| Storm surge is abnormal sea surface rise caused by strong atmospheric disturbances, such as strong winds (tropical and subtropical strom) or sudden changes in atmospheric pressure. If it encounters a high astronomical tide, the water level of the sea rises remarkably which may result in an inundation in the land, causing heavy loss of property and human life in coastal areas. China is one of the countries that suffer from storm surge severely, the coastal regions are attacked frequently by typhoons coming form Western Pacific Ocean, with serious disasters. Therefore, studies on the characters of storm surge and the factors which produce and affect storm surge are carried out to improve the precision of predictions of storm surge, may greatly avoid the loss of lives, and potentially reduce property damage.As wind stress plays a major role in the propagation and movement of storm surges and the wind stress effect is parameterized by the Drag coefficient(DC), it is critical to carry out investigation on DC. So far the following methods have been employed to deal with the DC in the simulation of storm surges. First, the DC has been taken as a constant over the whole computing domain, that indicates independent of sea surface roughness. Second, the DC can be expressed as a linear function of the 10-m wind speed(U10), and there is considerable discrepancy among the parameters in the linear parameterization proposed by different investigators. The above-mentioned two methods have been widely used in numerical simulation of storm surges, but have relatively low accuracy, and they should not be the best choice. Based on the observed sea level elevation, numerical experiments are performed to study the space varying DC with the adjoint assimilation method. We assume that the DC has spatially varying character: in the concerned sea, the DC at some grid points are uniformly selected as the independent DC, while the DC at all grid points ban be obtained through linear interpolation with the independent DC.In this paper, DC is treated with five different methods, and we simulated storm surges caused by 7203, 7303 and 8509, respectively. 1. The DC in the whole area is treated as a constant. 2. The DC is calculated as Smith formulae. 3. The CD is calculated as Wujin formulae. 4. The DC at grid points can be obtained through Cressman linear interpolation with the independent DC, and one independent DC is chosen form each 1°×1°area in the concerned sea, thus 128 independent DC are obtained. 5. one independent DC is chosen form each 0 .5°×0.5°area in the concerned sea, thus 486 independent DC are obtained. Of the last two experiments, the DC of all grid points are optimized with the independent DC are optimized by the adjoint method, with simulation precision increasing. The experimental results demonstrate that, 1. Compare to set DC as a constant and calculate DC according to an experiential formula, the spatially varying DC efficiently increases the simulation precision. Extraordinarily, the absolute mean deviation of storm surge caused by Typhoon 7203 is reduced from 22cm (constant),25cm(Smith formula) and 25cm (Wujin formula) to 16cm(case5), descends by 27.3%,36.0% and 36.0% respectively. 2. The absolute mean deviation of E5 of storm surge caused by Typhoon 8509 is a little higher than E4, which indicates that simulation precision is not increasing with the number of independent points gains. 3. A preliminary analysis on the inversion results of DC illustrates that, the DC of sea with tortuous coastline(the Bohai Sea and the Yellow Sea ) is much higher than that of open sea (the East China Sea).
|
PDF Full Text Request