Seasonal and longitudinal variations in the tropical tropopause observed with the GPS occultation technique (GPS/MET)

Abstract

The GPS/MET (Global Positioning System/Meteorology) experiment was carried out between April 1995 and February 1997. Height profiles of the atmospheric refractive index were obatined at 1-60 km from the active limb sounding of occulted radio signals from the GPS satellites. By assuming the hydrostatic relation for a dry atmosphere, a new global data set of high resolution temperature profiles has become available. This paper duals with the determination of the thermal structure near the tropopause in the equatorial region using the GPS/MET data. First, we have compared the GPS/MET profiles with nearby radiosonde results at two balloon launch sites in Indonesia; Bandung (6.9°S, 107.6°E) and Pontianak (0.03°N, 109.3°E), and determined that the rms deviation was approximately 1 K in the upper troposphere when effects of humidity are small, and about 2 K in the lower stratosphere. The GPS/MET profiles are found to be very unique in revealing detailed temperature structure, including sharp inversions and step-wise increase of temperature gradient near the tropical tropopause, which has not been achieved by a conventional satellite measurement. The monthly mean value of the minimum temperature (Tmin) near the tropopause agreed well between radiosonde profiles at Bandung and the GPS/MET results, showing an annual variation with warm enhancements in August-September, and broader cold periods from January to April. The altitude corresponding to Tmin became lower/higher for warmer/colder Tmin. Taking advantage of the global coverage of the GPS/MET data, we have investigated the longitude distribution of Tmin and Hmin, which generally agreed well with earlier studies.