Impacts of Terrain Slope and Surface Roughness Variations on Turbulence Generation in the Nighttime Stable Boundary Layer

Terrain-slope increases with and without upslope large surface roughness are found impacting downstream shear-generated turbulence differently in the nighttime stable boundary layer (SBL). Their different influences can be clearly identified in their different derivations in the relationship between turbulence and wind speed at a given height, known as the HOckey STick (HOST) transition, from the HOST relationship over a flat terrain. Due to transport of the cold surface air down from elevated uniform terrain in reducing the downstream air temperature not much stratification, the downstream hydrostatic imbalance increases with terrain slope resulting in enhanced turbulence for a given wind speed. The rate of turbulence increase with wind speed from this downslope flow, on the other hand, is independent of terrain slope. With turbulent mixing enhanced by upslope large surface roughness elements, the upslope cold surface air is elevated from the upslope terrain surface. Horizontal transport of this elevated cold turbulent air layer reduces the downstream upper warm air temperature, resulting in the increasing reduction of the downstream stable stratification with height. As the consequence of the effective wind-shear generation of turbulence with the reduced stratification, the downstream near-neutral turbulence increase with wind speed is enhanced with height in addition to the turbulence intensity enhancement from the cold downslope flow. The study demonstrates important physical mechanisms for turbulence generation captured by HOST and detection of terrain features for their impacts on those mechanisms through their deviations from the HOST relationship over a flat terrain.

This study demonstrates key physical mechanisms for turbulence generation captured by the HOST relationship. It also highlights the influence of terrain features on these mechanisms through deviations from the HOST relationship observed over flat terrain.

The study is supported by US National Science Foundation (NSF), AGS-2203248, AGS-2220664, and AGS-2231229 for JS;  AGS-1733877 and AGS-2220663 for JW, SB, and DK;  AGS-1733746, AGS-1843258, and AGS-2220662 as well as the University of South Carolina Department of Geography for AH; and AGS-1844426 for GP. SB was also partly supported by the NOAA cooperative agreement NA220AR4320151 for the Cooperative Institute for Earth System Research and Data Science (CIESRDS).