OROGENIC BLOCKING AND DEFLECTION OF STRATIFIED AIR-FLOW ON AN F-PLANE

The stationary, finite-amplitude, orogenic disturbance of a straight b arotrapic air current on the rotating earth is studied numerically on the basis of quasi-static equations. Three quantifiable characteristic s are defined in order to provide evaluation of the upstream Bow struc ture: (i) the point of incipient splitting; (ii) the strength of block ing; (iii) the extent of streamline displacement. Tile numerical resul ts have been used to investigate the dependence of these characteristi cs on the Rossby number, Ro, the non-dimensional mountain height, H, a nd the non-dimensional mountain shape factor, A (across-stream length/ along-stream length), with the non-dimensional stability fixed. The ex periments provide an indication of how the critical non-dimensional mo untain height for stagnation and for splitting, respectively, depends on the Ro and A. The experiments show that the proportion of flow that goes around the mountain increases as A becomes smaller. For the case s when Ro . H much greater than A, the surface-level flows upstream of the mountains are mostly or even fully diverted around it and partly blocked by the mountain. In agreement with the inferences drawn by Pie rrehumbert and Wyman (1985), it is shown that for an across-stream len gth of mountains greater or equal to the radius of deformation, i. e., Ro . H less than or equal to A, the maximum extent of the upstream in fluence is of the order of the radius of deformation, Ro . H. For Ro . H much greater than A, this extent is limited by horizontal dispersio n to a distance proportional to A.