A Statistical Study of Gravity Wave with Second-level Radiosonde Data in Sichuan

High vertical resolution radiosonde data of Sichuan Province from June 2014 to September 2017 are analyzed to derive important gravity wave parameters, such as wave energy, intrinsic frequencies, vertical and horizontal wavelengths, and propagation directions. The sampling period of these data is 1.2 s and the vertical resolution is 5-8 m. Five representative stations of Sichuan Province are investigated, including Ganzi, Hongyuan, Chengdu, Xichang and Dazhou. Data in troposphere (2-10 km) and stratosphere (18-25 km), and the latest upper-air wind measurement algorithm for L-band radiosonde sounding system are used to process the original data. Results show that there are obvious seasonal variabilities of gravity waves energy at various areas in Sichuan, strong in winter and weak in summer. In the troposphere, due to the influence of terrain, the energy in the western Sichuan and northern Sichuan regions is significantly smaller than that in other regions. And gravity wave activity is also affected by latitudes. There is no obvious spatial variation in vertical wavelength, and vertical wavelength in winter is slightly larger than summer. The vertical wavelength is concentrated at 1.5-3 km and 1.5-3.5 km in the troposphere and stratosphere, respectively. The horizontal wavelength is quite different, distributed in 0-300 km and 100-700 km, or averaged of 100 km and 350 km in the troposphere and stratosphere, respectively. The ratio of horizontal wavelength to vertical wavelength is 35:1 in the troposphere and 150:1 in the stratosphere. It indicates that gravity waves mainly propagate vertically in the troposphere and propagate horizontally in the stratosphere. In order to get more accurate intrinsic frequency of gravity waves, filtering is essential perform before calculation. There is a large regional difference for intrinsic frequency in the troposphere. The averaged intrinsic frequency in the Plateau regions in northwestern Sichuan is 3 (represents the Coriolis force parameter), while only 2.4 in other regions. There is no obvious spatial difference in the stratosphere, and the mean value is about 2. The vertical propagation directions of gravity waves at different stations in Sichuan are similar, with about 50% of the waves propagating upward in the troposphere and more than 90% in the stratosphere. Horizontal propagation direction of gravity wave is always influenced by background wind field, and it has significant uncertainty, especially in the troposphere. The horizontal propagation of gravity waves in stratosphere strongly depends on season, eastward in summer and westward in other seasons.