Royal Observatory of Belgium - Meridian Room
Ground-based remote sensing of the neutral Polar atmosphere by microwave spectrometers
Ground-based microwave remote sensing is extremely useful for monitoring the atmospheric composition and physical properties. It is effective during both daytime and nighttime, in clear sky or mild overcast weather, and is therefore particularly suited for observing the Polar regions where darkness (or direct sunlight) lasts throughout an entire winter (summer) season. Most techniques in this frequency range observe the spectral lines emitted by the chemical species present in the atmosphere and estimate their vertical distribution by studying the shape of the emitted lines, dominated by pressure broadening. Critical parameters of the specific microwave instrument employed are therefore the overall spectral passband and its frequency resolution (or number of independent channels). At INGV we recently developed a water vapor spectrometer, VESPA-22, capable of observing the 22 GHz water vapor emission line with a 500 MHz passband and a frequency resolution of 31 kHz. The collected spectra are inverted using an optimal estimation algorithm in order to retrieve water vapor vertical profiles from about 25 to 75 km with an overall uncertainty between 5 and 12%. Every 30 minutes the spectrometer also performs tipping curve measurements by observing the sky emission at various zenith angles. This procedure allows both the calibration of the spectrometer and the measurement of the sky opacity, which then provides an estimate of the water vapor total column with an uncertainty of 5%. In July 2016, VESPA was installed at the Thule High Arctic Atmospheric Observatory located at Thule Air Base (76.5° N, 68.8° W), Greenland (http://www.thuleatmos-it.it/). VESPA-22 has been operating in an autonomous mode since its installation, with very few short periods of data gaps.