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A. E. Hay and E. D. Kinsella

Abstract

The usual two-layer model for steady wind-driven upwelling along a uniform coastline is extended to incorporate the effects of an upper-layer jet trapped against the coast. The characteristic width of the jet is the internal deformation radius, so the jet Rossby number in the governing equations for the upper layer is order of unity, and the nonlinear term involving cross-stream shear must be retained. It is shown, however, that the equations can be reduced to a manageable form when the upper-layer thickness and equilibrium displacement of the interface are both much less than the total depth. Explicit solutions are obtained for equilibrium jet profiles for which the interface is either exponential, which corresponds to a frictionless jet with uniform potential vorticity, or parabolic. It is also shown that solutions should be obtainable when the jet profile can be expressed as an arbitrary polynomial in the offshore coordinate. The principal differences between our results and the usual ones for the no-jet case are that upwelling is reduced at the coast and amplified offshore. The differences are due to a reduction in the divergence of the on-offshore velocities within an internal Rossby radius of the coast and to increased divergence farther offshore. These changes in divergence are the result of the equilibrium displacement of the interface through the continuity equation and of advection of mean flow momentum by wind-induced offshore motion through the cross-stream shear.

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W. B. Rossow, F. Mosher, E. Kinsella, A. Arking, M. Desbois, E. Harrison, P. Minnis, E. Ruprecht, G. Seze, C. Simmer, and E. Smith

Abstract

The International Satellite Cloud Climatology Project (ISCCP) will provide a uniform global climatology of satellite-measured radiances and derive an experimental climatology of cloud radiative properties from these radiances. A pilot study to intercompare cloud analysis algorithms was initiated in 1981 to define a state-of-the-art algorithm for ISCCP. This study compared the results of applying six different algorithms to the same satellite radiance data. The results show that the performance of all current algorithms depends on how accurately the clear sky radiances are specified; much improvement in results is possible with better methods for obtaining these clear-sky radiances. A major difference between the algorithms is caused by their sensitivity to changes in the cloud size distribution and optical properties: all methods, which work well for some cloud types or climate regions, do poorly for other situations. Therefore, the ISCCP algorithm is composed of a series of steps, each of which is designed to detect some of the clouds present in the scene. This progressive analysis is used to retrieve an estimate of the clear sky radiances corresponding to each satellite image. Application of a bispectral threshold is then used as the last step to determine the cloud fraction. Cloudy radiances are interpreted in terms of a simplified model of cloud radiative effects to provide some measure of cloud radiative properties. Application of this experimental algorithm to produce a cloud climatology and field observation programs to validate the results will stimulate further research on cloud analysis techniques as part of ISCCP.

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