Ultraviolet Sky Radiance Distributions of Translucent Overcast Skies

Theor. Appl. Climatol. 58, 129-139 (1997) Theoretical and Applied Climatorogy © Springer-Vedag 1997 Printed in Austria 1 Department of Agonomy, Purdue University, W. Lafayette, IN, U.S.A. z USDA Forest Service, Northeastern Forest Experiment Station, Syracuse, NY, U.S.A. Ultraviolet Sky Radiance Distributions of Translucent Overcast Skies R. H. Grant 1, G. M. Heisler 2, and W. Gao 1 With 6 Figures Received August 5, 1996 Revised October 21, 1996 Summary 1. Introduction The diffuse sky radiation component in the ultraviolet wavelengths is often at least 50% of the global irradiance The recorded decreases in stratospheric ozone at under clear skies, and is the dominant component of all latitudes of the earth and the measured ultraviolet global radiation under translucent overcast skies. increases in ultraviolet radiation at some loca- The distribution of sky radiance was measured in a rural tions (Madronich and DeGruijl, 1994) has area and modeled for wavelength bands of ultraviolet-B increased the interest in understanding the (UVB, 280-320 nm) and ultraviolet-A (UVA, 320-400 nm). Sky radiance measurements were made during the summer distribution in time and space of solar ultraviolet of 1993 over a wide range of solar zenith angles using (UV) radiation (Nunez et al., 1994). Ultraviolet radiance sensors mounted on a hand-operated hemisphe- radiation in the wavelength bands of 280-320 nm rical rotation mount. UVB irradiance measurements were and 320-400nm are termed UVB and UVA also made during each scan. Since the ratio of measured radiation and together typically comprise about irradiance under overcast skies and that predicted for clear 3% of the total solar radiation reaching the skies was not correlated with cloud base height, opaque cloud fraction, or solar zenith angle, it was concluded that earth's surface. The UVB portion is less than 1% the scattering from the clouds dominated the global of the total. Although the amount of radiation irradiance, and this scattering was relatively unaffected by received at the earth's surface is relatively small, the scattering off opaque clouds in the translucent atmo- the photons at these wavelengths are highly sphere. energetic and detrimental to health of plants and Analysis of the translucent overcast sky UVA and UVB radiance measurements using a semi-empirical distribution animals. As a result, understanding the amount of model showed that the spectral influences on multiple UV radiation received by plant and animal scattering, single scattering, and horizon brightening com- organisms near the earth's surface is important ponents of the distributions agreed with basic atmospheric to a wide range of disciplines: tropospheric radiation theory. The best model used solar zenith, the sky chemistry, cancer research, forestry, agriculture, zenith, and the scattering angle with resultant coefficient of ophthalmology, and oceanography. determination values of 0.62 and 0.25 for the UVA and UVB respectively. The developed equations can be applied Ultraviolet solar radiation at the earth's surface directly to the diffuse sky irradiance on the horizontal to has two primary 'streams' of incoming radition: provide radiance distributions for the sky. direct beam and diffuse sky. The diffuse sky 130 R.H. Grant et al. radiation is in part a function of the scattering completely opaque but partial cloud decks of of beam and sky radiation from the earth's cumulus and stratus. surface, in part due to scattering by molecules and aerosols in the atmosphere, and in part due 2. Methods to scattering and transmission through clouds in the atmosphere. While our understanding of Sky radiance measurements in the UVA and the direct beam UV radiation reaching the earth's UVB were made under varying sky conditions surface is generally adequate for most studies, during the summer of 1993 at the Purdue our understanding of the diffuse sky UV radia- Agronomy Research Center located in West tion is still inadequate. The understanding of Lafayette, IN, USA (40.5°N latitude). The the diffuse sky radiation is particularly important surface UVA and UVB albedo was estimated at to estimating biological impacts of increases 2 to 5% based on the agricultural land use of the in UVB radiation, since the diffuse to direct ratio area (Blumthaler and Ambach, 1988). The local in the UVB is commonly greater than one for atmosphere is considered to be a "clean clear skies as well as cloudy skies (Garrison et continental atmosphere". al., 1978; Beaglehole and Carter, 1992). The Sky radiance measurements were made using relatively great importance of the diffuse sky silicon photodiode (SED033) and vacuum silicon UVB and the presence of many UV sensitive photodiode (SED240) sensors produced by Inter- organisms in environments that are partially- national Light (IL), Inc. (Newburyport, MA). sheltered from the sky predicates the need for a The UVB radiance was measured using a sensor function to describe the distribution of the sky with interference filters while the UVA radiance radiance for variable cloud cover. Analytical was measured using a sensor with a color approximations of the UVB radiation have been absorption filter. Since the spectral irradiance of developed for clear skies that are fundamentally solar radiation increases rapidly with increasing two-stream models (Green et al., 1974; Green et wavelength through the UVB wavelength band, al., 1980; Schippnick and Green, 1984). These an overall response of the sensor and filter in two-stream models however provide the sky sunlight is a more accurate indication of the diffuse irradiance as a bulk quantity. Measure- bandwidths of the sensor/filter combinations used ments of the diffuse sky radiance distribution to measure sky radiance (Fig. 1). The distribution shows that the sky radiance is not uniformly of solar radiation in the UVA wavelength band is distributed through the sky hemisphere (Grant more evenly distributed than in the UVB, et al., 1996a, b; Brunger and Hooper, 1991; resulting in little difference between the char- Coombes and Harrison, 1988). Sky radiance distributions of cloudy skies can be derived theoretically as scattering through equivalent ~-8 1.0000- homogeneous sheets of cloud (Stamnes et al., 1991; Nack and Green, 1974) or as fractional • 0.1000- unobstructed and cloud obstructed direct beam ,m (Rosen, 1992). Clouds have similar UV scattering thicknesses as the clear sky (Spinhirn and Green, 1978) ~ 0.0010 resulting in less difference in the UV sky radi- > ,n ance with or without clouds than found in the 0.0001 broadband short wave (SW) waveband. There I ' I ' I I~ 280 320 360 400 are, to the knowledge of the authors, no sky Wavelength (nm) radiance distribution functions for radiation in UV wavelength bands under cloudy skies. This Fig. 1. Spectral response of the sensors. The relative sensor paper reports on the development of sky radiance response of the UVA and UVB sensors are indicated as convolutions of the sensor response on the modeled clear- distribution functions for overcast skies consist- sky solar irradiance for 40 ° solar zenith at sea level with ing of translucent cloud sheets of stratus, cir- 0.32 Atm-cm of ozone, and a low rural aerosol load rostratus, and stratocumulus with or without (Schippnick and Green, 1988) Ultraviolet Sky Radiance Distributions of Translucent Overcast Skies 131 acteristic response bandwidth and the actual Coombes and Harrison (1988), Steven and Uns- bandwidth experienced under solar radiation. worth (1980), and Brunger and Hooper (1991, The temperatue coefficient of the SED240 and 1993). SED033 sensors was measured in the laboratory UVB irradiance measurements were made at 0.5% and 0.1% respectively (Grant, 1996). during each sky scan using a SED240 sensor The field of view of the UVA and UVB sensors with UVB filter and quartz diffuser as in Grant was measured at 16 ° (95% cutoff)(Grant et al., (1996). The UVB irradiance sensor had identical 1996a). temperature and spectral response characteristics The sensor/filter/hood combinations for the as the UVB radiance sensor. UVB irradiance UVA and UVB wavelength bands were factory measurements were corrected for temperature calibrated in February 1993 and November 1992 and cosine response of the sensor (Grant, 1996) respectively. The UVA sensor/filter/hood re- prior to analysis. The UVB irradiance and sponse was 5.77 × 10 -4 A/W cm -2 Sr for mono- radiance measurements and radiance sensor chromatic radiance at 360 nm. The UVB sensor/ orientation were sampled by a CR7X datalogger filter/hood response was 1.39 × 10 -4 A/Wcm -2 (Campbell Scientific, Logan UT) at two second Sr for monochromatic radiance at 297 nm and a intervals. reverse bias on the detector. The radiance sensors were mounted on a 2.1 Normalization of the Radiance platform that provided for rotational and inclina- Distributions tional motion, similar to that reported in Grant et al. (1996a). Alignment of the radiance sensors Normalization of the sky radiance measurements on the platform was estimated at approximately was identical to that used in Grant et al. (1996a). 2 ° . The hemispherical distribution of the sky Each set of measured sky radiance was averaged radiance was measured by inclining and rotating by 10 ° azimuth and zenith angle increments and the platform. Measurements were made at the standard deviation from the mean for each approximately 10 ° increments in zenith from 0 "zone" was determined. The averaged sky to 80 ° and approximately 3 ° increments through radiance distributions for each zenith-angle zone the full 360 ° azimuth. The position of the of the sky (Na(l~) in units of W m -2 Sr -1) were platform in polar coordinates was determined then normalized (Nm in units of 7rSr-1) by by excitation of potentiometers attached to the dividing the values of Na(O) by the integrated rotating axes of the platform. The positional error radiance Idiff according to: for the sensors was measured to be 5 ° in both azimuth and zenith.

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