Application of Variational Method to Correction of ASCAT Wind Field in the Bohai Sea

It is found that the wind velocity and wind direction of ASCAT have large errors comparing with observations of the Bohai Sea stations. Error analysis results show that the mean wind velocity deviations of ASCAT of the Bohai Bay, Bohai Strait, Laizhou Bay, Central Bohai Sea and Liaodong Bay areas are from the largest to the smallest. The mean wind direction deviation of ASCAT of the Laizhou Bay, Bohai Bay, Liaodong Bay, Bohai Strait and Central Bohai Sea area are from the largest to the smallest. The wind speed deviation of the Bohai Bay is the largest, and the wind direction deviation of the Laizhou Bay is the largest. According to error analysis results, the reliability of ASCAT wind field in the Bohai Sea needs to be improved. Therefore, the wind field of ASCAT is corrected using observations of the Bohai Sea stations in terms of variational method, which can obtain the corrected wind field with spatial resolution of 12.5 km×12.5 km and temporal resolution of 12 hours. In addition, to test the applicability of variational method in the sea area, the corrected wind field in five sea areas including the Liaodong Bay, Bohai Bay, Laizhou Bay, Central Bohai Sea and Bohai Strait are verified. Verification results show that the accuracy of ASCAT wind field is improved significantly after correction, the average deviation of wind velocity is reduced from 4 m·s^-1 to 1 m·s^-1, and the average deviation of wind direction is reduced from-30 °-30 ° to -7 °-4 °. The variational method is proved effective on ASCAT wind field correction. The ASCAT wind field revised by variational method from September 2017 to February 2018 shows that the distribution of the wind field is consistent with the wind field observed on the sea surface. The corrected wind speed of the Bohai Sea is proportional to the offshore distance and contour depth, which means areas further offshore, with deeper depth contour and wider surface are likely less influenced by submarine topography and the friction coefficient, and therefore the wind speed value is greater, and the wind direction is from the north to the northwest. The analysis of a gale process on 18 December 2017 shows that the corrected wind field can reflect coastal wind field information and the extreme value area of wind velocity in the process of wind, and it can monitor the process of gale changes dynamically. The variational method solves the problem of low spatial resolution of stations and low accuracy of ASCAT. It also has a good guiding significance for gale forecasting and can provide more accurate initial field for ocean models.