Modification of the physics and numerics in a third-generation ocean wave model

Abstract

The ocean wave model WAM was recently upgraded to improve the coupling between the sea state and the air flow and, in particular, enhance the growth of young wind sea over that of old wind sea. Prior to this change, numerous validations of the original wind input physics by the wave modeling community had shown a consistent underprediction of wave heights in the Southern Hemisphere. Preliminary tests of the upgraded version indicated that the change in physics improved wave height predictions but did not completely eliminate the bias. To specifically test the upgraded version, a one-month hindcast study for the Australian region was conducted using both the improved and the original physics. The hindcast significant wave heights were compared against observations from three locations around the Australian coast and were found to be overpredicted by the upgraded physics and underpredicted by the original physics. A series of experiments were then made that indicated the upgraded physics were sensitive to conditions unique to the Southern Ocean, that is, long stretches of open water and a continuous procession of strong frontal systems migrating from west to east. In particular, it is found that as part of the upgraded physics the modification to the dissipation term resulted in an excessive transfer of energy to the low frequencies by means of the nonlinear interaction term. Consequently, the wave energy continues to grow, resulting in excessive wave heights. While the use of the original physics has led to underpredicted wave heights in the past, the author shows that this bias can be reduced by improving the present first-order upwinding propagation numerics to third order. This dramatically reduces the numerical diffusion that causes excessive dissipation of swell traveling the long distances typical in the Southern Ocean. The author then reconducted the one-month hindcast study using the original physics and third-order upwinding numerics and showed that wave height predictions are improved.