CLARA: Aim of the Project

Climate model output plays an important role in present policy making discussions. The representation of clouds and of their impact on radiative transfer remains one of the greatest sources of uncertainty in present day climate models. To improve this representation, better parameterizations of clouds are needed, both of the macrophysics and dynamics (cloud cover, cloud structure and turbulence) and of the microphysics (droplet spectra, distinction between ice and water, role of condensation nuclei and precipitation formation). In addition, the relation between the micro- and macro properties of clouds and radiative transfer has to be clarified. For model improvements, dedicated measurements are needed.

In the Netherlands, KNMI has established a cloud detection system which focuses on the observation of macro properties of clouds and radiation, but which also provides some information on microphysics. This network is designed to operate continuously for a period of at least two years.

The CLARA-project focuses on microphysics, their relation with the macro properties of clouds and their importance for routine observations of clouds by satellite and ground based remote sensing. The information from the field campaigns will be used to improve routine retrieval methods and will also serve to test detailed models for clouds and radiative transfer. These case studies will further serve as a testbed for parameterizations, that will be included in general circulation models.

Anthropogenic aerosol particles serve as extra cloud condensation nuclei. This results in clouds with a larger number of droplets for a given liquid water content and thus a higher albedo. This "indirect" effect of aerosol on global albedo (also known as Twomey-effect) is very difficult to quantify. Measurements in the ECN cloud chamber revealed that the number of cloud droplets in "polluted" air is higher by a factor of four compared to that in relatively clean air. These findings provide an upper limit for the influence of anthropogenic aerosols. The CLARA-project offers the means to check if the cloud chamber results can indeed be extrapolated to the actual atmosphere.

Advanced ground based remote sensing techniques like radar and lidar, as well as satellites play a crucial role in climate monitoring of clouds. The algorithms which are used to derive physical parameters from these measurements (e.g. liquid water path, extinction profiles, total optical depth, albedo) are based on several crude assumptions about the micro-physical properties of the cloud and the relation between these properties and the measured (macro-physical) properties. This project offers the opportunity for validation of the different remote sensing techniques with in situ measurements.