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

The GPS/MET (Global Positioning System/Meteorology) experiment was carried out between April 1995 and February 1997. Height profiles of the atmospheri c refractive index were obtained at 1-60 km from the active limb sounding o f occulted radio signals from the GPS satellites. By assuming the hydrostat ic relation for a dry atmosphere, a new global data set of high resolution temperature profiles has become available. This paper deals with the determ ination of the thermal structure near the tropopause in the equatorial regi on using the GPS/MET data. First, we have compared the GPS/MET profiles wit h nearby radiosonde results at two balloon launch sites in Indonesia; Bandu ng (6.9 degreesS, 107.6 degreesE) and Fontianak (0.03 degreesN, 109.3 degre esE), and determined that the rms deviation was approximately 1 K in the up per troposphere when effects of humidity are small, and about 2 K in the lo wer stratosphere. The GPS/MET profiles are found to be very unique in revea ling detailed temperature structure, including sharp inversions and stepwis e 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 (T-min) near the tropopause agreed well b etween radiosonde profiles at Bandung and the GPS/MET results, showing an a nnual, variation with warm enhancements in August-September, and broader co ld periods from January to April. The altitude corresponding to T-min becam e lower/higher for warmer/colder T-min. Taking advantage of the global cove rage of the GPS/MET data, we have investigated the longitude distribution o f T-min and H-min, which generally agreed well with earlier studies.