Bird Simple Spectral Model
The Bird Simple Spectral Model computes clear sky spectral direct beam, hemispherical diffuse, and hemispherical total irradiances on a prescribed receiver plane (tilted or horizontal) at a single point in time.
For tilted planes, the user specifies the incidence angle of the direct beam or the tilt and azimuth of the plane. The wavelength spacing is irregular, covering 122 wavelengths from 305 nm to 4000 nm. Aerosol optical depth, total precipitable water vapor, and equivalent ozone depth must be specified by the user. No variations in atmospheric constituents or structure are available. There is no separate computation of circumsolar radiation. The direct beam spectral irradiance is assumed to contain the circumsolar radiation within a 5° solid angle. No smoothing functions are provided.
The Bird Simple Spectral Model was authored by Dr. Richard Bird and Dr. Carol Riordan.
Three versions of the Bird Simple Spectral Model are available. By accessing these files, you agree to abide by the NREL data disclaimer.
Users can find input data (aerosol optical depth, precipitable water vapor, ozone) for spectral modeling from NASA's AERONET site.
The spectrl2 v2 C code is based on the NREL technical report available below and available in a compressed file that contains:
- spectrl2_2.c – The code that computes the solar spectrum
- spectrl2_2.h – The header file that must be included in the code that calls spectrl2.c
- spectest.c – A test code that exercises spectrl2.c
- ReadMe.txt – Documentation for spectrl2 (text version of on-line documentation).
To compile and run spectrl2, users will need the solar position calculator code solpos.c and header file solpos00.h. These can be downloaded from the SOLPOS page.
This spreadsheet implementation uses equations from the technical report.
For SPCTRAL2_PC, the original Bird SPCTRAL2 code listed in the appendix of the technical report was compiled using Lahey FORTRAN V4.5 to create a compressed file, which is available with auxiliary files for running the program and includes:
- SPCTRAL2_PC.f – The FORTRAN code
- SPCTRAL2_PC.exe – The compiled FOTRAN executable
- SPCTRAL2_TAPE1.txt – A text file of required input parameters
- SPCTRAL2_TAPE2.txt – A text file of extraterrestrial spectral irradiance and absorption coefficients
- SPCTRAL2_OUTPUT.txt – A text file of example output.
Simple Solar Spectral Model for Direct and Diffuse Irradiance on Horizontal and Tilted Planes at the Earth's Surface for Cloudless Atmospheres, Journal of Climate and Applied Meteorology (1986)