Abstract
Abstract. A study was undertaken to improve upon the prognosticative capability of Environment and Climate Change Canada's (ECCC) UV Index forecast model. An aspect of that work, and the topic of this communication, was to investigate the use of the four UV broadband surface irradiance fields generated by ECCC's Global Environmental Multiscale (GEM) numerical prediction model to determine the UV Index. The basis of the investigation involves the creation of a suite of routines which employ high-spectral-resolution radiative transfer code developed to calculate UV Index fields from GEM forecasts. These routines employ a modified version of the Cloud-J v7.4 radiative transfer model, which integrates GEM output to produce high-spectral-resolution surface irradiance fields. The output generated using the high-resolution radiative transfer code served to verify and calibrate GEM broadband surface irradiances under clear-sky conditions and their use in providing the UV Index. A subsequent comparison of irradiances and UV Index under cloudy conditions was also performed. Linear correlation agreement of surface irradiances from the two models for each of the two higher UV bands covering 310.70–330.0 and 330.03–400.00 nm is typically greater than 95 % for clear-sky conditions with associated root-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 be significant for UV Index determination but would not impact weather forecasting. Corresponding empirical adjustments were applied to the broadband irradiances now giving a correlation coefficient of unity. From these, a least-squares fitting was derived for the calculation of the UV Index. The resultant differences in UV indices from the high-spectral-resolution irradiances and the resultant GEM broadband irradiances are typically within 0.2–0.3 with a root-mean-square relative error in the scatter of ∼ 6.6 % for clear-sky conditions. Similar results are reproduced under cloudy conditions with light to moderate clouds, with a relative error comparable to the clear-sky counterpart; under strong attenuation due to clouds, a substantial increase in the root-mean-square relative error of up to 35 % is observed due to differing cloud radiative transfer models.
Highlights
Throughout the late 1980s and early 1990s, extensive atmospheric studies in the polar regions of the planet revealed that stratospheric ozone (O3) concentrations were being depleted due to a variety of O3-destroying catalytic cycles driven by photochemical reactions liberating chlorine (Cl) and bromine (Br) atoms from chlorofluorocarbon (CFC) and hydrofluorocarbon (HCFC) molecules emitted into the atmosphere as airborne anthropogenic pollutants (Rowland, 1996).Ozone is an important atmospheric absorber of energetic short-wavelength radiation emitted by the Sun
The comparisons made between Global Environmental Multiscale (GEM) and Cloud-J broadband irradiances for clear-sky conditions shows a fairly good agreement in the 311–330 and 330–400 nm bands
This is further supported by the significantly improved agreement demonstrated in Fig. 6 where the cross sections of the correlated-k approach cited in Table 6 of Li and Barker (2005) and the solar broadband TOA fluxes employed by the GEM model were instead applied in the Cloud-J calculations
Summary
Throughout the late 1980s and early 1990s, extensive atmospheric studies in the polar regions of the planet revealed that stratospheric ozone (O3) concentrations were being depleted due to a variety of O3-destroying catalytic cycles driven by photochemical reactions liberating chlorine (Cl) and bromine (Br) atoms from chlorofluorocarbon (CFC) and hydrofluorocarbon (HCFC) molecules emitted into the atmosphere as airborne anthropogenic pollutants (Rowland, 1996). Ozone is an important atmospheric absorber of energetic short-wavelength radiation emitted by the Sun. Most critically, O3 is the primary absorber of ultraviolet (UV) radiation, which has wide-ranging implications for the health of Published by Copernicus Publications on behalf of the European Geosciences Union. Tereszchuk et al.: Optimizing UV Index determination from broadband irradiances the biosphere: both on a molecular level with the potential of damaging the cellular DNA of individual organisms (Ravanat et al, 2001) and the destabilization of entire biogeochemical cycles within a biome (Zepp et al, 1998)
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