Cloud thermodynamic phase detection with polarimetrically sensitive passive sky radiometers

Abstract

Abstract. The primary goal of this project has been to investigate if ground-based visible and near-infrared passive radiometers that have polarization sensitivity can determine the thermodynamic phase of overlying clouds, i.e., if they are comprised of liquid droplets or ice particles. While this knowledge is important by itself for our understanding of the global climate, it can also help improve cloud property retrieval algorithms that use total (unpolarized) radiance to determine cloud optical depth (COD). This is a potentially unexploited capability of some instruments in the NASA Aerosol Robotic Network (AERONET), which, if practical, could expand the products of that global instrument network at minimal additional cost.We performed simulations that found, for zenith observations, that cloud thermodynamic phase is often expressed in the sign of the Q component of the Stokes polarization vector. We chose our reference frame as the plane containing solar and observation vectors, so the sign of Q indicates the polarization direction, parallel (positive) or perpendicular (parallel) to that plane. Since the fraction of linearly polarized to total light is inversely proportional to COD, optically thin clouds are most likely to create a signal greater than instrument noise. Besides COD and instrument accuracy, other important factors for the determination of cloud thermodynamic phase are the solar and observation geometry (scattering angles between 40 and 60 • are best), and the properties of ice particles (pristine particles may have halos or other features that make them difficult to distinguish from water droplets at specific scattering angles, while extreme ice crystal aspect ratios polarize more than compact particles).We tested the conclusions of our simulations using data from polarimetrically sensitive versions of the Cimel 318 sun photometer/radiometer that compose a portion of AERONET. Most algorithms that exploit Cimel polarized observations use the degree of linear polarization (DoLP

Remote sensing of aerosols with small satellites in formation flight

“…Thus, S was chosen to be a diagonal matrix, with elements corresponding to I having an uncertainty of 3 %, and those corresponding to DoLP and uncertainty of 0.5 %. These values correspond to reasonable radiometric uncertainties for characterized orbital instruments (such as Eplee et al, 2012) and to desired polarimetric uncertainties cited for most future polarimetric instrument designs (Kokhanovsky et al, 2015).…”

“…The software performing these calculations was created at the NASA Goddard Institute for Space Studies (GISS) and has been validated against the results in de Haan et al (1987) to be within 1 % in radiance (average absolute deviation 0.03 %) and 0.08 % in DoLP (average absolute deviation 0.02 %). This software has been used for general tests of aerosol remote sensing with polarimeters (Cairns et al, 2003), incorporated into optimal estimation aerosol, cloud and land surface parameter retrieval algorithms (Knobelspiesse et al, 2011a, b;Ottaviani et al, 2012Ottaviani et al, , 2015van Diedenhoven et al, , 2014 and used for IC analyses such as this one (Knobelspiesse et al, 2012(Knobelspiesse et al, , 2015Ottaviani et al, 2013).…”

“…This is because most aerosol remote sensing is underdetermined, meaning there is less information contained in the observations than necessary to accurately extract the necessary aerosol descriptive parameters. The aerosol remote sensing community is therefore developing instruments that maximize "information content" (IC), by observing a scene at multiple wavelengths, viewing angles and polarimetric states (Kokhanovsky et al, 2015).…”

Analysis algorithm for sky type and ice halo recognition in all-sky images

“…The radiation balance of the Earth has been subject to much study and discussion (Trenberth et al, 2014;Fasullo and Kiehl, 2009;Kandel and Viollier, 2010;Trenberth et al, 2015). Global circulation models describe the influence of various parts of the Earth system in terms of radiative forcing factors (Kandel and Viollier, 2010;Kollias et al, 2007). Clouds may restrict the SW flux reaching the surface, but they also influence the LW emissions back into space.…”

Remote sensing of aerosols with small satellites in formation flight

“…The software performing these calculations was created at the NASA Goddard Institute for Space Studies (GISS), and has been validated against the results in de Haan et al (1987) to be within 1% in radiance (average absolute deviation 0.03%) and 0.08% in DoLP (average absolute deviation 0.02%). This software has been used for Ottaviani et al (2015), and used for information content analyses such as this one , Ottaviani et al (2013), Knobelspiesse et al (2015)). …”