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The Changing-Atmosphere Infra-Red Tomography Explorer (CAIRT) is one of the four candidates for ESA’s Earth Explorer 11, aimed to study the coupling between atmospheric composition, circulation and climate. By exploiting its imaging capabilities, CAIRT indeed can sound the atmosphere simultaneously from the middle troposphere to the lower thermosphere at 0.2 cm-1 spectral resolution and with horizontal sampling of 50 km along track, 50 km across track and vertical sampling of 1 km. Flying in loose formation with MetOp-SG mission allows to combine spatially resolved limb observations with horizontally resolved nadir measurements of IASI-NG and Sentinel-5 nadir spectrometers to extend limb observations down to the surface. An accurate knowledge of trace gas composition around the tropopause is particularly important to reduce significant uncertainties in projected future warming: changes in the surface air pollution and in the stratosphere-troposphere exchange may significantly affect it. Moreover quantifying the influence of stratospheric transport on near-surface tropospheric composition is important since it conditions attempts to derive emission fluxes from surface observations for important greenhouse gases. We evaluated advantages coming from the exploitation of the synergy between realistic simulations of CAIRT, IASI-NG and Sentinel 5 observations applying the rigorous approach of the two-dimensional complete data fusion. The synergistic products demonstrate a better quality in sounding both the troposphere and the Upper Troposphere - Lower Stratosphere region. Results will be shown for selected case studies involving ozone, methane and other pollutants., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The Far-infrared-Outgoing-Radiation Understanding and Monitoring (FORUM) mission concept, selected by ESA for the feasibility study of the Earth Explorer 9 (EE9) will have to evaluate of water vapor and temperature profile precision and accuracy from FORUM observations in the wavelength range of 7 - 100 micrometers. These observations are important because Earth's infrared emission to space is affected by water vapor and cirrus clouds, which, in turn, play key roles in Earth's temperature. The characterization of the radiative effects of the main atmospheric components in this spectral range is a new measure from space, as it fills the observational gap in the far-infrared (FIR) region from space. In the framework of the FORUM mission, the Kyoto protocoL Informed Management of the Adaptation (KLIMA) algorithm has been used to simulate the FORUM wide-band spectral radiance measurements, with the aim to assess the water vapor and temperature profile precision and accuracy obtained using the Fourier Transform Spectrometer (FTS) observations and to evaluate the quality of the retrieval. The algorithm consists of two distinct modules, the Forward Model (FM) and the Retrieval Model (RM). The FM is a line-by-line radiative transfer model, with capability to simulate wide-band spectral radiances from 100 to 1600 cm-1 (i.e. the FORUM spectral range requirement); the RM uses a constrained Nonlinear Least Square Fit (NLSF) approach and the cost function to be minimized takes into account the a priori information (optimal estimation method) and the Marquardt parameter. We performed several tests to retrieve temperature and water vapor profile, using different quantifiers to assess the quality of the retrieval. In this work, we describe the strategy adopted for the analysis and we show first results of these evaluation activities.

The knowledge of the water vapor distribution in the Earth's atmosphere is of great importance for weather prediction. Meteorological models, including the so called limited area models, can assimilate humidity measurements, increasing the reliability of the simulated atmospheric dynamics. Existing atmospheric remote sensing instruments are still insufficient to cover global scales with systematic WV measurements in the lowest part of the troposphere (5-6 km): this would indeed help to improve both climate modeling and numerical weather prediction capabilities at short time scale. In order to solve this problem, a new approach (the Normalized Differential Spectral Attenuation-NDSA) capable of estimating the integrated water vapor (IWV) through attenuation measurements in the Ku/K band along microwave is assessed. Nevertheless, the developed technique needs to be demonstrated with real measurements before moving toward space applications. A recent project (SWAMM-Sounding Water Vapor by Attenuation Microwave Measurements) had as main goal to develop low-cost instrumentation to perform the first NDSA measurements. The instrument was designed to operate with the transmitter located on the ground and the receiver placed on a moving platform (e.g. airborne, stratospheric balloon, HAPS), even though, as a preliminary test, the instrumentation was tested in a ground- to-ground link configuration. An instrument prototype, which includes a synthesized microwave transmitter and a Software Defined Radio (SDR) microwave receiver has been designed, assembled and tested in laboratory. Laboratory tests confirm that the transmitter generates two tones, respectively at 18.8 GHz and 19.2 GHz, with very high stability in both amplitude and frequency, and the receiver measures the amplitude (and its fluctuations) of the received signals with enough accuracy (< 0.1 dB) over a wide dynamic range. The differential attenuation is ±0.02dB then far below the theoretical value of the minimum sensitivity required to measure IWV. The instrument was then tested on a ground-to-ground link during an experimental campaign, which was carried out in the period June-October 2018. In order to obtain a transect in the troposphere of tens of km, (thus providing a value of differential attenuation high enough to be measurable by the instrument), the transmitter was installed at sea level and the receiver on the top of a mountain (around 2165m a.s.l.). In detail, the receiver and the transmitter were installed at the CNR laboratories, respectively in Bologna (central Italy) and Cimone mount in the Apennines; as a result, the length of the transect is 62 km. Data are acquired continuously for four months well covering different atmospheric conditions occurred in the summer/autumn seasons. Differential attenuation exhibits temporal variations, at both weekly and monthly scales, which can be correlated to the IWV. As first validation we compared the IWV calculated by NDSA measurements with the same quantity calculated, under the assumption of plane parallel atmosphere conditions, by using different hygrometers placed along the transect and daily radiosoundings of a nearby WMO station. The comparisons show that data are generally in good agreement as both absolute values and temporal variations. However, further investigations are necessary to better interpret the results also in terms of a few discrepancies found. Finally, Sentinel- 3 OLCI L2- IWV product will be used as additional term of comparison for the retrieved IWV over the same transect. A detailed analysis will be presented at the conference.

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MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) is operating on the ENVIronmental SATellite (ENVISAT) since March 2002. After two years of nearly continuous limb scanning measurements, at the end of March 2004, the instrument was stopped due to problems with the mirror drive of the interferometer. Operations with reduced maximum path difference, corresponding to both a reduced-spectral-resolution and a shorter measurement time, were resumed on January 2005. In order to exploit the reduction in measurement time, the measurement scenario was changed adopting a finer vertical limb scanning. The change of spectral resolution and of measurement scenario entailed an update of the data processing strategy. The aim of this paper is the assessment of the differences in the quality of the MIPAS ozone data acquired before and after the stop of the operations. Two sets of MIPAS ozone profiles acquired in 2003–2004 (full-resolution measurements) and in 2005–2006 (reduced-resolution measurements) are compared with collocated ozone profiles obtained by GOMOS (Global Ozone Monitoring by Occultation of Stars), itself also onboard ENVISAT. The continuity of the GOMOS data quality allows to assess a possible discontinuity of the MIPAS performances. The relative bias and precision of MIPAS ozone profiles with respect to the GOMOS ones have been compared for the measurements acquired before and after the stop of the MIPAS operations. The results of the comparison show that, in general, the quality of the MIPAS ozone profiles retrieved from reduced-resolution measurements is comparable or better than that obtained from the full-resolution dataset. The only significant change in MIPAS performances is observed at pressures around 2 hPa, where the relative bias of the instruments increases by a factor of 2 from the 2003–2004 to 2005–2006 measurements.

The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS), on-board the European ENVIronmental SATellite (ENVISAT) launched on 1 March 2002, is a middle infrared Fourier Transform spectrometer measuring the atmospheric emission spectrum in limb sounding geometry. The instrument is capable to retrieve the vertical distribution of temperature and trace gases, aiming at the study of climate and atmospheric chemistry and dynamics, and at applications to data assimilation and weather forecasting. MIPAS operated in its standard observation mode for approximately two years, from July 2002 to March 2004, with scans performed at nominal spectral resolution of 0.025 cm -1 and covering the altitude range from the mesosphere to the upper troposphere with relatively high vertical resolution (about 3 km in the stratosphere). Only reduced spectral resolution measurements have been performed subsequently. MIPAS data were re-processed by ESA using updated versions of the Instrument Processing Facility (IPF v4.61 and v4.62) and provided a complete set of level-2 operational products (geolocated vertical profiles of temperature and volume mixing ratio of H2O, O3, HNO3, CH4, N2O and NO2) with quasi continuous and global coverage in the period of MIPAS full spectral resolution mission. In this paper, we report a detailed description of the validation of MIPAS-ENVISAT operational ozone data, that was based on the comparison between MIPAS v4.61 (and, to a lesser extent, v4.62) O3 VMR profiles and a comprehensive set of correlative data, including observations from ozone sondes, ground-based lidar, FTIR and microwave radiometers, remote-sensing and in situ instruments on-board stratospheric aircraft and balloons, concurrent satellite sensors and ozone fields assimilated by the European Center for Medium-range Weather Forecasting. A coordinated effort was carried out, using common criteria for the selection of individual validation data sets, and similar methods for the comparisons. This enabled merging the individual results from a variety of independent reference measurements of proven quality (i.e. well characterized error budget) into an overall evaluation of MIPAS O3 data quality, having both statistical strength and the widest spatial and temporal coverage. Collocated measurements from ozone sondes and ground-based lidar and microwave radiometers of the Network for the Detection Atmospheric Composition Change (NDACC) were selected to carry out comparisons with time series of MIPAS O 3 partial columns and to identify groups of stations and time periods with a uniform pattern of ozone differences, that were subsequently used for a vertically resolved statistical analysis. The results of the comparison are classified according to synoptic and regional systems and to altitude intervals, showing a generally good agreement within the comparison error bars in the upper and middle stratosphere. Significant differences emerge in the lower stratosphere and are only partly explained by the larger contributions of horizontal and vertical smoothing differences and of collocation errors to the total uncertainty. Further results obtained from a purely statistical analysis of the same data set from NDACC ground-based lidar stations, as well as from additional ozone soundings at middle latitudes and from NDACC ground-based FTIR measurements, confirm the validity of MIPAS O3 profiles down to the lower stratosphere, with evidence of larger discrepancies at the lowest altitudes. The validation against O3 VMR profiles using collocated observations performed by other satellite sensors (SAGE II, POAM III, ODIN-SMR, ACE-FTS, HALOE, GOME) and ECMWF assimilated ozone fields leads to consistent results, that are to a great extent compatible with those obtained from the comparison with ground-based measurements. Excellent agreement in the full vertical range of the comparison is shown with respect to collocated ozone data from stratospheric aircraft and balloon instruments, that was mostly obtained in very good spatial and temporal coincidence with MIPAS scans. This might suggest that the larger differences observed in the upper troposphere and lowermost stratosphere with respect to collocated ground-based and satellite O3 data are only partly due to a degradation of MIPAS data quality. They should be rather largely ascribed to the natural variability of these altitude regions and to other components of the comparison errors. By combining the results of this large number of validation data sets we derived a general assessment of MIPAS v4.61 and v4.62 ozone data quality. A clear indication of the validity of MIPAS O3 vertical profiles is obtained for most of the stratosphere, where the mean relative difference with the individual correlative data sets is always lower than ±10%. Furthermore, these differences always fall within the combined systematic error (from 1 hPa to 5OhPa) and the standard deviation is fully consistent with the random error of the comparison (from 1 hPa to ∼~30-40hPa). A degradation in the quality of the agreement is generally observed in the lower stratosphere and upper troposphere, with biases up to 25% at 100 hPa and standard deviation of the global mean differences up to three times larger than the combined random error in the range 50-100 hPa. The larger differences observed at the bottom end of MIPAS retrieved profiles can be associated, as already noticed, to the effects of stronger atmospheric gradients in the UTLS that are perceived differently by the various measurement techniques. However, further components that may degrade the results of the comparison at lower altitudes can be identified as potentially including cloud contamination, which is likely not to have been fully filtered using the current settings of the MIPAS cloud detection algorithm, and in the linear approximation of the forward model that was used for the a priori estimate of systematic error components. The latter, when affecting systematic contributions with a random variability over the spatial and temporal scales of global averages, might result in an underestimation of the random error of the comparison and add up to other error sources, such as the possible underestimates of the p and T error propagation based on the assumption of a 1K and 2% uncertainties, respectively, on MIPAS temperature and pressure retrievals. At pressure lower than 1 hPa, only a small fraction of the selected validation data set provides correlative ozone data of adequate quality and it is difficult to derive quantitative conclusions about the performance of MIPAS O2 retrieval for the topmost layers.

On June 30th, 2005 the REFIR-PAD (Radiation Explorer in the Far InfraRed-Prototype for Application and Development) Fourier transform spectroradiometer performed the first wide-band spectral characterization of the top-of-atmosphere emitted radiation in the far-infrared with an uncooled instrument. The nadir emitted radiance has been measured down to 100 cm -1 , thus covering a spectral interval that, until now, was nearly unexplored, and up to 1400 cm -1 , including the well characterized atmospheric window region, in which it is possible to perform comparison and intercalibration with operative instruments. The measurements were performed at an altitude of 34 km, from a stratospheric balloon launched in tropical region, near Teresina (Brazil). The acquired spectra have a spectral resolution of 0.5 cm -1 . It should be noted that despite the operating spectral range extending to the far-infrared region, REFIR-PAD does not require any cooled components, thanks to the use of pyroelectric detectors and an optical scheme that compensates for the instrument self-emission. This work shows the results of the analysis of the spectra, focusing on the far infrared portion of the atmospheric emitted radiance. The retrieval of the vertical profiles of water vapour and temperature during the flight is presented. The vertical resolution of the retrieval is 2 km in the upper troposphere-lower stratosphere (UTLS) region, and lower at higher altitudes. The comparison with ECMWF for validation is also shown. Besides the characterization of temperature and water vapour, from the analysis of the emitted radiance useful information can be gathered about cloud and aerosol contribution to radiation budget.

Part of the abstract: The Michelson Interferometer for Passive AtmosphericSounding (MIPAS), on-board the European ENVIronmentalSATellite (ENVISAT) launched on 1 March 2002,is a middle infrared Fourier Transform spectrometer measuringthe atmospheric emission spectrum in limb sounding geometry.The instrument is capable to retrieve the vertical distributionMIPAS data were re-processed by ESA using updated versions ofthe Instrument Processing Facility (IPF v4.61 and v4.62) andprovided a complete set of level-2 operational products (geolocatedvertical profiles of temperature and volume mixingratio of H2O, O3, HNO3, CH4, N2O and NO2). MIPAS operated in its standard observation mode for approximately two years, from July 2002 to March 2004. MIPAS data were re-processed by ESA using updated versions of the Instrument Processing Facility (IPF v4.61 and v4.62) and provided a complete set of level-2 operational products (geolocated vertical profiles of temperature and volume mixing ratio of H2O, O3, HNO3, CH4, N2O and NO2). MIPAS operated in its standard observation mode from July 2002 to March 2004, covering the altitude range from the mesosphere to the upper troposphere with relatively high vertical resolution (about 3 km in the stratosphere). In this paper, we report a detailed description of the validation of MIPAS-ENVISAT operational ozone data, that was based on the comparison between MIPAS v4.61 (and, to a lesser extent, v4.62) O3 VMR profilesand a comprehensive set of correlative data, including observations from ozone sondes, ground-based lidar, FTIR and microwave radiometers, remote-sensing and in situ instruments on-board stratospheric aircraft and balloons, concurrent satellite sensors and ozone fields assimilated by theEuropean Center for Medium-range Weather Forecasting. A clear indication of the validity of MIPAS O3 vertical profiles is obtained for most of the stratosphere, where the mean relative difference with the individual correlative data sets is always lower than ±10%. Furthermore, these differences always fall within the combined systematic error (from1 hPa to 50 hPa) and the standard deviation is fully consistent with the random error of the comparison (from 1 hPa to 3040 hPa).

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The ENVISAT validation programme for the atmospheric instruments MIPAS, SCIAMACHY and GOMOS is based on a number of balloon-bone, aircraft and ground-based correlative measurements. In particular the activities of validation scientists were coordinated by ESA within the ENVISAT Stratospheric Aircraft and Balloon Campaign or ESABC. In parallel to the contribution of the individual validation teams, the present work provides a synthesis of comparisons performed between MIPAS CH4 and N2O profiles produced by the current ESA operational software (Instrument Processing Facility version 4.61 or IPF v4.61) and correlative measurements obtained from balloon and aircraft experiments as well as from satellite sensors or from ground-based instruments.