Atmosphere-Cryosphere Interaction over the Larsen Ice Shelf, Antarctica – the OFCAP (Orographic Flows and the Climate of the Antarctic Peninsula) Project

One of the most notable examples of recent environmental change in the polar regions has been the rapid collapse and retreat of ice shelves to the east of the Antarctic Peninsula since 1995. It has been suggested that this loss of ice has been driven by increased surface melting associated with a greater frequency of warm foehn winds blowing down the eastern side of the Antarctic Peninsula mountains and across the adjoining ice shelves in response to strengthening circumpolar westerlies. 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 foehn winds and studying their interaction with the atmospheric boundary layer over the Larsen Ice Shelf. The centrepiece of OFCAP was a one-month field campaign during January-February 2011. During this campaign atmospheric measurements were made across the Antarctic Peninsula mountains and over the Larsen Ice Shelf using an instrumented Twin Otter aircraft. Radiosondes were launched from a camp on the ice shelf and surface energy balance measurements were also made at this site. A chain of automatic weather stations was established across the Antarctic Peninsula and continued to operate for several months after the main field campaign. These observations were complemented by a programme of high-resolution (order 5 km grid spacing) regional atmospheric modelling using both the Weather Research and Forecasting (WRF) model and the UK Met Office Unified Model (UM). In this poster we describe the measurements made during the field campaign, and the associated modelling activities. Comparison of models and observations shows that both WRF and the UM are able to forecast the occurrence of foehn events and that both models produce a realistic simulation of the lee-side flow and boundary layer structure over the Larsen Ice Shelf. However, evaluation of modelled surface energy balance over Larsen Ice Shelf reveals significant biases in both models, possibly associated with a poor representation of the effects of clouds. The existence of these biases implies that care should be taken when calculating surface melt rates from model data.