AMT Atmospheric Measurement Techniques AMT Atmos. Meas. Tech. 1867-8548 Copernicus Publications Göttingen, Germany 10.5194/amt-8-633-2015 Impacts of cloud heterogeneities on cirrus optical properties retrieved from space-based thermal infrared radiometry Fauchez T. https://orcid.org/0000-0002-5967-9631 1 Dubuisson P. 1 Cornet C. 1 Szczap F. 2 Garnier A. 3 4 Pelon J. 5 Meyer K. https://orcid.org/0000-0001-5361-9200 6 7 Laboratoire d'Optique Atmosphérique, Université Lille 1, Villeneuve d'Ascq, France Laboratoire de Météorologie Physique, Université Blaise Pascal, Clermont Ferrand, France Science Systems and Applications, Inc., Hampton, Virginia, USA NASA Langley Research Center, Hampton, Virginia, USA Laboratoire Atmosphères, Milieux, Observations Spatiales, UPMC-UVSQ-CNRS, Paris, France Goddard Earth Sciences Technology and Research (GESTAR), Universities Space Research Association, Columbia, Maryland, USA NASA Goddard Space Flight Center, Greenbelt, Maryland, USA 09 02 2015 8 2 633 647 Copyright: © 2015 T. Fauchez et al. 2015 This work is licensed under the Creative Commons Attribution 3.0 Unported License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/3.0/ This article is available from https://amt.copernicus.org/articles/8/633/2015/amt-8-633-2015.html The full text article is available as a PDF file from https://amt.copernicus.org/articles/8/633/2015/amt-8-633-2015.pdf

This paper presents a study, based on simulations, of the impact of cirrus cloud heterogeneities on the retrieval of cloud parameters (optical thickness and effective diameter) for the Imaging Infrared Radiometer (IIR) on board CALIPSO. Cirrus clouds are generated by the stochastic model 3DCLOUD for two different cloud fields and for several averaged cloud parameters. One cloud field is obtained from a cirrus observed on 25 May 2007 during the airborne campaign CIRCLE-2 and the other is a cirrus uncinus. The radiative transfer is simulated with the 3DMCPOL code. To assess the errors due to cloud heterogeneities, two related retrieval algorithms are used: (i) the split-window technique to retrieve the ice crystal effective diameter and (ii) an algorithm similar to the IIR operational algorithm to retrieve the effective emissivity and the effective optical thickness. Differences between input parameters and retrieved parameters are compared as a function of different cloud properties such as the mean optical thickness, the heterogeneity parameter and the effective diameter. The optical thickness heterogeneity for each 1 km × 1 km observation pixel is represented by the optical thickness standard deviation computed using 100 m × 100 m subpixels. We show that optical thickness heterogeneity may have a strong impact on the retrieved parameters, mainly due to the plane-parallel approximation (PPA assumption). In particular, for cirrus clouds with ice crystal diameter of approximately 10 μm, the averaged error on the retrieved effective diameter and optical thickness is about 2.5 μm (~ 25%) and −0.20 (~ 12%), respectively. Then, these biases decrease with increasing effective size due to a decrease of the cloud absorption and, thus, the PPA bias. Cloud horizontal heterogeneity effects are greater than other possible sources of retrieval errors such as those due to cloud vertical heterogeneity impact, surface temperature or atmospheric temperature profile uncertainty and IIR retrieval uncertainty. Cloud horizontal heterogeneity effects are larger than the IIR retrieval uncertainty if the standard deviation of the optical thickness, inside the observation pixel, is greater than 1.

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