Accuracy assessments of cloud droplet size retrievals from polarized reflectance measurements by the research scanning polarimeter

Abstract

We present an algorithm for the retrieval of cloud droplet size distribution parameters (effective radius and variance) from the Research Scanning Polarimeter (RSP) measurements. The RSP is an airborne prototype for the Aerosol Polarimetery Sensor (APS), which was on-board of the NASA Glory satellite. This instrument measures both polarized and total reflectance in 9 spectral channels with central wavelengths ranging from 410 to 2260 nm. The cloud droplet size retrievals use the polarized reflectance in the scattering angle range between 135°and 165°, where they exhibit the sharply defined structure known as the rain-or cloud-bow. The shape of the rainbow is determined mainly by the single scattering properties of cloud particles. This significantly simplifies both forward modeling and inversions, while also substantially reducing uncertainties caused by the aerosol loading and possible presence of undetected clouds nearby. In this study we present the accuracy evaluation of our algorithm based on the results of sensitivity tests performed using realistic simulated cloud radiation fields.

Impact of cloud horizontal inhomogeneity and directional sampling on the retrieval of cloud droplet size by the POLDER instrument

“…The second step is to fit the observed polarized reflectances (OPRs) with the polarized phase functions (P P ) in the LUT, which is based on the following expression of Bréon and Doutriaux-Boucher (2005): where λ is the wavelength, is the scattering angle, and the empirical fitting parameters a, b, and c represent the polarized contributions from multiple scattering, Rayleigh scattering, aerosol extinction, ground surface reflectance of thin clouds, and effects caused by rotation to the scattering plane. The parameter cos 2 ( ) is used to account for the Rayleigh scattering contributions (Alexandrov et al, 2012a). We adopt the multiple linear regression fit method of Bevington and Robinson (1969) to calculate the parameters a, b, and c for every CDR and EV combination.…”

“…The Rayleigh optical thickness for different wavelengths was set according to the results of Bodhaine et al (1999). The solar zenith angle was assumed to be 20 • , and the relative azimuth angle was 180 • (the sensor was in front of the sun), which avoids potential error induced by the rotation to the scattering plane (Hansen and Travis, 1974;Alexandrov et al, 2012a).…”

“…Note that most scattering and reflection processes generate a polarization perpendicular to the scattering plane; thus, the definition of R p used here results in positive values. This choice of sign was adopted by Bréon and Doutriaux-Boucher (2005) and is opposite to that proposed by Alexandrov et al (2012a). The normalization term in the parentheses is used so that, in the single scattering approximation, R p is proportional to the polarized scattering phase function.…”

“…It is found that the vertical structure induced by drizzle and 3-D radiative effects operate together to cause dramatic differences between the 1.6, 2.1, and 3.7 µm retrievals (Zhang et al, 2012;Zhang, 2013;Nakajima et al, 2010a, b;Nagao et al, 2013). In addition, the water vapor absorption within a cloud and the presence of an absorbing aerosol layer above a cloud leads to a positive bias in the retrieval (Alexandrov et al, 2012a;Coddington et al, 2010;Haywood et al, 2004).…”

“…Surface albedo (surface type) can then be omitted in the retrieval algorithm (Bréon and Doutriaux-Boucher, 2005). Sensitivity studies based on simulated data sets demonstrate that the polarized technique is robust against uncertainties of 3-D radiative transfer, solarviewing geometry and aerosol layers above clouds (Alexandrov et al, 2012a). The polarized technique can also be applied to multi-modal cloud size distributions by means of the Rainbow Fourier transform (Alexandrov et al, 2012b(Alexandrov et al, , 2015.…”