5.7
Evolution of a Fresh Sea Surface Anomaly Produced by Tropical Rainfall: Model and Validation
H. W. Wijesekera, Oregon State University, Corvallis, OR; and C. A. Paulson and E. Skyllingstad
The time-dependent three-dimensional circulation of a localized, fresh, sea-surface anomaly produced by heavy tropical rainfall was simulated using the Blumberg-Mellor, finite difference, stratified, hydrostatic, primitive equation model. The motivation for the simulation arose from observations made during Coupled Ocean Atmosphere Response Experiment (COARE). The objective of this study is to understand the dynamical processes governing the evolution of low-salinity, surface anomalies formed by rainfall. The model consists of periodic channel with a flat bottom. The domain is about 125x125 km horizontally and 300 m vertically. The vertical resolution is 1 m in the upper 75 m. The horizontal resolution is 500 m. The initial density is horizontally homogeneous, and the vertical stratification is consistent with the COARE observations. The model was spun-up for 1.2 days with an along-channel wind stress of 0.08 N/m^2 and a upward heat flux of 250 W/m^2, and then a rain-induced buoyancy anomaly, equivalent to a rainfall of about 60 mm was applied as a surface boundary condition for a one time step. The size of the anomaly, 20x20 km, is similar to the spatial structure of radar rainfall. The model captures basic features of the observed event both qualitatively and quantitatively. Immediately after rainfall, the fresh lens trapped momentum in a shallow layer, while inhibiting turbulent mixing below the layer. Nearly 7 hours after the formation, the density anomaly mixed down to a depth near the mixed layer depth before rainfall, and there was a positive anomaly in velocity of 0.2 m/s in the direction of the wind. Simulations also showed upwelling at the upwind edge of the lens and downwelling at the downwind edge. The magnitude of vertical velocities at the base of the mixed layer is about 10 m/day.
Session 5, Air-Sea Interaction: Oceanic Processes
Monday, 14 May 2001, 3:30 PM-5:15 PM
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