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The Attenuation of Sunlight by High-Latitude Clouds: Spectral Dependence and Its Physical Mechanisms

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The Attenuation of Sunlight by High-Latitude Clouds: Spectral Dependence and Its Physical Mechanisms 15 DECEMBER 1997 FREDERICK AND ERLICK 2813 The Attenuation of Sunlight by High-Latitude Clouds: Spectral Dependence and Its Physical Mechanisms JOHN E. FREDERICK AND CARYNELISA ERLICK Department of the Geophysical Sciences, University of Chicago, Chicago, Illinois (Manuscript received 7 August 1996, in ®nal form 14 March 1997) ABSTRACT Measurements of the ground-level solar irradiance from Palmer Station, Antarctica, and Ushuaia, Argentina, reveal a systematic wavelength dependence in the attenuation provided by cloudy skies. As wavelength increases from 350 to 600 nm, the measured cloudy-sky irradiance, expressed as a fraction of the clear-sky value, decreases. Results from Ushuaia for a solar zenith angle of 458 show that a cloudy sky that reduces the spectral irradiance at 500 nm to 50% of that for clear skies is accompanied by irradiances at 350 and 600 nm, which are approximately 59% and 49%, respectively, of the clear sky value. A weaker wavelength dependence appears in the data for Palmer Station. The observed behavior can arise from Rayleigh backscattering of sunlight beneath the cloud, followed by re¯ection of this upwelling radiation from the cloud base back to the ground. This sequence of events is most effective at short wavelengths and leads to cloudy skies providing less overall attenuation as wavelength decreases. 1. Introduction the ultraviolet than in total wavelength-integrated sun- This paper addresses a question related to the atten- light (Frederick and Steele 1995). The authors suggest uation of sunlight by cloudy skies. Speci®cally, does a that the discrepancy between these results could arise cloudy sky attenuate incoming sunlight by the same from the very different sensitivities of the Robertson± percentage at all wavelengths, or is there a spectral de- Berger meter and Eppley ultraviolet sensor to ozone pendence in the attenuation? For example, if the solar located in the lower atmosphere. irradiance observed at a wavelength of 500 nm under In this work we analyze ground-level solar irradiances a cloudy sky is reduced to 50% of the value expected at selected wavelengths obtained by spectroradiometers for clear skies, is the irradiance at other wavelengths located at Palmer Station, Antarctica, latitude 64.88S, reduced by the same, or by a different, fraction? The and Ushuaia, Argentina, at latitude 55.08S. Each instru- spectral region considered extends from 350 nm in the ment is a Model SUV-100 scanning spectroradiometer near ultraviolet to 600 nm in the visible. provided by Biospherical Instruments, Inc. Once per Previously published results indicate that clouds pro- hour the sensors scan the solar spectrum from the ul- vide somewhat less attenuation in the ultraviolet than traviolet, shortward of 290 nm, into the visible and rec- in the visible. This comes from a comparison of data ord the sum of direct and diffuse irradiance striking a obtained by a total sunlight pyranometer and a Robert- horizontal surface. Lubin et al. (1992) have described son±Berger ultraviolet meter (Blumthaler and Ambach the instrumentation in more detail, so no further dis- 1988). These datasets were obtained in the Austrian cussion is required here. The measurement sites are, for Alps, at a site removed from sources of pollution. Using practical purposes, free of air pollution, allowing us to data from a large urban area, Frederick et al. (1993) focus on the effects of clouds alone. These data have found that cloudy skies provided essentially the same an advantage over previous work in that they refer to attenuation of irradiance as measured by a Robertson± speci®c wavelengths rather than being broadband mea- Berger meter and an Eppley pyranometer. However, sub- surements. We can therefore isolate any wavelength de- sequent data from an Eppley ultraviolet monitor, whose pendence in the attenuation associated with cloudy spectral response is shifted to longer wavelengths than skies. the Robertson±Berger meter, implied less attenuation in 2. The characterization of clouds We characterize the in¯uence of clouds on ground- Corresponding author address: Dr. John E. Frederick, Department level spectral irradiance E(l) at wavelength l by the of the Geophysical Sciences, University of Chicago, 5734 South Ellis ``attenuation factor'' T(l), de®ned by Avenue, Chicago, IL 60637. E-mail: [email protected] E(l) 5 T(l)ECLR(l), (1) q 1997 American Meteorological Society Unauthenticated | Downloaded 09/28/21 05:21 PM UTC 2814 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 54 TABLE 1. Regression coef®cients derived from irradiance mea- where ECLR(l) is the ground-level irradiance, including direct and diffuse components, that would have pre- surements at Palmer Station; t is the ratio of the standard error in derived coef®cient to the best estimate of that coef®cient. vailed under clear skies. All quantities in Eq. (1) are functions of the solar zenith angle (SZA). As de®ned l (nm) a0(l) t(a0) a1(l) t(a1) above, T(l) depends on the properties of the prevailing Solar zenith angle 5 408±508 clouds, the atmosphere in which the clouds reside, and 350 4.98 3 1021 203.4 21.20 3 1022 5.3 the albedo of the lower boundary. Physically, the at- 400 8.37 3 1021 317.4 25.51 3 1023 2.3 tenuation factor is simply the measured ground-level 600 7.82 3 1021 319.8 1.12 3 1022 5.0 irradiance expressed as a fraction of the clear-sky ir- Solar zenith angle 5 608±708 radiance. The analysis seeks to determine if a detectable 350 5.02 3 1021 199.5 23.16 3 1022 6.8 wavelength dependence exists in T(l). 400 8.41 3 1021 330.2 23.19 3 1022 6.8 1 2 The irradiance E(l) is measured by the spectroradi- 600 7.53 3 102 386.2 3.70 3 102 10.2 ometers, but the corresponding clear-sky irradiance, E (l), is not in general known. One approach would CLR 3. Measurements and statistical analysis be to compute ECLR(l) using a radiative transfer code, but this could introduce model-dependent uncertainties We sorted irradiances measured from Palmer Sta- into the analysis. Instead we develop an approach to tion and Ushuaia during September through Decem- detecting a wavelength dependence in T(l), which is ber 1990 into bins of SZA 108 wide. The smallest free of model-based assumptions. We select a reference range of SZA considered is 408±508, corresponding wavelength l* and consider the ratio to times close to local noon near summer solstice for Palmer Station. The various panels refer to the SZA [T(l)E (l)] E(l)/E(l*) 5 CLR . (2) ranges 408±508 and 608±708 for wavelengths of 350 [T(l*)ECLR(l*)] nm, 400 nm, and 600 nm. The regression model of Eq. (4) was ®tted to the data, and the resulting co- The ratio E (l)/E (l*) on the right-hand side of Eq. CLR CLR ef®cients appear in Table 1. The slopes derived for (2) is constant for a ®xed SZA, and when the sky is each panel in Fig. 1 are signi®cantly different from clear it is obvious that T(l)/T(l*) 5 1. If cloudy skies 0.0, as indicated by the t statistic associated with each provide the same attenuation at both wavelengths l and value of a exceeding a value of 2.0. l*, then we should also ®nd that T(l)/T(l*) 5 1 for 1 Although there is considerable scatter in the mea- all cloudy conditions. However, if a wavelength depen- surements, Fig. 1 and Table 1 show that the irradiance dence exists in the attenuation associated with cloudy ratios are dependent on the degree of cloudiness, in- skies, then the ratio T(l)/T(l*) should depart from unity dicating a wavelength dependence in the attenuation as the degree of cloudiness increases. factors. The irradiance ratios for 350 and 400 nm de- For a ®xed SZA, the absolute irradiance at l* is itself cline slightly as E(500 nm) increases, while the ratio a valid index of the degree of cloudiness. The simplest for 600 nm has the opposite behavior. This demon- relationship one could adopt in seeking a wavelength strates that cloudy skies over Palmer Station provide dependence associated with cloudiness is then less attenuation as wavelength decreases. Further- T(l)/T(l*) 5 c 0(l) 1 c1(l)E(l*), (3) more, as the clouds become thicker, cover a greater fraction of the sky, or preferentially block the direct where the wavelength-independent case corresponds to solar beam, as indicated by a shrinking value of E(500 c0(l) 5 1 and c1(l) 5 0. The combination of Eqs. (2) nm), the difference in attenuations experienced at two and (3) plus that ECLR(l)/ECLR(l*) is a constant for ®xed wavelengths increases. SZA yields Figure 2 and Table 2 present the measurements and E(l)/E(l*) 5 a (l) 1 a (l)E(l*), (4) derived regression coef®cients for Ushuaia. The pat- 0 1 tern is qualitatively similar to that for Palmer Station, where the regression coef®cients a0(l) and a1(l) are to and all of the slopes, a1 , are signi®cantly different be determined by a least squares ®t of measured irra- from zero. However, a comparison of the plots and diances to Eq. (4). We use the three wavelengths l 5 coef®cients for the two locations reveals a stronger 350, 400, and 600 nm with the reference wavelength wavelength dependence at Ushuaia than at Palmer l* 5 500 nm. A wavelength dependence in T(l) will Station.
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