c) Radiation boundary condition 



The effects of the open boundary condition employed in this 

 study on tjq was of primary concern. In order to evaluate this 

 effect, a pure radiation boundary condition was imposed in the 

 simulations of HUR5, HUR23 and hurricane Allen. The names of these 

 storms modified by (R) are used in the following discussions to 

 distinguish them from their corresponding original simulations. 



Figures 106 through 108 show the time histories of tjq obtained 

 from the simulations of HUR5(R), HUR23(R) and hurricane Allen(R), 

 respectively. Only the 77^ signal obtained by averaging water levels 

 in the Gulf is shown in the figures (note the changes in height 

 scale) . The radiation condition at the open boundaries effectively 

 radiates tjq out of the Gulf. The peaks of tjq, sequentially, are 0.28 

 m, 0.14 m and 0.22 m for HUR5(R), HUR23(R) and hurricane Allen(R) . 

 The estimated e-folding times of the damping rate are 34 h for 

 HUR5(R), 24 h for HIIR23(R) and less than 10 h for hurricane Allen(R) . 

 The radiation condition not only drastically damps the j?q signal but 

 also changes their periods. Reid and Whitaker (1981) experimentally 

 determined the damping rate of the volume mode in the GOMT model to 

 be 2.81 X 10"^ s"-'- or an e-folding time of 2.6 days. The radiation 

 condition employed in the GOMT model has a complex admittance 

 coefficient in which the imaginary part governed the effective added 

 mass of adjoining seas. The radiation condition employed in this 

 study is a special case where only the real part of the admittance is 

 considered. Exclusion of the imaginary part results in a much larger 

 damping rate of the volume mode as found in this study. However, the 



175 



