Development and recent evaluation of the MT_CKD model of continuum absorption

Water vapour continuum absorption is an important contributor to the Earth's radiative cooling and energy balance. Here, we describe the development and status of the MT_CKD (MlawerTobinCloughKneizysDavies) water vapour continuum absorption model. The perspective adopted in developing the MT_CKD model has been to constrain the model so that it is consistent with quality analyses of spectral atmospheric and laboratory measurements of the foreign and self continuum. For field measurements, only cases for which the characterization of the atmospheric state has been highly scrutinized have been used. Continuum coefficients in spectral regions that have not been subject to compelling analyses are determined by a mathematical formulation of the spectral shape associated with each water vapour monomer line. This formulation, which is based on continuum values in spectral regions in which the coefficients are well constrained by measurements, is applied consistently to all water vapour monomer lines from the microwave to the visible. The results are summed-up (separately for the foreign and self) to obtain continuum coefficients from 0?to 20?000?cm(-1). For each water vapour line, the MT_CKD line shape formulation consists of two components: exponentially decaying far wings of the line plus a contribution from a water vapour molecule undergoing a weak interaction with a second molecule. In the MT_CKD model, the first component is the primary agent for the continuum between water vapour bands, while the second component is responsible for the majority of the continuum within water vapour bands. The MT_CKD model should be regarded as a semi-empirical model with strong constraints provided by the known physics. Keeping the MT_CKD continuum consistent with current observational studies necessitates periodic updates to the water vapour continuum coefficients. In addition to providing details on the MT_CKD line shape formulation, we describe the most recent update to the model, MT_CKD_2.5, which is based on an analysis of satellite- and ground-based observations from 2385 to 2600?cm(-1) (approx. 4?μm).     

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.     

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.     

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.     

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.     

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.     

Spectroscopic measurement of the vapour pressure of ice

We present a laser absorption technique to measure the saturation vapour pressure of hexagonal ice. This method is referenced to the triple-point state of water and uses frequency-stabilized cavity ring-down spectroscopy to probe four rotation-vibration transitions of at wavenumbers near 7180?cm(-1). Laser measurements are made at the output of a temperature-regulated standard humidity generator, which contains ice. The dynamic range of the technique is extended by measuring the relative intensities of three weak/strong transition pairs at fixed ice temperature and humidity concentration. Our results agree with a widely used thermodynamically derived ice vapour pressure correlation over the temperature range 0°C to -70°C to within 0.35 per cent.     

Portable Fourier transform infrared spectroradiometer for field measurements of radiance and emissivity

A hand-held, battery-powered Fourier transform infrared spectroradiometer weighing 12.5 kg has been developed for the field measurement of spectral radiance from the Earth's surface and atmosphere in the 3-5-μm and 8-14-μm atmospheric windows, with a 6-cm(-1) spectral resolution. Other versions of this instrument measure spectral radiance between 0.4 and 20 μm, using different optical materials and detectors, with maximum spectral resolutions of 1 cm(-1). The instrument tested here has a measured noise-equivalent delta T of 0.01 °C, and it measures surface emissivities, in the field, with an accuracy of 0.02 or better in the 8-14-μm window (depending on atmospheric conditions), and within 0.04 in accessible regions of the 3-5-μm window. The unique, patented design of the interferometer has permitted operation in weather ranging from 0 to 45 °C and 0 to 100% relative humidity, and in vibration-intensive environments such as moving helicopters. The instrument has made field measurements of radiance and emissivity for 3 yr without loss of optical alignment. We describe the design of the instrument and discuss methods used to calibrate spectral radiance and calculate spectral emissivity from radiance measurements. Examples of emissivity spectra are shown for both the 3-5-μm and 8-14-μm atmospheric windows.     

Deposition of tritiated water vapour to water surface in an outdoor field

Deposition of tritiated water vapour in the atmosphere to a water surface was studied in an outdoor field, where elevated concentrations of tritiated water vapour existed in the atmosphere over twelve days. Exchange velocities of tritiated water vapour between air and water were evaluated from tritium concentrations in air and water obtained in the field experiment. It was found that the average of outdoor exchange velocities was about three times greater than that observed in a nuclear reactor room previously. Relationships between the outdoor exchange velocities and meteorological conditions were analysed to derive a multiple correlation equation. The wind speed was strongly correlated with the exchange velocity and the air temperature appeared to have an enhancing effect on the velocity. These observations were supported by a follow-up experiment conducted on a laboratory scale.     

Enhanced field emission from injector-like ZnO nanostructures with minimized screening effect

Injector-like zinc oxide (ZnO) nanostructures have been synthesized on Si substrate by the vapour phase transport method. Samples with different areal densities were obtained by controlling the temperature. The field emission properties of the injector-like ZnO nanostructures showed a clear dependence on the areal density of the nanostructures, which is due to the screening effect. The samples with a needle length of 850?nm and an areal density of 1 × 10(8)?cm(-2) showed the lowest field emission turn-on field of 1.85?V?μm(-1) at a current density of 10?μA?cm(-2), and the current density reaches 1?mA?cm(-2) at an applied field of 4.7?V?μm(-1).     

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.     

Temperature dependence of light absorption in water at holmium and thulium laser wavelengths

A simple experimental setup is described that facilitates accurate measurements of the temperature-dependent water absorption coefficient in the mid-infrared spectral region. With this setup, the absorption of holmium and thulium laser radiation in water was quantified to a precision of 0.5%. In the 20-100 degrees C temperature range, a linear decrease of the absorption coefficient with temperature is observed. The slope coefficients amount to -0.104 +/- 0.001 and -0.259 +/- 0.003 l/(K cm) for 2090-nm holmium and 2014-nm thulium radiation, respectively. At both wavelengths, this bleaching reduces the absorption coefficients of water at 100 degrees C by one third when compared with room temperature. A numerical simulation shows that the variable absorption has a noticeable influence on peak temperatures in laser heating of water.     

Global spectroscopy of the water monomer

Given the large energy required for its electronic excitation, the most important properties of the water molecule are governed by its ground potential energy surface (PES). Novel experiments are now able to probe this surface over a very extended energy range, requiring new theoretical procedures for their interpretation. As part of this study, a new, accurate, global spectroscopic-quality PES and a new, accurate, global dipole moment surface are developed. They are used for the computation of the high-resolution spectrum of water up to the first dissociation limit and beyond as well as for the determination of Stark coefficients for high-lying states. The water PES has been determined by combined ab initio and semi-empirical studies. As a first step, a very accurate, global, ab initio PES was determined using the all-electron, internally contracted multi-reference configuration interaction technique together with a large Gaussian basis set. Scalar relativistic energy corrections are also determined in order to move the energy determinations close to the relativistic complete basis set full configuration interaction limit. The electronic energies were computed for a set of about 2500 geometries, covering carefully selected configurations from equilibrium up to dissociation. Nuclear motion computations using this PES reproduce the observed energy levels up to 39?000?cm(-1) with an accuracy of better than 10?cm(-1). Line positions and widths of resonant states above dissociation show an agreement with experiment of about 50?cm(-1). An improved semi-empirical PES is produced by fitting the ab initio PES to accurate experimental data, resulting in greatly improved accuracy, with a maximum deviation of about 1?cm(-1) for all vibrational band origins. Theoretical results based on this semi-empirical surface are compared with experimental data for energies starting at 27?000?cm(-1), going all the way up to dissociation at about 41?000?cm(-1) and a few hundred wavenumbers beyond it.     

DRIFT studies of thick film un-doped and Pd-doped SnO2 sensors: temperature changes effect and CO detection mechanism in the presence of water vapour

Diffuse reflectance infrared Fourier transform measurements were performed on tin oxide based thick film gas sensors operated in normal working conditions. We characterised SnO₂ sensors at different temperatures between room temperature and 300 °C. The results show the presence of different surface OH groups as well as coordinated water on the SnO₂ sensor surface. Their intensity changes with temperature. During the temperature cycles the bands’ peak positions are reversibly changed but their intensity is not. CO measurements were performed at 300 °C at different humidity levels (0 and 50% r.h.) on un-doped and Pd-doped sensors. In the presence of CO we observed in the spectra: a decrease of the OH groups on the SnO₂ surfaces, the appearance of gaseous CO₂ and CO in the pores of the sensitive layer and an increase of hydrated protons and of the free charge concentration. The effects are dramatically influenced by the water vapour concentration, temperature, dopands (Pd) and can be correlated with simultaneously performed sensor resistance measurements.     

Pressure effects on water vapour lines: beyond the Voigt profile

A short overview of recent results on the effects of pressure (collisions) regarding the shape of isolated infrared lines of water vapour is presented. The first part of this study considers the basic collisional quantities, which are the pressure-broadening and -shifting coefficients, central parameters of the Lorentzian (and Voigt) profile and thus of any sophisticated line-shape model. Through comparisons of measured values with semi-classical calculations, the influences of the molecular states (both rotational and vibrational) involved and of the temperature are analysed. This shows the relatively unusual behaviour of H(2)O broadening, with evidence of a significant vibrational dependence and the fact that the broadening coefficient (in cm(-1) atm(-1)) of some lines increases with temperature. In the second part of this study, line shapes beyond the Voigt model are considered, thus now taking 'velocity effects' into account. These include both the influence of collisionally induced velocity changes that lead to the so-called Dicke narrowing and the influence of the dependence of collisional parameters on the speed of the radiating molecule. Experimental evidence of deviations from the Voigt shape is presented and analysed. The interest of classical molecular dynamics simulations, to model velocity changes, together with semi-classical calculations of the speed-dependent collisional parameters for line-shape predictions from 'first principles', are discussed.     

Infrared collision-induced absorption by O2 near 6.4 microm for atmospheric applications: measurements and empirical modeling

Accurate measurements of collision-induced absorption by O(2) and O(2)-N(2) mixtures in the fundamental band near 6.4 mum have been made. A Fourier-transform spectrometer was used with a resolution of 0.5 cm(-1). Absorption has been investigated in the 0-20-atm and 193-293 K pressure and temperature ranges, respectively. The current measurements confirm that the broad O(2) continuum carries small features whose attribution is not yet clear. Available experimental data in the 190-360 K temperature range have been used to build a simple, low cost computer, empirical model that is well adapted for computation of atmospheric O(2) absorption. Tests show that it is accurate, contrary to predictions of widely used atmospheric transmission codes.     

大气垂直探测仪光谱定标

为了大气垂直探测仪能够进行精确的光谱测量,对其进行光谱定标.大气垂直探测仪使用面阵探测器,由于离轴像元接收的光线不平行于光轴,使仪器函数发生变化.本文针对大气垂直探测仪使用的16x4元面阵探测器理论分析给出仪器函数,讨论其对光谱定标的影响,并将利用理论分析结果及利用气体吸法测得的光谱,对大气垂直探测仪进行了光谱标定,光谱定标精度达10-5.采用面阵探测器的干涉仪,边缘视场像元所测光谱会向低频漂移,因此必需研究干涉仪的仪器函数,从而消除光谱漂移对边缘视场光谱定标的影响.     

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.     

Infrared collision-induced absorption by N(2) near 4.3 μm for atmospheric applications: measurements and empirical modeling

Accurate measurements of collision-induced absorption by pure nitrogen in the fundamental band near 4.3 μm have been made in the 0-10 atm and 230-300 K pressure and temperature ranges, respectively. A Fourier-transform spectrometer was used with a resolution of 0.5 cm(-1). The current measurements, which agree well with previous ones but are more precise, reveal that weak features are superimposed on the broad N(2) continuum. These features have negligible temperature dependence, and their origin is not clear at the present time. Available experimental data in the 190-300 K temperature range have been used to build a simple empirical model that is suitable for use to compute atmospheric N(2) absorption. Tests indicate that this model is accurate unlike the estimates produced by widely used atmospheric transmission codes.