CLOUD-RADIATION STUDIES DURING THE EUROPEAN CLOUD AND RADIATION EXPERIMENT (EUCREX)

The dominant role of clouds in modulating and interacting with radiati ve energy transports within the atmosphere, in providing precipitation , transporting water and influencing air-chemical processes is still n ot understood well enough to be accurately represented within atmosphe ric circulation and climate models over all regions of the globe. Also the extraction of real-world cloud properties from satellite measurem ents still contains uncertainties. Therefore, various projects have be en developed within the Global Energy and Water Cycle Experiment (GEWE X), to achieve more accurate solutions for this problem by direct meas urements within cloud fields and other complementary studies. They are based on the hypothesis, that most relevant properties of cloud field s can be parametrized on the basis of the prognostic field variables o f atmospheric circulation models, and that the cloud microphysical pro perties can directly be related - with additional parameters on the pa rticle shapes etc. - to the radiative transfer properties. One of thes e projects has been the European Cloud and Radiation Experiment (EUCRE X) with its predecessor ICE (International Cirrus Experiment). The EUC REX and ICE provided a common platform for research groups from France , Germany, Sweden and the United Kingdom to concentrate their efforts primarily on high, cold cirrus. They showed, with data from satellites , that this cloud species enhances the atmospheric greenhouse-effect. Numerical mesoscale models were used in sensitivity studies on cloud d evelopments. In-situ measurements of cloud properties were made during more than 30 aircraft missions, where also insight comparisons of var ious instruments were made to ensure the quality of data sets measured from different aircraft. The particle sampling probes, used for in-cl oud measurements, showed a disagreement in total number density in all ranges between about 20-50%, while all other instruments agreed quite satisfactorily A few measured holographic data provided information o n typical ice-crystal shapes, which were used in numerical simulations of their absorption and scattering properties. Several new instrument s for both in-situ and remote measurement, such as a polar nephelomete r, a chopped pyrgeometer and an imaging multispectral polarimeter (POL DER) for cloud and radiation measurements were tested and improved. Ne w algorithms were developed for cloud classifications in multispectral satellite images and also for simulations of the scattering of radiat ion by non-spherical particles. This paper primarily summarizes the EU CREX results obtained between 1989 and 1996, and provides examples of the many results which have been obtained so far. It is not a complete review of the world-wide state in this field, but it tries to place t he EUCREX results into the world-wide development. Therefore many refe rences are made to the results of other groups, which in turn influenc ed the work within EUCREX.