Statistical analysis of inertial gravity wave parameters in the lower stratosphere over Northern China

To determine the morphology of inertial gravity wave (IGW) activity in the lower stratosphere (18-25km) over Northern China and provide observational data constraints for IGW parameterization in atmospheric circulation models, the seasonal variation and longitudinal distribution of IGW parameters were analyzed statistically using 4years (2010-2013) of radiosonde data from 20 sites distributed throughout Northern China (80.2 degrees E-122.3 degrees E). The results are as follows. (1) The seasonal variation in the monthly mean IGW energy (strong in winter and weak in summer) is consistent with the results for other regions in the Northern Hemisphere. However, the energy shows a significant longitudinal increase from west to east in winter and the opposite pattern in summer, which has rarely been reported. (2) The monthly mean intrinsic frequency exhibits clear seasonal variation (large in winter and small in summer). The annual average ratio with the Coriolis frequency is 2.47, while the degree of polarization exhibits the opposite seasonal variation, with an annual average of 0.71. The monthly mean vertical and horizontal wavelengths are 2.5 and 481.3km in Northern China, respectively. (3) The annual mean magnitude of the intrinsic horizontal group speed (7.0m/s) is much larger than that of the vertical group speed (0.08m/s), and both have maxima in winter and minima in summer. The frequency of occurrence of the westward-propagated IGW is larger than that of the eastward-propagated IGW year round, and it is isotropic in the north-south direction. This phenomenon is related to the filtering effect of the tropospheric westerly jet. The annual mean fraction of upward propagation is 75.9%, with a minimum in winter. (4) The momentum flux exhibits a similar temporal and spatial distribution to energy, showing preferred propagation in the northwest direction. (5) The correlation of the monthly mean total energy of the IGW with the westerly jet and the dynamic instability were studied. The results suggest that the westerly jet near the tropopause has a strong influence on the IGWs activity in the lower stratosphere in Northern China and the height region of the westerly jet is a possible source region for IGWs, the maxima regions of the occurrence rates of dynamic instability at the upper and lower edges of the westerly jet in winter are also the possible source region for IGWs.