Development of A 22‐GHz ground‐based spectrometer for middle atmospheric water vapour monitoring

Abstract

Water vapour is a crucial element of the climate system. Accurate observations of stratospheric humidity are needed in the equatorial belt, where most water vapour crosses the tropopause, and in the polar regions, that are most affected by climate change trends. Satellite-based observations provide atmospheric composition data with extensive spatial and temporal coverage, but these need to be validated and integrated by ground-based networks like GAW (Global Atmospheric Watch) and NDACC (Network for Detection of Atmospheric Composition Change). This contribution presents a new ground-based spectrometer for the observation of middle atmospheric humidity profiles being currently developed at INGV - Istituto Nazionale di Geofisica e Vulcanologia. The instrument will detect the water vapour spectral line at 22.235 GHz by using the balanced beam-switching observation technique. The receiver antenna system has a parabolic mirror and a corrugated horn with an overall HPBW of 3.5°. Preliminary tests of the horn performed at the Table Mountain Facility of the Jet Propulsion Laboratory, California are presented. An uncooled GaAsFET low-noise amplifier was custom-assembled for the receiver front-end. The back-end will be a FFT spectrometer with a 1 GHz bandwidth and a 63 kHz resolution which, given the pressure broadening coefficient of the H2O line, will allow to retrieve concentration profiles from about 15 to 80 km altitude. Since the retrieval altitude range is also limited by the spectral signal-to-noise ratio and baseline artifacts, special care is taken in minimizing receiver noise temperature. Low noise temperatures will imply integration times short enough to be able to observe diurnal changes in the lower stratosphere. In order to extend unmanned operation time and limit LN2 supplies at remote stations, calibrated noise sources will be used as cold load reference on a daily basis. The control interface, which is also under development at INGV, will be based on reconfigurable hardware (USB-CPLD). Several different sites are proposed for permanent installation. Among these the GAW sites of Thule Airbase, Greenland for polar monitoring, or Mount Chacaltaya, Bolivia, for tropical tropopause observations.