Strong Wind Characteristics of the Lower Boundary Layer (0–300 m) during the Landfall of a Typhoon

Based on in situ gradient observations from wind profiler radars, WindCubeV2 lidar, and masts during the landfall of typhoons Mangkhut (1822) and Lekima (1909), combined with the simulated winds of typhoon Mangkhut (1822) with the finest horizontal resolution of 2 km and vertical resolution of 50 model levels in which the lower levels were densified, the strong wind structure in the lower level (below 300 m height over sea level or terrain) were analyzed, and the following results were obtained. (1) Within the range of 0–200 km from the typhoon center, the maximum wind speed height and wind shear index increased outward along the radial direction, and the wind shear index on the land surface was generally higher than 0.12. Because of the weak drag effect on the ocean surface, the wind shear index was generally small, except for island areas. (2) The strong wind shear on the right front quadrant of the moving direction of the typhoon remained stable at approximately 0.17, which was insensitive to distance and altitude. The wind profile was similar to the jet stream on the left rear quadrant, and a previous study pointed out that the changes of supergradient/subgradient wind in the vertical direction were responsible for the jet-type profiles. The vertical variations of strong winds on the left front quadrant showed nonlinear characteristics, indicating the complex, strong wind structure over this area. (3) The gust factor and turbulence intensity decreased with the mean wind speed and altitude. (4) The maximum wind direction variation during the landfall of a typhoon decreased outward along the radial direction and exhibited statistically significant spatial asymmetry, with the largest variation near the right rear quadrant. Over some areas of the right rear quadrant, the wind direction changed more than 30° in half an hour, and most of them occurred before or during the typhoon’s landfall. This information could be useful for microscale wind simulation as well as the prevention and mitigation of typhoon disasters over offshore wind farms in China.