coarse resolution of several 10s of kilometers. The effects of mountainous terrain on PBL depths are poorly simulated at these coarse resolution, partly because of weaknesses in the representation of boundary layer processes and partly because of terrain smoothing (unresolved terrain). Our previous investigations show that coarser scale models with the same physical parameterizations tend to result in larger PBL depths over mountainous terrain, especially at mountain tops. In this presentation, we further investigate these PBL depth differences for other regions in the mountainous western USA, and for a variety of grid spacings. By performing semi-idealized simulations, we also investigate the sensitivity to different PBL derivation methodologies, PBL parameterization, and the relative contributions of physical processes and terrain representation to the grid spacing in a systematic way. We show that terrain smoothing in the coarse-grid domain is the largest contributor for the differences in PBL depths between a coarse- and a fine-grid domain in mountainous terrain. Including information of the underlying terrain, either by using PBL height (equal to PBL depth + terrain elevation), or the usage of interpolated terrain, improves the comparison between coarse- and fine-grid domains. These results are especially important to consider when performing evaluations of simulated PBL depths with observed PBL depths from in-situ or remote sensing measurements.