Predicting precipitation in a mesoscale mountain range

Quantitative precipitation forecasting in meso-scale mountains that are not resolved in NWP models is not an easy task. In order to develop some guidelines, observations in SW-Iceland are studied to investigate the connection between precipitation in a 700 m high mountain ridge and precipitation in an airmass that is not influenced by the mountain ridge. A stong correlation is found between the difference in precipitation in the mountains and in the lowland and the low level wind The connection between these factors is described by the M-curve. At very low wind speeds, the mean precipitation in the mountain is similar to the mean precipitation in the lowland. As the low level winds increase and the flow enters the ‘flow-over' regime, so does the ratio of precipitation in the mountains to precipitation in the lowland. For a range of higher wind speeds, the ratio of precipitation in the mountains to precipitation in the lowland remains close to constant, while for wind speeds of 15-18 m/s, this ratio increases rapidly. At winds greater than 20 m/s the ratio drops rapidly again.