Radiosonde Observations Of Lower Atmospheric Gravity Wave Source Spectra

Gravity waves (GWs) are important atmosphere waves. GWs in the lower atmosphere are usually activated in the troposphere and can propagate into the middle and upper atmosphere under certain conditions. During the propagating process, GWs would make impact on local and global atmospheric circulations by interacting with the background atmosphere and deposit energy and momentum into it. Therefore, gravity wave source spectra including momentum flux spectra and energy density spectra would provide critical input information to General Circulation models (GCMs).The inertial gravity wave source spectra both in the troposphere and stratosphere are calculated by analyzing the data from the American radiosonde observations for10years (1998-2007). And then the seasonal and latitude distribution are acquired. The primary results are summarized as follow:1. The lower atmospheric momentum flux spectra are calculated by statistically analyzing the gravity wave parameters derived from the radiosonde observations. Based on the observational results, we fit the momentum flux spectra with a Gaussian function and the seasonal variations of the fitting parameters are analyzed. It is found that the strongest gravity wave activities occur in winter. In almost all months the westward Gaussian heights are larger than the eastward ones in the troposphere while the same appears only at the mid and high latitudes in the lower stratosphere. The westward intrinsic phase speeds at Gaussian center are larger than the eastward ones and the southward ones are larger than the northward ones through the whole year in the troposphere while it is the same cases for the lower-stratospheric spectra in spring, autumn and winter. The lower-stratospheric Gaussian heights are smaller by about two orders of magnitude than the tropospheric ones. This is because the GWs deposit part of their momentum into the background atmosphere during the process of propagation. Furthermore, the lower-stratospheric Gaussian centers are larger than the tropospheric ones as a result of the Doppler effects by the background winds.2. This is the first time to analyze the latitude distribution of the gravity wave momentum flux spectra parameters based on observations. We found that the tropospheric gravity wave activities are strongest in the mid latitudes. The lower-stratospheric Gaussian heights reach their maximum near40°N and it is because of the strong upward propagating waves activated by mid-latitude jet in the tropopause. The full widths at half maximum (FWHM) in the troposphere are smaller than those in the lower stratosphere in the mass while the Gaussian centers in the troposphere are smaller than the lower-stratospheric ones. The lower-stratospheric Gaussian heights are smaller by about two orders of magnitude than the tropospheric ones. The latitudinal variation of the lower-stratospheric spectra is more fluctuant than that of tropospheric ones because the former is impacted both by wave sources and propagation effects while the latter is only impacted by sources.30°N is a boundary line for both the tropospheric spectra and the lower-stratospheric ones, which reflects the difference between the low-latitude GWs and mid-latitude GWs. However, the essential cause for this latitudinal difference of waves is the latitudinal variation of background winds.3. This is the first time to derive the total energy density spectra, potential energy density spectra and kinetic energy density spectra in the lower atmosphere and to analyze the latitudinal variation of those spectra on an observational basis. It is found that the latitudinal variation of the tropospheric total energy density spectra is almost the same with the counterpart of the tropospheric momentum flux spectra. The lower-stratospheric total energy density spectra are different from the momentum flux spectra only for the Gaussian heights, namely their sharp decrease in the low latitudes and the second maximum in the mid latitudes. The maxima of the tropospheric potential energy density spectra and kinetic energy density spectra are found near60°N and30°N-40°N respectively, which indicates that the wave source do make impact on the distribution of wave potential energy and kinetic energy. In almost all latitudes, the westward Gaussian heights of the tropospheric potential energy density spectra and kinetic energy density spectra are larger than the eastward ones, which is the same with the total energy density spectra. The total energy density spectra, potential energy density spectra and kinetic energy density spectra in the lower stratosphere all have a tendency of poleward decrease. Furthermore, the potential energy density spectra and kinetic energy density spectra have second maxima in the mid latitudes.4. The symmetry parameters of the source spectra and their latitudinal variation are analyzed. In the lower stratosphere, the zonal symmetry is relatively low at the low latitudes. The main cause is that the westward winds are dominant in the zonal direction at the low latitudes. The symmetry in the troposphere is greater than the lower-stratospheric one on the whole, which may be caused by the asymmetry of the tropospheric wave sources. At the same latitude, the symmetry of the momentum flux spectra is lower than that of the total energy density spectra, which suggests that the asymmetry-related factors induced by source and propagation effects have a more far-reaching influence on the momentum flux spectra than on the total energy density spectra.