Cirrus clouds are an important part of the Earth radiation budget but an accurate assessment of their role remains highly uncertain. Cirrus optical properties such as Cloud Optical Thickness (COT) and ice crystal effective particle size are often retrieved with a combination of Visible/Near InfraRed (VNIR) and ShortWave-InfraRed (SWIR) reflectance channels. Alternatively, Thermal InfraRed (TIR) techniques, such as the Split Window Technique (SWT), have demonstrated better accuracy for thin cirrus effective radius retrievals with small effective radii. However, current global operational algorithms for both retrieval methods assume that cloudy pixels are horizontally homogeneous (Plane Parallel Approximation (PPA)) and independent (Independent Pixel Approximation (IPA)). The impact of these approximations on ice cloud retrievals needs to be understood and, as far as possible, corrected. Horizontal heterogeneity effects can be more easily estimated and corrected in the TIR range because they are mainly dominated by the PPA bias, which primarily depends on the COT subpixel heterogeneity. For solar reflectance channels, in addition to the PPA bias, the IPA can lead to significant retrieval errors if there are large transport effects between cloudy columns in addition to brightening and shadowing effects that are more difficult to quantify. Furthermore TIR retrievals techniques have demonstrated better retrieval accuracy for thin cirrus having small effective radii over solar reflectance techniques. The TIR range is thus particularly relevant in order to characterize, as accurately as possible, thin cirrus clouds.
In the first part of the presentation, heterogeneity effects in the VNIR and TIR are evaluated as a function of spatial resolution in order to estimate the optimal spatial resolution for VNIR/TIR cirrus retrieval applications. In the second part, differences between 3D and 1D TIR cloud scattering inside cirrus are discussed and a new model to approach the 3D RT in the TIR from a simple parameterization is presented. These investigations are performed using a cirrus 3D cloud generator (3DCloud) and a 3D radiative transfer code (3DMCPOL).
*T. Fauchez(1;2), S. Platnick(1), K. Meyer(1;3), Z. Zhang(4;5), C. Cornet(6), F. Szczap(7) and P. Dubuisson(6)
1NASA Goddard Space Flight Center, Greenbelt, MD, USA
2Universities Space Research Association (USRA), Columbia, MD, USA
3Goddard Earth Sciences Technology and Research (GESTAR), Universities Space Research Association (USRA), Columbia, MD, USA
4Joint Center Earth Systems & Technology (JCET), UMBC, Baltimore, MD, USA
5Department of Physics, University of Maryland, Baltimore County (UMBC), Baltimore, MD, USA
6Laboratoire d'Optique Atmosphérique, UMR 8518, Université Lille 1, Villeneuve d'Ascq, France
7Laboratoire de Météorologie Physique, UMR 6016, Université Blaise Pascal, Clermont Ferrand, France
Seminar Series Coordinators