VESPA-22: a ground-based microwave spectrometer for long-term measurements of Polar stratospheric water vapor

摘要

The new ground-based 22 GHz spectrometer, VESPA-22 (water Vapor Emission Spectrometer for Polar Atmosphere at 22 GHz) measures the 22.23 GHz water vapor emission line with a bandwith of 500 MHz and a frequency resolution of 31 kHz. The integration time for a measurement is of the order of hours, depending on season and weather conditions. Water vapor spectra are collected using the beam switching technique. VESPA-22 is designed to operate automatically with minimum need of maintenance; it employs an uncooled front-end characterized by a receiver temperature of about 180 K and its quasi-optical system presents a half power full beam angle of 3.5°. VESPA-22 measures also the sky opacity with a temporal resolution of two measurements an hour using the tipping curve technique. The instrument calibration is performed automatically by a noise diode; the emission temperature of this element is measured two times an hour through the observation of a black body at ambient temperature and of the sky at 60° of elevation. The retrieved profiles obtained inverting a 24-hour integration spectra present a sensitivity higher than 0.8 from about 25 to 72 km of altitude, a vertical resolution from about 12 to 23 km (depending on altitude) and an overall 1σ uncertainty between 5 and 12 %.In July 2016, VESPA-22 was installed at the THAAO (Thule High Arctic Atmospheric Observatory) located at Thule Air Base (76.5° N, 68.8° W), Greenland, and has been operating almost continuously since then, with very few interruption periods characterized by poor weather. The VESPA-22 water vapor mixing ratio vertical profiles discussed in this work cover the period from July 2016 to May 2017 and are compared with Version 4.2 of concurrent Aura/MLS (Waters et al., 2006) water vapor vertical profiles. In the sensitivity range of VESPA-22 retrievals, the intercomparison between the VESPA-22 dataset and Aura/MLS dataset convolved with VESPA-22 averaging kernels reveals a correlation coefficient of about 0.9 or higher and an average difference reaching its maximum of −6 % or −0.2 ppmv at the top of the sensitivity range.