Evolution of Cold-Air-Pooling Processes in Complex Terrain

Elucidating cold-air-pooling processes forms part of the longstanding problem of 5 parametrizing the effects of complex terrain in larger-scale numerical models. The Weather 6 Research and Forecasting model has been set-up and run at high resolution over an ideal7 ized alpine-valley domain with a width of order 10 km, to investigate the four-dimensional 8 variation of key cold-air-pooling forcing mechanisms, under decoupled stable conditions. 9 Results of the simulation indicated that the total average valley-atmosphere cooling is driven 10 by a complex balance/interplay between radiation and dynamics effects. Three fairly dis11 tinct regimes in the evolution of cold-air-pooling processes have been identified. Starting 12 about 1 hr before sunset, there is an initial 30-min period when the downslope flows are 13 initiated and the total average valley-atmosphere cooling is dominated by radiative heat loss. 14 A period of instability follows, when there is a competition between radiation and dynam15 ics effects, lasting some 90 min. Finally, there is a gradual reduction of the contribution of 16 radiative cooling from 75 to 37 %. The maximum cold-air-pool intensity corresponds to the 17 time of minimum radiative cooling, within the period of instability. Although, once the flow 18 is established, the valley atmosphere cools at broadly similar rates by radiation and dynam19 ics effects, overall, radiation effects dominate the total average valley-atmosphere cooling. 20 Some of the intricacies of the valley mixing have been revealed. There are places where the 21 dynamics dominate the cooling and radiation effects are minor. Characteristics of internal 22 gravity waves propagating away from the slopes are discussed. 23