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3. Vertical wavenumber spectrum of temperature perturbations

Temperature profiles are re-analyzed by using a shorter data smoothing range (about 200 m) with an improved retrieval algorithm [Hocke, 1997]. Using 83 GPS/MET profiles collected on Feruary 2-10, 1997 in a latitude band between 20o N and 20oS, we have calculated vertical wavenumber spectrum of the normalized temperature fluctuations, and compared them with the radiosonde result determined from 78 profiles over Pontianak (0o, 109oE), Indonesia, in January-February, 1997. The observed spectra are also compared with a model spectrum assuming a linear saturation of gravity waves, N4/(10g2m3), where m is the vertical wavenumber. We have found that the GPS/MET profiles seem to resolve gravity waves with a vertical scale as small as about 400 m at 20-30 km altitude. However, above 30 km, the spectral density from GPS/MET profiles could be suppressed because the data retrieval algorithm artificially reduces large temperature deviation from a standard model atmosphere. In particular, the observed spectral density at 40-50 km obviously exceeds a theoretical model probably due to spurious temperature fluctuations caused by ionosphereic correction and receiver noise [Hocke, 1997].

We have analyzed the spectra at 20-30 km and 30-40 km during three prime times under A/S-off condition in June/July 1995, October 1995 and February 1997. At 20-30 km the spectral slope for short wavelengths (>2 km) is about ?3 as predicted by a saturated gravity wave model. But, the spectral density is sometimes smaller than the model. At 30-40 km both spectral slope and density agree well with the model, especially for wavelengths shorter than about 1.5 km.

We have calculated the normalized temperature variance, (T’/T)2, integrating the spectra for long (10-2.5 km) and short (2-0.4 km) wavelength ranges. The seasonal and latitudinal variations of (T’/T)2 are evident. In particular, (T’/T)2 is largely enhanced near the equator for both long and short wavelength ranges at 20-30 km.

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