Ground-based millimeter-wave observations of water vapor emission (183Â GHz) at Atacama, Chile

We report the first results of ground-based millimeter-wave measurements of 183 GHz atmospheric water vapor spectra from Atacama highland (4800 m alt.), Chile. The measurements were carried out in December 2005 by using a spectroscopic radiometer equipped with a superconductive heterodyne receiver. A conspicuous H2O spectrum at 183 GHz was detected with an integration time of only 1.5 min, and this is the first high frequency-resolution H2O spectrum at 183 GHz obtained in the southern subtropical region. The vertical profile of H2O volume mixing ratio between 40 and 64 km were retrieved from the spectrum by using the modified optimal estimation method.