Evening transition in inland and coastal mountainous terrain

The evening transition between up and down slope/valley flows in complex terrain remains one of the least understood aspects of mountain meteorology. Non-stationary and spatially inhomogeneous nature of processes involved and possible front formation have stymied the data analysis, delineation of physical processes and numerical modeling of evening transition. Following the theoretical, laboratory and observational insights gained from VTMX, we have conducted an evening transition experiment in the complex terrain of Phoenix airshed (TANSFLEX-2007) and participated in Meteo-Diffusion studies in the Biferno Valley of Italy, organized by ENEA. The results of former show that evening transition is associated with the formation of a front, immediately followed by its downward propagation to initiate the katabatic flow. Convective instabilities in the front also generated intense localized turbulence, thus causing strong aerosol entrainment. Once the downslope flow is established, the near surface stable layer cuts off the communication of ground with the upper layers, thus increasing the longevity of upslope flow aloft. On the other hand, the Biferno valley case is complicated by the contiguous ocean, and differential cooling rates between oceans and valley complicate the transition mechanism. The front formation may still be present, but the mean flow is dominated by the horizontal pressure gradients induced by the variation of land cover.