Evaluation of a sky/cloud formula for estimating UV-B irradiance under cloudy skies

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. D24, PAGES 29,685-29,691, DECEMBER 27, 2000 Evaluation of a sky/cloud formula for estimating UV-B irradiance under cloudy skies J. Sabburg, and J. Wong Centre for Medical and Health Physics, Queensland University of Technology, Brisbane, Australia Abstract. A new sky/cloud formula is evaluated using results collected over 1 year from an integrated sky camerand irradiance measurement system. Sky properties were recorded every 6 min, simultaneously alongside solaradiation measurements at Toowoomba, Australia (27.6 øs, øe), over the solar zenith angle range of 4.2 ø to 64.3 ø. This is the first sky formula that incorporates a complement of parameters based on sky properties around the Sun. The sky properties included cloud cover, cloud brokenness, cloud brightness variation, the angle of maximum cloud cover and aureole brightness. The sky formula was developed by correlating the ratio of the measured UV-B irradiance to the modeled clear sky UV-B irradiance for the same atmospheric conditions, with each of the five sky properties listed above. Variation of the UV-B ratio for each individual sky property is examined. This sky formula, used in conjunction with a clear sky semiempirical UV-B model, was compared to three published cloud formulas based on cloud cover alone. Using a data set restricted to cases of cloud obscuring the solar disk, the new formula was found to have the highest r 2 value of Further comparison with a fourth published formula using a subset of data containing predominately cirrus cloud type, produced an explained variance of 0.91 between the measured and the modeled data. 1. Introduction Australia has recorded the highest incidence rate of skin cancer of any country in the world [National Health and Medical Research Council (NHMRC), 1996]. A number of researchers, such as Roy et al. [1996], have concluded that this high incidence rate relates to Australia's high levels of ambient ultraviolet (UV) radiation. They suggested that these levels are a result of Australia's geographic location, relatively unpolluted skies and low levels of stratospheric ozone. Comparisons between corresponding Northern and Southern Hemisphere locations have confirmed higher levels of UV in the Southern Hemisphere [e.g., Sabburg et al., 1997; Seckmeyer and McKenzie, 1992]. However, J. Sabburg et al. (UV, ozone and cloud comparison between southern and northern hemisphere, sub-tropical latitude sites during 1997, submitted to dournal of Atmospheric and Solar-Terrestrial Physics, 2000) have also shown that cloud cover plays an important role in making such comparisons. This was the case for a Northern Hemisphere site producin greater average UV levels compared to a Southern Hemisphere site with a similar latitude. The main reason was the difference in the average cloud cover between sites. Also at Centre for Astronomy and Atmospheric Research, Faculty of Sciences, University of Southern Queensland, Toowoomba 4350, Australia. Fax: Copyright 2000 by the American Geophysical Union. Paper number 2000JD /00/2000JD ,685 Sabburg [2000] reviewed the effect of cloud on UV irradiance measured at the Earth's surface. He found that a number of theoretical, empirical, and combined approaches have been used to approximate clear sky UV levels [e.g., Forster and Shine, 1995; McKenzie, 1991; Green et al., 1974]. UV models have also been developed to directly incorporate sky properties, aiming to better understand the interaction of radiation and cloud. However, theoretical models [e.g., Kylling et al., 1995; Tsay and Stamnes, 1992; Charache et al., 1994] require knowledge of physical cloud properties which are often difficult to measure from the ground, for example, cloud droplet size. In addition, broken cloud is difficult to simulate with these theoretical models. For example, Wang and Lenoble [1996] used a radiative transfer model and found that nonuniform broken cloud could not be handled using the plane-parallel geometry of their model. Empirical UV/cloud models, or sky formula, as referred to in this paper, have been obtained by measuring sky properties that can be quantifiedirectly from ground observation, either manually or automatically. Three such formulas will be presented below and referred to again in section 4. Essentially, they are cloud formulas because the only parameters included the formul are cloud cover and, in one case, the addition of cloud type. It is assumed that these models will also be valid for the Australian site chosen for the present research. In Greece, Bais et al. [1993] found that overcast skies were capable of attenuating UV in the wavelength range of 290 to 325 nm, by as much as 80%, irrespective of wavelength. They produced a sky formula in the form of the second-degree expression:

2 29,686 SABBURG AND WONG: CLOUD FORMULA FOR UV-B ESTIMATION O.06*f- O.02*f, (1) 190 nm. A broadband filter (Davis UV, Sydney, Australia) was selected to give a wavelength range of 275 to 310 nm. The area beneath the filter was "Matt black" to reduce where f is cloud cover measured in tenths of the whole sky (1 to 10). This expression represents the fraction of equivalent reradiation back to the gallium arsenide material. A diffuser clear sky UV that would be measured under a cloudy sky. (Davis UV, Sydney, Australia) was placed above the filter to Schafer et al. [1996] undertook a series of ground-based improve the sensor cosine response. The UV-B sensor was measurements of UV-B and cloud conditions in the United intercompared with a spectroradiometer (Jobin-Yvon, Paris, States. They used a wide-angle lens video system for France), around local noon on September 15, 1997 and assessment of the cloud conditions. For a solar zenith angle January 21, March 21, August 11 and September 14, (SZA) of 50 ø they reported the following sky formula: The standard lamp, used by the spectroradiometer, was itself calibrated by trained staff at the National Measurement 2 3 Laboratory in Sydney, Australia, on October 27, The *f 'f 'f, (2) accuracy of the lamp calibration was reported as less than + 3%. The UV-B sensor had an overall, absolute irradiance where, once again, f is cloud cover in tenths. accuracy of between + 15 and 22% mw/cm 2, dependent on Finally, Thiel et al. [1997] presented cloud transmission SZA, when cosine and temperature corrections were applied. data obtained from erythemal UV radiation measurement and simultaneous manual cloud observation in Germany. They The greatest error was found to occur for SZA less than 25 ø. produced the sky formula: The irradiance was an average over the 6-min period. Daily, average, total column ozone was measured at 1 -a*b 3, (3) Brisbane, approximately 100 km east of Toowoomba, using a Dobson spectrophotometer operated by staff of the Australian where a was the attenuation coefficient for each cloud type Bureau of Meteorology. The ozone levels were considered and b the fraction of whole sky cloud (0 to 1). For cloud cover constanthroughouthe day and uniform from Brisbane to in the range of 1 to 4 okta (eighths of the whole sky) the Toowoomba. The uncertainty of the ozone data was estimated attenuation was virtually independent of cloud type. At values to be within + 3% Dobson units (DU) [Sabburg, 2000]. The greater than or equal to 7 okta, a difference of 40 to 50 % was UV model, presented in section 3, used this ozone data as observed between high-altitude clouds, such as cirrus, and input. The variation of the ozone data over this time was as lower-altitude clouds, such as cumulus. Cloud type was also expected, with the highest levels occurring during spring 1997 to mid-january The maximum and minimum ozone considered by Josej son and Landelius [2000] using 10 years values were 354 and 244 DU, respectively. of Robertson-Berger (RB) meter data collected in Sweden; however, no sky formula is given. The sky camera comprised a color video camera (ISSCO, The development and evaluation of a new sky formula Sydney, Australia) with a ø field of view (FOV) lens incorporating the effect of a unique set of measured sky (Computar, Commac, United States) as well as two stepper properties based on the distribution of cloud in proximity to motors (ARC, Texas, United States). These motors controlled the solar disk is presented in this paper. These sky properties the solar tracking of the camera housing and the rotation of a include cloud cover, cloud brokenness, cloud brightness, the filter wheel assembly across the lens of the camera. Images of angle of maximum cloud cover, and aureole brightness. the sky were captured every 6-min following a UV-B Selection of these sky properties was based on the premise measurement, approximately midway between subsequent UV-B measurements. The images were automatically that cloud distributed across and around the Sun has a major effect on UV levels measured at the Earth's surface, analyzed by image-processing software to measure a number compared to cloud distributed closer to the observed horizon of sky properties that are defined below. The software was [Sabburg and Wong, 2000; Sabburg and kvong, 1999a]. developed using the building blocks of a commercially available image-processing toolbox [Thompson and Shure, 1995]. The properties included those of solar disk obstruction, 2. Instrumentation and Data cloud cover (CA), aureole brightness (AR), cloud brokenness Complete details of the instrumentation are presented by (CB), cloud brightness variation (CT), and the angle of Sabburg [2000] and Sabburg and kvong [1999b], and for maximum cloud cover (MA). The sky camera also measured convenience are also summarized as follows: An integrated other sky properties; however, these properties are not radiation and sky-camera system was located on the roof of a considered, because they were not found to increase the building at the campus of the University of Southern correlation between the final sky formula and the measured UV-B data. Queensland (USQ), Toowoomba, Australia (27.6 øs, øe, 696 m above sea level). This location had relatively A correlation technique was developed to determine the unpolluted rural skies, which was considered an important state of solar obstruction by cloud. The technique was based factor, as the concentration of aerosol content was not on comparison between the sky image to be analyzed and a measured. Simultaneous measurements of both UV-B clear sky reference image containing a clear view of the solar irradiance and sky images were taken at 6-min time intervals disk. The data set analyzed in section 3 consisted of diskfrom September 1, 1997 to August 31, obscured (DO) cases only. The accuracy of determination The radiation instrumentation was based on a was estimated to be less than %. The measurement of commercially available environmental data logger (Monitor cloud cover was based on the cloud identified by the software, Sensors, Caboolture, Australia) with associated radiation in the angularegion of between 5 ø and 37.5 ø and centered on sensors. The UV-B sensor was constructed from a gallium the disk of the Sun. Cloud cover was measured in the range of arsenide material that had a spectral response of 550 to 0 to 100% to an accuracy of to 29.9%, dependent on

3 SABBURG AND WONG' CLOUD FORMULA FOR UV-B ESTIMATION 29, I ß Cloud Cover (%) Figure 1. Graph of cloud perimeter/cloud cover, called cloud brokenness (CB), versus cloud cover in the field of view of the camera. The continuous line shows the trend of the ideal relationship assuming that the clouds were a circular shape. SZA. The greatest error was found to occur for SZA greater than 42.5 ø. Aureole brightness is the normalized brightness of the Sun. It was calculated by dividing the average brightness of the Sun and aureole regions by the average brightness of the whole image. Aureole brightness was measured in the range from one to zero (not including undefined cases, i.e., 0/0) and determined to an accuracy of less than + 5%. Figure 1 presents a graph of cloud brokenness (ratio of cloud perimeter to cloud cover) versus cloud cover. The continuous line is the ideal relationship (with an offset of- 0.3), assuming that the cloud behaved as a perfect circular shape. It can be inferred from this graph that as cloud cover decreases less than 20%, an individual cloud, or a collection of cloud perimeter, decreases faster than the ideal rate. Scatter in the data shows the variability of the shape and size of clouds that occurred in the sky over Toowoomba in 1 year. Cloud brokenness was measured in the range of zero to one (not including undefined values) and determined to an accuracy of between and 51.4%, dependent on SZA. Cloud brightness variation was measured as the standard deviation in the brightness of the cloudy pixels identified in an image. It was measured in the range of zero to one, to an accuracy of + 1%. Finally, MA was defined as the angle of maximum cloud cover. This angle subtended an arc from the center of the Sun to the outer edge of a circular sector of width 2.5 ø. This sector contained the greatest area of cloud of 10 possible concentric sectors whose angles ranged from 12.5 ø to 37.5 ø. The angle of maximum cloud cover was measured to an accuracy of + 2.3%. This measurement was made even when clouds obscured the solar disk. The accuracy quoted for these properties were determined using a detailed comparison with test and sky images of known properties [Sabburg, 2000]. The percentage uncertainty is a measure of the variance between the automated and manual observations. 3. Development of Sky Formula This section describes the development of the sky formula using the sky properties introduced in section two. These properties were found to affect UV irradiance at ground level ß and may be characterized in order of decreasing effect as follows: Angular distance from the Sun (including solar obstruction), cloud thickness (particularly opacity of clouds obscuring the Sun), cloud cover, size distribution of the cloud, and finally the uniformity of cloud brightness. In terms of these properties one can introduce parameters into a sky formula which can then be used in conjunction with any clear sky UV model. The choice of parameters was based on the physical properties measured by the sky camera and the significance of those parameters determined by statistical methods. The statistical method that was chosen correlated the UV-B ratio with each individual parameter using the analysis of variance data analysis tool of Microsoft Excel 97 SR-2. Mathematically speaking, each unique sky property was allowed to vary in turn, while the remaining properties were held in a constant range. This enabled the effect on UV- B levels of one sky property to be examined, while all other properties remained as constant as possible, allowing for a sufficient number of data points. The UV-B ratio consisted of the measured broadband irradiance divided by the corresponding clear sky modeled data (section 4). Typical uncertainty in the UV-B ratio was + 20% [Sabburg, 2000]. This ratio normalized the effects of physical propertie simulated by the clear sky model that included SZA, Sun/Earth distance, and altitude. The greatest variation between minimum and maximum UV-B irradiance was found when clouds obscured the disk of the Sun. To maximize the correlations of the UV-B ratios with each sky property, a data set (size of 11,527 data points) was formed which contained only data for DO, as well as cases when the UV-B ratio did not exceed 1.2 (allowing for measurement and modeling error). Obvious outliers were removed from the data set. These had been manually confirmed to correspond to disk not obscured cases. In other words, some data had been incorrectly identified as DO by the image-processing software. Each of the five sky properties is discussed, in turn, in the following subsections. It should be noted that the mathematical forms of the expressions presented below were based on the best fit to the data, i.e., producing the largest explained variance (r2) Aureole Brightness Figure 2a presents the graph of the UV-B ratio versus aureole brightness. The data ranges for the remaining four sky properties were, MA (37.5ø), CA (100%), CB (0.06 to 0.09), and CT (0.09 to 0.1). The ranges were selected on the availability of sufficient numbers of data points (in this case 11) to allow a statistically valid investigation of each property in turn. An explained variance of 0.90 was found between the UV-B ratio and the aureole brightness, producing the following quadratic expression: 'AR 'AR (4) It can be seen from the graph that as the brightness of the aureole region around the Sun increased so did the measured UV-B irradiance (i.e., a UV-B ratio approaching or exceeding 1). This could be explained if it is assumed that the cloud had a uniform thickness covering the aureole region of the Sun. One might expect that as the cloud thickness decreases that there would be less back scattering of both UV and visible

4 29,688 SABBURG AND WONG: CLOUD FORMULA FOR UV-B ESTIMATION 1.2 -[ O Aureole Brightness (AR) (a) MA (37.5 ø), CA (100%), CB (0.06 to 0.09), CT (0.09 to 0.!) Angle of Maximum Cloud Cover (MA) ' (b) AR (0.2 to ), CA (30 to 40%), CB (0.2 to 0.3), CT (0.1 to 0.5) o o 50 6o Cloud Cover (CA) % Cloud Brokenness (CB) (C) AR (0.295 to 0.349), MA (37.5ø), CB (0.2 to 0.3), CT (0.087 to 0.097) (d) AR (0.295 to 0.349), MA (37.5ø), CA (40 to 50%), CT (0.087 to 0.097) Variation of Cloud Brightness (CT) (e) AR (0.3 to ), MA (37.5Q), CA (100%), CB (0.06 to 0.09) Figure 2. UV-B ratio versus (a) aureole brightness (AR), (b) angle of maximum cloud cover (MA), (c) cloud cover (CA), (d) cloud brokenness (CB), and (e) variation of cloud brightness (CT) for a restricted range of sky properties (see text). The curves are the best fit to the data. radiation, thus the UV-B level and the Sun's visible brightness would increase at the ground level Angle of Maximum Cloud Cover CT (0.1 to 0.5). Again, a quadratic expression produced the highest explained variance, this time equal to 0.86:, MA 'MA (5) Figure 2b presents the correlation of the UV-B ratio with the angle of maximum cloud cover using a total of 11 data It can be seen from the graph that when maximum cloud area, in the FOV of the sky camera, is farthest away from the points. The data ranges of the remaining four sky properties Sun, a reduction in UV-B irradiance occurred. One were, AR (0.2 to ), CA (30 to 40%), CB (0.2 to 0.3), and explanation is that forward scattering from the sides of cloud

5 SABBURG AND WONG: CLOUD FORMULA FOR UV-B ESTIMATION 29,689 closer to the Sun (particularly for the angle of maximum cloud cover between 22 ø and 23ø), or refraction of direct sunlight through cirrus clouds, causes an increase in UV-B levels recorded at ground level. This angular range corresponds to that of the visible halo that can often be seen around the Sun, when cirrus cloud or haze obscures the Sun's disk Cloud Cover Figure 2c correlates the UV-B ratio with cloud cover using 14 data points, using data ranges of AR (0.295 to 0.349), MA (37.5ø), CB (0.2 to 0.3), and CT (0.087 to 0.097). A quadratic expression produced the highest explained variance of 0.76: *CA 'CA (6) It can be seen that the greater the cloud cover, the smaller the UV-B irradiance. The quadratic form of the expression would suggesthat optically thicker clouds are also associated with increasing cloud cover. However, it should be remembered that these data apply to a selected range of sky properties, including that of aureole brightness that would be associated with cloud opacity and hence cloud thickness for a uniform cloud Cloud Brokenness Figure 2d correlates the UV-B ratio with cloud brokenness using nine data points for the ranges AR (0.295 to 0.349), MA (37.5ø), CA (40 to 50%), and CT (0.087 to 0.097). A quadratic expression was produced with an explained variance equal to 0.73: as cloud brightness variation increases, the resulting UV-B level decreases. This trend would also suggest that higher UV-B levels (UV-B ratios approaching or exceeding 1) are associated with uniform cloud brightness around the Sun, indicated by smaller values of cloud brightness variation. Conversely, as the variation of visible light from a cloud increases, the UV-B level received at the ground decreases. This may be a consequence of the difference between the processes of Mie scattering for visible light and Raleigh scattering of the UV-B radiation Empirical Sky Formula Data analysis was used to develop an empirical sky formula based on the five sky properties discussed above. A number of forms of the sky formula were considered, including combinations resulting from multiple regression of the properties versus UV-B irradiance. The formula that gave the highest explained variance when compared to the measure data and was considered to be in the least complex form as possible is considered below: Expression (4 to 7) * i.3 Z Expression (4 to 8) This formula consisted of the numerical addition of expressions (4) to (7), adjusted to clear sky values by a factor of 1.3 in the numerator, and divided by the addition of expressions (4) to (8) in the denominator. This normalization, compared to the addition of the expressions only, made a considerable difference when evaluating this sky formula as presented in section 4. (9) 'CB 'CB (7) 4. Results and Discussion This data set contained the least number of data points of the five sky properties. Nevertheless, the graph clearly illustrates that the UV-B levels increased as the ratio of cloud perimeter to cloud cover, around the Sun, increased. This increase in UV-B level is due to the increased number of "gaps"(clear sky), which would exist between the larger number of clouds, corresponding to a larger cloud brokenness ratio. Conversely, for the same area of cloud and a smaller cloud brokenness ratio, fewer clouds would exist. Thus less clear sky between the clouds would result in less UV-B reaching the ground Variation of Cloud Brightness Finally, Figure 2e presents the correlation of the UV-B ratio with cloud brightness variation using 51 data points. Cloud brightness variation represents the variation of cloud brightness in the FOV of the camera. The ranges for the other properties were AR (0.3 to ), MA (37.5ø), CA (100%), and CB (0.06 to 0.09). A linear expression was produced corresponding to an explained variance of 7: 'CT (8) This correlation is the lowest of the sky properties. It can be seen from the graph that the trend of the data indicates that A semiempirical radiation model, based on that of Rundel [1986], was used to generate clear sky UV-B data corresponding to the same time period as the measured UV-B irradiance. The model incorporated average, daily ozone levels, and an aerosol optical depth of zero. It was found that the modeled data equaled that of clear sky measured data to an accuracy of less than + 5%. This model was also used, in conjunction with each of four-sky formula (expressions(1) to (3) and the new formula (9)), to estimate UV-B irradiance under cloudy skies (uv-gmodeled)'. UV-BModele d -- UV-B from clear sky model * sky formula. (1 O) Figure 3 shows a time series graph of the percentage difference between the measured UV-B irradiance and the corresponding clear sky modeled data, and the modeled data including the new sky formula (equation(10)), for the 12- month period. It can be seen that there is improved agreement between the measured and the new UV-modeled data compared to the clear sky modeled data alone. This is a direct result of including the new sky formula. On days of overcast skies the difference between the two models exceeded 100%. Cases of measured UV-B data points exceeding the corresponding modeled data are due to measurement error of the UV-B irradiance (typically_+ 20%) and also uncertainties in the clear

6 , 29,690 SABBURG AND WONG: CLOUD FORMULA FOR UV-B ESTIMATION The corresponding explained variance of the linear best fits for Thiel et al. [ 1997] and the new sky formula were 0.92 and 0.91, respectively. The variation between the results was less than + 10%. These high r 2 values indicate thathe formula of Thiel et al. [ 1997] compares favorably to the new formula for cases of high altitude, light textured cloud. However, it is apparent from Figure 4 that the Thiel et al. [1997] formula generally overestimated UV-B irradiance, whereas the new formula tended to under estimate UV-B. This underestimation is most likely a result of the development of the formula using cloud cover measured in the region centered on the Sun, in contrast to all previous empirical models that were developed 12 - Nov -97? - Feb Apr May Jul - 98 Date using whole sky cloud cover. The clear sky factor of 1.3 (formula(9)) could also have contributed to this offset. Figure 3. UV-B time series where the measured UV data are compared to the clear sky modeled data (squares) and the modeledata using the new sky formula (circles). See text for 5. Conclusions and Future Research details. The primary aim of this research was to quantify the effects of sky properties on global UV-B irradiance received at the surface of the Earth. Specifically, an empirical sky sky UV model (typically_+ 5%). A few cases would also be formula was soughthat could be used in conjunction with due to cloud enhancement of UV-B irradiance to levels existing clear sky UV-B models to correct the irradiance equivalento clear sky conditions or up to 20% above clear sky. The average UV-B ratio was values for cloudiness and cloud geometry. From an investigation of a data set containing DO cases, UV-B ratios less than 1.2 and a limited range of five sky 4.1. Evaluation of the Sky Formula properties, it was found that as the brightness of the region around the Sun increased (AR), so did the measured UV-B This section summarizes the results of a comparison of the irradiance. As cloud cover (CA) increased, UV-B irradiance correlation between measured and modeled UV-B levels decreased. UV-B levels increased as the ratio of the cloud using the four-sky formulas (1, 2, 3, and 9) in conjunction perimeter to the area of cloud around the Sun (CB) increased. with the clear sky UV model (equation(10)). The data set used for the comparison included a restrictedata range for three of the five sky properties. This was because formulas 1 and 2 were based on one property alone, that of cloud cover. When maximum cloud area, measured in 10 sectors located in the FOV of the sky camera (MA), was farthest away from the Sun, a reduction in UV-B irradiance took place. The clear sky UV model was used to compare the new sky The ranges for the fixed sky properties were MA (37.5ø), CB formula to three published formulas containing up to two (0.2 to 0.3), and CT (0.087 to 0.097). This resulted in the cloud properties. The new sky formula produced the closest ranges of AR (0.3 to 0.8) and CA (15 to 100%). The coefficient a, used in formula 3, was set to 0.72, which results to the measuredata. This was predominantly due to the inclusion of a greater number of sky properties in the corresponded to all cloud types. formula. The explained variance of the new sky formula The explained variance of the linear best fits were compared favorably with the only reported empirical formula calculated to be 5, 0.74, 0.76, and 0.82, corresponding to Bais et al. [1993], Schafer et al. [1996], Thiel et al. [1997], and the new sky formula, respectively. These values quantify 0.3 the progressive improvement of empirical sky formula over the last 7 years of published results. The new formula produced the closest results to the measured data. This was predominantly due to the selection of sky properties in the sky formula than had been previously used. 0.2 Finally, Figure 4 presents a graph comparing the new sky formula to the cloud-type-dependent formula of Thiel et al. [ 1997]. This comparison was undertaken to evaluate the new 0.16 formula with an existing formula based on one cloud-type alone. The coefficient a, used in formula (3), was set to 0, 0.12 which corresponded to cirrus cloud [Sabburg, 2000]. Again, the data set was restricted, this time with the ranges of AR (0.57 to 1.0), which corresponded to mostly cirrus clouds, UVB Model (mwicm ) MA (37.5ø), and CB (0.2 to 0.3). This resulted in the range for Figure 4. Measured versus modeled UV-B i adiance for CT of 0.04 to 0.05 and CA of 22 to 67 %. Semiautomatic cases of disk obs cted (DO) and ci s clouds. The graph analysis of the images also ensured that the clouds across the compares the cle sky model inco orating the new sky Sun were either of light texture, such as cirrus cloud, or of fo ula (diamonds, solid line) and the ci s cloud fo of haze. This cloud identification was estimated to an accuracy fo ula (3) [Thie/et a/., 1997] (circles, dashed-doted). The of less than _+ 10% [Sabburg, 2000]. lines are the linear best fit to the co esponding data points.

7 SABBURG AND WONG: CLOUD FORMULA FOR UV-B ESTIMATION 29,691 that included cloud type [Thiel et al., 1997]. The comparison solar UV-B radiation, in Stratospheric Ozone Reduction, Solar was based on cirrus cloud; however, the new formula tended Ultraviolet Radiation and Plant Life, edited byr. C. Wormst and to underestimate UV-B levels, whereas the existing formula M.M. Caldwell, Springer-Verlag, New York, tended to overestimate. Sabburg, J., Quantification of cloud around the sun and its correlation It is recommended that future research in improving the to global UV measurement, Ph.D. thesis, Queensland Univ. of sky formula should include (1) alternative propertiesuch as Technol., Brisbane, Australia, cloud altitude and the distinction of cloud layers; (2) Sabburg, J., and J. Wong, Measurement of cloud angle for enhanced investigation of disk not obscured and enhanced UV-B ratios; UV-B at the earth's surface, Photochemistry and Photobiology (3) the effects of erythemal, diffuse, and/or spectral UV Internet Conf, Jan 18 to Feb 5, 1999a. ratios; and (4) comparison of whole sky cloud properties with Sabburg, J., and J. Wong, Evaluation of a ground-based sky camera relevant Sun centered properties. system for use in surface irradiance measurement, Journal of Atmospheric and Oceanic Technology, 16, , 1999b. Acknowledgments. The authors would like to thank JimEasson Sabburg, J., and J. Wong, The effect of clouds on enhancing UV-B (Australian Bureau of Meteorology) for supplying the raw ozone irradiance at the earth's surface: aone year study, Geophys. Res. data. Both QUT and USQ Physics staff played a vital role in Lett., 27, , establishing the instrumentation used for this study. Sabburg, J., A.V. Parisi, and J.C.F. Wong, Ozone, cloud, solar and UV-B levels at a low pollution, southern hemisphere, sub-tropical References site for winter/spring 1995, Aust. Phys. Eng. Sci. Medicine, 20, , Bais, A.F., C.S. Zerefos, C. Meleti, I.C. Ziomas, and K. Tourpali, Schafer, J.S., V.K. Saxena, B.N. Wenny, W. Barnard, and J.J. De Spectral measurements of solar UV-B radiation and its relations to Luisi, Observed influence of clouds on ultraviolet-b radiation, total ozone, SO2, and clouds, J. Geophys. Res., 98, , Geophys. Res. Lett., 23, , Seckmeyer, G., and R.L. McKenzie, Elevated ultraviolet radiation in Charache, D.H., V.J. Abreu, W.R. Kuhn, and W.R. Skinner, New Zealand (45 øs) contrasted with Germany (48øN), Nature, Incorporation of multiple cloud layers for ultraviolet radiation 359, , modeling studies, J. Geophys. Res., 99, 23,031-23,039, Thiel, S., K. Steiner, and H.K. Seidlitz, Modification of global Forster, P.M.F., and K.P. Shine, A comparison of two radiation erythemally effective irradiance by clouds, Photochem. and schemes for calculating ultraviolet radiation, Q. J. R. Meteorol. Photobiol., 65, , Soc., 121, , Thompson, C.M., and L. Shure, Image-Processing TOOLBOX for Green, A.E.S., T. Sawanda, and E.P. Shettle, The middle ultraviolet Use With MATLAB, The MATH WORKS Inc., Massachusetts, reaching the ground, Photochem. Photobiol., 19, , Josefsson, W., and T. Landelius, Effect of clouds on UV radiance: As Tsay, S.-C., and K. Stamnes, Ultraviolet radiation in the Arctic: The estimated from cloud amount, cloud type, precipitation, global impact of potential ozone depletions and cloud effects, J. radiation, and sunshine duration, J. Geophys. Res., 105, Geophys. Res., 97, , , Wang, P., and J. Lenoble, Influence of clouds on UV irradiance at Kylling, A., K. Stamnes, and S.-C. Tsay, A reliable and efficientwoground level and backscattered exitance,ddv. Atmos $ci., 13, 217- stream algorithm for spherical radiative transfer: Documentation 28, of accuracy in realistic layered media,j. Atmos. Chem., 21, , McKenzie, R.L., Application of a simple model to calculate J. Sabburg, National UV Monitoring Center, Department of latitudinal and hemispheric differences in ultraviolet radiation, Physics & Astronomy, University of Georgia, Athens, GA Weather Clim., 11, 3-14, (sabburg hal.physast.uga.edu) National Health and Medical Research Council (NHMRC), Primary J. Wong, Centre for Medical and Health Physics, Queensland prevention of skin cancer in Australia, Report of the Sun University of Technology, GPO Box 2434, Brisbane 4001, Australia. Protection Programs Working Party, Pub. 2120, Aust. Govt. Publ. Fax: Serv., Canberra, Roy, C., H. Gies, and S. Toomey, Monitoring UV-B at the Earth's surface, Cancer Forum, 20, , (Received April 26, 2000; revised August 11, 2000; accepted August Rundel, R., Computation of spectral distribution and intensity of 25, 2000.)