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4. Summary

Profiles of atmospheric temperature in the upper troposphere and stratosphere have been obtained by means of radio occultation observations of GPS (Global Positioning System) signals by the GPS/MET (GPS Meteorology) experiment from April 1995 to February 1997.

• From each GPS/MET temperature profile we have extracted mesoscale temperature perturbations with a vertical scale ranging from 2 to 10 km, as well as the background N². Then, we have defined the potential energy, Ep, from the temperature variance.
• The seasonal variation of Ep determined with the GPS/MET data in the lower stratosphere is consistent with the climatological behavior of the kinetic energy of gravity waves, E_k, observed with ST radars in Japan and New Mexico.
• We have analyzed the latitude-longitude distribution of Ep at 20-30 km from November to February. Ep values are particularly enhanced over convectively active regions around the equator, which strongly suggests that gravity waves are generated by tropical convection.
• Using GPS/MET data without anti-spoofing, latitudinal variations of Ep were determined between 15 and 45 km altitude. The results indicate that large Ep values are concentrated near the equator at 20-30 km. But, Ep becomes larger at midlatitudes at 30-40 km and higher altitude regions. At midlatitudes, Ep was larger in winter months in each hemisphere, although differences also existed between northern and southern hemispheres. During equinoxes, a remarkable latitudinal symmetry of Ep relative to the equator was found.
• The longitudinal distribution of Ep at latitudes between 30^oN and 60^oN during November and February indicates an enhanced gravity wave activity over continents, possibly suggesting orographic generation of mountain lee waves.
• We have also determined height variations of Ep at 15-45 km. The Ep values decrease between 20 km and 25-30 km altitude, and show a gradual and rapid increase at 25-35 km and above 35 km, respectively.
• We have employed an improved retrieval software so as to obtain temperature profiles with a better height resolution (about 200 m) from Level 2 data of the GPS/MET experiment. We have analyzed vertical wavenumber spectrum of the normalized temperature fluctuations at 20-30 km, and found that the observed spectra agree with a saturated gravity wave model with a vertical scale as small as about 400 m.
• Latitude variations of the spectra are investigated at 20-30 km and 30-40 km by using GPS/MET data during a prime time in June-July, October and February. The spectral slope is nearly ?3, consistent with themodel. However, the spectral density at 20-30 km is slightly smaller than the model value.
• Temperature variance caluculated by integrating the spectra in wavelength ranges of 10-2.5 km and 2-0.4 km is largely enhanced in the equatorial region.

This study has clarified that the GPS occultation technique provides important and unique data for studies of the global distribution of atmospheric gravity waves.

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