Interpretation of the mesospheric and lower thermospheric mean winds observed by MF radar at about 30°N with the 2D-SOCRATES model

Data obtained by Wuhan (30.5°N, 114.4°E) MF radar and Yamagawa (31.2°N, 130.6°E) MF radar have been used to study the mean winds in the mesosphere and lower thermosphere (MLT) at about 30°N. The observed mean zonal and mean meridional winds show obviously seasonal variations. Westerly wind prevails in winter, and decreases with the increasing height above 76 km, even reverses above 96 km sometimes. The summer mean zonal wind is westward in the mesosphere and eastward in the lower thermosphere, with the reversal height of about 80 km. From 70 to 95 km, the mean meridional wind blows northward in winter and southward in summer. Northerly wind prevails between 95 and 98 km throughout seasons. These wind features have similar patterns to those of the empirical HWM93 wind model. 2D-SOCRATES model is used to try to give physical interpretations of the observed wind fields, with which dynamic contributions to the MLT wind structures are analyzed. Simulations show that the planetary waves play an unimportant role in the MLT region since they have relatively small magnitudes during winter and even cannot propagate upward into the upper atmosphere during summer. The gravity waves play a crucial role in determining the wind structures in the MLT region, providing forcing of about 40 m/s/day and diffusion coefficients of about 50 m2/s at 30°N. The atmospheric tidal waves have significant influences in the wind structures with forcing of about 10 m/s/day and diffusion coefficients of about several m2/s in the MLT at 30°N. Breakings of these atmospheric waves tend to close off the westerly jet in winter and easterly jet in summer, to produce strong wind shear in the mesopause, and to drive the meridional wind directed from the summer hemisphere to the winter hemisphere.