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Investigation of clouds from ground based and airborne radar and lidar


The atmospheric branch of the hydrological cycle and its interactions with energy fluxes have been identified as playing an important role in the climate system. Clouds are one part of the hydrological cycle whose representation in general circulation models still remains unreliable.
In recent years millimeter wavelengths radars have proven to be invaluable for studying cloud because of their sensitivity to small cloud particles and their ability to penetrate multilayered cloud system. Currently, a 95 GHz polarimetric cloud Doppler radar is taken into operation at GKSS Research Center in Germany. In parallel, airborne lidars have been used during several field campaigns for studying the structural variability and microphysics of clouds, in relation with radiometric and in-situ measurements. These lidars generally operate at 0.5 or 1 mm. as does the airborne backscatter lidar LEANDRE 1 jointly developed by CNRS and CNES in France.
To validate and to improve cloud parameterisations in general circulation models, a detailed information about the cloud characteristics must be available on large scales. Therefore further measurements must also be carried out from airborne platforms which, in opposite to ground-based ones, allow cloud sampling on large horizontal scales. A crucial cloud characteristics is the particle size distribution (liquid or ice), which is known to have a strong influence on the radiative property of the cloud. Currently, particle size distributions can only be obtained from in situ airborne probes. An important step towards remote sensing of particle size distributions can be done by the combination of a cloud radar and a backscattering lidar on an aircraft platform. This "dual-wavelength" mesurements in optically thin clouds will lead to informations about size distribution along vertical profile, helping to understand the cloud microphysics and its link to larger-scale cloud properties (in particular with respect to radiative budget) and tropospheric dynamics. In the same spirit, in optically thick clouds, the "dual beam technique" considered for the radar (already used with success in ASTRAIA, a dual beam airborne weather radar), aims going far beyond the description of cloud layering obtained from a single beam radar, by measuring two integrated parameter of the particle size distribution: the radar reflectivity and the specific attenuation, from which it is possible to derive the water content W and the equivalent radius re of droplets. Mapping W and re within the cloud suffices to fully characterise the radiative property of the cloud. Also, using the Doppler capability of the GKSS radar, the dual beam technique will allow to better describe the cloud dynamics by measuring two independent components of the air velocity.
Therefore, this proposal concentrates on the simultaneous airborne operation of a 95 GHz cloud radar and a backscattering lidar. The work will consist of:
1. Installation of the 94 GHz cloud radar owned by GKSS, into a french research aircraft (ARAT) already equipped with a backscattering lidar (system LEANDRE operated by LMD and SA), and with classical radiative measurement devices.
2. Deployment of the ARAT in a field experiment, to observe various lower level cloud fields with both lidar and radar instruments: data analysis with respects to above mentioned parameters.
3. Combined analysis of the data of the two instruments with respect to above mentioned parameters: interpretation using a three-dimensional atmospheric model able to represent the cloud process.
This programme will remain in close relation with EUCREX (EUropean Cloud and Radiation EXperiment).
KEYWORDS: Climate system, cloud radiative properties, cloud microphysicsand dynamics.


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