Validation of the CALIPSO/CALIOP extinction coefficients from in situ observations in midlatitude cirrus clouds during the CIRCLE-2 experiment

International audienceThis paper presents a comparison of combined Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) extinction retrievals with airborne lidar and in situ cirrus cloud measurements. Specially oriented research flights were carried out in western Europe in May 2007 during the Cirrus Cloud Experiment (CIRCLE-2) with the German Deutsches Zentrum für Luft- und Raumfahrt (DLR) and the French Service des Avions Français Instrumentés pour la Recherche en Environnement (SAFIRE) Falcon aircraft equipped for remote and in situ measurements, respectively. Four cirrus cloud situations including thin cirrus layers and outflow cirrus linked to midlatitude fronts and convective systems were chosen to perform experimental collocated observations along the satellite overpasses. The measurements were carried out with temperatures ranging between −38°C and −60°C and with extinction coefficients no larger than 2 km−1. Comparisons between CALIOP and airborne lidar (LEANDRE New Generation (LNG)) attenuated backscatter coefficients reveal much larger CALIOP values for one frontal cirrus situation which could be explained by oriented pristine ice crystals. During the four selected cases the CALIOP cirrus extinction profiles were compared with in situ extinction coefficients derived from the Polar Nephelometer. The results show a very good agreement for two situations (frontal and outflow cases) despite very different cloud conditions. The slope parameters of linear fittings of CALIOP extinction coefficients with respect to in situ measurements are 0.90 and 0.94, with correlation coefficients of 0.69 and only 0.36 for the latter case because of a small number of measurements. On the contrary, significant differences are evidenced for two other situations. In thin frontal cirrus at temperatures ranging between −58°C and −60°C, systematic larger CALIOP extinctions can be explained by horizontally oriented ice crystals with prevalent planar-plate shape as revealed by the Cloud Particle Imager instrument. This nicely explains the disagreements between CALIOP and LNG observations for that case. For the last cirrus situation related to dense outflow cirrus, CALIOP extinctions are systematically lower than the in situ observations. No clear explanations can be drawn to assess this feature, but the shattering of ice crystals on probe tips may enhance the measured extinction because numerous large ice crystals are observed during this cirrus situation. Finally, relationships between the ice water content and the extinction coefficient, the effective diameter, and the temperature are determined from this in situ measurements data set