[1] Observations of very low amounts of precipitable water vapor (PWV) by means of the Ground-Based Millimeter wave Spectrometer (GBMS) are discussed. Low amounts of column water vapor (between 0.5 and 4 mm) are typical of high mountain sites and polar regions, especially during winter, and are difficult to measure accurately because of the lack of sensitivity of conventional instruments to such low PWV contents. The technique used involves the measurement of atmospheric opacity in the range between 230 and 280 GHz with a spectral resolution of 4 GHz, followed by the conversion to precipitable water vapor using a linear relationship. We present the intercomparison of this data set with simultaneous PWV observations obtained with Vaisala RS92k radiosondes, a Raman lidar, and an IR Fourier transform spectrometer. These sets of measurements were carried out during the primary field campaign of the Earth Cooling by Water vapor Radiation (ECOWAR) project which took place at Breuil-Cervinia (45.9°N, 7.6°E, elevation 1990 m) and Plateau Rosa (45.9°N, 7.7°E, elevation 3490 m), Italy, from 3 to 16 March 2007. GBMS PWV measurements show a good agreement with the other three data sets exhibiting a mean difference between observations of '9%. The considerable number of data points available for the GBMS versus lidar PWV correlation allows an additional analysis which indicates negligible systematic differences between the two data sets.

The Radiation Explorer in the Far InfraRed‐Prototype for Applications and Development (REFIR‐PAD) spectroradiometer was operated from the Testa Grigia Italian‐Alps station in March 2007 during the Earth Cooling by Water Vapour Radiation (ECOWAR) measurement campaign, obtaining downwelling radiance spectra in the 100–1100 cm−1 range, under clear‐sky conditions and in the presence of cirrus clouds. The analysis of these measurements has proven that the instrument is capable of determining precipitable water vapor with a total uncertainty of 5–7% by using the far‐infrared rotational band of water. The measurement is unaffected by the presence of cirri, whose optical depth can be instead retrieved as an additional parameter. Information on the vertical profiles of water vapor volume mixing ratio and temperature can also be retrieved for three altitude levels. The ability to measure the water vapor column with a simple, uncooled instrument, capable of operating continuously and with a time resolution of about 10 min, makes REFIR‐PAD a very valuable instrument for meteorological and climatological studies for the characterization of the water vapor distribution

The 2009 Arctic sudden stratospheric warming (SSW) was the most intense event of this kind ever observed. Unique ground‐based measurements of middle atmospheric profiles for temperature, O3, CO, and N2O obtained at Thule (76.5°N, 68.8°W), Greenland, in the period January to early March are used to show the evolution of the 2009 SSW in the region of its maximum intensity. The first sign of the SSW was detected at θ ∼ 2000 K on 19 January, when a rapid decrease in CO mixing ratio took place. The first evidence of a temperature increase was observed at the same level on 22 January, the earliest date on which lidar measurements reached above ∼50 km. The warming propagated from the upper to the lower stratosphere in 7 days and the record maximum temperature of 289 K was observed between 1300 and 1500 K potential temperature on 22 January. A strong vortex splitting was associated with the SSW. Stratospheric backward trajectories indicate that air masses arriving at Thule during the warming peak underwent a rapid compression and an intense adiabatic warming of up to 50 K. The rapid advection of air from the extratropics was also occasionally observed to produce elevated values of N2O mixing ratio. Starting in mid‐February the temperature profile and the N2O mixing ratio returned to the prewarming values in the mid and upper stratosphere, indicating the reformation of the vortex at these levels. In late winter, vertical descent from starting altitudes of ∼60 km is estimated from CO profiles to be 0.25 ± 0.05 km/day.

Abstract. The Ground-Based Millimeter-wave Spectrometer (GBMS) was designed and built at the State University of New York at Stony Brook in the early 1990s and since then has carried out many measurement campaigns of stratospheric O 3 , HNO 3 , CO and N 2 O at polar and mid-latitudes. Its HNO 3 data set shed light on HNO 3 annual cycles over the Antarctic continent and contributed to the validation of both generations of the satellite-based JPL Microwave Limb Sounder (MLS). Following the increasing need for long-term data sets of stratospheric constituents, we resolved to establish a long-term GMBS observation site at the Arctic station of Thule (76.5 • N, 68.8 • W), Greenland, beginning in January 2009, in order to track the long-and short-term interactions between the changing climate and the seasonal processes tied to the ozone depletion phenomenon. Furthermore, we updated the retrieval algorithm adapting the Optimal Estimation (OE) method to GBMS spectral data in order to conform to the standard of the Network for the Detection of Atmospheric Composition Change (NDACC) microwave group, and to provide our retrievals with a set of averaging kernels that allow more straightforward comparisons with other data sets. The new OE algorithm was applied to GBMS HNO 3 data sets from 1993 South Pole observations to date, in order to produce HNO 3 version 2 (v2) profiles. A sample of results obtained at Antarctic latitudes in fall and winter and at mid-latitudes is shown here. In most conditions, v2 inversions show a sensitivity (i.e., sum of column elements of the averaging kernel matrix) of 100 ± 20 % from 20 to 45 km altitude, with somewhat worse (better) sensitivity in the Antarctic winter lower (upper) stratosphere. The 1σ uncertainty on HNO 3 v2 mixing ratio vertical profiles depends Correspondence to: I. Fiorucci (irene.fiorucci@ingv.it) on altitude and is estimated at ∼15 % or 0.3 ppbv, whichever is larger. Comparisons of v2 with former (v1) GBMS HNO 3 vertical profiles, obtained employing the constrained matrix inversion method, show that v1 and v2 profiles are overall consistent. The main difference is at the HNO 3 mixing ratio maximum in the 20-25 km altitude range, which is smaller in v2 than v1 profiles by up to 2 ppbv at mid-latitudes and during the Antarctic fall. This difference suggests a better agreement of GBMS HNO 3 v2 profiles with both UARS/ and EOS Aura/MLS HNO 3 data than previous v1 profiles.

a ground-based millimeter-wave spectrometer (GBMS) was installed at Thule Air Base (76.5 N, 68.8 W), Greenland, for long-term winter monitoring of several stratospheric and mesospheric trace gases in the framework of the Network for the Detection of Atmospheric Composition Change. This work is aimed at characterizing the GBMS O 3 vertical profiles between 35 and 80 km altitude obtained by applying the optimal estimation method to O 3 pressure-broadened spectral line measurements carried out during three winters. In this altitude range, GBMS O 3 retrievals are highly sensitive to variations of the atmospheric state, and their accuracy is estimated to be the larger of 11% or 0.2 ppmv. Comparisons of GBMS O 3 profiles with colocated satellite-based measurements from Aura Microwave Limb Sounder (MLS) and Thermosphere Ionosphere Mesosphere Energetics and Dynamics Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) show a good agreement below 65 km altitude once the known 10%-20% high bias of SABER O 3 profiles is considered, with the GBMS displaying an averaged low bias of $9% and 17% with respect to MLS and SABER. In the nighttime mesosphere, the GBMS detects the ozone tertiary maximum within 0.1 ppmv (6%) on average with respect to the convolved MLS, SABER, and global 3-D ROSE model profiles but shifts its position to lower altitudes by 4-5 km compared to the height obtained by the other three data sets. In the 50-80 km altitude range, estimates of mesospheric O 3 diurnal variation obtained from the GBMS and the convolved satellite measurements agree well within the AE1 standard deviation ($0.6 ppmv) of the GBMS mean profile.Citation: Muscari, G., C. Cesaroni, I. Fiorucci, A. K. Smith, L. Froidevaux, and M. G. Mlynczak (2012), Strato-mesospheric ozone measurements using ground-based millimeter-wave spectroscopy at Thule, Greenland,

Abstract. In response to the need for improving our understanding of the evolution and the interannual variability of the winter Arctic stratosphere, in January 2009 a GroundBased Millimeter-wave Spectrometer (GBMS) was installed at the Network for the Detection of Atmospheric Composition Change (NDACC) site in Thule (76.5 • N, 68.8 • W), Greenland. In this work, stratospheric GBMS O 3 and HNO 3 vertical profiles obtained from Thule during the winters 2010 (HNO 3 only), 2011 and 2012 are characterized and intercompared with co-located measurements of the Aura Microwave Limb Sounder (MLS) experiment. Using a recently developed algorithm based on Optimal Estimation, we find that the GBMS O 3 retrievals show good sensitivity (> 80 %) to atmospheric variations between ∼ 17 and ∼ 50 km, where their 1σ uncertainty is estimated to be the larger of ∼ 11 % or 0.2 ppmv. Similarly, HNO 3 profiles can be considered for scientific use between ∼ 17 and ∼ 45 km altitude, with a 1σ uncertainty that amounts to the larger of 15 % or 0.2 ppbv. Comparisons with Aura MLS version 3.3 observations show that, on average, GBMS O 3 mixing ratios are biased negatively with respect to MLS throughout the stratosphere, with differences ranging between ∼ 0.3 ppmv (8 %) and 0.9 ppmv (18 %) in the 17-50 km vertical range. GBMS HNO 3 values display instead a positive bias with respect to MLS up to 26 km, reaching a maximum of ∼ 1 ppbv (10 %) near the mixing ratio profile peak. O 3 and HNO 3 values from the two datasets prove to be well correlated at all altitudes, although their correlations worsen at the lower end of the altitude ranges considered. Column contents of GBMS and MLS O 3 (from 20 km upwards) and HNO 3 (from 17 km upwards) correlate very well and indicate that GBMS measurements can provide valuable estimates of column interannual and seasonal variations for these compounds.

Abstract. 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 1σ 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.

In response to the need for improving our understanding of the evolution and the interannual variability of the winter Arctic stratosphere, in January 2009 a ground-based millimeter-wave spectrometer (GBMS) was installed at the Network for the Detection of Atmospheric Composition Change (NDACC) site in Thule (76.5° N, 68.8° W), Greenland. In this work, stratospheric GBMS O₃ and HNO₃ vertical profiles obtained from Thule during winters 2010 (HNO₃ only), 2011 and 2012 are characterized and intercompared with co-located Aura MLS measurements. Using a recently developed algorithm based on Optimal Estimation, we find that the GBMS O₃ retrievals show good sensitivity (> 80%) to atmospheric variations between ~ 17 and ~ 50 km, where their 1σ uncertainty is estimated to be the larger of ~ 11% or 0.2 ppmv. Similarly, HNO₃ profiles can be considered for scientific use between ~ 17 and ~ 45 km altitude, with a 1σ uncertainty that amounts to the larger of 15% or 0.2 ppbv. Comparisons with Aura MLS version 3.3 observations show that, on average, GBMS O₃ mixing ratios are biased low with respect to MLS throughout the stratosphere, with differences ranging between ~ 0.3 ppmv (8%) and 0.9 ppmv (18%) in the 17–50 km vertical range. GBMS HNO₃ values display instead a high bias with respect to MLS up to 26 km, reaching a maximum of ~ 1 ppbv (10%) near the mixing ratio profile peak. O₃ and HNO₃ values from the two data sets prove to be well correlated at all altitudes, although their correlations worsen at the lower end of the altitude ranges considered. Column contents of GBMS and MLS O₃ (from 20 km upwards) and HNO₃ (from 17 km upwards) correlate very well and indicate that GBMS measurements can provide valuable estimates of column interannual and seasonal variations for these compounds