Improved information about the vertical location and extent of monolayer clouds from POLDER3 measurements in the oxygen A-band

This paper describes new advances in the exploitation of oxygen A-band measurements from POLDER3 sensor onboard PARASOL, satellite platform within the A-Train. These developments result from not only an account of the dependence of POLDER oxygen parameters to cloud optical thickness τ and to the scene's geometrical conditions but also, and more importantly, from the finer understanding of the sensitivity of these parameters to cloud vertical extent. This sensitivity is made possible thanks to the multidirectional character of POLDER measurements. In the case of monolayer clouds that represent most of cloudy conditions, new oxygen parameters are obtained and calibrated from POLDER3 data colocalized with the measurements of the two active sensors of the A-Train: CALIOP/CALIPSO and CPR/CloudSat. From a parameterization that is (μs, τ) dependent, with μs the cosine of the solar zenith angle, a cloud top oxygen pressure (CTOP) and a cloud middle oxygen pressure (CMOP) are obtained, which are estimates of actual cloud top and middle pressures (CTP and CMP). Performances of CTOP and CMOP are presented by class of clouds following the ISCCP classification. In 2008, the coefficient of the correlation between CMOP and CMP is 0.81 for cirrostratus, 0.79 for stratocumulus, 0.75 for deep convective clouds. The coefficient of the correlation between CTOP and CTP is 0.75, 0.73, and 0.79 for the same cloud types. The score obtained by CTOP, defined as the confidence in the retrieval for a particular range of inferred value and for a given error, is higher than the one of MODIS CTP estimate. Scores of CTOP are the highest for bin value of CTP superior in numbers. For liquid (ice) clouds and an error of 30 hPa (50 hPa), the score of CTOP reaches 50% (70%). From the difference between CTOP and CMOP, a first estimate of the cloud vertical extent h is possible. A second estimate of h comes from the correlation between the angular standard deviation of POLDER oxygen pressure σPO2 and the cloud vertical extent. This correlation is studied in detail in the case of liquid clouds. It is shown to be spatially and temporally robust, except for clouds above land during winter months. The analysis of the correlation's dependence on the scene's characteristics leads to a parameterization providing h from σPO2. For liquid water clouds above ocean in 2008, the mean difference between the actual cloud vertical extent and the one retrieved from σPO2 (from the pressure difference) is 5 m (−12 m). The standard deviation of the mean difference is close to 1000 m for the two methods. POLDER estimates of the cloud geometrical thickness obtain a global score of 50% confidence for a relative error of 20% (40%) of the estimate for ice (liquid) clouds over ocean. These results need to be validated outside of the CALIPSO/CloudSat track

Three-way comparison between OMI and PARASOL cloud pressure products

The cloud pressures determined by three different algorithms, operating on reflectances measured by two spaceborne instruments in the "A" train, are compared with each other. The retrieval algorithms are based on absorption in the oxygen A-band near 765 nm, absorption by a collision induced absorption in oxygen near 477 nm, and the filling in of Fraunhofer lines by rotational Raman scattering near 350 nm. A Lambertian reflector as cloud model is assumed in the retrievals. The first algorithm operates on data collected by the POLDER instrument on board PARASOL, while the latter two operate on data from the OMI instrument on board EOS-Aura. The satellites sample the same air mass within about 15 min. We compare the retrieval algorithms using synthetic spectra to give the comparison realistic baseline expectations. It appears that these cloud pressures are not the pressure of the cloud top, but of a level inside the cloud. This is corroborated by comparisons with MODIS and CloudSat data: while the top of the cloud is seen by MODIS using emitted IR radiation, both OMI and PARASOL algorithms retrieve a pressure near the midlevel of the cloud. The three cloud pressure products are compared using 1 month of data. The cloud pressures are found to show a similar behavior, with correlation coefficients larger than 0.85 between the data sets for high effective cloud fractions. The average differences in the cloud pressure are small, between 2 and 45 hPa, for the whole data set, with an RMS difference of 65 to 93 hPa. This falls within the science requirement for the OMI cloud pressure to have an accuracy of 100 hPa. For small to medium effective cloud fractions, the cloud pressure distribution found by PARASOL is very similar to that found by OMI using the O2–O2 absorption. Somewhat larger differences are found for very high effective cloud fractions

Cloud heterogeneity on cloud and aerosol above cloud properties retrieved from simulated total and polarized reflectances

International audienceSimulations of total and polarized cloud re-flectance angular signatures such as the ones measured by the multi-angular and polarized radiometer POLDER3/PARASOL are used to evaluate cloud hetero-geneity effects on cloud parameter retrievals. Effects on optical thickness, albedo, effective radius and variance of the cloud droplet size distribution and aerosol parameters above cloud are analyzed. Three different clouds that have the same mean optical thicknesses were generated: the first with a flat top, the second with a bumpy top and the last with a fractional cloud cover. At small scale (50 m), for oblique solar incidence , the illumination effects lead to higher total but also polarized reflectances. The polarized reflectances even reach values that cannot be predicted by the 1-D homogeneous cloud assumption. At the POLDER scale (7 km × 7 km), the angular signature is modified by a combination of the plane-parallel bias and the shadowing and illumination effects. In order to quantify effects of cloud heterogeneity on operational products, we ran the POLDER operational algorithms on the simulated reflectances to retrieve the cloud optical thickness and albedo. Results show that the cloud optical thickness is greatly affected: biases can reach up to −70, −50 or +40 % for backward, nadir and forward viewing directions , respectively. Concerning the albedo of the cloudy scenes, the errors are smaller, between −4.7 % for solar incidence angle of 20° and up to about +8 % for solar incidence angle of 60°. We also tested the heterogeneity effects on new algorithms that allow retrieving cloud droplet size distribution and cloud top pressures and also aerosol above clouds. Contrary to the bi-spectral method, the retrieved cloud droplet size parameters are not significantly affected by the cloud heterogeneity, which proves to be a great advantage of using polarized measurements. However, the cloud top pressure obtained from molecular scattering in the forward direction can be biased up to about 60 hPa (around 550 m). Concerning the aerosol optical thickness (AOT) above cloud, the results are different depending on the available angular information. Above the fractional cloud, when only side scattering angles between 100 and 130° are available, the AOT is underestimated because of the plane-parallel bias. However, for solar zenith angle of 60° it is overestimated because the polarized reflectances are increased in forward directions

Retrieval of aerosol microphysical and optical properties above liquid clouds from POLDER/PARASOL polarization measurements

Most of the current aerosol retrievals from passive sensors are restricted to cloud-free scenes, which strongly reduces our ability to monitor the aerosol properties at a global scale and to estimate their radiative forcing. The presence of aerosol above clouds (AAC) affects the polarized light reflected by the cloud layer, as shown by the spaceborne measurements provided by the POlarization and Directionality of Earth Reflectances (POLDER) instrument on the PARASOL satellite. In a previous work, a first retrieval method was developed for AAC scenes and evaluated for biomass-burning aerosols transported over stratocumulus clouds. The method was restricted to the use of observations acquired at forward scattering angles (90–120°) where polarized measurements are highly sensitive to fine-mode particle scattering. Non-spherical particles in the coarse mode, such as mineral dust particles, do not much polarize light and cannot be handled with this method. In this paper, we present new developments that allow retrieving also the properties of mineral dust particles above clouds. These particles do not much polarize light but strongly reduce the polarized cloud bow generated by the liquid cloud layer beneath and observed for scattering angles around 140°. The spectral attenuation can be used to qualitatively identify the nature of the particles (i.e. accumulation mode versus coarse mode, i.e. mineral dust particles versus biomass-burning aerosols), whereas the magnitude of the attenuation is related to the optical thickness of the aerosol layer. We also use the polarized measurements acquired in the cloud bow to improve the retrieval of both the biomass-burning aerosol properties and the cloud microphysical properties. We provide accurate polarized radiance calculations for AAC scenes and evaluate the contribution of the POLDER polarization measurements for the simultaneous retrieval of the aerosol and cloud properties. We investigate various scenes with mineral dust particles and biomass-burning aerosols above clouds. For clouds, our results confirm that the droplet size distribution is narrow in high-latitude ocean regions and that the droplet effective radii retrieved from both polarization measurements and from total radiance measurements are generally close for AAC scenes (departures smaller than 2 μm). We found that the magnitude of the primary cloud bow cannot be accurately estimated with a plane parallel transfer radiative code. The errors for the modeling of the polarized cloud bow are between 4 and 8% for homogenous cloudy scenes, as shown by a 3-D radiative transfer code. These effects only weakly impact the retrieval of the Aerosol Optical Thickness (AOT) performed with a mineral dust particle model for which the microphysical properties are entirely known (relative error smaller than 6%). We show that the POLDER polarization measurements allow retrieving the AOT, the fine-mode particle size, the Ångström exponent and the fraction of spherical particles. However, the complex refractive index and the coarse-mode particle size cannot be accurately retrieved with the present polarization measurements. Our complete and accurate algorithm cannot be applied to process large amounts of data, so a simpler algorithm was developed to retrieve the AOT and the Ångström exponent above clouds in an operational way. Illustrations are provided for July–August 2008 near the African coast. Large mean AOTs above clouds at 0.865 μm (>0.3) are retrieved for oceanic regions near the coasts of South Africa that correspond to biomass-burning aerosols, whereas even larger mean AOTs above clouds for mineral dust particles (>0.6) are also retrieved near the coasts of Senegal. For these regions and time period, the direct AAC radiative forcing is likely to be significant. The final aim of this work is the global monitoring of the AAC properties and the estimation of the direct aerosol radiative forcing in cloudy scenes

Review of capabilities of multi-angle and polarization cloud measurements from POLDER

International audiencePolarization and directionality of the Earth's reflectances (POLDER) is a multispectral imaging radiometer-polarimeter with a wide field-of-view, a moderate spatial resolution, and a multi-angle viewing capability. It functioned nominally aboard ADEOS1 from November 1996 to June 1997. When the satellite passes over a target, POLDER allows to observe it under up to 14 different viewing directions and in several narrow spectral bands of the visible and near-infrared spectrum (443-910 nm). This new type of multi-angle instruments offers new opportunity for deriving cloud parameters at global scale. The aim of this short overview paper is to point out the main contributions of such an instrument for cloud study through its original instrumental capabilities (multidirectionality, multipolarization, and multispectrality). This is mainly illustrated by using ADEOS 1-POLDER derived cloud parameters which are operationally processed by CNES and are available since the beginning of 1999

Scientific results from the Polarization and Directionality of the Earth's Reflectances (POLDER)

International audienceThe POLDER (POlarization and Directionality of the Earth's Reflectances) instrument, developed by the French Space Agency (CNES) has flown on board the ADEOS-1/NASDA platform from November 1996 until June 1997. The sensor has a wide field of view (2400km swath) for collecting global daily data and has multi-angle viewing capability. It measures the solar radiation reflected by the Earth in eight spectral bands. For three of these bands (0.443, 0.670 and 0.865 μm), measurements include the polarization ratio by the use of 3 polarizers. This measurement strategy provides unique information on aerosols, clouds and surfaces