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 front the experimental literature or, in a preprocessed form, from MODTRAN, a state-of-the-art rigorous code. The NO, 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 Angstrom 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. (C) 2001 Elsevier Science Ltd. All rights reserved.