The impact of Föhn conditions across the Antarctic Peninsula in AWS measurements and model simulations.

Föhn winds are warm, strong, downslope winds on the lee side of mountains, which can last from several hours to a few days. 1995 and 2002 saw the dramatic break-up of huge parts of the Larsen Ice Shelf (LIS) on the east of the Antarctic Peninsula (AP). Hydrofracturing, the widening of crevasses due to the excess hydrostatic pressure exerted by meltwater which accumulates inside them, is the mechanism behind the break-up of the Larsen A and Larsen B ice shelves. On the LIS, in the lee of the mountain range that runs along the spine of the AP, Föhn winds provide the atmospheric conditions for significant warming over the ice shelf, leading to the initial firn densification, and subsequently providing the melt water for hydrofracturing. Measurements provide evidence that in some cases Föhn events reach an Automatic Weather Station (AWS) on the LIS at over 100 km distance. We examine the representation of Föhn events during 2011 as they were observed in measurements by an AWS, and in simulations with the Weather Research and Forecasting Model (WRF) as run for the Antarctic Mesoscale Prediction System (AMPS). We find that, while the model generally simulates meteorological parameters very well, and shows good skill in capturing the occurrence, frequency and duration of Föhn events, it underestimates the temperature increase and humidity decrease during the Föhn significantly, and may thus underestimate the contribution of Föhn to driving surface melt on LIS.