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

• Published in: Atmospheric Measurement Techniques
• Main Authors: Mevi, Gabriele, Muscari, Giovanni, Bertagnolio, Pietro Paolo, Fiorucci, Irene, Pace, Giandomenico
• Format: Text
• Language: English
• Published: 2019
• Subjects: Arctic; Greenland; New Ground; Thule Air Base; Thule Air; Thule
• Online Access: https://doi.org/10.5194/amt-11-1099-2018; https://amt.copernicus.org/articles/11/1099/2018/

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 bandwidth of 500 MHz and a frequency resolution of 31 kHz . The integration time for a measurement ranges from 6 to 24 h, depending on season and weather conditions. Water vapor spectra are collected using the beam-switching technique. VESPA-22 is designed to operate automatically with little maintenance; it employs an uncooled front-end characterized by a receiver temperature of about 180 K and its quasi-optical system presents a full width at half maximum of 3.5 ∘ . Every 30 min VESPA-22 measures also the sky opacity using the tipping curve technique. The instrument calibration is performed automatically by a noise diode; the emission temperature of this element is estimated twice an hour by observing alternatively a black body at ambient temperature and the sky at an elevation of 60 ∘ . The retrieved profiles obtained inverting 24 h integration spectra present a sensitivity larger than 0.8 from about 25 to 75 km of altitude during winter and from about 30 to 65 km during summer, a vertical resolution from about 12 to 23 km (depending on altitude), and an overall 1 σ uncertainty lower than 7 % up to 60 km altitude and rapidly increasing to 20 % at 75 km . In July 2016, VESPA-22 was installed at the Thule High Arctic Atmospheric Observatory located at Thule Air Base (76.5 ∘ N, 68.8 ∘ W), Greenland, and it has been operating almost continuously since then. The VESPA-22 water vapor mixing ratio vertical profiles discussed in this work are obtained from 24 h averaged spectra and are compared with version 4.2 of concurrent Aura/Microwave Limb Sounder (MLS) water vapor vertical profiles. In the sensitivity range of VESPA-22 retrievals, the intercomparison from July 2016 to July 2017 between VESPA-22 dataset and Aura/MLS dataset convolved with VESPA-22 averaging kernels shows an average difference within 1.4 % up to 60 km altitude and increasing to about 6 % (0.2 ppmv ) at 72 km .