Gerit Birnbaum

A new parameterization of surface drag in the marginal sea ice zone

A parameterization of subgridscale surface fluxes over the marginal sea ice zone which has beenused earlier in several studies is modified and applied to a nonhydrostatic mesoscale model.The new scheme accounts for the form drag of ice floes and is combined with a so-called fluxaveraging method for the determination of surface fluxes over inhomogeneous terrain. Individualfluxes over ice and water are calculated as a function of the blending height. It is shown bycomparison with observations that the drag coefficients calculated with the new parameterizationagree well with data. The original scheme strongly overestimates the form drag effect.An improvement is mainly obtained by an inclusion of stratification and by use of a moreadequate coefficient of resistance for individual ice floes. The mesoscale model is applied to officeflows over the polar marginal sea ice zone. The model results show that under certainmeteorological conditions the form drag can have a strong influence on the near-surface windvelocity and on the turbulent fluxes of momentum. Four case studies are carried out. Themaximum influence of form drag occurs in the case with strong unstable stratification and withwind oblique to the ice edge. Under these conditions the wind stress on sea ice is modified byat least 100% for ice concentrations less than 50% if form drag is taken into account.

Strong-wind events and their influence on the formation of snow dunes: observations from Kohnen station, Dronning Maud Land, Antarctica

Analyses of shallow cores obtained at the European Project for Ice Coring in Antarctica (EPICA) drilling site Kohnen station (75800 0 S, 00804 0 E; 2892 m a.s.l.) on the plateau of Dronning Maud Land reveal the presence of conserved snow dunes in the firn. In situ observations during three dune formation events in the 2005/06 austral summer at Kohnen station show that these periods were characterized by a phase of 2 or 3 days with snowdrift prior to dune formation which only occurred during high wind speeds of >10 m s -1 at 2 m height caused by the influence of a low-pressure system. The dune surface coverage after a formation event varied between 5% and 15%, with a typical dune size of (4 � 2) m � (8 � 3) m, a maximum height of 0.2 � 0.1 m and a periodicity length of about 30 m. The mean density within a snow dune varied between 380 and 500 kg m -3 , whereas the mean density at the surrounding surface was 330 � 5k g m -3 . The firn cores covering a time-span of 22 � 2 years reveal that approximately three to eight events per year occurred, during which snow dunes had been formed and were preserved in the firn.

Synoptic situations causing high precipitation rates on the Antarctic plateau: observations from Kohnen Station, Dronning Maud Land

Kohnen Station (75°S, 0°E, 2892 m) is one of the two drilling sites of the European Project for Ice Coring in Antarctica. Snow falls at Kohnen only a few times a year with comparatively high precipitation rates of 1 mm to over 5 mm water equivalent per event. These events contribute considerably to the total annual accumulation of which the long-term mean value is 62 mm water equivalent per year. For ice core interpretation, it is important to understand synoptic processes leading to such high precipitation rates. Our investigation is based on visually observed periods of heavy snowfall at Kohnen during summer campaigns since 2001/2002. The corresponding synoptic situations can be grouped into three categories. Category I is where occluding fronts of eastward-moving low pressure systems reach the plateau, a fairly frequent occurrence. Category II is where lows or secondary lows formed east of the Greenwich Meridian move to the west (retrograde movement), and frontal clouds influence the plateau. In Category III, large-scale lifting processes (due to an upper air low west of Kohnen Station) lead to cloud formation over the plateau of Dronning Maud Land.

Mesoscale modelling of the Arctic atmospheric boundary layer and its interaction with sea ice

This chapter summarises mesoscale modelling studies, which were carried out during the ACSYS decade until 2005. They were aiming at the parameterisation and improved understanding of processes in the Arctic boundary layer over the open ocean and marginal sea ice zones and over the Greenland ice sheet. It is shown that progress has been achieved with the parameterization of fluxes in strong convective situations such as cold-air outbreaks and convection over leads. A first step was made towards the parameterization of the lead-induced turbulence for high-resolution, but non-eddy resolving models. Progress has also been made with the parameterization of the near-surface atmospheric fluxes of energy and momentum modified by sea ice pressure ridges and by ice floe edges. Other studies brought new insight into the complex processes influencing sea ice transport and atmospheric stability over sea ice. Improved understanding was obtained on the cloud effects on the snow/ice surface temperature and further on the near-surface turbulent fluxes. Finally, open questions are addressed, which remained after the ACSYS decade for future programmes having been started in the years after 2005.