Algorithms and uncertainties for the determination of multispectral irradiance components and aerosol optical depth from a shipborne rotating shadowband radiometer

Abstract

Abstract. The 19-channel rotating shadowband radiometer GUVis-3511 built by Biospherical Instruments provides automated shipborne measurements of the direct, diffuse and global spectral irradiance components without a requirement for platform stabilization. Several direct sun products, including spectral direct beam transmittance, aerosol optical depth, Ångström exponent and precipitable water, can be derived from these observations. The individual steps of the data analysis are described, and the different sources of uncertainty are discussed. The total uncertainty of the observed direct beam transmittances is estimated to be about 4 % for most channels within a 95 % confidence interval for shipborne operation. The calibration is identified as the dominating contribution to the total uncertainty. A comparison of direct beam transmittance with those obtained from a Cimel sunphotometer at a land site and a manually operated Microtops II sunphotometer on a ship is presented. Measurements deviate by less than 3 and 4 % on land and on ship, respectively, for most channels and in agreement with our previous uncertainty estimate. These numbers demonstrate that the instrument is well suited for shipborne operation, and the applied methods for motion correction work accurately. Based on spectral direct beam transmittance, aerosol optical depth can be retrieved with an uncertainty of 0.02 for all channels within a 95 % confidence interval. The different methods to account for Rayleigh scattering and gas absorption in our scheme and in the Aerosol Robotic Network processing for Cimel sunphotometers lead to minor deviations. Relying on the cross calibration of the 940 nm water vapor channel with the Cimel sunphotometer, the column amount of precipitable water can be estimated with an uncertainty of ±0.034 cm.

Measurements of spectral irradiance during the solar eclipse of 21 August 2017: reassessment of the effect of solar limb darkening and of changes in total ozone

“…These theoretical calculations also agree with the conclusion by Sharp et al (1971) that "sky light may be considered as attenuated sunlight up to at least 99.8 % obscuration." These theoretical results disagree with the results by Zerefos et al (2000Zerefos et al ( , 2001, who suggest that the erythemal (sunburning) diffuse irradiance was declining at a slower rate than the erythemal direct irradiance during the solar eclipse observed in Thessaloniki, Greece, on 11 August 1999. The largest difference was observed at the time of the eclipse maximum, when 88 % of the solar disk was obscured and the ratio of diffuse to direct irradiance was increased by approximately 50 % relative to the change expected from calculations with a one-dimensional radiative transfer model.…”

“…For comparison, the increase in the diffuseto-direct ratio calculated at 315 nm from our measurements was only 31 % and did not peak at the time of totality. The available evidence from our observations and 3-D RT model calculations suggests that a measurement artifact may have contributed to the large change in the diffuse-to-direct ratio reported by Zerefos et al (2000Zerefos et al ( , 2001.…”

“…The band obstructs a wedge of the sky with a width of 15 • when the band is vertical and 13 • when it is horizontal. The band can either be "stowed" below the instrument's diffuser for measuring global spectral irradiance or move slowly over the instrument to collect data for calculating the direct spectral irradiance from the solar beam with an algorithm developed for the BioSHADE (Morrow et al, 2010;Witthuhn et al, 2017). In brief, the algorithm first determines the spectral irradiance when the centers of diffuser, shadowband, and Sun are collinear.…”

“…Our algorithm alleviates this problem. The uncertainty of our method was estimated by Witthuhn et al (2017). The AOD can be retrieved with an uncertainty of 0.02 for all channels within a 95 % confidence interval.…”

“…Data collected during this campaign can be used for studying processes initiated in the atmosphere by the passing of the Moon's shadow and for validating three-dimensional (3-D) radiative transfer (RT) calculations that simulate the irradiance at Earth's surface during the period of totality (Emde and Mayer, 2007). Similar observations have been used during previous solar eclipses to study wavelength-dependent changes in spectral irradiance (Blumthaler et al, 2006;Kazantzidis et al, 2007), the change in the ratio of diffuse and direct irradiance over the period of the eclipse (Zerefos et al, 2000(Zerefos et al, , 2001), short-term and longer-lasting fluctuations in the total ozone column (TOC; Antón et al, 2010;Kazantzidis et al, 2007;Mateos et al, 2014;Mims and Mims, 1993;Zerefos et al, 2000Zerefos et al, , 2001Zerefos et al, , 2007, variations in the NO 2 column (Adams et al, 2010), and comparison of measurements during totality with 3-D RT model results (Kazantzidis et al, 2007). We revisit some of these issues by focusing on the wavelength dependence of solar limb darkening (LD), ozone observations, and the changing direct and diffuse radiation.…”