Publications acceptées, mars 2013

Quelques publications reliées aux thématiques d’EECLAT ayant été acceptées récemment :

Tropical islands, such as Reunion Island (21°S, 55.5°E) in the southwestern Indian Ocean, have significant solar resource that is highly variable in both spatial and temporal scales because of heterogeneous and rapidly changing cloudiness. The characterization of this variability is essential to enhance penetration of solar energy systems, such as photovoltaic or thermal farms. This work focuses on the large-scale, meso-scale and local-scale variability in cloudiness and surface solar irradiance at different temporal scales. Vertical velocity at 500 hPa from ERA-Interim reanalyses are used to study large-scale subsidence. CALIPSO, MODIS and Meteosat-7 satellite observations are used to study cloud properties and associated irradiances at the meso-scale. Solar irradiance measurements at seven Meteo-France stations around Reunion Island are used to investigate three meteorologically-distinct regions, namely the windward and leeward coasts, and the coasts parallel to the general trade wind direction. Day-to-day variations in daily irradiation values and diurnal-scale variability of solar irradiances are seasonally dependent. Winter seasons are characterized by large-scale atmospheric subsidence and broken low-level cloudiness, while in summer, clouds are found both at low and high altitudes. Three parameters are introduced to characterize solar irradiance diurnal cycle regimes. Five physically-sensible regimes are found, identified as clear, morning clear, overcast, afternoon clear, and random cloudiness. Regime occurrences have marked seasonal dependencies and vary significantly between the windward, leeward and lateral coasts. In winter and early summer, when cloudiness is driven predominantly by local (thermal, orographic) processes, the aggregated solar daily irradiation (average of the seven ground stations) remains near 80% of the clear-sky irradiation. In late summer, this values drops below 65% as large-scale overcast systems regularly affect the entire island. The station-to-station dis-correlation distance in terms of daily clear-sky index, defined as the distance for which the correlation coefficient drops to 0.5, is 62 km in summer and 29 km in winter. For hourly clear-sky indexes, the station-to-station dis-correlation distances is 4 and 3 km, for summer and winter, respectively. Accordingly, aggregate analyses show that compensation effects are more important at hourly than daily time scales. The measurements also reveal that the atmosphere over the island tends to be clearer in the morning than in the afternoon while over the ocean surrounding the island, the opposite is true. These results provide insights and tools to help develop improved diurnal-scale solar irradiance forecast systems for tropical islands

Four years (2007-2010) of co-located 94 GHz CloudSat radar reflectivities and 532 nm CALIPSO Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) backscattering coefficients are used to globally characterize snow precipitating clouds. CALIOP is particularly useful for the detection of mixed and supercooled liquid water (SLW) layers. Liquid layers are ubiquitous in snow precipitating clouds: overall/over sea/over land 49%/57%/33% of the snowy profiles present SLW or mixed-phase layers. The spatial and seasonal dependencies of our results -with snowing clouds more likely to be associated with mixed phase during summer periods- are related to snow layer top temperatures. SLW occurs within the majority (> 80%) of snow-precipitating clouds with cloud tops warmer than 250 K, and is present 50% of the time when the snow-layer top temperature is about 240 K. There is a marked tendency for such layers to occur close to the top of the snow-precipitating layer (75% of the times within 500 m). Both instruments can be synergetically used for profiling ice-phase-only snow, especially for light snow (Z < 0 dBZ, S < 0.16 mm/h) when CALIOP is capable of penetrating, on average, more than half of the snow layer depth. These results have profound impact for deepening our understanding of ice nucleation and snow growth processes, for improving active and passive snow remote sensing techniques and for planning snow precipitation missions.

A ground-based Rayleigh lidar has provided continuous observations of tropospheric water vapour profiles and cirrus cloud using a preliminary Raman channels setup on an existing Rayleigh lidar above La Reunion over the period 2002–2005. With this instrument, we performed a first measurement campaign of 350 independent water vapour profiles. A statistical study of the distribution of water vapour profiles is presented and some investigations concerning the calibration are discussed. Analysis regarding the cirrus clouds is presented and a classification has been performed showing 3 distinct classes. Based on these results, the characteristics and the design of a future lidar system, to be implemented at the new Reunion Island altitude observatory (2200 m) for long-term monitoring, is presented and numerical simulations of system performance have been realised to compare both instruments.

Spaceborne lidar observations have recently revealed a previously undetected significant population of Subvisible Cirrus (SVC). We show them to be colder than −74 °, with an optical depth below 0.0015 on average. The formation and persistence over time of this new cloud population could be related to several atmospheric phenomena. In this paper, we investigate if these clouds follow the same formation mechanisms as the general tropical cirrus population (including convection and in-situ ice nucleation), or if specific nucleation sites and trace species play a role in their formation. The importance of three scenarios in the formation of the global SVC population is investigated through different approaches that include comparisons with data imaging from several spaceborne instruments and back-trajectories that document the history and behavior of air masses leading to the point in time and space where subvisible cirrus were detected. In order to simplify the study of their formation, we singled out SVC with coherent temperature histories (mean variance lower than 4 K) according to back-trajectories along 5, 10 or 15 days (respectively 58, 25 and 11% of SVC). Our results suggest that external processes, including local increases in liquid and hygroscopic aerosol concentration (either through biomass burning or volcanic injection forming sulfate-based aerosols in the troposphere or the stratosphere) have very limited short-term or mid-term impact on the SVC population. On the other hand, we find that ~20% of air masses leading to SVC formation interacted with convective activity 5 days before they led to cloud formation and detection, a number that climbs to 60% over 15 days. SVC formation appears especially linked to convection over Africa and Central America, more so during JJA than DJF. These results support the view that the SVC population observed by CALIOP is an extension of the general upper tropospheric ice clouds population with its extreme thinness as its only differentiating factor.

Clouds cover about 70% of the Earth’s surface and play a dominant role in the energy and water cycle of our planet. Only satellite observations provide a continuous survey of the state of the atmosphere over the entire globe and across the wide range of spatial and temporal scales that comprise weather and climate variability. Satellite cloud data records now exceed more than 25 years; however, climate data records must be compiled from different satellite datasets and can exhibit systematic biases. Questions therefore arise as to the accuracy and limitations of the various sensors and retrieval methods. The Global Energy and Water cycle Experiment (GEWEX) Cloud Assessment, initiated in 2005 by the GEWEX Radiation Panel (GEWEX Data and Assessment Panel since 2011), provides the first coordinated intercomparison of publically available, standard global cloud products (gridded, monthly statistics) retrieved from measurements of multi-spectral imagers (some with multi-angle view and polarization capabilities), IR sounders and lidar. Cloud properties under study include cloud amount, cloud height (in terms of pressure, temperature or altitude), cloud thermodynamic phase, cloud radiative and bulk microphysical properties (optical depth or emissivity, effective particle radius and water path). Differences in average cloud properties, especially in the amount of high-level clouds, are mostly explained by the inherent instrument measurement capability for detecting and/or identifying optically thin cirrus, especially when overlying low-level clouds. The study of long-term variations with these datasets requires consideration of many factors. The monthly, gridded database presented here facilitates further assessments, climate studies and the evaluation of climate models.