The thermal emission of cirrus clouds, spectrally resolved in the 100–1400 cm−1 range (100–7.1 μm), has been modeled and compared with measurements performed during two field campaigns from the ...ground‐based site of Testa Grigia on the Italian Alps at 3480 m of altitude. The analysis of cirrus microphysics, through spectral fitting, shows the importance of using also the far infrared portion of the emitted spectrum at wave numbers below the 667 cm−1 carbon dioxide absorption band, where only a few measurements exist because of the high opacity of the atmosphere caused by the strong water vapor absorption. The resulted distribution of the fitted cloud parameters is in good agreement with the typical statistical distribution of the midlatitude cirrus cloud parameters.
Key Points
Water vapor and temperature profiles, and cirrus cloud microphysics are better simultaneously retrieved using wide band infrared spectra
The far infrared spectral region increases the accuracy for the retrieval of the cirrus particle effective diameter
The retrieved cirrus parameters follow the statistical distribution reported by Heymsfield
FORUM Palchetti, L.; Brindley, H.; Bantges, R. ...
Bulletin of the American Meteorological Society,
12/2020, Letnik:
101, Številka:
12
Journal Article
Recenzirano
Odprti dostop
The outgoing longwave radiation (OLR) emitted to space is a fundamental component of the Earth’s energy budget. There are numerous, entangled physical processes that contribute to OLR and that are ...responsible for driving, and responding to, climate change. Spectrally resolved observations can disentangle these processes, but technical limitations have precluded accurate space-based spectral measurements covering the far infrared (FIR) from 100 to 667 cm−1 (wavelengths between 15 and 100 μm). The Earth’s FIR spectrum is thus essentially unmeasured even though at least half of the OLR arises from this spectral range. The region is strongly influenced by upper-tropospheric–lower-stratospheric water vapor, temperature lapse rate, ice cloud distribution, and microphysics, all critical parameters in the climate system that are highly variable and still poorly observed and understood. To cover this uncharted territory in Earth observations, the Far-Infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission has recently been selected as ESA’s ninth Earth Explorer mission for launch in 2026. The primary goal of FORUM is to measure, with high absolute accuracy, the FIR component of the spectrally resolved OLR for the first time with high spectral resolution and radiometric accuracy. The mission will provide a benchmark dataset of global observations which will significantly enhance our understanding of key forcing and feedback processes of the Earth’s atmosphere to enable more stringent evaluation of climate models. This paper describes the motivation for the mission, highlighting the scientific advances that are expected from the new measurements.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The Far-infrared Earth Harries, J.; Carli, B.; Rizzi, R. ...
Reviews of geophysics (1985),
December 2008, Letnik:
46, Številka:
4
Journal Article
Recenzirano
The paper presents a review of the far‐infrared (FIR) properties of the Earth's atmosphere and their role in climate. These properties have been relatively poorly understood, and it is one of the ...purposes of this review to demonstrate that in recent years we have made great strides in improving this understanding. Seen from space, the Earth is a cool object, with an effective emitting temperature of about 255 K. This contrasts with a global mean surface temperature of ∼288 K and is due primarily to strong absorption of outgoing longwave energy by water vapor, carbon dioxide, and clouds (especially ice). A large fraction of this absorption occurs in the FIR, and so the Earth is effectively a FIR planet. The FIR is important in a number of key climate processes, for example, the water vapor and cloud feedbacks (especially ice clouds). The FIR is also a spectral region which can be used to remotely sense and retrieve atmospheric composition in the presence of ice clouds. Recent developments in instrumentation have allowed progress in each of these areas, which are described, and proposals for a spaceborne FIR instrument are being formulated. It is timely to review the FIR properties of the clear and cloudy atmosphere, the role of FIR processes in climate, and its use in observing our planet from space.
The outgoing long-wave radiation from the Earth's atmosphere in the far infrared spectral region is mostly unexplored, while is well recognized that the water vapour contribution to greenhouse ...trapping is dominant in this region. The Radiation Explorer in the Far InfraRed (REFIR) study has proven the feasibility of a space-borne Fourier transform spectrometer able to perform the measurement in the 100–1100 cm−1 range with a resolution of 0.5 cm−1. Following this work a prototype of the spectrometer named REFIR-PAD (Prototype for Applications and Development) has been developed to observe the atmospheric radiance from both ground-based sites and from stratospheric balloon platforms. In this work we describe the REFIR-PAD level 1 data analysis procedure, that, starting from raw instrumental data produces the calibrated atmospheric spectral radiance. Performances of the procedure are also described.
The latest release of a high-resolution transmission molecular absorption database along with two improved models of water vapor continuum absorption are used to check their impact on the improvement ...of state-of-art radiative transfer. Radiative transfer performance has been assessed using high mountains atmospheric emitted spectral downwelling radiance observations in the 360–1200cm−1 spectral regions. These high mountains observations are particularly suited to check the behavior and performance in the water vapor rotation band. In addition, they also have allowed us to gain insight into understanding the quality of recent new compilation of lines and related treatment for the ν2 CO2 band and the O3 band at 9.6μm. Comparisons are made between forward calculations of atmospheric transmission spectra and spectral radiances measured using two ground-based Fourier transform instruments. The results demonstrate that water vapor absorption largely benefits from the recent improvement in the related continuum (both self and foreign). In addition, ozone absorption is very accurately reproduced and, although to a less extent, this is also the case of CO2 absorption in the long wave ν2 band.
► Acquisition and presentation of new spectrally resolved radiances. ► Measurement recorded in high mountains sites. ► Observations covering the far to mid infrared, range 360–1200cm−1. ► Validation of the newest water vapor continuum MT_CKD version 2.5.2. ► Validation of radiative transfer in the O3(9.6μm) and CO2(15μm) bands.
The second Radiative Heating in Underexplored Bands Campaign (RHUBC‐II) was conducted in 2009 by the U.S. Department of Energy Atmospheric Radiation Measurement program to improve water vapor ...spectroscopy in the far‐infrared spectral region. RHUBC‐II was located in an extremely dry region of Chile to ensure very low opacities in this spectral region. Spectrally resolved measurements by a far‐infrared spectrometer and a submillimeter interferometer from RHUBC‐II are compared with line‐by‐line radiative transfer model calculations. Water vapor amounts and temperatures used in the calculations come from collocated radiosondes, with extensive adjustments to correct for issues due to the campaign's dry conditions and mountainous terrain. A reanalysis is also performed of far‐infrared measurements taken at the Atmospheric Radiation Measurement North Slope of Alaska site before and during the first RHUBC campaign. These analyses determine that differences between the measurements and model calculations using existing spectroscopic parameters are significant in the far‐infrared and submillimeter regions, leading to the derivation of improved water vapor continuum absorption coefficients and air‐broadened widths of 74 water vapor lines. The foreign continuum is increased by more than 50% in part of the far‐infrared and the widths of more than 20 lines are changed by more than 10%. The uncertainty in the foreign continuum coefficients is estimated as greater than 20% in some spectral regions, primarily a consequence of the uncertainty in the specification of water vapor. The improved far‐infrared spectroscopic parameters have a notable impact on calculated spectral radiances and a modest impact on broadband radiative fluxes and heating rates.
Key Points
Radiomteric measurements from two Atmospheric Radiation Measurement program field campaigns, held in dry locations, have been analyzed
Improved spectroscopic parameters for the water vapor continuum and lines in the far‐infrared and submillimeter regions have been derived
The improved far‐infrared spectroscopic parameters have a notable impact on calculated spectral radiances and a modest impact on broadband radiative fluxes and heating rates
This paper analyses high spectral resolution downwelling radiance measurements in the far infrared in the presence of cirrus clouds taken by the REFIR-PAD interferometer, deployed at 3500m above the ...sea level at the Testa Grigia station (Italy), during the Earth COoling by WAter vapouR emission (ECOWAR) campaign. Atmospheric state and cloud geometry are characterised by the co-located millimeter-wave spectrometer GBMS and by radiosonde profile data, an interferometer (I-BEST) and a Raman lidar system deployed at a nearby location (Cervinia). Cloud optical depth and effective diameter are retrieved from REFIR-PAD data using a limited number of channels in the 820–960cm−1 interval. The retrieved cloud parameters are the input data for simulations covering the 250–1100cm−1 band in order to test our ability to reproduce the REFIR-PAD spectra in the presence of ice clouds. Inverse and forward simulations are based on the same radiative transfer code. A priori information concerning cloud ice vertical distribution is used to better constrain the simulation scheme and an analysis of the degree of approximation of the phase function within the radiative transfer codes is performed to define the accuracy of computations. Simulation-data residuals over the REFIR-PAD spectral interval show an excellent agreement in the window region, but values are larger than total measurement uncertainties in the far infrared. Possible causes are investigated. It is shown that the uncertainties related to the water vapour and temperature profiles are of the same order as the sensitivity to the a priori assumption on particle habits for an up-looking configuration. In case of a down-looking configuration, errors due to possible incorrect description of the water vapour profile would be drastically reduced.
•We analyze down-welling spectral radiances in the far infrared (FIR) spectrum.•Discuss the scattering in the fir and the ice crystals phase function reconstruction.•Compare radiative transfer simulations with interferometric data in the presence of cirri.•Residuals cannot be interpreted as caused by use of unrealistic cloud properties.•Best experimental setup for ice clouds study in the fir is down-looking.
Our understanding of global warming depends on the accuracy with which the atmospheric components that modulate the Earth's radiation budget are known. Many uncertainties still exist as regards the ...radiative effect of water in the different spectral regions, among which is the far infrared, where very few observations have been made. An assessment is shown of the atmospheric outgoing flux obtained from a balloon-borne platform with wideband spectrally-resolved nadir measurements at the top of the atmosphere over the full spectral range, from 100 to 1400 cm−1, made by a Fourier transform spectrometer with uncooled detectors. From these measurements, we retrieved 15 pieces of information regarding water vapour and temperature profiles and surface temperature, with a major improvement in our knowledge of water vapour in the upper troposphere. The retrieved atmospheric state made it possible to calculate the emitted radiance also at frequencies and zenith angles that have not been observed and to determine the outgoing spectral radiation flux. This proves that spectrally resolved observations can be used to derive accurate information on the integrated flux. While the retrieved temperature was in agreement with ECMWF analysis, the retrieved water vapour profile differed significantly; depending on the time and the location, the derived flux in the far infrared (20–600 cm−1) differed by 2–3.5 W/m2 from that calculated using ECMWF. The error with which the far infrared flux is determined by REFIR-PAD is about 0.4 W/m2 and is caused mainly by calibration uncertainties, while detector noise has a negligible effect. This proves that uncooled detectors are adequate for top-of-the-atmosphere radiometry.
A field experiment was conducted in northern Chile at an altitude of 5.3 km to evaluate the accuracy of line‐by‐line radiative transfer models in regions of the spectrum that are typically opaque at ...sea level due to strong water vapor absorption. A suite of spectrally resolved radiance instruments collected simultaneous observations that, for the first time ever, spanned the entire terrestrial thermal spectrum (i.e., from 10 to 3000 cm−1, or 1000 to 3.3 μm). These radiance observations, together with collocated water vapor and temperature profiles, are used to provide an initial evaluation of the accuracy of water vapor absorption in the far‐infrared of two line‐by‐line radiative transfer models. These initial results suggest that the more recent of the two models is more accurate in the strongly absorbing water vapor pure rotation band. This result supports the validity of the Turner et al. (2012) study that demonstrated that the use of the more recent water vapor absorption model in climate simulations resulted in significant radiative and dynamical changes in the simulation relative to the older water vapor model.
Key PointsFirst ground‐based spectrally‐resolved observations of entire infrared spectrumExtremely dry conditions result in semi‐transparent regions in far‐infraredObservations used to evaluate line‐by‐line radiative transfer models
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.
PDF
