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

摘要

The new ground-based22 GHz spectrometer, VESPA-22 (water Vapor Emission Spectrometer forPolar Atmosphere at 22 GHz) measures the 22.23 GHz watervapor emission line with a bandwidth of 500 MHz and a frequencyresolution of 31 kHz. The integration time for a measurement rangesfrom 6 to 24 h, depending on season and weather conditions. Water vaporspectra are collected using the beam-switching technique. VESPA-22 isdesigned to operate automatically with little maintenance; it employs anuncooled front-end characterized by a receiver temperature of about180 K and its quasi-optical system presents a full width at halfmaximum of 3.5°. Every 30 min VESPA-22 measures also the sky opacityusing the tipping curve technique. The instrument calibration is performedautomatically by a noise diode; the emission temperature of this element isestimated twice an hour by observing alternatively a black body at ambienttemperature and the sky at an elevation of 60°. The retrievedprofiles obtained inverting 24 h integration spectra present a sensitivitylarger than 0.8 from about 25 to 75 km of altitude during winter andfrom about 30 to 65 km during summer, a vertical resolution fromabout 12 to 23 km (depending on altitude), and an overall 1σuncertainty lower than 7 % up to 60 km altitude and rapidlyincreasing to 20 % at 75 km.In July 2016, VESPA-22 was installed at the Thule High Arctic AtmosphericObservatory located at Thule Air Base (76.5° N, 68.8° W),Greenland, and it has been operating almost continuously since then. TheVESPA-22 water vapor mixing ratio vertical profiles discussed in this workare obtained from 24 h averaged spectra and are compared with version 4.2 ofconcurrent Aura/Microwave Limb Sounder (MLS) water vapor vertical profiles.In the sensitivity range of VESPA-22 retrievals, the intercomparison fromJuly 2016 to July 2017 between VESPA-22 dataset and Aura/MLS datasetconvolved with VESPA-22 averaging kernels shows an average difference within1.4 % up to 60 km altitude and increasing to about 6 %(0.2 ppmv) at 72 km.