Study of Mean Wind Variations and Gravity Wave Forcing Via a Meteor Radar Chain and Comparison with HWM-07 Results

We present an analysis of long-term observation of the mean winds in the mesosphere and lower thermosphere region. The neutral wind data are collected over the period of January 2009 to November 2017 from a chain of meteor radars along the 120 degrees E meridian in the Northern Hemisphere which consists of four stations at Mohe (MH; 53.5 degrees N, 122.3 degrees E), Beijing (BJ; 40.3 degrees N, 116.2 degrees E), Wuhan (WH; 30.5 degrees N, 114.6 degrees E), and Sanya (SY; 18.3 degrees N, 109.6 degrees E). The annual oscillation dominates the neutral wind pattern at midlatitudes while both the annual oscillation and semiannual oscillations are important at low latitudes. The mean zonal wind pattern in midlatitudes exhibits two eastward jets. One is in the period from April to October above 85km and the other is from November to March below 90km. A reversal of the mean zonal winds from eastward to westward is observed in midlatitudes around the spring equinox. Gravity wave forcing during this season shows a rapid change from westward to eastward, which is consistent with the reversals. A three-cell southward wind pattern is observed over WH and SY. A strong vertical wind shear with northward flow below 90km and southward flow above is observed over SY from September to April. Composite-year comparisons between the observations and the Horizontal Wind Model-07 show large discrepancies during winter time. HWM-07 predictions agree with the meteor radar observations better in the zonal component than in the meridional component. The model predictions at low latitude are not as accurate as the middle latitudes.