Friday 08 Apr 2016: The effects of terrain length scales on cold-air pooling processes
Dr. Paul Burns - University College Dublin
The Weather Research and Forecasting (WRF) model was used to investigate the effects of terrain depth on cold-air pooling processes in decoupled, poorly drained and saturation free conditions. WRF was run in Large Eddy Simulation mode for U-shaped valleys with depths from 1000 to 100 m. Intermediate valley depths were found to induce elevated inversions close to the plateaux height, adding a new factor for the development of these very stable layers. Small-scale terrain (hills) were engulfed by deep ground-based inversions (GBIs) with these inversions merging smoothly with those over the plateaux. In contrast to previous research, cooling from the dynamics was found to be important for average valley cooling in both large and small-scale terrain. The top height of the region of enhanced cooling, above the GBI, was effectively scaled by valley depth. The depth of the GBI was reasonably well scaled by valley depth for depths from 1000 to 400 m, but not for smaller depths. The ratio of valley surface area to valley volume can be used to effectively scale the average valley cooling later in the night, with errors of about 10 % of the cooling in the smallest scale terrain. Larger errors of about 30 % were apparent in the early night. Consideration of average flow speeds in the cooling factor can be used to reduce scaling errors in the early night to less than 20 %.