Airborne and satellite remote sensing of the mid-infrared water vapour continuum

Remote sensing of the atmosphere from space plays an increasingly important role in weather forecasting. Exploiting observations from the latest generation of weather satellites relies on an accurate knowledge of fundamental spectroscopy, including the water vapour continuum absorption. Field campaigns involving the Facility for Airborne Atmospheric Measurements research aircraft have collected a comprehensive dataset, comprising remotely sensed infrared radiance observations collocated with accurate measurements of the temperature and humidity structure of the atmosphere. These field measurements have been used to validate the strength of the infrared water vapour continuum in comparison with the latest laboratory measurements. The recent substantial changes to self-continuum coefficients in the widely used MT_CKD (Mlawer-Tobin-Clough-Kneizys-Davies) model between 2400 and 3200?cm(-1) are shown to be appropriate and in agreement with field measurements. Results for the foreign continuum in the 1300-2000?cm(-1) band suggest a weak temperature dependence that is not currently included in atmospheric models. A one-dimensional variational retrieval experiment is performed that shows a small positive benefit from using new laboratory-derived continuum coefficients for humidity retrievals.     

The water vapour self- and water-nitrogen continuum absorption in the 1000 and 2500 cm(-1) atmospheric windows

The pure water vapour and water-nitrogen continuum absorption in the 1000 and 2500?cm(-1) atmospheric windows has been studied using a 2?m base-length White-type multi-pass cell coupled with a BOMEM DA3-002 Fourier transform infrared spectrometer. The measurements were carried out at the National Institute of Standards and Technology (NIST, Gaithersburg, MD) over the course of several years (2004, 2006-2007, 2009). New data on the H(2)O:N(2) continuum in the 1000?cm(-1) window are presented and summarized along with the other experimental results and the continuum model. The experimental data reported on the water vapour continuum in these atmospheric windows basically agree with the most reliable laboratory data from the other sources. The MT_CKD (Mlawer-Tobin-Clough-Kneizys-Davies) continuum model significantly departs from the experimental data in both windows. The deviation observed includes the continuum magnitude, spectral behaviour and temperature dependence. In the 2500?cm(-1) region, the model does not allow for the nitrogen fundamental collision-induced absorption (CIA) band intensity enhancement caused by H(2)O:N(2) collisions and underestimates the actual absorption by over two orders of magnitude. The water vapour continuum interpretation as a typical CIA spectrum is reviewed and discussed.     

Absolute high spectral resolution measurements of surface solar radiation for detection of water vapour continuum absorption

Solar-pointing Fourier transform infrared (FTIR) spectroscopy offers the capability to measure both the fine scale and broadband spectral structure of atmospheric transmission simultaneously across wide spectral regions. It is therefore suited to the study of both water vapour monomer and continuum absorption behaviours. However, in order to properly address this issue, it is necessary to radiatively calibrate the FTIR instrument response. A solar-pointing high-resolution FTIR spectrometer was deployed as part of the 'Continuum Absorption by Visible and Infrared radiation and its Atmospheric Relevance' (CAVIAR) consortium project. This paper describes the radiative calibration process using an ultra-high-temperature blackbody and the consideration of the related influence factors. The result is a radiatively calibrated measurement of the solar irradiation at the ground across the IR region from 2000 to 10?000?cm(-1) with an uncertainty of between 3.3 and 5.9 per cent. This measurement is shown to be in good general agreement with a radiative-transfer model. The results from the CAVIAR field measurements are being used in ongoing studies of atmospheric absorbers, in particular the water vapour continuum.     

Water vapour foreign-continuum absorption in near-infrared windows from laboratory measurements

For a long time, it has been believed that atmospheric absorption of radiation within wavelength regions of relatively high infrared transmittance (so-called 'windows') was dominated by the water vapour self-continuum, that is, spectrally smooth absorption caused by H(2)O--H(2)O pair interaction. Absorption due to the foreign continuum (i.e. caused mostly by H(2)O--N(2) bimolecular absorption in the Earth's atmosphere) was considered to be negligible in the windows. We report new retrievals of the water vapour foreign continuum from high-resolution laboratory measurements at temperatures between 350 and 430?K in four near-infrared windows between 1.1 and 5?μm (9000-2000?cm(-1)). Our results indicate that the foreign continuum in these windows has a very weak temperature dependence and is typically between one and two orders of magnitude stronger than that given in representations of the continuum currently used in many climate and weather prediction models. This indicates that absorption owing to the foreign continuum may be comparable to the self-continuum under atmospheric conditions in the investigated windows. The calculated global-average clear-sky atmospheric absorption of solar radiation is increased by approximately 0.46?W?m(-2) (or 0.6% of the total clear-sky absorption) by using these new measurements when compared with calculations applying the widely used MTCKD (Mlawer-Tobin-Clough-Kneizys-Davies) foreign-continuum model.     

Recent advances in measurement of the water vapour continuum in the far-infrared spectral region

We present a new derivation of the foreign-broadened water vapour continuum in the far-infrared (far-IR) pure rotation band between 24?μm and 120?μm (85-420?cm(-1)) from field data collected in flight campaigns of the Continuum Absorption by Visible and IR radiation and Atmospheric Relevance (CAVIAR) project with Imperial College's Tropospheric Airborne Fourier Transform Spectrometer (TAFTS) far-IR spectro-radiometer instrument onboard the Facility for Airborne Atmospheric Measurement (FAAM) BAe-146 research aircraft; and compare this new derivation with those recently published in the literature in this spectral band. This new dataset validates the current Mlawer-Tobin-Clough-Kneizys-Davies (MT-CKD) 2.5 model parametrization above 300?cm(-1), but indicates the need to strengthen the parametrization below 300?cm(-1), by up to 50 per cent at 100?cm(-1). Data recorded at a number of flight altitudes have allowed measurements within a wide range of column water vapour environments, greatly increasing the sensitivity of this analysis to the continuum strength.     

Measurements of the foreign-broadened continuum of water vapor in the 6.3 microm band at -30 degrees C

The foreign-broadened continuum of water vapor in the nu2 band (5-7.7 microm, 1300-2000 cm(-1)) is important for satellite-based retrievals of water vapor in the upper troposphere, where temperatures are below -25 degrees C. Continuum coefficients have previously been measured mostly at or above +23 degrees C. We present continuum coefficients in the nu(2) band retrieved from measurements made in Antarctica at temperatures near -30 degrees C: atmospheric transmission at South Pole Station and atmospheric emission at Dome C. The continuum coefficients derived from these measurements are generally in agreement with the widely used Mlawer, Tobin-Clough, Kneizys, Davies continuum. Differences are at most 30%, corresponding to a 6% relative error in retrieved upper-tropospheric humidity.     

Horizontal lidar measurements for the proof of spontaneous Rayleigh-Brillouin scattering in the atmosphere

Several atmospheric lidar techniques rely on the exact knowledge of the spectral line shape of molecular scattered light in air, which, however, has not been accurately measured in real atmosphere up to now. In this paper we report on the investigation of spontaneous Rayleigh-Brillouin scattering within the atmosphere, utilizing horizontal lidar measurements (λ=355 nm, θ=180°) performed from the mountain observatory Schneefernerhaus (2650?m), located below Germany's highest mountain, the Zugspitze. These lidar measurements give proof of the effect of Brillouin scattering within the atmosphere for the first time to our knowledge. The measurements confirm that the Tenti S6 model can be used to adequately describe spontaneous Rayleigh-Brillouin spectra of light scattered in air under real atmospheric conditions. The presented results are of relevance for spectrally resolving lidars like those deployed on the Atmospheric Dynamics Mission Aeolus (ADM-Aeolus) andthe Earth Clouds, Aerosols, and Radiation Explorer Mission (EarthCARE).     

Effective frequency technique for finite spectral bandwidth effects

A technique that uses a single effective frequency to represent the effects of finite spectral bandwidth for active and passive measurements centered on an absorption line, a trough region, or a slowly varying spectral feature is described. For Gaussian and rectangular instrumental line shapes, the effective frequency is shown to have a simple form that depends only on the instrumental line shape and bandwidth and not on the absorption line profile. The technique is applicable to a large class of active and passive measurements and simulations in both the laboratory and the atmosphere. Simulations show that the technique yields accuracies better than 0.1% for bandwidths less than 0.2 times the atmospheric linewidth for a rectangular line shape or better than 0.2% for a Gaussian.     

H(2)O--N(2) collision-induced absorption band intensity in the region of the N(2) fundamental: ab initio investigation of its temperature dependence and comparison with laboratory data

The present paper aims at ab initio and laboratory evaluation of the N(2) collision-induced absorption band intensity arising from interactions between N(2) and H(2)O molecules at wavelengths of around 4?μm. Quantum chemical calculations were performed in the space of five intermolecular coordinates and varying N--N bond length using M?ller-Plesset perturbation and CCSD(T) methods with extrapolation of the electronic energy to the complete basis set. This made it possible to construct the intermolecular potential energy surface and to define the surface of the N--N dipole derivative with respect to internal coordinate. The intensity of the nitrogen fundamental was then calculated as a function of temperature using classical integration. Experimental spectra were recorded with a BOMEM DA3-002 FTIR spectrometer and 2?m base-length multipass White cell. Measurements were conducted at temperatures of 326, 339, 352 and 363?K. The retrieved water-nitrogen continuum significantly deviates from the MT_CKD model because the relatively strong nitrogen absorption induced by H(2)O was not included in this model. Substantial uncertainties in the measurements of the H(2)O-N(2) continuum meant that quantification of any temperature dependence was not possible. The comparison of the integrated N(2) fundamental band intensity with our theoretical estimates shows reasonably good agreement. Theory indicates that the intensity as a function of temperature has a minimum at approximately 500?K.     

Polarimetric measurements of long-wave infrared spectral radiance from water

Polarimetric measurements of the thermal infrared spectral radiance from water are reported and are compared with calculations from a recently published model over the spectral range of 600-1600 cm(-1) (6.25-16.67-mum wavelength). In this spectral range, warm water viewed under a dry, clear atmosphere appears vertically polarized by 6-12%. The measured spectral degree of polarization agrees with calculations within the measurement uncertainty (~0.5% polarization in spectral regions with high atmospheric transmittance and 1.5% polarization in spectral regions with low atmospheric transmittance). Uncertainty also arises from temporal changes in water and air temperatures between measurements at orthogonal polarization states, indicating the desirability of simultaneous measurements for both polarization states.     

End-to-end sensor simulation for spectral band selection and optimization with application to the Sentinel-2 mission

An end-to-end sensor simulation is a proper tool for the prediction of the sensor's performance over a range of conditions that cannot be easily measured. In this study, such a tool has been developed that enables the assessment of the optimum spectral resolution configuration of a sensor based on key applications. It employs the spectral molecular absorption and scattering properties of materials that are used for the identification and determination of the abundances of surface and atmospheric constituents and their interdependence on spatial resolution and signal-to-noise ratio as a basis for the detailed design and consolidation of spectral bands for the future Sentinel-2 sensor. The developed tools allow the computation of synthetic Sentinel-2 spectra that form the frame for the subsequent twofold analysis of bands in the atmospheric absorption and window regions. One part of the study comprises the assessment of optimal spatial and spectral resolution configurations for those bands used for atmospheric correction, optimized with regard to the retrieval of aerosols, water vapor, and the detection of cirrus clouds. The second part of the study presents the optimization of thematic bands, mainly driven by the spectral characteristics of vegetation constituents and minerals. The investigation is performed for different wavelength ranges because most remote sensing applications require the use of specific band combinations rather than single bands. The results from the important "red-edge" and the "short-wave infrared" domains are presented. The recommended optimum spectral design predominantly confirms the sensor parameters given by the European Space Agency. The system is capable of retrieving atmospheric and geobiophysical parameters with enhanced quality compared to existing multispectral sensors. Minor spectral changes of single bands are discussed in the context of typical remote sensing applications, supplemented by the recommendation of a few new bands for the next generation of optical Sentinel sensors.     

Control of spectral aberrations in a monochromator using a plane holographic chirped grating

A method aimed to minimize the impact of spectral aberrations in a monochromator is proposed in which the spectrum of the source of radiation under study is scanned by the rectilinear translation of a plane chirped grating. The chirped grating, which has a spatially variable groove spacing, is used to diffract and to spectrally focus the radiation. Imaging properties of the chirped grating were analyzed in order to develop the expression of the aberration coefficients of the system and the expression of the width of the instrument line shape due to aberrations. The optimal rectilinear trajectory required to operate the monochromator without significant spectral aberrations in measurements has been obtained numerically and tested in the laboratory. Experimental measurements of the emission spectrum of a seven-wavelength helium-neon laser are presented, as well as the sensitivity of the monochromator performance to different geometrical parameters.     

Remote sensing of sea surface temperature through infrared radiometry — a review

A review on the estimation of sea surface temperature (SST) from space using infrared radiometry is presented. The principle of the remote sensing technique, the infrared radiometers and the theory of SST measurement have been explained briefly. The absorption of infrared radiation from earth by the atmospheric constituents has been a major problem in the retrieval of SST from space-borne sensors. The effect of clouds and other atmospheric constituents, especially moisture content/water vapour, has been discussed and the various techniques used for evaluating the atmospheric correction and their limitations are summarised. Development of separate algorithms for each oceanic area coupled with validation by realistic sea truth measurements has been suggested for improving the accuracy of SST measurements from space. The first author was supported by a CSIR fellowship.     

Analysis of the instrumental line shape of high-resolution fourier transform IR spectrometers with gas cell measurements and new retrieval software

The instrumental line shape (ILS) of two commercial high-resolution Fourier transform IR spectrometers has been analysed with gas cell measurements and a new ILS retrieval software LINEFIT. The instruments are used for atmospheric remote sounding, and the compatibility of the ILS deduced from laboratory gas cell measurements with the ILS in the atmospheric measurement itself is examined.     

Heat transfer during vapour blanket stage of quench

Abstract

Although the production of a vapour blanket during the quenching of steel components is well known, there has been little quantitative study of this phenomenon. At the same time there has been intensive investigation of film boiling under other, better controlled conditions. The present work has applied certain mathematical models of film boiling to the initial stages of a quench in both water and a polymer solution. This has allowed a comparison of the surface heat transfer coefficients obtained using these models with experimental data. For all models the calculated coefficients underestimate the experimental values, although the reduction produced by the introduction of sodium polyacrylate into the quenchant is demonstrated. Refinements to the original model, involving the use of a moving vapour/liquid interface and temperature dependent physical properties, produced only modest reductions in the discrepancy between experiment and calculation. However, the available models of film boiling assume laminar flow, whereas the experimental evidence suggests that the liquid/vapour interface is turbulent. It is suggested that such a model would provide closer agreement with experiment. Of critical importance is the velocity of the interface between the vapour and liquid, experimental measurements of which are lacking. Further progress towards a quantitative understanding of the phenomena involved requires an experimental investigation of this parameter.

MST/1815     

Interferometer for ground-based observations of emitted spectral radiance from the troposphere: evaluation and retrieval performance

We evaluate the spectral quality, radiometric noise, and retrieval performance of a Fourier transform infrared spectrometer, which has been developed for recording spectrally resolved observations in a region of the spectrum which is important both for the science of Earth's climate and applications, such as the remote sensing of temperature and atmospheric gas species. This spectral region extends from 100 to 1600 cm(-1) and encompasses the two fundamental, rotation and vibration, absorption bands of water vapor. The instrument is a customized version of a Bomem AERI (Atmospheric Emitted Radiance Interferometer) spectrometer, whose spectral coverage has been extended in the far infrared with the use of uncooled pyroelectric detectors. Retrieval examples for water vapor and temperature profiles are shown, which also allow us to intercompare the retrieval performance of both H(2)O vibration and rotation bands.     

Design of a photowritten cavity neodymium-doped fiber laser tunable around an absorption molecular line

Experiments have shown that it is possible to realize laser cavities photowritten within rare-earth-doped silicate fibers. Experimental results on a particular application, namely, the detection of gas traces by identification of one of the gas's absorption lines, are presented. Experiments on an absorption line of atmospheric water have been carried out to illustrate this purpose. Two lasers have been designed with particular characteristics (spectral linewidths, slope efficiencies, thresholds). Preliminary experimental results on the detection of an absorption line of atmospheric water have been obtained with fiber lasers and optoacoustic detection.     

On modeling anisotropy in deformation processes involving textured polycrystals with distorted grain shape

A mathematical formulation is presented which uses rate-dependent polycrystalline plasticity to model the development of plastic anisotropy in bulk forming processes. The formulation assumes that underlying a material point on a continuum scale is a collection of anisotropic, contiguous grains. The mechanical response at a continuum level is derived from the response of individual grains suitably averaged over all grains in the aggregate. The effects of preferred orientation (texture) and of the evolving grain shape on the directionality to the flow properties of the polycrystal are included. A general numerical framework is described for incorporating this complex material behavior in a finite element formulation. As an application, texture development during the flat rolling of aluminum sheets is presented. The simulation predictions have been compared with reported experimental data and with a previous study where the effects of grain shape were neglected.     

Fast transmittance model for satellite sounding

Through the use of new line-by-line spectral calculations in both the infrared and microwave regions, coefficients have been generated for the transmittance stage of the fast radiative transfer model used by the United Kingdom Meteorological Office. These permit the fast model to calculate the transmittance for the high-resolution infrared sounder and the microwave sounding unit instruments aboard the National Oceanic and Atmospheric Administration polar-orbiting satellite for a given atmospheric profile, simply by taking these coefficients in linear combination with a set of predictors. These are expressed in terms of the deviation of the profile from a reference. However, the method can be applied to any instrument within the range of the spectral calculations, thereby permitting new coefficients to be calculated as soon as the spectral response details for the instrument become available. It also permits effective consideration to be given in the longer term to new line data or improvements in line-shape theory. The process by which these coefficients have been obtained is described, along with a discussion of some of the tests carried out on their installation into the fast model; these tests show that they are suitable for operational use. The predictors employed by the fast model are discussed, and changes are proposed for those that relate to the water-vapor transmittance. In this respect it was found that the inclusion of predictors that depend primarily on the zenith angle of the radiation path leads to improvements in the transmittance calculation.     

Submillimeter fourier-transform spectrometer measurements of atmospheric opacity above mauna kea

We present accurately calibrated submillimeter atmospheric transmission spectra obtained with a Fourier-transform spectrometer at the Caltech Submillimeter Observatory on Mauna Kea, Hawaii. These measurements cover the 0.9-0.3-mm wavelength range and are the first in a series aimed at defining the terrestrial long-wave atmospheric transmission curve. The 4.1-km altitude of the Mauna Kea site provides access to extremely low zenith water-vapor columns, permitting atmospheric observations at frequencies well above those possible from sea level. We describe the calibration procedures, present our first well-calibrated transmission spectra, and compare our results with those of a single-layer atmospheric transmission model, AT. With an empirical best-fit continuum opacity term included, this simple single-layer model provides a remarkably good fit to the opacity data for H(2)O line profiles described by either van Vleck-Weisskopf or kinetic shapes.