The windfield and rainfall distribution induced within a small valley: Field observations and 2-D numerical modelling

The distribution of rainfall over small-scale topography of uniform roughness was studied both numerically and from field observations. Field data have shown that small-scale topographic inhomogeneities (hills and valleys of a few tens of metres in height) substantially influence the rainfall distribution over the inhomogeneity itself and also some distance downwind from it. According to comprehensive rainfall measurements carried out in a small watershed, windward slopes received less conventionally-measured rain than lee-sides. Moreover, on the windward slope rainfall decreased uphill, to a pronounced minimum near the crest. In addition, 'directional' raingages, situated at the bottom of both slopes revealed a pattern suggesting a reversed surface flow on the lower slopes. This pattern persisted through a variety of background conditions. To explain the observed data, a numerical simulation of both the windfield and of drop trajectories was carried out. The windfield was generated in simulations using the Colorado State University RAMS model with a very high resolution (5 m in the horizontal). Drop trajectories were calculated using an equation for the motion of drops in a flow field that incorporates current experimental results. A strong effect of small-scale topography on drop trajectories was found. The simulation results are encouraging and help to explain many characteristic features of precipitation distributions observed in field experiments.