GPS radio occultation with CHAMP: Global monitoring of climate change parameters

Abstract

The availability of Global Positioning System (GPS) radio signals has introduced a promising remote sensing technique for the Earth's atmosphere. GPS based radio occultation (RO) exploits these signals received onboard Low Earth Orbiting (LEO) satellites for atmospheric limb sounding. Refractivity and temperature profiles can be derived from this measurements with high accuracy and vertical resolution. The GPS RO technique requires no calibration, is not affected by clouds, aerosols or precipitation, and the occultations are almost uniformly distributed over the globe.CHAMP RO data are available since 2001 with up to 200 high-resolution vertical refractivity and temperature profiles per day. The bias between CHAMP temperature profiles and radiosonde data as well as ECMWF analyses is less than 0.5 K between 300 and 30 hPa with root-mean-square deviations between 1-2 K.As an example for global monitoring the structure and temporal and spatial variability of the tropical tropopause are discussed. In the CHAMP RO temperature data clear evidence of the stratospheric quasi-biennial oscillation (QBO) was found. Because of the accuracy, high vertical resolution, and global distributed temperature data in the tropopause region the RO technique is suitable for global temperature monitoring of the Upper Troposphere/Lower Stratosphere as an important part of the atmosphere.Apart from temperature measurements water vapour profiles can be determined in the troposphere. Due to the ambiguity with respect to the dry and wet part in the refractivity additional temperature information from meteorological analyses or forecasts is necessary. Results of a 1DVAR retrieval scheme to derive tropospheric water vapour profiles using ECMWF data as background will be shown and discussed.The CHAMP RO experiment generates the first long-term RO data set. Other satellite missions will follow (GRACE, TerraSAR-X, COSMIC, METOP) providing some thousand profiles of refractivity and temperature daily.