Spatial and Temporal Variations of Gravity Wave Parameters. Part I: Intrinsic Frequency, Wavelength, and Vertical Propagation Direction

Five years (1998–2002) of U.S. high vertical resolution radiosonde data are analyzed to derive important gravity wave parameters, such as intrinsic frequencies, vertical and horizontal wavelengths, and vertical propagation directions in the lower stratosphere and troposphere. Intrinsic frequencies ω̂ increase with increasing latitude, with larger values in the troposphere. In the lower stratosphere, ω̂ is higher in winter than in summer, especially at mid- and high latitudes. Intrinsic frequencies divided by the Coriolis parameter f are ∼4 in the troposphere, and ∼2.4–3 in the lower stratosphere. The lower-stratospheric ω̂/f generally decreases weakly with increasing latitude. The latitudinal distributions of the lower-stratospheric ω̂/f are explained largely by the propagation effects. The seasonal variations of ω̂ in the lower stratosphere are found to be related to the variations of the background wind speeds. Dominant vertical wavelengths decrease with increasing latitude in the lower stratosphere, and maximize at midlatitudes (35°–40°N) in the troposphere. They are generally longer in winter than in summer. The variations of the dominant vertical wavelengths are found to be associated with the similar variations in gravity wave energies. Dominant horizontal wavelengths decrease with increasing latitude, with larger values in the lower stratosphere. Approximately 50% of the tropospheric gravity waves show upward energy propagation, whereas there is about 75% upward energy propagation in the lower stratosphere. The lower-stratospheric fraction of upward energy propagation is generally smaller in winter than in summer, especially at mid- and high latitudes. The seasonal variation of upward fraction is likely an artifice due to the analysis method, although a small part of it may be interpreted by the variations in background wind speeds. Results suggest that propagation effects are much more important than source variations for explaining the large-scale time-average properties of waves observed by radiosondes.