Monday, 7 July 2014
Clouds radiation feedback processes in polar regions have been identified as key uncertainties in the prediction of global climate in GCMs. To better understand cloud-radiation interactions in these regions, knowledge of arctic clouds properties has to be improved. Among the large diversity of clouds, Arctic is particularly well-known for the frequent occurrence of mixed phase clouds (MPC). Nevertheless, since MPC have been studied for years from numerous observations, as well as numerical modeling, results still suffer of large uncertainties and important discrepancies are observed between observations and GCM simulations. Characterization of MPC on the whole Arctic region, including space and time variability, as well as macrophysical and microphysical properties, is not yet accurate enough to fully understand the complexity of interactions between processes responsible for their life cycle. In particular, the quantification of the so-called mixed-phase state at large and small scales and the understanding of processes governing liquid and ice particles formation, coexistence and evolution must be achieved. One typical MPC structure very common at such high latitudes, involving important influence on radiation budget, is the presence of a supercooled liquid layer at cloud top and ice crystals below, precipitating down to the surface. This frequent arctic cloud structure has been frequently observed but is not yet completely understood, and its impact on radiation budget should be determined with better accuracy. In order to improve the knowledge of arctic clouds properties, the Laboratoire de Météorologie Physique (LaMP, France) is involved since several years in airborne measurement campaigns dedicated to the study of arctic clouds (ASTAR 2004, ASTAR 2007, POLARCAT 2008, SORPIC 2010, RACEPAC 2014), providing optical and microphysical in situ measurements from a unique combination of airborne probes (CPI, Polar nephelometer, PMS probes, 2D-S, Nevzorov ). While these in situ observations remain very localized in time and space, it is important to assess their representativity, in order to know in which extent small scale properties and processes deduced from these local observations could be generalized to Arctic clouds at regional scale. So, a description of Arctic MPC at regional scale is first necessary. The aim of this study is the characterization of MPC properties from regional scale to small scale. This work is based on the synergy of spaceborne remote sensing and airborne in situ observations. Such a multi-observational strategy presents the great advantage to study together large and small scale properties of clouds and MPC over Arctic. The talk will present first a study devoted to the time and space variability and vertical distribution of clouds and MPC on the Arctic regional scale, as shown in Figure 1, with a focus on the Svalbard region, located between the Barents, Norvegian and Greenland seas (78°N, 15°E). This will allow the assessment of the representativity of in situ measurements made in this area. Cloud thermodynamic phase and cloud distribution on the whole Arctic region are investigated from CALIPSO/CloudSat measurements (spaceborne active remote sensing formed by of 532nm and 1064nm lidar and 94Ghz radar, forming part to the A-Train constellation) and DARDAR retrievals products, providing since 2006 an unprecedented dataset concerning vertical structure of clouds, continuously in time and space, near the North pole (82°N). Then properties over Svalbard region are investigated and compared to those at regional scale in order to establish a link between local and regional scale. Then, the talk will focus on a statistical analysis of MPC clouds based on in situ measurements carried out during airborne campaigns in Svalbard region. This will provide a detailed characterization of microphysical and optical properties of MPC, discriminating liquid and ice thermodynamic phases, allowing an investigation of small scale processes occurring in such arctic clouds. Accurate profiles of relevant clouds parameters will be provided to contribute to the improvement of clouds representation in global and mesoscale models and to improve airborne and spatial remote sensing retrievals algorithms (CALIPSO, CloudSat, EarthCare ). These relevant parameters are for example the thermodynamic phase, geometrical characteristics (height, thickness), and optical and microphysical properties (asymmetry parameters, optical depth, liquid/water fraction, ice crystals morphology, size and concentration, IWC ).
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