Source spectra information derived from US high-resolution radiosonde data

A ray-tracing model is used to study the climatology of the source properties for gravity waves in the lower stratosphere using 8 years of U. S. high-resolution radiosonde data. If the simulated total energy density and zonal momentum flux correlate well with those two parameters derived from the observations simultaneously, we claim a "best fit'' is found. Eighty-five stations with sufficient completeness and quality are analyzed, and we are able to obtain "best fit'' source spectra for 61 of them. Two conditions are found to be necessary to achieve a "best fit.'' First, the kinetic energy density and the zonal momentum flux derived from observations need to be highly correlated. Second, the source altitude needs to be located right below the lower stratosphere since the observed gravity waves represent the average climatology of the cumulative effect on a monthly average basis of the various time-varying physical sources together with their propagation from different altitudes. All the sources we found are anisotropic, with most of momentum flux directed upstream of the dominant wind direction. This might be caused by wave propagation effects or the nature of the sources themselves. The latitudinal variation of the horizontal wavelength of the source spectra corresponds well with that derived independently from the observations, but other variable parameters do not show obvious latitudinal dependence. A source spectrum type representing convection works well for lower latitudes, while another more general source works better for middle to high latitudes. The central U. S. and the North American monsoon region are two regions where it is difficult to obtain "best fit'' source spectra. We suspect that the reason for this is that during the summer, mesoscale convective systems are dominant sources for inertia-gravity waves, which may not be well represented by our chosen spectral source functions.