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Effect of precipitation and winds on sea surface elevation and storm surges

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Title: Effect of precipitation and winds on sea surface elevation and storm surges
Authors: Wong, Benjamin
Item Type: Thesis or dissertation
Abstract: Ocean circulation and storm surge models have neglected mass contributions from precipitation and can have a systematic bias in sea surface height (SSH). Here, a new rainfall scheme has been set up in the ocean circulation model Regional Ocean Modelling System (ROMS) to incorporate the effects of precipitation mass. When precipitation is added to the sea surface, it spreads out via surface gravity waves that increase in propagation speed with increasing water depth. Over several days, the increase in SSH due to the precipitation mass added created a geostrophic adjustment, generating clockwise-rotating geostrophic currents around the SSH increase. The transfer of momentum from precipitation to the sea surface, or rain stress, is investigated in ROMS. An error in the existing implementation of rain stress has been uncovered and corrected. The existing ROMS code generated an error in the direction of rain stress by up to 45 and systematically overestimated its magnitude by 41%. The SSH response to wind stress is examined. Positive and negative surges are generally generated by onshore and offshore winds respectively. While positive surges are widely studied, negative surges are less well understood. Negative surges are larger in magnitude and extend further across the coastline than positive surges. It is shown for the first time that the alongshore component of the wind stress is the main contributor to the asymmetrical surge response. Without this component, the ratio of negative to positive surge can decrease by more than half. This asymmetry also increases with increasing latitude and decreasing depth. In the case study of a real tropical cyclone, Monica, the effect of incorporating precipitation mass is compared with other processes affecting storm surge: surface wind, inverse barometer effect and rain stress. The maximum SSH response is 170.6 cm for the wind effect, 61.5 cm for the inverse barometer effect, 7.5 cm for the effect of rain stress and 6.4 cm for the effect of rain mass. Each process has been shown to have different spatial influences. The effect of rain mass has a strong remote influence compared to the inverse barometer effect and the effect of rain stress. This is particularly seen in semi-enclosed bays.
Content Version: Open Access
Issue Date: Jul-2015
Date Awarded: Jan-2016
URI: http://hdl.handle.net/10044/1/29413
DOI: https://doi.org/10.25560/29413
Supervisor: Toumi, Ralf
Department: Physics
Publisher: Imperial College London
Qualification Level: Doctoral
Qualification Name: Doctor of Philosophy (PhD)
Appears in Collections:Physics PhD theses



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