A new multispectral cloud retrieval method for ship‐based solar transmissivity measurements

Brückner, M.; Pospichal, Bernhard; Macke, Andreas; Wendisch, Manfred

doi:10.1002/2014jd021775

Cited by 41 publications

(33 citation statements)

References 63 publications

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“…After this transition, the slope of normalized radiance in the visible varies less and depends on τ , r e , and φ, rather than on molecular scattering. Similar results are also observed by Brückner et al (2014), where instead of a slope in the mid-visible, they used a ratio of transmittance at 450 and 680 nm. The normalized spectra are modified by φ and r e , especially in the three wavelength ranges which show variations with τ .…”

Section: Variability In Normalized Radiance Spectra Due Tosupporting

confidence: 79%

A spectral method for discriminating thermodynamic phase and retrieving cloud optical thickness and effective radius using transmitted solar radiance spectra

et al. 2015

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Abstract.A new retrieval scheme for cloud optical thickness, effective radius, and thermodynamic phase was developed for ground-based measurements of cloud shortwave solar spectral transmittance. Fifteen parameters were derived to quantify spectral variations in shortwave transmittance due to absorption and scattering of liquid water and ice clouds, manifested by shifts in spectral slopes, curvatures, maxima, and minima. To retrieve cloud optical thickness and effective particle radius, a weighted least square fit that matched the modeled parameters was applied. The measurements for this analysis were made with the ground-based Solar Spectral Flux Radiometer in Boulder, Colorado, between May 2012 and January 2013. We compared the cloud optical thickness and effective radius from the new retrieval to two other retrieval methods. By using multiple spectral features, we find a closer fit (with a root mean square difference over the entire spectra of 3.1 % for a liquid water cloud and 5.9 % for an ice cloud) between measured and modeled spectra compared to two other retrieval methods which diverge by a root mean square of up to 6.4 % for a liquid water cloud and 22.5 % for an ice cloud. The new retrieval introduced here has an average uncertainty in effective radius ( ± 1.2 µm) smaller by factor of at least 2.5 than two other methods when applied to an ice cloud.

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Section: Variability In Normalized Radiance Spectra Due Tosupporting

confidence: 79%

A spectral method for discriminating thermodynamic phase and retrieving cloud optical thickness and effective radius using transmitted solar radiance spectra

et al. 2015

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“…Especially surface albedo values close to unity are uncertain due to the small differences between upward and downward irradiance. To minimize uncertainties, the retrieval algorithm was adapted to spectral ratio measurements as introduced by Werner et al (2013) and Brückner et al (2014). Using ratios of measured snow albedo at different wavelengths decreases the retrieval error as the impact of measurement uncertainties is reduced.…”

Section: Retrieval Of Optical Snow Grain Size From Spectral Albedo Mementioning

confidence: 99%

Comparison of different methods to retrieve optical-equivalent snow grain size in central Antarctica

et al. 2017

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Abstract. The optical-equivalent snow grain size affects the reflectivity of snow surfaces and, thus, the local surface energy budget in particular in polar regions. Therefore, the specific surface area (SSA), from which the optical snow grain size is derived, was observed for a 2-month period in central Antarctica (Kohnen research station) during austral summer 2013/14. The data were retrieved on the basis of ground-based spectral surface albedo measurements collected by the COmpact RAdiation measurement System (CORAS) and airborne observations with the Spectral Modular Airborne Radiation measurement sysTem (SMART). The snow grain size and pollution amount (SGSP) algorithm, originally developed to analyze spaceborne reflectance measurements by the MODerate Resolution Imaging Spectroradiometer (MODIS), was modified in order to reduce the impact of the solar zenith angle on the retrieval results and to cover measurements in overcast conditions. Spectral ratios of surface albedo at 1280 and 1100 nm wavelength were used to reduce the retrieval uncertainty. The retrieval was applied to the ground-based and airborne observations and validated against optical in situ observations of SSA utilizing an IceCube device. The SSA retrieved from CORAS observations varied between 27 and 89 m 2 kg −1 . Snowfall events caused distinct relative maxima of the SSA which were followed by a gradual decrease in SSA due to snow metamorphism and wind-induced transport of freshly fallen ice crystals. The ability of the modified algorithm to include measurements in overcast conditions improved the data coverage, in particular at times when precipitation events occurred and the SSA changed quickly. SSA retrieved from measurements with CORAS and MODIS agree with the in situ observations within the ranges given by the measurement uncertainties. However, SSA retrieved from the airborne SMART data slightly underestimated the ground-based results.

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“…For the TROPOS instrument, three custom wavelengths were chosen to optimize the information content for atmospheric retrievals and had to be realized using less durable soft-coated interference filters for cost reasons. Specifically, this applies to the channels at 750 nm as absorption-free reference for the 765 nm oxygen A-band channel, the 940 nm channel to measure the atmospheric water vapor column (Halthore et al, 1997) and the 1550 nm channel for cloud microphysics retrievals (Brückner et al, 2014). Data analysis suggests that the transmission of these soft-coated filters has changed significantly during the deployment of the instrument (Sect.…”

Section: Instrumentationmentioning

confidence: 99%

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

et al. 2017

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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.

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A new multispectral cloud retrieval method for ship‐based solar transmissivity measurements

Cited by 41 publications

References 63 publications

A spectral method for discriminating thermodynamic phase and retrieving cloud optical thickness and effective radius using transmitted solar radiance spectra

A spectral method for discriminating thermodynamic phase and retrieving cloud optical thickness and effective radius using transmitted solar radiance spectra

Comparison of different methods to retrieve optical-equivalent snow grain size in central Antarctica

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

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