Transmittance Model Research Articles

A method for predicting the direct transmittance of atmospheric aerosol for solar radiation models

This paper presents a proposed method for predicting the transmittance of aerosol in the atmosphere for solar radiation models. The results from the application of the developed expression have been used to check its consistency with that of a universal direct solar transmittance model due to King and Buckius (1979) for ‘clear atmosphere' and ‘haze atmosphere' conditions. The trends in the variations of the two models are identical but the disparity in the magnitudes of the transmittances tends to widen with increasing value of air mass under the specified conditions, particularly for air masses 20. Appropriate reasons have been given for the observed differences. It is proposed that the developed model may be relevant for the prediction of the transmittances of gases at different strata of the atmosphere for solar radiation models, provided the ‘scaled heights' of the gases are known. KEY WORDS: Transmittance, atmospheric aerosols, clear atmosphere, haze atmosphere, scaled heights. Global Jnl of Pure and Applied Sciences Vol.10(4) 2004: 619-622

Experimental evaluation of an anisotropic scattering model of a slab geometry

A model has been developed based on random walk theory that allows the diffusion of light along a principal axis to differ from that in orthogonal directions. We present expressions that describe the time-resolved intensity measured across a slab and on the surface of a semi-infinite medium for a principal axis oriented parallel or perpendicular to the surface. The model of time-resolved transmittance is compared with experimental data acquired by use of a phantom consisting of wax fibers arranged within a solid cube of resin. It is shown that a single set of optical parameters is sufficient to model the experimental measurements acquired across all three orientations of the cube.

An Examination of the Crystallization of Urea from Supersaturated Aqueous and Aqueous−Methanol Solutions as Monitored In-Process Using ATR FTIR Spectroscopy

In-process ATR FTIR spectroscopy combined with optical turbidometry is applied to the measurement of reactant supersaturation during temperature-programmed studies of the batch crystallization of urea from aqueous and mixed aqueous/methanolic solvent systems at the 400 mL scale size (pitched blade impeller) for solute compositions in the 35−65 w/w% range. The metastable zone widths were found to be in the range 3−12 °C and critical supersaturations in the 1.05−1.18 range, respectively, inversely dependent upon solute concentration. Only a slight dependency of the metastable zone width with reactor heating/cooling rates, indicative of fairly matched rates of nucleation and solubility change, is found. Cross-correlation between FTIR and turbidometic measurements reveal, as expected, the former technique to be more sensitive to the detection of the crystallization onset. No evidence was found for phase separation (oiling-out) prior to nucleation, in contrast to previous studies of citric acid (Groen, H.; Roberts, K. J. J. Phys. Chem. B 2001, 105, 10723−10730). Crystallization enthalpic release at the onset point is found to affect transient crystal dissolution and regrowth following nucleation. Good FTIR calibration, with excellent correlation to known solubility data, using a univariant IR transmittance (peak intensity) model is achieved for both single and mixed solvent systems, this despite slight variation in the IR peak positions with temperature and concentration. Attempts to use the calibration data at higher solute concentrations beyond the calibration range were not successful, this being associated with a significant underestimation of the solution supersaturation. A forward look to improvements in FTIR calibration using multivariant chemometric approaches is highlighted.

Correlation of lattice distortion with optical and electrical properties of In2O3:Sn films

Polycrystalline In2O3:Sn (ITO) films were prepared by direct current magnetron sputtering at substrate temperatures between 150 and 400 °C and at various oxygen admixtures to the sputter gas. X-ray diffraction peaks (position and width) were evaluated with respect to lattice distortion (net increase and fluctuations of the lattice constant). The characteristic optical parameters of the films were obtained by dielectric modeling of experimental transmittance and reflectance spectra. The plasma edge was evaluated with respect to electron density and mobility. The electron density decreases linearly with the lattice distortion. This confirms a model of oxygen incorporation into the lattice during sputtering and allows an estimation of the density of oxygen interstitials from x-ray spectra. The electron mobility inside the grains decreases with decreasing electron density. This is well explained by a model based on scattering at ionized Sn donors that are partially compensated by interstitial oxygen ions. The direct current conductivity is smaller than the conductivity derived from the optically determined parameters because it is hampered by badly conducting grain boundaries. The refractive index decreases linearly with the free electron density which is explained, via the Kramers–Kronig relation, by increasing absorption in the plasma edge and the Burstein–Moss shift of the band edge.

Assimilating GOES Brightness Temperatures. Part I: Upper-Tropospheric Moisture

Abstract Imager channel 3 (at 6.7 μm) on the Geostationary Operational Environmental Satellite (GOES) is particularly sensitive to water vapor in the atmosphere. Channel-3 data from both clear and cloudy regions are used in a new assimilation scheme to improve the initial upper-tropospheric moisture fields for modeling and numerical weather prediction purposes. In this assimilation, the navigated and calibrated radiance (brightness temperature) observations from GOES are used in combination with a forward radiative transmittance model and a numerical optimization procedure to produce modifications to the upper-tropospheric moisture field. All modifications are made proportional to the contribution weighting function, which is associated with the forward radiative model. Cloudy regions are given special consideration. When processed by a forward radiative transfer model, the assimilated moisture fields are shown to correlate better with GOES observations both initially and in 24- and 48-h forecasts. Additi...

Analysis of a full spectrum hybrid lighting system

Hybrid lighting is a new approach to lighting that integrates light from natural and electric sources. A two-axis tracking concentrator collects beam radiation which is reflected onto a mirror that divides the solar radiation into infrared and visible spectra. The visible light is distributed through optical fibers and combined with fluorescent lighting in specially designed luminaires. The infrared portion of the spectrum is used to generate electricity using a thermal photovoltaic array. A simulation of a hybrid lighting system has been created using the TRNSYS transient simulation program. The simulation incorporates the spectral properties of the hybrid lighting components as well as the spectral distribution of the incoming solar radiation that is based upon output from the SMARTS atmospheric transmittance model. An office building model is coupled with the hybrid lighting simulation to predict the annual energy impact upon lighting, heating, and cooling loads. Simulations were performed in six locations within the United States. Hybrid lighting systems performed best in Honolulu, HI and Tucson, AZ justifying system capital costs of $2410 and $1995 per module, respectively, based on a 10 year payback period.

Water vapour line and continuum absorption in the thermal infrared—reconciling models and observations

AbstractHigh‐spectral‐resolution thermal infrared radiance observations made with the Airborne Research Interferometer Evaluation System instrument on the Met Office C130 aircraft in tropical and sub‐arctic atmospheres are used to evaluate the water vapour continuum and line absorption. Coincident microwave radiometer measurements at 183 GHz are used to help constrain the water vapour profile. Through careful selection of wavelengths where the water vapour continuum has varying impacts on the observed radiances, analysis shows that the continuum is too strong; this is using the General Line‐by‐line Atmospheric Transmittance and Radiance Model with the CKD2.4 water vapour continuum and High Resolution Transmission Molecular Absorption 2000 spectral database. Zenith observations from low altitude indicate that the continuum requires reducing by between 6% and 15% depending on the frequency, which is in good agreement with recent laboratory data. A modified continuum is presented and used in modelling to compare against upper‐troposphere nadir measurements obtained in tropical and sub‐arctic atmospheres. Copyright © Crown copyright, 2003

A simplified but accurate spectral solar irradiance model

¶The paper deals with the computation of solar energy available in a specific location. First, a new formula describing the relation between global solar irradiation and the duration of bright sunshine is established. The analysis of its use shows that global solar clear sky irradiance models are essential tools for daily computation of global irradiation. An integrated spectral atmospheric transmittance model is presented, it can be used to compute beam and diffuse clear sky irradiance for all applications where broadband solar energy input is needed. Since it is desirable to use simplified estimation methods for many applications, a parametric global solar irradiance model, derived from the spectral model, is also presented. This model needs only surface meteorological data as input. The influence of the averaging method used for the input parameters on the model accuracy is evaluated. Comparison of model results with the measurements shows an acceptable level of accuracy with the new model. Finally, an application of daily global solar energy computation is presented.

Direct solar transmittance and irradiance predictions with broadband models. Part I: detailed theoretical performance assessment

A thorough investigation on the performance of broadband direct irradiance predictions using solar radiation models is detailed here. Nineteen models were selected from an extensive literature survey. In addition, two new models were specifically developed for this study to provide state-of-the-art modelling of the broadband transmittances associated with the most important extinction processes in the atmosphere. The SMARTS spectral radiative code has been selected to provide 2064 reference transmittance and irradiance values, corresponding to as many combinations of solar position and varied atmospheric conditions. Inconsistencies or errors in the modelling of different transmittance functions from existing models were found, and could be corrected in some cases. As a result of this theoretical assessment, it is concluded that detailed transmittance models normally perform better than bulk models, and that models using Linke’s turbidity coefficient in intermediate calculations performed poorly. Four high-performance models can be recommended as a result of this detailed investigation: CPCR2, MLWT2, REST and Yang (in alphabetical order). The new MLWT2 model provides the best performance in all tests, thanks to its elaborate multi-layer spectral weighting scheme.

Determination of thin-film parameters from high accuracy measurements of spectral regular transmittance

The importance of reliable spectral transmittance measurements for reliable thin-film characterization is demonstrated. By using a model of the spectral transmittance of thin-film samples, the effect of various uncertainty components in transmittance measurements on the determined thin-film parameters is analysed. The experimental results for aluminium dioxide and tantalum pentoxide thin-film samples show good agreement with the model.

Rapid radiative transfer model for AMSU/HSB channels

The atmospheric transmittance model for the Advanced Microwave Sounding Unit-A (AMSU-A) and the Humidity Sounder for Brazil (HSB) channels on the Aqua spacecraft uses a polynomial approximation to the temperature dependence of oxygen-band opacity within atmospheric layers. It uses lookup tables to calculate local water-vapor line intensity and pressure-broadening parameters as well as contributions to absorption from the water-vapor continuum, distant lines, and cloud liquid water. The algorithm includes water-line self-broadening and the magnetic-field effect on AMSU-A channel 14. The microwave surface emission model is based on a preliminary classification of the surface type, with subsequent adjustments to the emissivity spectrum that are obtained from the retrieval algorithm. A simple approximate correction for surface nonspecularity is included. The algorithm has been tested by comparisons to a line-by-line calculation and to measurements made by the NOAA-15 AMSU-A.

Will ASTER see the Etna SO 2 plume?

The aim of this paper is to show the feasibility of evaluating the sulphur dioxide (SO 2 ) flux emitted by Mt Etna volcano, using remotely sensed data of Advanced Spaceborne Thermal Emission and Reflection radiometer (ASTER) in the thermal infrared (TIR). ASTER is one of the instruments on board the first Earth Observation System (EOS) satellite launched on 18 December 1999. To simulate the ASTER data, Multispectral Infrared and Visible Imaging Spectrometer (MIVIS) images, acquired during the 'Sicily '97' campaign, were used. The SO 2 column abundance maps shown in this paper have been obtained by means of a simple algorithm based on the split-window technique. The parameters of the algorithm were computed using Moderate Resolution Atmospheric Radiance and Transmittance Model (MODTRAN) simulations performed with atmospheric profiles acquired during MIVIS flights. The mean SO 2 flux on 12 and 16 June 1997, obtained using the ASTER simulated images, results in 42 and 33 kg s m 1 respectively; these values are in good agreement with the results obtained by the inversion procedure used by Teggi et al . (1999) applied to the MIVIS data and with the Correlation Spectrometer (COSPEC) measurements performed during the campaign by other teams.

An updated parameterization for infrared emission and absorption by water vapor in the National Center for Atmospheric Research Community Atmosphere Model

An updated parameterization for the absorption and emission of infrared radiation by water vapor has been developed for the Community Atmosphere Model (CAM) from the National Center for Atmospheric Research (NCAR). The CAM is the latest version of the NCAR Community Climate Model (CCM). This updated treatment preserves the formulation of the radiative transfer equations using the absorptivity/emissivity method. However, the components of the absorptivity and emissivity related to water vapor have been replaced with new terms calculated with the General Line‐by‐line Atmospheric Transmittance and Radiance Model (GENLN2). The mean absolute errors in the surface and top‐of‐atmosphere clear‐sky longwave fluxes for standard atmospheres are reduced to less than 1 W/m2. Mean absolute differences between the cooling rates from the original method and GENLN2 are typically 0.2 K/d. These differences are reduced by at least a factor of 3 using the updated parameterization. The updated parameterization increases the longwave cooling at 300 mbar by 0.4 to 0.7 K/d, and it decreases the cooling near 800 mbar by 0.2 to 0.6 K/d. The increased cooling is caused by line absorption and the foreign continuum in the rotation band, and the decreased cooling is caused by the self‐continuum in the rotation band. These changes in the vertical profile of longwave cooling interact strongly with the parameterization of convection. The effects on the fluxes, diabatic cooling rates, and climate state are illustrated using simulations with the new climate model.

Radiative transfer codes applied to hyperspectral data for the retrieval of surface reflectance

The present investigation evaluates surface reflectance retrieved from Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) and Compact Airborne Spectrographic Imager ( casi™) data using the atmospheric radiative transfer (RT) codes: atmospheric removal program (ATREM), Canadian advanced modified 5S (CAM5S) and moderate atmospheric radiance and transmittance model (MODTRAN4). The retrieved surface reflectances were compared with ground-based reflectances acquired with a GER3700™ spectroradiometer for a playa and canola target. The results showed that the best overall performance was achieved with MODTRAN4 (average relative error of 2.3%), followed by ATREM (3.6%) and CAM5S (4.2%). Major differences occur in the stronger gas absorption regions. At wavelengths unaffected by strong gaseous absorption, the performance was similar for the three RT codes even though ATREM and CAM5S make extensive use of analytical expressions and, therefore, have faster execution times.

All-digital impulse radio with multiuser detection for wireless cellular systems

Impulse radio is an ultrawideband system with attractive features for baseband asynchronous multiple-access, multimedia services, and tactical wireless communications. Implemented with analog components, the continuous-time impulse radio multiple-access model utilizes pulse-position modulation and random time-hopping codes to alleviate multipath effects and suppress multiuser interference. We introduce a novel continuous-time impulse radio transmitter model and deduce from it an approximate one with lower complexity. We also develop a time-division duplex access protocol along with orthogonal user codes to enable impulse radio as a radio link for wireless cellular systems. Relying on this protocol, we then derive a multiple-input/multiple-output equivalent model for full continuous-time model and a single-input/single-output model, for the approximate one. Based on these models, we finally develop design composite linear/nonlinear receivers for the downlink. The linear step eliminates multiuser interference deterministically and accounts for frequency-selective multipath while a maximum-likelihood receiver performs symbol detection. Simulations are provided to compare performance of the different receivers.

Comparison of linear forms of the radiative transfer equation with analytic Jacobians.

Determining the Jacobians of the radiative transfer equation (RTE) is important to the qualities of the simultaneous retrieval of geophysical parameters from satellite radiance observations and the assimilation of radiance data into a numerical weather prediction system. Two linear forms of the RTE with analytic Jacobians are formulated. The first linear form has approximate analytic Jacobians, which involves some monochromatic approximation applied to a fast transmittance model. Unlike previous research, which lacks the transmittance Jacobian with respect to the atmospheric temperature profile, this form is complete in the sense that the transmittance Jacobians with respect to atmospheric temperature and absorbing constituent profiles are both present. The second linear form has exact analytic Jacobians derived consistently from the same fast transmittance model without using any monochromatic approximation. By numerical comparison between the two linear forms for the NOAA-12 High-Resolution Infrared Sounder, we show significant errors in the linear form with approximate analytic Jacobians. The relative absolute linearization error from the linear form with approximate analytic Jacobians is shown to be 2-4 orders of magnitude larger than that from the linear form with exact analytic Jacobians, even for the case of a 0.1% perturbation of the U.S. Standard Atmosphere. The errors unnecessarily complicate the ill-posed retrieval problem of atmospheric remote sensing and can be avoided if the correct linear form of the RTE with exact analytic Jacobians is adopted.

Satellite remote sensing of land surface temperatures: Application of the atmospheric correction method and split-window technique to data of ARM-SGP site

The performance of algorithms for the satellite remote sensing of land surface temperatures based on an atmospheric correction method and a split-window technique was examined. For this purpose, we employed comprehensive data collected over the Atmospheric Radiation Measurement Program (ARM) Southern Great Plain (SGP) Cloud and Radiation Testbed (CART) site for a number of manually determined clear and mostly clear dates during 1997 and 1999 with cloud coverage of less than 10%. The data sources included brightness temperature data measured by the Advanced Very High Resolution Radiometer (AVHRR) on-board National Oceanic and Atmospheric Administration (NOAA)-12 and NOAA-14, collocated and coincident observed surface temperatures derived from the solar and IR radiation observing system (SIROS) and solar and IR station (SIRS) data, and balloon-borne atmospheric sounding profiles. For the application of the atmospheric correction method, we used the Moderate Resolution Transmittance Model (MODTRAN) 3.7 to compute transmittances and atmospheric radiances. In the present study, comparison of the retrieved surface temperatures with observations obtained at the central and external facility sites over the SGP area shows that the accuracy of the atmospheric correction scheme is closer to the requirement that is acceptable for estimating surface latent heat fluxes than the split-window method.

Parameterized transmittance model for direct beam and circumsolar spectral irradiance

An upgraded spectral radiation model called SMARTS2 (Simple Model of the Atmospheric Radiative Transfer of Sunshine) is introduced. The solar shortwave direct beam irradiance is calculated from spectral transmittance functions for the main extinction processes in the cloudless atmosphere: Rayleigh scattering, aerosol extinction, and absorption by ozone, uniformly mixed gases, water vapor, and nitrogen dioxide. Temperature-dependent or pressure-dependent extinction coefficients have been developed for all these absorbing gases, based on recent spectroscopic data obtained either directly from the experimental literature or, in a preprocessed form, from MODTRAN, a state-of-the-art rigorous code. The NO 2 extinction effect, in both the UV and visible, is introduced in detail for the first time in a simple spectral model by taking into account temperature-dependent absorption coefficients. Aerosol extinction is evaluated using a two-tier Ångström approach. Parameterizations of the wavelength exponents and single-scattering coefficient for different aerosol models (proposed by Shettle and Fenn, Braslau and Dave, and also in the Standard Radiation Atmosphere) are provided as a function of both wavelength and relative humidity. Moreover, aerosol turbidity can now be estimated from airport visibility data using a function based on the Shettle and Fenn aerosol model. SMARTS2 also has an optional circumsolar correction function and two filter smoothing functions which together allow the simulation of actual spectroradiometers. This facilitates comparison between modeled results and measured data. Preliminary performance assessment indicates that the direct-beam irradiance predicted by the proposed model compares well to published reference spectra obtained with rigorous radiative codes, and to measured spectroradiometric data.

Step-discontinuity approach for non-paraxial diffractive optics

Abstract An approximate method based on local field perturbations caused by abrupt transitions of the surface profile is described, which enables rapid approximate calculation of diffraction patterns of diffractive elements in the non-paraxial domain of diffractive optics. In this domain the simple complex-amplitude transmittance model based on scalar diffraction theory is no longer valid and rigorous computations are exceedingly time-consuming. Comparisons with rigorous diffraction theory show that the method is accurate for binary and multilevel profiles provided that the minimum feature size is larger than approximately one optical wavelength. The method is applied to the evaluation of the uniformity error of binary, four-level, and analogue array illuminators.

Step-discontinuity approach for non-paraxial diffractive optics

An approximate method based on local field perturbations caused by abrupt transitions of the surface profile is described, which enables rapid approximate calculation of diffraction patterns of diffractive elements in the non-paraxial domain of diffractive optics. In this domain the simple complex-amplitude transmittance model based on scalar diffraction theory is no longer valid and rigorous computations are exceedingly time-consuming. Comparisons with rigorous diffraction theory show that the method is accurate for binary and multilevel profiles provided that the minimum feature size is larger than approximately one optical wavelength. The method is applied to the evaluation of the uniformity error of binary, four-level, and analogue array illuminators.