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. A study was undertaken to improve upon the prognosticative capability ofEnvironment and Climate Change Canada's (ECCC) UV Index forecast model. Anaspect of that work, and the topic of this communication, was to investigatethe use of the four UV broadband surface irradiance fields generated byECCC's Global Environmental Multiscale (GEM) numerical prediction model todetermine the UV Index. The basis of the investigation involves the creation of a suite of routineswhich employ high-spectral-resolution radiative transfer code developed tocalculate UV Index fields from GEM forecasts. These routines employ amodified version of the Cloud-J v7.4 radiative transfer model, whichintegrates GEM output to produce high-spectral-resolution surface irradiancefields. The output generated using the high-resolution radiative transfercode served to verify and calibrate GEM broadband surface irradiances underclear-sky conditions and their use in providing the UV Index. A subsequentcomparison of irradiances and UV Index under cloudy conditions was alsoperformed. Linear correlation agreement of surface irradiances from the two models foreach of the two higher UV bands covering 310.70–330.0 and 330.03–400.00 nmis typically greater than 95 % for clear-sky conditions with associatedroot-mean-square relative errors of 6.4 and 4.0 %. However,underestimations of clear-sky GEM irradiances were found on the order of ∼  30–50 % for the 294.12–310.70 nm band and by a factor of ∼  30 for the 280.11–294.12 nm band. This underestimation can besignificant for UV Index determination but would not impact weatherforecasting. Corresponding empirical adjustments were applied to thebroadband irradiances now giving a correlation coefficient of unity. Fromthese, a least-squares fitting was derived for the calculation of the UVIndex. The resultant differences in UV indices from the high-spectral-resolution irradiances and the resultant GEM broadband irradiances aretypically within 0.2–0.3 with a root-mean-square relative error in thescatter of ∼  6.6 % for clear-sky conditions. Similar results arereproduced under cloudy conditions with light to moderate clouds, with arelative error comparable to the clear-sky counterpart; under strongattenuation due to clouds, a substantial increase in the root-mean-squarerelative error of up to 35 % is observed due to differing cloud radiativetransfer models.

Optimizing UV Index determination from broadband irradiances