Abstract
Abstract. Cirrus are cloud types that are recognized to have a strong impact on the Earth-atmosphere radiation balance. This impact is however still poorly understood, due to the difficulties in describing the large variability of their properties in global climate models. Consequently, numerous airborne and space-borne missions have been dedicated to their study in the last decades. The satellite constellation A-Train has for instance proven to be particularly helpful for the study of cirrus. More particularly, the Infrared Imaging Radiometer (IIR) carried onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite shows a great sensitivity to the radiative and microphysical properties of these clouds. Our study presents a novel methodology that uses the thermal infrared measurements of IIR to retrieve the ice crystal effective size and optical thickness of cirrus. This methodology is based on an optimal estimation scheme, which possesses the advantage of attributing precise uncertainties to the retrieved parameters. Two IIR airborne validation campaigns have been chosen as case studies for illustrating the results of our retrieval method. It is observed that optical thicknesses could be accurately retrieved but that large uncertainties may occur on the effective diameters. Strong agreements have also been found between the products of our method when separately applied to the measurements of IIR and of the airborne radiometer CLIMAT-AV, which consolidates the results of previous validation studies of IIR level-1 measurements. Comparisons with in situ observations and with operational products of IIR are also discussed and appear to be coherent with our results. However, we have found that the quality of our retrievals can be strongly impacted by uncertainties related to the choice of a pristine crystal model and by poor constraints on the properties of possible liquid cloud layers underneath cirrus. Simultaneous retrievals of liquid clouds radiative and microphysical properties and/or the use of different ice crystal models should therefore be considered in order to improve the quality of the results.
Highlights
Two Imaging Radiometer (IIR) airborne valida- composed of ice crystals with highly complex habits, which tion campaigns have been chosen as case studies for illustrat- implies the necessity to deal with large ranges of microphysing the results of our retrieval method
The choice of the method was determined by its clear treatment of experimental errors, which is made possible by using a rigorous mathematical framework
Optimal estimation is perfectly appropriate for posterior analyses of errors linked to the non-retrieved parameters and for prior analyses of information content (Shannon and Weaver, 1949)
Summary
The two airborne campaigns CIRCLE-2 and Biscay ’08 were conducted with the principal objective to validate measurements of space-borne instruments, such as the Infrared Imaging Radiometer (IIR) (Corlay et al, 2000) This radiometer is carried onboard CALIPSO together with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) and the Wide Field Camera (WFC). CIRCLE-2 and Biscay ’08 took place during the periods of 12 to 26 May 2007 and 2 September to 18 October 2008, respectively Both campaigns involved a Falcon-20 aircraft (hereafter referred to as FF20) that performed remote sensing measurements over cloud decks. The study by Sourdeval et al (2012) presents direct comparisons between brightness temperatures simultaneously measured by IIR and CLIMAT-AV during the three “optimal” campaign days These comparisons showed strong similarities, yet small deviations could be observed between the measurements made by both instruments, especially in their 8.6μm channels. This paper seeks to consolidate the validation of IIR level-1 measurement by showing that coherent retrievals of ice cloud properties can be obtained when applying our retrieval method to the measurements of CLIMAT-AV and IIR
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