92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Tuesday, 24 January 2012: 3:30 PM
The Effect of Bottom Boundary Layer Dynamics on the Forerunner Simulation During Hurricane Ike in the Gulf of Mexico
Room 337 (New Orleans Convention Center )
Harry Wang, Virginia Institute of Marine Science, Gloucester Point, VA

It has long been recognized that forerunner plays an important role in generating large hurricane-induced storm surge in the Gulf of Mexico. The forerunner is a phenomenon whereby water elevation in the vast coast was lifted days before Hurricanes make the landfall. It can contribute significantly to the total water level subsequently during primary surge when the hurricanes made the landfall. The 2008 Hurricane Ike, which devastating the Galveston Bay in the Texas coast, was a good example, in which 1.8 m forerunner occurred beforehand and contributed to the subsequent maximum surge of 4.5 m in the Gulf of Mexico where normal tidal range is less than 1 m.

A collective effort for improving modeling skill for storm surge and inundation including forerunner in the Gulf of Mexico was recently been funded by NOAA IOOS program and carried out by a team of modelers on a testbed paradigm using open source codes. The coupled tide, surge, and wind wave models in two and three dimension were tested and compared systematically. The consensus from initial results of multiple models indicated that the forerunner occurred as a result of Ekman set up along the broad Louisiana-Texas shelf by the shore parallel wind field. It was found that the cross-shore Ekman transport is highly sensitive to the bottom boundary layer dynamics especially to the drag coefficient used. The Manning' roughness n required in a 2D model, for example, is as small as 0.01.

Given the fact that Gulf of Mexico is known to be rich in fluid mud, we postulate that, during the stormy condition, the suspended sediment-induced density stratification is likely to be ubiquitously present at the bottom boundary layer. An empirical-derived bottom boundary layer sub-model including the sediment-induced stratification effect was coupled to the unstructured grid circulation and wind wave model: SELFE-WWM for simulating the forerunner during Hurricane Ike. The model results demonstrates that the bottom boundary layer has an commanding effect on the velocity veering orthogonal to the wind direction and the Ekman transport toward the shore. The idea presented is consistent with Jelesnianski's work (1967) on incorporating slip bottom boundary condition as the bottom stress for the storm surge simulation.

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