Latitude-altitude cross-sections of middle atmospheric water vapour obtained by airbourne millimeter-wave spectroscopy

Abstract

The observation of rotational transitions from minor constituents in the mm-wave range permits the retrieval of the corresponding molecule abundance in the altitude range of 15 to 70 km. This work describes the latitude-altitude cross-sections of water vapour which were derived from measurements of the 183 GHz emission line and were are obtained during different seasons with the Learjet of the Swiss Air Force. Several flights from Spitzbergen (78N) to the Cap Verde islands (15N) have been performed in the last three years. A maximum VMR of 7 ppm was found around 60 km altitude at low latitudes during October 1995 and March 1996. At high winter-latitudes up to 6 ppm VMR in the lower stratosphere associated with a decreased mesospheric water vapour abundance has been measured, both indicating downward transport. The measurements obtained in August 1994 were compared with 2-D models showing the typical seasonal and latitudinal features as expected from the current understanding of the large scale transport processes. However, particular dynamical features of our measurements in October and March such as the confinement and descent of air within the polar vortex cannot be resolved by a 2-D model. A persistent local maximum around 10 mb was observed by AMSOS but was not found in current models.