Polarimetric retrievals of surface and cirrus clouds properties in the region affected by the Deepwater Horizon oil spill

Abstract

In 2010, the Goddard Institute for Space Studies (GISS) Research Scanning Polarimeter (RSP) performed several aerial surveys over the region affected by the oil spill caused by the explosion of the Deepwater Horizon offshore platform. The instrument was deployed on the NASA Langley B200 aircraft together with the High Spectral Resolution Lidar (HSRL), which provides information on the distribution of the aerosol layers beneath the aircraft, including an accurate estimate of aerosol optical depth. This work illustrates the merits of polarization measurements in detecting variations of ocean surface properties linked to the presence of an oil slick. In particular, we make use of the degree of linear polarization in the glint region, which is severely affected by variations in the refractive index but insensitive to the waviness of the water surface. Alterations in the surface optical properties are therefore expected to directly affect the polarization response of the RSP channel at 2264 nm, where both molecular and aerosol scattering are negligible and virtually all of the observed signal is generated via Fresnel reflection at the surface. The glint profile at this wavelength is fitted with a model which can optimally estimate refractive index, wind speed and direction, together with aircraft attitude variations affecting the viewing geometry. The retrieved refractive index markedly increases over oil-contaminated waters, while the apparent wind speed is significantly lower than in adjacent uncontaminated areas, suggesting that the slick dampens high-frequency components of the ocean wave spectrum. The constraint on surface reflectance provided by the shortwave infrared channels is a cornerstone of established procedures to retrieve atmospheric aerosol microphysical parameters based on the inversion of the RSP multispectral measurements. This retrieval, which benefits from the ancillary information provided by the HSRL, was in this specific case hampered by prohibitive variability in atmospheric conditions (very inhomogeneous aerosol distribution and cloud cover). Although the results presented for the surface are essentially unaffected, we discuss the results obtained by typing algorithms in sorting the complex mix of aerosol types, and show evidence of oriented ice in cirrus clouds present in the area. In this context, polarization measurements at 1880 nm were used to infer ice habit and cirrus optical depth, which was found in the subvisual/threshold-visible regime, confirming the utility of the aforementioned RSP channel for the remote sensing of even thin cold clouds.

Aerosol classification from airborne HSRL and comparisons with the CALIPSO vertical feature mask

“…A qualitative classification of aerosol type along with the quantitative profile measurements of aerosol backscatter and extinction provided by the airborne HSRL-1 has many useful applications. HSRL-1 aerosol classification results have been used to support and interpret coincident aircraft in situ and satellite measurements made by other research groups (Molina et al, 2010;Warneke et al, 2010;Ottaviani et al, 2012;Zaveri et al, 2012;Patadia et al, 2013). In addition, the products can be used to apportion aerosol optical thickness (AOT) by type and vertical location in the column.…”

Investigation of ice particle habits to be used for ice cloud remote sensing for the GCOM-C satellite mission

“…There is increasing evidence that the ice particle model should contain some degree of surface roughness (Foot, 1988;Baran et al, 2001Baran et al, , 2003Ottaviani et al, 2012;van Diedenhoven et al, , 2013van Diedenhoven et al, , 2014Cole et al, 2013Cole et al, , 2014Holz et al, 2016). In particular, using an ensemble ice particle model, Baran and C.-Labonnote (2007) and Baran et al (2014) showed that featureless phase functions best fitted their multi-angle satellite measurements at solar wavelengths.…”

Photopolarimetric retrievals of snow properties

“…Our approach is based on the recognition that hexagonal plates and columns can be effectively used as radiative proxies for more complex crystal habits (Fu, 2007;McFarquhar, 2007, 2009;Baran, 2009;Ottaviani et al, 2012a). As demonstrated by , matching measured polarized reflectances with a look-up table of values simulated by adjusting the aspect ratios and roughness parameters of simple hexagonal particles yields an estimate of the asymmetry parameter of more complex ice crystals.…”

“…However, the limited information content of such measurements translates into uniqueness problems during the retrieval process, with the result that operational snow products are still limited to snow covered area and albedo (with the exception of the recent addition to MODIS of the dust radiative forcing product as described in Painter et al, 2012). Polarimetric remote sensing (Diner et al, 2007;Cairns et al, 2009;Dubovik et al, 2011;Hasekamp et al, 2011;Chowdhary et al, 2012;Ottaviani et al, 2012a;Knobelspiesse et al, 2012) adds to MODIS-type observations because it can provide information on the ice grain shape Ottaviani et al, 2012b) and an accurate characterization of the optical and microphysical properties of aerosols present in the scene, including possible impurities embedded in the snowpack. The benefits of passive polarimetric observations originate from both their extreme sensitivity to particle microphysics and the specifics of surface polarized reflectance.…”