Mesoscale Eddies in Wake of Islands

Abstract

The properties of mesoscale eddies in the wake of islands are investigated in the light of their apparent resemblance to the Kármán vortex-street pattern. By use of the theoretical results from drag theory and the observed quantities such as cross-stream diameter of the obstacle, wind speed and the dimensions of the pattern, estimates are made for the rate of shedding of eddy pairs, the lateral and longitudinal spacing between the vortices, and the life-time of the eddies and drag of the island. These estimates for the various characteristic parameters of the eddy pattern are in good agreement with their corresponding values in the laboratory experiments where stable vortex streets become discernible. Effects of the tip, ground, and neighboring obstacles are neglected and the problem is treated essentially as two-dimensional flow. Use of this theory makes possible a computation of the lateral coefficient of eddy viscosity. The results provide support for the usual assumption made in meteorological studies that viscosity which is used in the equations of motion and which enters into various dimensionless parameters such as Reynolds number, is the eddy viscosity rather than the kinematic viscosity used in hydrodynamic theory. General meteorological considerations applied to these results indicate that the vertical flux of momentum must be included to account for the lifetime of the eddies.