critical Richardson number much closer to 0.25. In addition, such a 

 formulation allows the definition of finite eddy coefficients In the 

 unstable range (R1 < 0). In order to utilize these relationships, a 

 turbulence length scale which varies with depth and Richardson number 

 has to be prescribed empirically. 



SIMULATION OF TIDE- AND WIND DRIVEN CURRENTS 



The three-dimensional numerical model of coastal currents has been 

 applied to simulate the tide- and wind-driven currents in the 

 Mississippi Sound and adjacent continental shelf waters of the Gulf of 

 Mexico. As shown in Figure 4, the horizontal grid Is composed of 116 

 grid points in the y-direction and 60 grid points in the x-direction. 

 The smallest grid spacing In the computational domain is on the order 

 of 1 km. The water depth varies from only a few meters within the 

 Mississippi Sound and the Mobile Bay to over 1000 m along the southern 

 boundary. 



fx.i 



Figure 4. Computational Grid for Mississippi Sound Simulation. 



Tides in the Gulf of Mexico 



Gulf tides differ from tides in most other places in the world due 

 to the dominance of the diurnal components Kl, 01 and PI collectively 

 over the semi-diurnal components M2 and S2, except along the west 

 Florida coast. Reid and Whitaker (1981) developed a numerical tide 

 model for the Gulf based on the vertically-integrated, linearized tidal 

 equations to portray the barotropic response of the Gulf to tidal 

 forcing. Forcing at ports was also Included with an impedance type 

 condition. Detailed data from 20 tidal gages located in open coastal 



275 



Sheng 



