Y. Qin, Boston U.

This study aims to investigate the morning transition of the thermally-driven slope flow system over the Eastern Snake River Plain, which is northeast-southwest oriented and bordered by large mountain ranges. A better understanding of such physical processes is important in local weather forecasting and the dispersion and transport of air pollutants. In this study, WRF is used to perform mesoscale simulations of a typical diurnal cycle without synoptic disturbance, which is dominated by a northeasterly (downslope) wind at night and a southwesterly (upslope) wind during the day. Sensitivity tests in grid spacing, spin-up time, land surface models, PBL schemes, initial soil conditions, and boundary conditions are conducted and evaluated by eddy covariance data measured by a 62m tower and the Mesonet observing systems at Idaho National Laboratory (INL). Although a generally good agreement between simulations and observations is found across the diurnal cycle in terms of radiation components and surface fluxes, the model underestimates drainage flow while overestimating air temperatures at night, and predicts a faster wind speed acceleration and an earlier wind direction shift in the morning transition. These results seem to suggest that the boundary layer schemes provide too much mixing from the warmer atmosphere above, leading to a higher near-surface temperature at night. Also, the mesoscale grids could fail to capture the local “cold pool” effect induced by the elevation gradient from the north and northeast down to the south and southwest. Therefore, the parameterization of the nocturnal stably-stratified boundary layer over complex terrain is worth more attention numerically.