Momentum Budget of the East Antarctic Atmospheric Boundary Layer: Results of a Regional Climate Model

M. R. van den Broeke Institute for Marine and Atmospheric Research, Utrecht University, Utrecht, Netherlands

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N. P. M. van Lipzig Royal Netherlands Meteorological Institute, de Bilt, Netherlands

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E. van Meijgaard Royal Netherlands Meteorological Institute, de Bilt, Netherlands

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Abstract

Output of a regional atmospheric climate model is used to quantify the average January and July momentum budget of the atmospheric boundary layer (ABL) over the East Antarctic ice sheet and the surrounding oceans. Results are binned in nine elevation intervals over the ice sheet and six distance intervals over the ocean. In January, when surface cooling is weak, the large-scale pressure gradient force dominates the ABL momentum budget. In July, under conditions of strong surface cooling, a shallow katabatic jet develops over the gentle slopes of the interior ice sheet and a strong, deep jet over the steep coastal slopes. In the coastal regions the ABL thickens considerably, caused by the piling up of cold air over the adjacent sea ice and ice shelves. This represents the main opposing force for the katabatic winds. Horizontal and vertical advection are generally small. In the cross-slope direction the momentum budget represents a simple balance between surface drag and Coriolis turning. Intraseasonal variability of the large-scale wind field in the ABL can be explained in terms of the strength of the polar vortex, the background baroclinicity, and the topography of the ice sheet. Subsidence is found over the interior ice sheet and rising motion in the coastal zone, reflecting the acceleration and deceleration of the katabatic circulation. However, vertical velocities are generally small, because the downslope mass flux in the ABL is confined to a shallow layer below the wind speed maximum.

Corresponding author address: Dr. Michiel R. van den Broeke, Institute for Marine and Atmospheric Research (IMAU), Utrecht University, P.O. Box 80 005, 3508 TA Utrecht, Netherlands. Email: broeke@phys.uu.nl

Abstract

Output of a regional atmospheric climate model is used to quantify the average January and July momentum budget of the atmospheric boundary layer (ABL) over the East Antarctic ice sheet and the surrounding oceans. Results are binned in nine elevation intervals over the ice sheet and six distance intervals over the ocean. In January, when surface cooling is weak, the large-scale pressure gradient force dominates the ABL momentum budget. In July, under conditions of strong surface cooling, a shallow katabatic jet develops over the gentle slopes of the interior ice sheet and a strong, deep jet over the steep coastal slopes. In the coastal regions the ABL thickens considerably, caused by the piling up of cold air over the adjacent sea ice and ice shelves. This represents the main opposing force for the katabatic winds. Horizontal and vertical advection are generally small. In the cross-slope direction the momentum budget represents a simple balance between surface drag and Coriolis turning. Intraseasonal variability of the large-scale wind field in the ABL can be explained in terms of the strength of the polar vortex, the background baroclinicity, and the topography of the ice sheet. Subsidence is found over the interior ice sheet and rising motion in the coastal zone, reflecting the acceleration and deceleration of the katabatic circulation. However, vertical velocities are generally small, because the downslope mass flux in the ABL is confined to a shallow layer below the wind speed maximum.

Corresponding author address: Dr. Michiel R. van den Broeke, Institute for Marine and Atmospheric Research (IMAU), Utrecht University, P.O. Box 80 005, 3508 TA Utrecht, Netherlands. Email: broeke@phys.uu.nl

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