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Bird Clear Sky Model | |
The Bird Clear Sky Model, authored by Richard Bird, is a broadband algorithm which produces estimates of clear sky direct beam, hemispherical diffuse, and total hemispherical solar radiation on a horizontal surface. The Excel@ Spreadsheet implementation was done by Daryl Myers. The model is based on comparisons with results from rigourous radiative transfer codes. It is composed of simple algebraic expressions with 10 user provided inputs. Model results should be expected to agree within ± 10% with rigourous radiative transfer codes. The model computes hourly average solar radiation for every hour of the year, based on the 10 user input parameters; however variable atmospheric parameters such as Aerosol Optical Depth, Ozone, Water vapor are fixed for the entire year.
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ERROR NOTE:
BEFORE 1600 GMT 05-22-2009 The equation in the spreadsheet column K for Rayleigh Transmittance is incorrect. There is a missing AIR MASS term in the numerator of the pressure correction terms: THUS THE EQUATION IN COLUMN K (for example cell K3) should be IF(J3>0,EXP(-0.0903*($A$15*J3/1013)^0.84*(1+$A$15*J3/1013-($A$15*J3/1013)^1.01)),0) And copied down the entire K column. The error has the greatest impact on direct beam at high latitudes. We have corrected the download version of the spreadsheet as of 1600 GMT on 05-22-2009. Users who previously downloaded the model can make the correction themselves; or download the corrected version below. To correct a version downloaded before 1600 GMT on 05-22-2009: UNPROTECT the sheet (Password BIRD); edit K3 by inserting the J3 product term where indicated, and copying down the column. Then Re-protect the sheet (best to use the same password, BIRD). Our apologies for any inconvenience.
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