F. Waquet

Analysis of the spectral and angular response of the vegetated surface polarization for the purpose of aerosol remote sensing over land

A precise estimate of the polarization induced by the surface in reflected radiation is crucial for remote sensing applications dedicated to monitoring the atmosphere. Here we present airborne observations acquired during a field campaign in the North of France over vegetated surfaces. Polarized reflectances were measured in four spectral bands in the range between 0.67 and 2.2 μm and for scattering angles between 75° and 145°. Our results confirm that the polarization generated by the reflection of vegetated surfaces can be understood as being primarily a specular reflection process. It is not possible from our measurements to see any spectral dependence of the surface polarization in the given spectral channels. The surface polarization is well fitted by existing surface models which have two degrees of freedom that allow the magnitude and angular behavior of the surface-polarized reflectance to be adjusted.

How Do A-train Sensors Intercompare in the Retrieval of Above-cloud Aerosol Optical Depth? A Case Study-based Assessment

We intercompare the above-cloud aerosol optical depth (ACAOD) of biomass burning plumes retrieved from A-train sensors, i.e., Moderate Resolution Imaging Spectroradiometer (MODIS), Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), Polarization and Directionality of Earth Reflectances (POLDER), and Ozone Monitoring Instrument (OMI). These sensors have shown independent capabilities to retrieve aerosol loading above marine boundary layer clouds—a kind of situation often found over the southeast Atlantic Ocean during dry burning season. A systematic comparison reveals that all passive sensors and CALIOP-based research methods derive comparable ACAOD with differences mostly within 0.2 over homogeneous cloud fields. The 532 nm ACAOD retrieved by CALIOP operational algorithm is underestimated. The retrieved 1064 nm AOD however shows closer agreement with passive sensors. Given the different types of measurements processed with different algorithms, the reported close agreement between them is encouraging. Due to unavailability of direct measurements above cloud, the validation of satellite-based ACAOD remains an open challenge. The intersatellite comparison however can be useful for the relative evaluation and consistency check.

Comparison of aerosol optical properties above clouds between POLDER and AeroCom models over the South East Atlantic Ocean during the fire season

Aerosol properties above clouds have been retrieved over the South East Atlantic Ocean during the fire season 2006 using satellite observations from POLDER (Polarization and Directionality of Earth Reflectances). From June to October, POLDER has observed a mean Above-Cloud Aerosol Optical Thickness (ACAOT) of 0.28 and a mean Above-Clouds Single Scattering Albedo (ACSSA) of 0.87 at 550 nm. These results have been used to evaluate the simulation of aerosols above clouds in five Aerosol Comparisons between Observations and Models (Goddard Chemistry Aerosol Radiation and Transport (GOCART), Hadley Centre Global Environmental Model 3 (HadGEM3), European Centre Hamburg Model 5-Hamburg Aerosol Module 2 (ECHAM5-HAM2), Oslo-Chemical Transport Model 2 (OsloCTM2), and Spectral Radiation-Transport Model for Aerosol Species (SPRINTARS)). Most models do not reproduce the observed large aerosol load episodes. The comparison highlights the importance of the injection height and the vertical transport parameterizations to simulate the large ACAOT observed by POLDER. Furthermore, POLDER ACSSA is best reproduced by models with a high imaginary part of black carbon refractive index, in accordance with recent recommendations.

Global detection of absorbing aerosols over the ocean in the red and near infrared spectral region

The spatial and temporal variability of the aerosol Single Scattering Albedo (SSA at 865 nm) has been estimated over clear-sky ocean for 2006 by using measurements acquired by POLDER (Polarization and Directionality of Earth Reflectances). Our estimates are correlated with sun-photometer retrievals (R = 0.63). Differences in SSA are generally around 0.05 and systematically fall below 0.055 for optical thicknesses ≥ 0.3 (at 865 nm) and modeling errors ≤ 3.0 %. Fine absorbing aerosols (radius ≤ 0.16 μm) are detected in many coastal regions. The lowest SSAs are retrieved over the southeast Atlantic during summer (0.80) whereas non-absorbing fine particles (≥ 0.98) are observed over the North Pacific. During winter, fine absorbing aerosols are detected together with mineral dust near the coasts of western Africa (0.90), over the tropical Atlantic (0.88) and around India (0.88). Long–range transport of absorbing species is also detected, as for instance over the Arctic. This study could help to constrain aerosol absorption and radiative forcing in models.

Remote Sensing of Cloud and Aerosol over Cloud from Multi-Viewing Polarized Measurements

We are revisiting the use of multi-viewing polarized reflectances to retrieve atmospheric component properties. We will focus first on cloud microphysical retrieval, then will demonstrate the potentiality of such measurements to detect aerosol above clouds.