Date of Award

12-2001

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

School of Geography and Geology

Supervisor

J. A. Davies

Abstract

This is a study concerning the modeling of UV-B irradiance at the earth's surface. It is timely because stratospheric ozone depletion has occurred globally as a result of increasing chlorofluorocarbons in the stratosphere. This reduction allows more UV-B irradiance (290-325 nm) to reach the earth's surface and cause detrimental biological effects. Presently there are few spectral UV-B radiation measurements. Therefore, irradiance models are useful tools for estimating UV-B irradiances in areas where measurements are not made. A numerical model to calculate spectral and broadband irradiances for all sky conditions is described and the results are validated with measurements for nine Canadian stations (Alert, Resolute Bay, Churchill, Edmonton, Regina, Winnipeg, Montreal, Halifax and Toronto). The model uses either the discrete ordinate radiative transfer (DISORT) or the delta-Eddington algorithms to solve the radiative transfer equation for a 49-layer, vertically inhomogeneous, plane-parallel atmosphere, with cloud inserted between the 2 and 3 km heights. Spectral calculations are made at 1 nm intervals. The model uses extraterrestrial spectral irradiance, spectral optical properties for each atmospheric layer for ozone, air molecules, and aerosol and surface albedo. Cloud optical depths τ c were calculated separately for overcast irradiance measurements for nine stations from 26 years of data. The delta-Eddington method performed well for producing τc and overcast broadband irradiances. A fixed τc value of 18.7 was found to be accurate for calculating cloudy sky irradiances at all stations except in the arctic. Twenty-six station years of irradiance measurements and model estimates are compared. Comparisons are made both for daily totals and for monthly averaged spectral and broadband irradiances. It is shown that the delta-Eddington method is not suitable for calculating spectral irradiances under clear skies, at short wavelengths (<305 nm), where absorption by ozone is high, and at large solar zenith angles. The errors are smaller for overcast conditions. The method was found to be adequate for daily total spectral (≥305 nm) and for broadband calculations for all sky conditions, although consistently overestimating the irradiances. There is a good agreement between broadband measurements and calculations for both daily totals and monthly averages with mean bias error (MBE) mainly less than 5% of the mean measured daily irradiance and root mean square error (RMSE) less than 26%, decreasing to below 15% for monthly averages. Agreement between mean monthly measured and calculated spectral irradiances is also good for wavelengths ≥305 nm. The accuracy of the Brewer instrument is questioned at wavelengths <305 nm at most stations. Comparison of the model broadband irradiances with simultaneous satellite-based results and Brewer measurements at six stations shows that the model performs as well as the satellite model but with the advantage that it can provide irradiance estimates throughout the day and, therefore, daily totals.

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