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 1-60km 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 deals 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 troposphere when effects of humidity are small, and about 2K 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.

Assessing the Climate Monitoring Utility of Radio Occultation Data: From CHAMP to FORMOSAT-3/COSMIC

“…RO data with their high ver ti cal res o lu tion are well suited for the de ter mi na tion of tropo pause pa ram e ters. Pre vi ous studies used RO data from GPS/Met (Nishida et al 2000;Randel et al 2003), CHAMP Borsche et al 2007) and CHAMP + SAC-C .…”

GPS radio occultation with CHAMP and SAC-C: global monitoring of thermal tropopause parameters

“…In the deep tropics (10 • S-10 • N) the LRT pressure reaches in the mean values as low as about 100 hPa. For a more detailed discussion of tropical LRT parameters from GPS RO we refer to Nishida et al (2000), Randel et al (2003), and Schmidt et al (2004b). The strongest gradients in tropopause pressure occur between 30 • -60 • on both hemispheres.…”

“…First investigations of the thermal structure and variability in the tropical upper troposphere and lower stratosphere (UTLS) region based on GPS RO measurements were performed by Nishida et al (2000) and Randel et al (2003) on the basis of the GPS/MET data from 1995-1997 focused on the so-called "prime-times" (June-July 1995, DecemberFebruary 1996/97) (Rocken et al, 1997). Applications of CHAMP RO data to that region with a 1-and 2.5-year data set were shown by Schmidt et al (2004a,b).…”

Global temperature estimates in the troposphere and stratosphere: a validation study of COSMIC/FORMOSAT-3 measurements

“…The GPS-RO technique has the advantages of global coverage, high accuracy, high vertical resolution (less than 1 km), long-term stability, self-calibration and capability to operate in all-weather conditions (Kursinski et al, 1997;Wickert et al, 2001;Hajj et al, 2002). performed by Nishida et al (2000) and Randel et al (2003). The CHAMP and SAC-C profiles occurring within 30 min and 200 km are compared and agreed to better than 0.86 K and to within 0.1 K in the mean between 5 and 15 km altitude, after removing the expected variability of the atmosphere (Hajj et al, 2004 (Schreiner et al, 2007).…”