Vorticity, Momentum and Divergence Budgets of Synoptic-Scale Wave Disturbances in the Tropical Eastern Atlantic

Duane E. Stevens Department of Atmospheric Sciences, University of Washington. Seattle 98195

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Abstract

The budgets of vorticity, momentum and divergence are evaluated for the average synoptic-scale wave composited by Thompson et al. (1979) from Phase III data of the GARP Atlantic Tropical Experiment (GATE). The data are analyzed into a Phase III mean component and a time-varying wave component. The geopotential field which is required for the momentum and divergence budgets is obtained from vertical integration of the hydrostatic equation.

It is found that nonlinear terms in all wave budgets tend to be small, so that the disturbances are governed by linear dynamics. In contrast with the composite wave analyzed by Shapiro (1978), the waves are not approximately advected by the horizontal wind. Accelerations have the same amplitude as the Coriolis force in the momentum balances of the disturbances, indicating that the disturbances are not quasi-geostrophic. However, thermal wind balance appears to be a good approximation for the mean zonal wind. None of the usual simplifications to the divergence equation is satisfactory for the traveling waves.

Each of the large-scale budgets has a significant residual imbalance, suggesting that subsynoptic-scale circulations strongly affect the wave dynamics. Cumulus transports must be parameterized in the dynamic budgets for a proper treatment of the dynamics and energetics of the wave disturbances. Neither Rayleigh friction nor a simple vertical momentum exchange is an adequate parameterization of the apparent momentum sources.

Abstract

The budgets of vorticity, momentum and divergence are evaluated for the average synoptic-scale wave composited by Thompson et al. (1979) from Phase III data of the GARP Atlantic Tropical Experiment (GATE). The data are analyzed into a Phase III mean component and a time-varying wave component. The geopotential field which is required for the momentum and divergence budgets is obtained from vertical integration of the hydrostatic equation.

It is found that nonlinear terms in all wave budgets tend to be small, so that the disturbances are governed by linear dynamics. In contrast with the composite wave analyzed by Shapiro (1978), the waves are not approximately advected by the horizontal wind. Accelerations have the same amplitude as the Coriolis force in the momentum balances of the disturbances, indicating that the disturbances are not quasi-geostrophic. However, thermal wind balance appears to be a good approximation for the mean zonal wind. None of the usual simplifications to the divergence equation is satisfactory for the traveling waves.

Each of the large-scale budgets has a significant residual imbalance, suggesting that subsynoptic-scale circulations strongly affect the wave dynamics. Cumulus transports must be parameterized in the dynamic budgets for a proper treatment of the dynamics and energetics of the wave disturbances. Neither Rayleigh friction nor a simple vertical momentum exchange is an adequate parameterization of the apparent momentum sources.

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