Air-ice interaction on the Larsen Ice Shelf during a westerly foehn event across the Antarctic Peninsula

A 2-day westerly foehn event was observed across the Antarctic Peninsula during early February 2011 in aircraft measurements carried out as part of the OFCAP (Orographic Flows and Climate of the Antarctic Peninsula) field campaign. The event was associated with the drawdown of relatively warm and dry foehn air from aloft to near surface levels above the Larsen Ice Shelf. The warming was most pronounced towards the base of the Peninsula's eastern slopes, subsiding eastward across the Larsen. Within this region a series of westerly jets (the downwind continuation of gap flows funnelling through passes in the Peninsula ridge) emanate from the mouths of inlets. These jets (peak wind speeds ~20 m/s) are separated by regions of calmer flow. Within one such jet, downwind of Whirlwind Inlet, stable, moist air is observed at near surface level. Time series derived from both aircraft observations of the Whirlwind Inlet jet and adjacent calm regions, along with model simulations, reveal that at near surface level within the jet regions nighttime cooling is apparent, whereas within the calm regions the diurnal signature is weak or completely obscured.

Model potential temperature budget and heat flux analysis is used to investigate these characteristics. Within the jet, massive sensible heat loss to the surface (associated with ice sublimation due to the strong surface winds) is largely balanced by warm air advection, associated with the foehn flow, and downward turbulent heat flux through the air column. In the warmer calm regions there is more often an approximate balance in the heat budget, with incoming turbulent heat flux from above cancelling out radiative heat loss.

The enhanced air-ice interaction within the jet regions leads to a greater connectivity between near surface air and ice temperatures. Accordingly, the stronger sensitivity of ice (relative to air) temperature to radiative forcing accounts for the enhanced diurnal fluctuations in near surface air temperature within the jet regions, relative to the calm regions. Equally, ice beneath the jets exhibits a reduced amplitude diurnal temperature cycle. Sublimation of the ice surface accounts for the stable, moist near surface air observed within Whirlwind Inlet.