Quantification of the importance of wind drift to the surface distribution of orographic rain on the occasion of the extreme Cockermouth flood in Cumbria

The 1.5 km grid length version of the Met Office Unified Model has been used to investigate a case of persistent orographic rain. The event gave up to almost 400 mm of rain locally in 48 h and led to severe flooding in the vicinity of the Cumbrian hills of northwest England in November 2009. A strong and moist low-level jet (LLJ) was responsible for the heavy rain by the Bergeron seeder-feeder mechanism. The LLJ led to strong ascent and high liquid water content in the low-level feeder cloud above the hills. The LLJ also caused the raindrops to drift a considerable horizontal distance as they fell through the feeder cloud. The version of the model that was used incorporates prognostic rain and it was run with and without the effect of wind drift. This showed that the area of rainfall exceeding 250 mm in 48 h was displaced some 8 km, taking it from a position largely upwind of a major watershed to a position largely downwind of it. Although there was flooding on both the windward and leeward sides of the hills, the most severe flooding occurred on the leeward side, notably in the catchments feeding the rivers through Cockermouth. The shift in the position of the maximum rainfall gave a 70% increase in the rainfall in these catchments when wind drift was included.