Effect of moist processes on the flow of air within complex terrain

Analyses of the flow of air within major river valleys on on the south side of the European Alps, using ground-based and airborne Doppler radar, surface, and upper-air data taken in the fall of 1999 during the Mesoscale Alpine Program (MAP) Special Observing Period (SOP), indicate that precipitation can strongly modify the airflow within valleys. Observations made during the MAP SOP show that during persistent widespread orographic precipitation events a down-valley drainage flow can develop underneath an opposite-directed flow of moist air that is lifted onto the topographic barrier. Such drainage flows may reach a maximum depth limited primarily by the height of the melting layer and secondarily by the valley confines. The drainage flow strength and depth are found to be related to the rainfall amount, and within the valley appear to be disconnected from the larger-scale upslope flow.

Idealized simulation experiments have been carried out using the new Weather Research and Forecasting (WRF) model to further a fundamental understanding of the effect of atmospheric moisture on the flow of air past terrain. The results of two-dimensional simulations show that moist processes, such as condensation, melting, and evaporation, can significantly alter the thermodynamic stratification and stability, and thus modify a flow pattern obtained otherwise based on dry conditions. For example, latent heat released by condensation tends to decrease the thermodynamic stability and thus ease an airparcel to cross over a topographic barrier. Melting and evaporation, on the other hand, result in a cooling of the air and thus tend towards stabilization.

The presentation will cover the results of systematic two- and three-dimensional analyses, highlighting under what conditions down-slope and down-valley flows may be obtained.