Typhoon Wave Assimilation Model Establishment And South China Sea Typhoon Wave Characteristics Under The Background Of Winter Monsoon

In the present operational wave forecast in China has not assimilated satellitealtimeter observations into wave modeling. The initial field without the improvementof data assimilation will affect the accuracy of forecasting results. We established anassimilation module using the optimal interpolation and added it into the wave modelWAVEWATCH III v3.14(NWW3) to improve the initial field.Considering monsoon prevails in the South China Sea (SCS), we combinemonsoon winds (as background) with the empirical model typhoon winds to constructthe forcing wind field. The empirical typhoon wind model was proposed byJelesnianski in1996, and the maximum wind speed radius is calculated using theformula proposed by Willoughby and Rahn (2004). Using typhoon Chanthu(2010) asa case, results show that the blended wind field we established is highly related withthe measured winds from the altimeter, the correlation coefficient being0.96. For thetyphoon Yutu(2001), the correlation coefficients between the blended winds and themeasured winds in Dongsha and Xisha station are0.94and0.91, respectively. Theseverifications show that the forcing winds we established can be used in the typhoonwave modeling in SCS.Using the above method of blended winds to form typhoon Conson(2010) andChanthu(2010) wind field, respectively, we run the NWW3model using these windfields. During the model integration, significant wave heights by Jason-2altimeterwere assimilated into the model. We compare the assimilating analysis of significantwave heights with the altimeter measured data during the typhoon’s passage throughthe SCS. Results show that root mean square errors between them can be decreasedby32%compared with those without data assimilation.In the SCS, monsoon winds prevail, and winter monsoon is stronger thansummer monsoon. Taking typhoon Durian (2006) as an example, we investigated theunique wave characteristics during the typhoon’s passage in conjunction with steadymonsoon wind forcing. Also we analyzed the basic wave characteristics and the role of monsoon in determining the typhoon wave.Along the path of typhoon, the maximal significant wave height on the right sideof the typhoon center is larger than that on the left side, and the width of the coreregion of the maximal significant wave heights on the right side is larger than that onthe left side. For the direction spectrum, the spectrum appears to be single peakstructure with low frequency on the right, while double peak on the left and back. Thespectra energy on the right side is bigger than that on the left side, and the spectraenergy on the circle of two times the max wind radius is smaller than that of at thecircle of max wind radius.The background winds of monsoon do not affect the directional wave spectrumduring the typhoon’s passage but they do affect the directional wave spectrum beforeits arrival and after its departure. Before the typhoon’s arrival, SCS waves are forcedby monsoon winds only, and the dominant waves are down-wind directional swells.Around the typhoon’s arrival, the dominant waves along the typhoon’s track aretyphoon-generated waves. As the typhoon moved out, the typhoon waves decayedgradually and swell generated by monsoon winds appeared again.