Katabatic flow / marine atmospheric boundary layer interactions around Antarctica

Supervisers - Dr Ian Renfrew (UEA), Dr Phil Anderson (BAS) and Dr John King (BAS)

The surface wind field over the Antarctic continent is strongly influenced by katabatic winds, downslope density-driven flows which occur as a result of the persistent long-wave radiational cooling that take place in the polar regions. When these katabatic flows reach the coast (in an area with a narrow, or non-existent, ice shelf) they will interact with the marine atmospheric boundary layer that is present over the sea-ice zone. For example, over an area of open water (a polynya) a convective boundary layer will be in place and the katabatic flow may well be strengthend through an “ice-breeze” circulation (analogous to a sea-breeze circulation). In this case, stronger winds will lead to higher surface heat and moisture fluxes, leading to a rapid freeze over the polynya and considerable brine rejection as sea ice is formed. Upon freeze-over the nature of the marine boundary layer will change to become more stable (less convective); until the next high wind-speed event redistributes the sea ice and opens new leads and polynyas. It is apparent that the atmosphere-ocean-sea ice system is tightly coupled and that katabatic flows play a key part in air-sea interaction and the production of sea ice.

In this project a state-of-the-art numerical weather prediction model (RAMS) will be used to carry out simulations of the interaction between katabatic flows and the atmospheric boundary layer over the sea ice zone. A unique set of year-round near-surface katabatic wind profiles from Coats Land, Antarctica and Halley Research Station that was gathered during 2002 & 2003 will be used as validation of the numerical simulations over the continent. What sparse atmospheric observations are available over the coastal sea-ice zone will be used as validation over the ocean. The RAMS model has already successfully been used to simulate “idealised” katabatic flows over Coats Land (Renfrew 2004 – see Figure: a simulation of downslope wind field after 24 h over Coats Land, the Brunt Ice Shelf and the adjoining sea ice zone). The first aim of this project will be the simulation of a number of “case studies” where specific observational data are available. Once thoroughly tested, the model domain will be relocated to other parts of the continent where katabatic flow/marine boundary layer interactions are thought to be more important. The interaction will be examined through a series of sensitivity experiments, for example, changing the continental slope (and so the strength of the downslope winds); changing the background atmospheric forcing; changing the model’s sea-ice distribution; and changing parameterisations important to the surface fluxes in the model. Generic conclusions drawn from the atmospheric boundary-layer modelling work will be used to assess the importance of  katabatic flows in the exchange of heat, moisture and momentum in the sea-ice zone.
 
Renfrew, I. A., 2004: The dynamics of  idealized katabatic flow over a moderate slope and ice shelf, Quart. J. Roy. Meteor. Soc., 130, 1023-1045.