A modeling study on the response of Chesapeake Bay to hurricane events of Floyd and Isabel

摘要

The response of Chesapeake Bay to forcing from two hurricanes is investigated using an unstructured-gridthree-dimensional hydrodynamic model SELFE. The model domain includes Chesapeake Bay, its tributar-ies, and the extended continental shelf in the mid-Atlantic Bight. The hurricanes chosen for the study areHurricane Floyd (1999) and Hurricane Isabel (2003), both of which made landfall within 100 km of themouth of the Bay. The model results agree reasonably well with field observations of water level, velocity,and salinity. From the Bay's water level response to the hurricanes, it was found that the storm surge inthe Bay has two distinct stages: an initial stage set up by the remote winds and the second stage - aprimary surge induced by the local winds. For the initial stage, the rising of the coastal sea level was setupby the remote wind of both hurricanes similarly, but for the second stage, the responses to the twohurricanes' local winds are significantly different. Hurricane Floyd was followed by down-Bay winds thatcanceled the initial setup and caused a set-down from the upper Bay. Hurricane Isabel, on the other hand,was followed by up-Bay winds, which reinforced the initial setup and continued to rise up against thehead of the Bay. From the perspective of volume and salt fluxes, it is evident that an oceanic saltwaterinflux is pushed into the Bay from the continental shelf by the remote wind fields in the initial stagesof the storm surge for both Floyd and Isabel. In the second stage after the hurricane made landfall, theBay's local wind plays a key role in modulating the salinity and velocity fields through vertical mixingand longitudinal salt transport. Controlled numerical experiments are conducted in order to identifyand differentiate the roles played by the local wind in stratified and destratified conditions. Down-estuarylocal wind stress (of Hurricane Floyd-type) tends to enhance stratification under moderate winds, butexhibits an increasing-then-decreasing stage when the wind stress increases. The up-estuary local windstress (of Hurricane Isabel-type) tends to penetrate deeper into the water column, which reduces strati-fication by reversing gravitational circulation. To characterize mixing conditions in the estuary, a modifiedhorizontal Richardson number that incorporates wind stress, wind direction, horizontal salinity gradient,and vertical eddy viscosity is used for both hurricanes. Finally, the direct precipitation of rainfall into theBay during Hurricane Floyd appears to create not only a thin surface layer of low salinity but also a sea-ward barotropic pressure gradient that affects the subsequent redistribution of salinity after the storm.