C. Cornet

Evaluation of cloud heterogeneity effects on total and polarized visible radiances as measured by POLDER/PARASOL and consequences for retrieved cloud properties

Several sensors are dedicated to cloud observations. Among them, POLDER/PARASOL measures total and polarized visible radiances in up to 16 directions. As for other sensors, the POLDER cloud retrieval algorithm is based on the assumption that clouds are plan-parallel, homogeneous and infinite. To assess the cloud heterogeneities impacts on POLDER radiances and thus on retrieved cloud parameters, we developed a tri-dimensional radiative transfer model called 3DMCPOL allowing the computation of total and polarized radiances of 3D cloud fields. The input cloud properties are simulated with a model called 3Dcloud, which is based on a simplified dynamic/thermodynamic scheme to get cloud characteristic shape coupled with a Fourier stochastic approach to enforce cloud scale invariance. The POLDER algorithm is next applied to the simulated radiances to assess the 3D errors on the retrieved cloud parameters.

Cirrus Heterogeneity Effects on Cloud Optical Properties Retrieved with an Optimal Estimation Method from MODIS VIS to TIR Channels.

This study presents preliminary results on the effect of cirrus heterogeneities on top-of-atmosphere (TOA) simulated radiances or reflectances for MODIS channels centered at 0.86, 2.21, 8.56, 11.01 and 12.03 µm, and on cloud optical properties retrieved with a research-level optimal estimation method (OEM). Synthetic cirrus cloud fields are generated using a 3D cloud generator (3DCLOUD) and radiances/reflectances are simulated using a 3D radiative transfer code (3DMCPOL). We find significant differences between the heterogeneity effects on either visible and near-infrared (VNIR) or thermal infrared (TIR) radiances. However, when both wavelength ranges are combined, heterogeneity effects are dominated by the VNIR horizontal radiative transport effect. As a result, small optical thicknesses are overestimated and large ones are underestimated. Retrieved effective diameter are found to be slightly affected, contrarily to retrievals using TIR channels only.This study presents preliminary results on the effect of cirrus heterogeneities on top-of-atmosphere (TOA) simulated radiances or reflectances for MODIS channels centered at 0.86, 2.21, 8.56, 11.01 and 12.03 µm, and on cloud optical properties retrieved with a research-level optimal estimation method (OEM). Synthetic cirrus cloud fields are generated using a 3D cloud generator (3DCLOUD) and radiances/reflectances are simulated using a 3D radiative transfer code (3DMCPOL). We find significant differences between the heterogeneity effects on either visible and near-infrared (VNIR) or thermal infrared (TIR) radiances. However, when both wavelength ranges are combined, heterogeneity effects are dominated by the VNIR horizontal radiative transport effect. As a result, small optical thicknesses are overestimated and large ones are underestimated. Retrieved effective diameter are found to be slightly affected, contrarily to retrievals using TIR channels only.

A 3D polarized Monte Carlo LIDAR system simulator for studying effects of cirrus inhomogeneities on CALIOP/CALIPSO measurements

To estimate cirrus inhomogeneity effects on the apparent backscatter and on the apparent depolarization ratio measured by CALIOP/CALIPSO, a 3D polarized Monte Carlo LIDAR simulator was developed. Comparisons were done with the fast Hogans LIDAR simulator. Early results show that clouds inhomogeneous effects seem to be negligible on the apparent backscatter but not on the apparent depolarization ratio.