The directional‐dependence of the enhancement of rainfall over complex orography

Rainfall over Scotland is analysed to determine the distributions of rainfall amount associated with flow from different directions. The dominant effects of orography are dearly displayed, with large amounts of rainfall over the first area of high ground encountered and a rapid decline downwind; indeed, amounts are already declining before the highest ground is reached. This general behaviour applies to all flow directions, including those in which showery precipitation predominates. A simple model of mean orographic enhancement is developed which incorporates cloudwater generation by orographic and synoptic-scale lifting, the conversion of cloud-water to rainwater and wind-drift of falling precipitation. This model allows spatially detailed patterns of rainfall to be inferred. The dependence of rainfall enhancement on wind speed is investigated with the model: over the first area of high ground there is an almost linear increase of rainfall with increasing wind speed, hut over subsequent high ground rainfall amounts decrease with increasing speed, in accord with observations.