Radar and raingauge observations of orographic rain over south Wales

Eight detailed case studies are summarized to clarify the structure and mechanism of orographically enhanced frontal rain over hills of modest height. the observations were obtained as part of a field project in south Wales in which data from a 3‐dimensionally scanning radar were combined with autographic raingauge data. the results show that the generation of orographic rain is consistent with Bergeron's seeder‐feeder mechanism, according to which raindrops from upper‐level (seeder) clouds wash out small droplets within low‐level (feeder) clouds formed over the hills. It is demonstrated that the orographic enhancement is strongly influenced by the low‐level wind speed. the largest enhancement of rainfall occurred in association with strong winds, and also high relative humidity, below 2km. the radar showed that over 80% of the enhancement occurred in the lowest 1.5 km above the hills. It also showed that the periods of enhanced rainfall were associated with the passage of pre‐existing areas of precipitation. the precise value of the upwind rainfall rate was rather unimportant in influencing the orographic increment provided the rainfall rate upwind exceeded about 0.5mmh −1 , These findings are compared with the results of theoretical calculations based upon the washout model of Bader and Roach.