Validation of Water Vapour Profiles from GPS Radio Occultations in the Arctic

The relevance of water vapour in atmospheric physics and climate research contrasts strongly with the availability of humidity data in the Arctic. The most extensive humidity data set is based on approx. 80 radiosonde stations north of 60°N, but suffers from two major problems: Sensor diversity and sensor limitations under Arctic conditions, on the one hand, and lacking radiosonde launch sites in the Arctic Ocean and the Greenland iceshield, on the other hand. Calibration free satellite borne instruments like the GPS receiver onboard CHAMP prevent from both handicaps. Comparisons between radiosonde data and GPS-based humidity profiles are presented for single occultations as well as averaged data from the proof-of-concept experiment GPS/MET and the recent CHAMP satellite mission. The effects of low absolute humidity and uncertain meteorological analyses are examined using additional information from a regional climate model. For the observations of CHAMP in summer 2001, a general dry bias has been found if compared with radiosonde data, apparent both in single and mean profile intercomparisons. In contrast, during February 1997 GPS/MET data show slightly higher humidity in the midtroposphere, if compared with model data and objective analyses.