Global Forecast Model to Predict the Daily Dose of the Solar Erythemally Effective UV Radiation

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  • Photochemistry and Photobiology, 2005, 81 : 154-1 62

    Global Forecast Model to Predict the Daily Dose of the Solar Erythemally Effective UV Radiation?

    Alois W. Schmalwieser*', Gunther Schauberger', Michal Janouch2, Manuel Nunez3, Tapani Koskela4, Daniel Berger5 and Gabriel Karamanian' 'Institute of Medical Physics and Biostatistics, University of Veterinary Medicine, Vienna, Austria 'Solar and Ozone Observatory Hradec Kralove, Czech Hydrometeorological Institute, Hradech Kralove, Czech Republic 3School of Geography and Environmental Studies, University of Tasmania, Hobart, Tasmania, Australia 4Finnish Meteorological Institute, Helsinki, Finland 5Solar Light Inc., Philadelphia, PA 'GAW Station-Ushuaia, Ushuaia, Argentina

    Received 1 December 2003; accepted 24 September 2004.

    ABSTRACT

    A worldwide forecast of the erythemally effective ultraviolet (UV) radiation is presented. The forecast was established to inform the public about the expected amount of erythemally effective UV radiation for the next day. Besides the irradiance, the daily dose is forecasted to enable people to choose the appropriate sun protection tools. Following the UV Index as the measure of global erythemally effective irradiance, the daily dose is expressed in units of UV Index hours. In this study, we have validated the model and the forecast against measurements from broadband UV radiometers of the Robertson-Berger type. The measurements were made at four continents ranging from the northern polar circle (67.4"N) to the Antarctic coast (61.1's). As additional quality criteria the frequency of underestimation was taken into account because the forecast is a tool of radiation protection and made to avoid overexposure. A value closer than one minimal erythemal dose for the most sensitive skin type 1 to the observed value was counted as hit and greater deviations as underestimation or overestimation. The Austrian forecast model underestimates the daily dose in 3.7% of all cases, whereas 1.7% results from the model and 2.0% from the assumed total ozone content. The hit rate could be found in the order of 40 % .

    INTRODUCTION The solar radiation reaching down to the earth's surface consists of only a few percentage of ultraviolet (UV) radiation. Despite being

    YPosted on the website on 28 September 2004 *To whom correspondence should be addressed: Institute of Medical

    Physics and Biostatistics, University of Veterinary Medicine, Veter- inaerplatz 1, A-1210 Vienna, Austria. Fax: 0043-1-25077-4390 e-mail: alois.schmalwieser@vu-wien.ac.at

    Abbreviations: asl, above sea level; EPTOMS, Earth Probe Total Ozone Mapping Spectrometer; MED, minimal erythemal dose; SED, standard erythemal dose; SPF, sun protection factor; TOC, total ozone content of the atmosphere; UV, ultraviolet; UVI, UV Index used as unit; UVIh, UV Index hours used as unit; WHO, World Health Organization; WMO, World Meteorological Organization.

    0 200.5 American Society for Photobiology 0031 -8655/05

    just a small fraction, the UV part is responsible for various effects on the humans. UV radiation is the main cause for many damaging effects by penetrating the skin and the eye. Sunburn, tanning, snow blindness and immune suppression are the acute and most obvious reaction to UV radiation overexposure. Cataracts and skin cancer are examples for latent damages.

    Fair-skinned populations have associated tanned skin as a sign of good health and well-being during the past decades. This has resulted in excessive exposure to both sun and artificial sources (solaria) of UV to obtain and maintain a tan over the whole year. The exposure of humans to UV radiation depends strongly on the individual behavior. Schauberger et al. (1) have shown that in the case of the Austrian population, approximately 50% of UV radiation exposure is job related, whereas 36% occurs during leisure time, 13% through outdoor vacation activities and 1 % is caused by artificial UV sources in tanning devices. The UV radiation exposure of the skin can therefore be reduced greatly in many cases through public education and information addressing essential radiation protection.

    Since 1995, International Commission on Nonionizing Radia- tion Protection, World Health Organization (WHO) and World Meteorological Organization (WMO) (2) jointly recommend the UV Index as the quantity for informing the public about the UV intensity. Following WMO (3) the UV Index should be defined as the global solar irradiance weighted by the Commission Inter- nationale de 1'Eclairage action spectrum (4) of the erythema. Thus, the UV Index is a unit of intensity, not alone the maximum value of the day. The UV Index is a dimensionless quantity normalized to unity by dividing the effective irradiance (W/m2) by 0.025 W/m2, which corresponds also to a multiplication by 40.

    For quantifying biological effects, the radiant exposure (or dose) is the relevant parameter and not the irradiance. Under the assumption of the Bunsen-Roscoe law, the caused effect is proportional to the radiant exposure. Integration of the daily course of irradiance leads to the daily dose that can be expressed in units of UV Index hours (UVIh) following a suggestion of Saxebal(5). An advantage of the daily dose is that it is not dominated by short-term changes of atmospheric conditions, e.g. attenuation by clouds, like the UV Index, and it takes into account the length of the day.

    In this article the global UV forecast model for the daily dose, expressed in units of UV Index hours (UVIh), is presented, which was developed and operationally used at the Institute of Medical

    154

  • Photochemistry and Photobiology, 2005, 81 155

    Physics and Biostatistics, University of Veterinary Medicine in Vienna, Austria. Model and forecast calculations of daily doses are validated by a comparison with measurements. These measure- ments were made at six sites on four continents ranging from 67"N to 60S, including extreme locations during periods up to 5 years. The validation is done with special attention to radiation protection and health care, using hit rate and underestimation of UV radiation as main quality criteria.

    MATERIALS AND METHODS The forecast procedure was established to inform people about the expected daily dose for the next day. From the forecasted dose the user may choose the necessary sun protection tools. Therefore, the quality of such a forecast must be validated to prove its claim: radiation protection. In the first part of this section the model is described, as well as its input parameters and output. In the second part the methodology and data used for validation are presented.

    Austrian forecast model. The core procedure of the Austrian forecast model is a fast spectral model, also called physical model with simple parameterization. The general idea traces back to a suggestion of Diffey (6): spectral measurements are parameterized to solar height and total ozone content of the atmosphere (TOC). The basing spectral measurements were made by Bener (7) over many years at the alpine observatory (46"48'N, 9"49'E, 1590 m above sea level [asl]) of Davos, Switzerland. Splines are introduced for parameterization. This procedure delivers the global spectral irradiance as the sum of the diffuse and direct component. From this the irradiance is calculated for 16 discrete wavelengths between 297.5 and 400 nm. To correspond to the altitude dependence of UV radiation, a wavelength-dependent factor gained by Blumthaler et al. (8) is applied. This factor leads to an increase in the erythemally effective irradiance of 15% per 1000 m. A further improvement was done by adding the eccentricity factor of the earth path following Sonntag (9). Also from Sonntag (9), formulas are taken for calculating the sun's zenith angle, correspondingly solar height. Cloudiness, aerosols and albedo can be handled by attenuation or enhancement factors.

    Following the definition of the UV Index the spectral irradiance E(h) from the model has to be weighted by the action spectrum of the human erythema w ( 3 ) . Integration over the spectral range and normalization by 2.5 mW/mirYdeliver the dimensionless, erythemally effective irradiance E'uv" as the UV index:

    Since the UV Index was introduced as minimum denominator for public information (2,3,10,11,12) in 1995, detailed research has already been done regarding measurements (13), models (14) and forecasts (15). The UV Index is therefore a well-determined measure of irradiance. But biological effects due to UV radiation depend on duration of exposure, the radiant exposure or dose O'uv'hl(At). The radiant exposure is calculated by the erythemally effective irradiance E[""'] and the exposure time At as

    where f, and tb are starting and ending time of exposure, determining At = tb - ta. Following this, the daily dose DruVlhl(At) is calculated by integrating the daily course of erythema1 effective irradiance from sunrise (t, = tsunrise) to sunset (tb = t,,,,,t) and expressed in units of UV Index hours (UVIh).

    Input parameters. The forecast is done for clear skies. Input parameters are time and date, geographical coordinates, elevation as1 and the TOC. Time and date determine the eccentricity factor and together with the geographical coordinates also solar height. Besides solar height the path length of irradiance through the atmosphere is determined by topography. For gridded data, we use a digital elevation model with high spatial resolution of 30 arcsec, respectively higher than 1 km. This data set is called GTOP030 and was built up by a collaborative effort led by the U.S. Geological Survey EROS Data Centre. Detailed description can be found in Gesch and Larson (16).

    Atmospheric absorption results mainly from the total ozone content. Because of showing both, spatial and temporal variability, appropriate TOC values have to be provided to ensure high accuracy of UV model

    calculations (17). As shown by Schmalwieser et al. (18), TOC measure- ments from satellites can be prepared to deliver TOC values appropriate for a forecast of the erythemally effective UV radiation. For the presented forecast, the basis is TOC data from National Aeronautics and Space Adminstration's Earth Probe Total Ozone Mapping Spectrometer (EP- TOMS) (19). Details on this simple global TOC forecast scheme can be found in Schmalwieser et al. (18). By its use, daily mean TOC values for the day of forecast, for certain sites or on a global grid were gained. The latter consists of 360 X 180 values, which correspond to a spatial resolution of 1.0" in latitude and longitude.

    Output. The Austrian forecast model generates a global forecast of the erythemally effective UV radiation as maximum irradiance around solar noon (UVI) and as daily dose (UVIh), both for clear sky. The forecast is made daily at 0O:OO h, for up to 36 h in advance. Data visualization is done by maps, showing the geographical distribution of the expected UV radiation. According to the recommended labels of the WHO (12) for the UV Index, 11 levels of intensity were used, indicated by colors between green, yellow, orange, red and purple. Values are also assessed verbally following the suggestion of the WHO. Verbal assessment and colors are adopted for dose levels. Besides the presentation by maps the forecast is also made for selected sites. This forecast is done using TOC values prepared by the TOC forecast scheme mentioned above, and for certain geographical coordinates and altitudes.

    Validation of the Austrian forecast model. The quality of the forecast model is estimated by comparisons with measurements. Deviations AD:" are defined as

    (3) mpak = D Y - DdC

    Whereas denotes the measurements of daily dose made at day i and the Dfalc the corresponding calculated value, which may result either from the model DTd or from the forecast Of".

    Further, the deviations ,fa1' of the calculated daily dose from the measurements can be divided into three groups depending on their sign: (1) hits, where -E 5 ALI;" 5 +& with E describing the preselected accuracy; (2) overestimation, where < - E; ( 3 ) underestimation, where AD;dc > + E.

    Occurring deviations can be divided into two types of errors. One error type is the pure model error myd. This error results from parameter- izations and approximations within the chosen radiation model. The other error type is the total error of the forecast AD:, which is the sum of the pure model error and the error due to uncertainties of the forecasted input parameters, in our case the forecast error of the TOC and the uncertainties related to atmospheric aerosols and surface albedo.

    The statistical description of deviations was done using quartiles (minimum [Min], first quartile [Ql], median [Med], third quartile [Q3] and maximum [Max]) and the mean deviation (MD) defined as:

    1 " MD = . ( AD?") i=1

    (4)

    whereas n is the number of measurements. Measurements for validation. The measurements for validation were

    taken from broadband UV radiometers of the Robertson-Berger type (20) at six different sites (Table 1) in four continents. The spectral sensitivity of these devices (UV-Biometer model SL501, Solar Light Inc., Philadelphia, PA [21]; hereafter, UV-Biometer) is similar to the sensitivity of the human skin for the acute effect of the erythema (13). The measurements were stored as mean values over a certain period ranging from 1 to 60 min. Their sum from sunrise to sunset delivers the daily dose. The measured values are observed in the upper panels of Figs. 1-6; a statistical description for each station is given in Table 1. These doses are used to validate the model and the forecast on all days, cloudy or not.

    The first data set was obtained from a site north of the polar circle, Sodankyla (67.367"N, 26.650"E 179 m asl), Finland, where the sun does not rise above horizon for a week in December and does not set from the end of May till the middle of July. The operational measurements were carried out there since 1993, with a temporal resolution of 1 min. In absolute scale, the measurements at the observatory are traceable to the WMO intercomparison of 1995 (22). At Sodankyla the lowest values in dose were measured. During summer the daily dose values reach up to 37 UVIh. These result from high TOC values during summer (-370 DU) and the northern location. During spring, short-term increases in TOC can be measured, forcing decreases in UV radiation.

  • 156 Alois W. Schmalwieser et a/.

    Table 1. Specifications of measuring sites and statistical description of TOC measurements (maximum [03,,], mean [03,,,] and minimum [03,,,]) and maximum measured daily dose(D,,,)*

    Altitude 03,,, 03,, 03,i, D,,, Station Latitude Longitude (masl) (DU) (DU) (DU) (UVIh)

    Sodankyla,

    Vienna,

    Philadelphia,

    Hobart,

    Ushuaia,

    Arctowski,

    Finland 67.367"N 26.650"E 179 513.5 336.8 230.8 37.0

    Austria 48.258"N 16.434"E 153 512.5 334.3 198.2 50.6

    USA 40.050"N 75.130"W 20 475.9 325.0 225.9 61.2

    Australia 42.444's 147.904"E...

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