The effect of the Welsh mountains on rainfall distribution over Wales and England: a case study of a severe extratropical storm.

We performed a set of numerical experiments for a severe extratropical storm that hit the United Kingdom in December 2013 using the Weather Research and Forecasting (WRF) model in a nested setup. Four experiments were performed, using the original orography (ORG), a 50% increase in orography (T50), a 100% increase (i.e. double; T100), and a total removal (flattening) of the mountainous terrain (RMV) over Wales. The results show that rainfall amounts and distribution are sensitive to the modifications made. Moreover, the analysis of the vertical distribution of water vapor, as well as vertical velocities, indicates that higher mountains act as a windbreak and confine moisture to the windward side. Lower mountains let moisture flow and contribute to rain patterns in England (leeward side). Changes in Mean Sea Level Pressure (MSLP) were also observed, and the results agree with rainfall amounts. Furthermore, the time series of the spatial average of Convective Available Potential Energy (CAPE) suggests that orography has a stronger influence on storm dynamics than it does on storm genesis. Increased orography results in deeper storms and intensifies vertical flows, changing the dynamics and the energy of the environment. [ABSTRACT FROM AUTHOR]