Browsing by Author "Wind, G."
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Item A framework for quantifying the impacts of sub-pixel reflectance variance and covariance on cloud optical thickness and effective radius retrievals based on the bispectral method(American Institute of Physics (AIP), 2017-02-22) Zhang, Z.; Werner, F.; Wind, G.; Platnick, S.; Ackerman, A. S.; Girolamo, L. Di; Marshak, A.; Meyer, Kerry; Cho, H. M.The so-called bi-spectral method retrieves cloud optical thickness (τ) and cloud droplet effective radius (rₑ) simultaneously from a pair of cloud reflectance observations, one in a visible or near infrared (VIS/NIR) band and the other in a shortwave-infrared (SWIR) band. A cloudy pixel is usually assumed to be horizontally homogeneous in the retrieval. Ignoring sub-pixel variations of cloud reflectances can lead to a significant bias in the retrieved τ and rₑ. In this study, we use the Taylor expansion of a two-variable function to understand and quantify the impacts of sub-pixel variances of VIS/NIR and SWIR cloud reflectances and their covariance on the τ and rₑ retrievals. This framework takes into account the fact that the retrievals are determined by both VIS/NIR and SWIR band observations in a mutually dependent way. In comparison with previous studies, it provides a more comprehensive understanding of how sub-pixel cloud reflectance variations impact the τ and rₑ retrievals based on the bi-spectral method. In particular, our framework provides a mathematical explanation of how the sub-pixel variation in VIS/NIR band influences the rₑ retrieval and why it can sometimes outweigh the influence of variations in the SWIR band and dominate the error in rₑ retrievals, leading to a potential contribution of positive bias to the rₑ retrieval.Item Improving cloud optical property retrievals for partly cloudy pixels using coincident higher‐resolution single band measurements: A feasibility study using ASTER observations(American Geophysical Union, 2018-10-09) Werner, F.; Zhang, Z.; Wind, G.; Miller, D.J.; Platnick, S.; Girolamo, L. DiClear‐sky contamination is a challenging and long‐lasting problem for cloud optical thickness (τ) and effective droplet radius (rₑ𝒻𝒻) retrievals using passive satellite sensors. This study explores the feasibility of improving both _ and rₑ𝒻𝒻retrievals for partly cloudy (PCL) pixels by using available subpixel samples in a visible to near‐infrared (VNIR) band, which many satellite sensors offer. Data is provided by high‐resolution reflectance (R) observations and cloud property retrievals by the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) at horizontal resolutions between 30‐960m. For partly cloudy 960‐m observations, the clear‐sky component of the pixels induces significant underestimations of up to 58% for τ, while overestimations in rₑ𝒻𝒻 can exceed 41%. This yields underestimations in the derived liquid water path and cloud droplet number concentration of up to 68% and 72%, respectively. By means of three different assumptions it is shown that subpixel R observations in the VNIR can be used to estimate higher‐resolution R for the second band in the retrieval scheme, as well as the subpixel cloud cover. The estimated values compare well to actually observed ASTER results and are used to retrieve cloud properties, which are unbiased by the clear‐sky component of PCL pixels. While the presented retrieval approach is only evaluated for marine boundary layer clouds, it is computationally efficient and can be easily applied to observations from different imagers. As an example, the PCL retrieval scheme is applied to data by the Moderate Resolution Imaging Spectroradiometer (MODIS), where similar biases for PCL pixels are observed.Item Quantifying the Impacts of Subpixel Reflectance Variability on Cloud Optical Thickness and Effective Radius Retrievals Based On High-Resolution ASTER Observations(American Geophysical Union, 2018-04-26) Werner, F.; Zhang, Z.; Wind, G.; Miller, D. J.; Platnick, S.Recently, Zhang et al. (2016, https://doi.org/10.1002/2016JD024837) presented a mathematical framework based on a second-order Taylor series expansion in order to quantify the plane-parallel homogeneous bias (PPHB) in cloud optical thickness (τ) and effective droplet radius (r ₑ𝒻𝒻) retrieved from the bispectral solar reflective method. This study provides observational validation of the aforementioned framework, using high-resolution reflectance observations from the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) over 48 marine boundary layer cloud scenes. ASTER reflectances at a horizontal resolution of 30 m are aggregated up to a scale of 1,920 m, providing retrievals of τ and r ₑ𝒻𝒻 at different spatial resolutions. A comparison between the PPHB derived from these retrievals and the predicted PPHB from the mathematical framework reveals a good agreement with correlation coefficients of r > 0.97 (for Δτ) and r > 0.79 (for Δr ₑ𝒻𝒻 ). To test the feasibility of PPHB predictions for present and future satellite missions, a scale analysis with varying horizontal resolutions of the subpixel and pixel-level observations is performed, followed by tests of corrections with only limited observational high-resolution data. It is shown that for reasonably thick clouds with a mean subpixel τ larger than 5, correlations between observed and predicted PPHB remain high, even if the number of available subpixels decreases or just a single band provides the information about subpixel reflectance variability. Only for thin clouds the predicted Δr ₑ𝒻𝒻 become less reliable, which can be attributed primarily to an increased retrieval uncertainty for r ₑ𝒻𝒻 .Item The retrieval of cloud properties based on spectral solar light diffuse transmittance measurements under optically thick cloud cover conditions(Elsevier, 2020-05-25) Kokhanovsky, A.A.; Smirnov, A.; Korkin, Sergey; Wind, G.; Slutsker, I.We propose a simple and fast cloud retrieval technique based on the spectral diffuse transmittance measurements under optically thick clouds. The technique is aimed at retrieval of cloud optical thickness, liquid water path and effective radius of particles in cloudy media. It is based on the asymptotic radiative transfer solutions valid at cloud optical thicknesses above 10. Also we use the parametrizations of the Mie theory results for local optical properties of clouds such as the single scattering albedo, extinction coefficient, and asymmetry parameter. This makes it possible to reduce the inverse problem solution to finding a root of a simple transcendent equation.