A westerly foehn event across the Antarctic Peninsula: flow dynamics and downwind response

A westerly foehn event across the Antarctic Peninsula (AP) has been analysed via observations from a series of research aircraft flights undertaken as part of the OFCAP (Orographic Flows and Climate of the Antarctic Peninsula) field campaign, and high-resolution model simulations using the UK Met Office Unified Model (MetUM). The foehn event is associated with lee-side warming and drying above the Larsen Ice Shelf (Larsen IS) over a 2-day period during which near-surface temperatures are found to exceed 0°C across the breadth of the Larsen IS, implying considerable surface melt.. The most intense warming is found close to the base of the lee slope. Downwind re-ascent of foehn air and increasingly laminar flow encourage stratification in the lower atmosphere and the development of a near-surface temperature inversion further east across the Larsen IS. Embedded within the foehn flow previously unknown fjord-scale jets have been discovered. A number of westerly jets are found to flow downwind of the mouths of inlets at the base of the AP's eastern slopes. The jets are separated by regions of weak westerly and near-stagnant flow. The jets exhibit a cool and moist signature relative to the calm regions. Back trajectory analysis reveals the jets to be the downwind continuation of gap flows, the constituent air being sourced from lower down on the upwind side of the AP and thus from potentially cooler and moister air. Downwind of the topographic gaps (i.e. passes) little Bernoulli loss is apparent as the jets are associated with downslope windstorms which are able to penetrate to the base of the lee slope before ‘rebounding' upwards. Downwind of higher terrain these windstorms are simulated to extend only part way down the lee slope before a hydraulic jump rebounds the flow upwards and the strong winds dissipate in its lee. Weak descending foehn flow continues to the Larsen IS at the base of the lee slope, to form the ‘calm' regions, advecting in relatively warm and dry air sourced from comparatively high altitudes upwind of the AP.