Ground‐based measurements of middle atmospheric water vapor at 183 GHz

We present here the first ground‐based observations of stratospheric/mesospheric H 2 O emission at the 3 1,3 → 2 2,0 resonance (183.3101 GHz of central frequency). The spectra were obtained in January 1991 and January 1994 using the Institut de Radioastronomie Millimétrique (IRAM) 30‐m radiotelescope located on Pico Veleta, Spain, (altitude 2870 m, latitude 37°N), during very dry tropospheric conditions (total precipitable water within the range 0.2–0.3 mm above the observation site). Microwave studies of stratospheric and mesospheric H 2 O profile have been carried out so far mostly by means of observations of the weak 6 1,6 → 5 2,3 resonance at 22.2351 GHz. Some measurements at 183 GHz were previously obtained from airborne observations. The water vapor abundance in the troposphere is, in most cases, so high that the atmosphere appears optically thick at 183 GHz. However, we show that emission from the stratosphere and mesosphere can be sometimes detected from the ground (allowing upper stratospheric and mesospheric water vapor profiles to be obtained). We use a generalized non linear radiative transfer code in order to perform a least squares fit of the observed spectra. The derived H 2 O abundances are consistent with most measurements at 22 GHz. We suggest the possibility of carrying out this kind of observation in some selected sites during a significant fraction of the whole winter season. The required integration times to get a given signal‐to‐noise ratio are much shorter at 183 GHz than at 22 GHz.