The influence of Föhn events on the boundary layer over the Larsen Ice Shelf, Antarctica: A comparison of observations and model simulations

It has been shown that circumpolar westerly winds around Antarctica have increased in strength and frequency over the past decades. This is thought to in turn increase the occurrence of Föhn events on the lee (eastern) side of the Antarctic Peninsula. Föhn events lead to the influx of relatively warm and dry air, thus contributing to the extreme warming that has been observed in this area over the last 30 years. It has been suggested that these episodes and the strong surface melting they can cause, contributed significantly to the collapse of parts of the Larsen Ice Shelf in 1995 and 2002. The objective of the OFCAP (Orographic Flows and the Climate of the Antarctic Peninsula) project was to test this hypothesis by investigating the structure of Antarctic Peninsula Föhn winds and studying their interaction with the atmospheric boundary layer over the Larsen Ice Shelf. During the OFCAP field campaign an Automatic Weather Station (AWS) was deployed at the foot of the Antarctic Peninsula Mountains on the eastern side of this mountain range. Measurements from this AWS on Cole Peninsula (-66°51'S, 63°48'W, 424m asl) will be introduced, and it will be discussed how Föhn events are captured by the data, and what criteria may be used to automatically identify Föhn events in the observations. One year of AWS data will be compared with simulations with the Weather Research and Forecasting Model WRF, which are available through the Antarctic Mesoscale Prediction System AMPS. For this comparison, an artificial time series has been created from the model output and an independent algorithm was applied to identify Föhn events. The Föhn detection algorithms and their scoring rate will be presented. We will also present how the model simulations compare with the observations in general, and how its performance changes in the case of Föhn events. The model shows biases in near surface parameters, which imply that the modelled surface energy balance will not capture the influence of these events on the interactions between surface and atmosphere in their full extent.