Spectral Radiative Model Research Articles

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.

Photochemical activity over the eastern mediterranean under variable environmental conditions

The present paper presents some of the results from the EU-funded Photochemical Activity and solar Ultraviolet Radiation (PAUR) and Photochemical Activity and solar Ultraviolet Radiation Modulation Factors (PAUR II) projects that are relevant to the photochemistry of the Eastern Mediterranean under variable conditions. Compared to S-SW flows, northern flows result in higher ozone values above the PBL. Usually during N flow the aerosol optical depth is also lower, thus resulting in higher NO2 photodissociation rates. In one case of northern flow, the Etesian synoptic regime, N-NE flows are associated with high wind speeds and result to even lower aerosol optical depth. S-SW flow during springtime in some cases brings air loaded with Saharan dust particles over the region that is poor in tropospheric ozone and precursors. Often, these cases are associated with southern stratospheric circulation, resulting in reduced total ozone amounts and hence increased ozone photolysis rates, whereas the dust load reduces nitrogen dioxide photolysis rates. Results are presented from a spectral radiation model, which simulates the spectral UV radiation during some of the above cases. Measurement results from the Athens basin and rural Aegean sites are presented, that compare the ozone levels between normal N flow conditions and Etesian flow.

Estimating atmospheric turbidity from climate data

Aerosols have several important influences on the climate system. Among the more important of these are their roles in absorbing and scattering radiation, and as condensation nuclei in cloud-forming processes. Despite their importance, knowledge of their spatial and temporal variability and, in turn, their influence on climate, is incomplete. Constraints associated with conventional approaches to measuring atmospheric turbidity – including the requirements for clear skies and costly equipment – have contributed to a paucity of turbidity data. This paper presents a methodology for estimating atmospheric turbidity from readily available surface-weather data, regardless of cloud cover. Using a high-resolution spectral radiation model, clear-sky beam irradiance is parameterized as a function of atmospheric attenuation processes, including scattering and absorption by aerosols. The model is integrated over the day to obtain an expression for estimating potential daily clear-sky beam irradiation. Turbidity can then be estimated by forcing the model with monthly averaged climate data. The methodology can be applied at any location where the requisite climate data are available and therefore holds promise for a more complete, and possibly global, climatology of aerosols.

Satellite retrieval of the absorption coefficient of chromophoric dissolved organic matter in continental margins

The absorption coefficient of chromophoric dissolved organic matter at 443 nm, aCDOM(443), has been retrieved from coastal zone color scanner (CZCS) normalized water‐leaving radiances. For a wintertime, low‐chlorophyll condition in the Middle Atlantic Bight the aCDOM(443) is computed by mathematical inversion of an oceanic spectral radiance model. The CZCS aCDOM(443) retrieval is validated by a recent ship‐aircraft aCDOM(443) retrieval experiment within the same oceanographic region.

Solar cells efficiency variations with varying atmospheric conditions

The resuls of model simulations and analyses of operating solar cell efficiencies are reported. Defined as the ratio (%) between the electrical output power of the device and the total incident radiant power, this efficiency is somewhat ambiguous as it ignores the effects of temperature, total irradiance, and the spectral distribution of the light source. Since the total intensity and spectral distribution of sunlight varies with atmospheric conditions such as cloudiness, total column ozone, turbidity, and precipitable water, the efficiency of a cell operating in a field installation may also be a function of the above factors. This paper examines variation of modeled and observed cell efficiencies with atmospheric variations. Variations in turbidity and water vapor content drive the SPCTRAL2 solar spectral radiation model of Bird and Riordan (1986). Model output, coupled with prescribed spectral response functions representing monocrystalline and amorphous solar cells, is used to model the efficiency of these cell types. The modeled efficiency rises with increasing humidity, especially for the simulated amorphous cell. Simulated efficiency decreases with increasing turbidity for both modeled cell types. The amount of increase or decrease dependents on the spectral response specified. The measured performance of different solar cells operating at the PVUSA site in Davis, California is also analyzed. The major factors found to cause variations in operational efficiency are ambient temperature and total irradiance intensity. The authors also observed increases of the apparent efficiency of amorphous cells with decreasing energy in the red portion of the solar spectrum which are consistent with model predictions.

Spectral solar radiation data base, models, and instrumentation

Abstract This paper summarizes recent results of the SERI Resource Assessment and Instrumentation Branch on a spectral solar radiation data base, models, and instrumentation. The objective of such research has been to support the photovoltaics community by helping to develop a thorough understanding of the relationships between the natural spectral solar radiation environment and the design, performance, and performance testing of photovoltaic devices. A measured spectral data base consisting of about 3000 spectra has been produced. A new instrument, called the Atmospheric Optical Calibration Systems has been developed and patented. This instrument will provide real-time data comparing outdoor atmospheric optical conditions, during PV device testing, with reference conditions. A plan has been developed for possibly improving SERI's current simple spectral solar radiation model (SPCTRAL2) by comparing it with the measured spectral data base and with a recently upgraded rigorous model (LOW-TRAN) produced by the Air Force Geophysics Laboratory. A spectro-radiometer has been under development to provide better measurements of the complete solar spectrum from 0.3 to 3.0 μm.

Quantitative laboratory spectra and spectral line parameters for the v 2 and v 4 bands of PH 3 applicable to spectral radiative models of the atmosphere of Jupiter

Quantitative laboratory PH 3 absorption spectra were obtained in the 800–1350 cm -1 region, at ∼0.05 cm -1 resolution, with gas amounts corresponding to observed PH 3 absorptions in the atmosphere of Jupiter. A compilation of spectral line positions, intensities and ground state energies has been generated for the v 2 and v 4 bands of PH 3. Line-by-line calculations have been compared with the experimental spectra.

A clear‐sky spectral solar radiation model for snow‐covered mountainous terrain

A clear‐sky spectral solar radiation model for direct and diffuse fluxes, combined with topographic calculations from digital terrain data, computes either incident, net, or reflected solar radiation at any point on a snow surface in mountainous terrain. The radiation may be integrated over any wavelength range from 250 to 5000 nm, or over any time step. Atmospheric attenuation parameters are ozone, water vapor, the Angstrom turbidity coefficient and exponent, and the absorptance to reflectance ratio of the atmospheric aerosols. The model derives these, from measurements which may contain both systematic and random errors, by finding the least squares solution to an overdetermined set of nonlinear equations. For calculations over a specified area, it employs table look‐up procedures, so that computation speed for the spectral model approaches that for a lumped model. Thus it may be useful as part of a snow surface energy budget calculation over a drainage basin.

Time and height variability of the sensible heat flux in the surface layer

The energy-budget concept is used together with a high resolution spectral radiation model to study the time and height variation of the sensible heat flux and of the eddy thermal diffusivity for an actual case up to 50 m above ground. A data set of 32 h is chosen for this study. Time-height cross-sections of the local enthalpy changes, and the radiative and the sensible heat flux divergences are presented, which show the interaction of the two energy fluxes. It can be seen, for example, that during the transient hours from night to day the sensible heat flux divergence plays an outstanding role in the warming of the atmosphere, while during the clear night radiative cooling exceeds the cooling caused by the sensible heat flux. For the layer under consideration (0-50 m) a constancy of the sensible heat flux within 10% is obtained for seven hours only, namely for the time around noon and in the early afternoon. Selected night-time profiles of the thermal diffusivity are shown, which have their maximum below 48.3 m.