In fluence parameters for a polar mesocyclone development
Dierer, Silke; Schluenzen, Katharina Heinke
Polar mesocyclones are small-scale, short-living phenomena in the Arctic and Antarctic. Their forecast is difficult due to a lack of knowledge of initial and boundary conditions. Idealised sensitivity studies are performed with the mesoscale atmosphere - sea ice model METRAS-MESIM to evaluate the relevance of the large-scale meteorological situation and sea ice cover for surface heat fluxes and a mesocyclone development. The evaluation area is 340 km × 340 km, and a sea ice distribution typical for the Fram Strait is used. The area averaged surface heat flux strongly depends on the ice distribution. The surface heat flux is about 95 W m−2 for a mesocyclone moving close to the ice edge and a high sea ice concentration of 97 % in the ice covered region. Ice thickness and ocean currents do not significantly influence the average surface heat fluxes. A large-scale situation that alters the cyclone track and forces the cyclone to mostly move over ice (cover 97 %) reduces the average surface heat flux by 30 %. An increase of the heat flux by 81 % is found for a homogeneous ice distribution with the same total sea ice mass. A decrease of sea ice concentration from 97 % to 75 % increases the average surface heat flux by 44 %. Thus, for the forecast of average surface heat fluxes during a cyclone passage the knowledge of the sea ice distribution is more relevant than the knowledge of the cyclone track. The break up of sea ice distant from the ice edge might trigger the development of enhanced baroclinicity over ice covered regions and, thus, favour conditions for a mesocyclone intensification. The mesocyclone development is mainly determined by intensi fication at the ice edge resulting from enhanced surface heat fluxes and horizontal temperature gradients in that region. If the cyclone moves over densely ice covered regions the central pressure is up to 6 hPa higher than when it passes close to the ice edge. A homogeneous sea ice cover leads to a maximum pressure difference of 4 hPa. A 22 % reduction of sea ice concentration in the ice covered regions results in maximum pressure deviations of 2 hPa and an increase of the area averaged wind speed by 2 m s−1. Thus, for the forecast of minimum central pressure and wind speed of a polar mesocyclone the cyclone track with respect to the ice distribution needs to be known. The actual pressure may be influenced by the cyclone passage itself by changing the sea ice distribution.