OROGRAPHIC RAINFALL ENHANCEMENT IN THE MOUNTAINS OF THE LAKE DISTRICT AND SNOWDONIA

A two-dimensional model of the seeder-feeder effect has been compared with rainfall data from a network of daily-read rain gauges located in the Lake District and the Snowdonain mountains. Two days in the winter of 1980, when persistent heavy orographic rainfall occurred, have been considered. The rain gauge data indicated that rainfall near the hill summits was enhanced by a factor of between 3.2 and 7.6. It was found that the model was capable of predicting accurately the areas of maximum rainfall but tended to underpredict the magnitude of the orographic rainfall enhancement. This was attributed to orographic effects on the seeding rainfall rates. Over certain topography three-dimensional airflow effects were thought to play an important role in determining the locations of the rainfall maxima. Using typical values for sulphate aerosol loading and the concentration of sulphate in seeding rain, the model predicted a greater deposition of sulphate over the hills accompanying the rainfall enhancement. Sulphate deposition was enhanced by a factor of approximately 4 relative to the coastal rates.