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 uniformroughness was studied both numerically and from field observations. Field data haveshown that small-scale topographic inhomogeneities (hills and valleys of afew tens of metres in height) substantially influence the rainfalldistribution over the inhomogeneity itself and also some distance downwind from it.According to comprehensive rainfall measurements carried out in a smallwatershed, windward slopes received less conventionally-measured rain than lee-sides.Moreover, on the windward slope rainfall decreased uphill, to apronounced minimum near the crest. In addition, directional raingages,situated at the bottom of both slopes revealed a pattern suggesting a reversedsurface flow on the lower slopes. This pattern persisted through a variety ofbackground conditions.To explain the observed data, a numerical simulation of both the windfieldand of drop trajectories was carried out. The windfield was generated insimulations using the Colorado State University RAMS model with a very highresolution (5 m in the horizontal). Drop trajectories were calculated usingan equation for the motion of drops in a flow field that incorporates currentexperimental results.A strong effect of small-scale topography on drop trajectories wasfound. The simulation results are encouraging and help to explain manycharacteristic features of precipitation distributions observed in fieldexperiments.