The 4D-clouds project aims at capturing the radiative influence of inhomogeneous clouds and at implementing these influences in the modelling of transport and exchange processes in dynamical atmospheric models. Especially, the project wants to contribute to the major scientific questions concerning enhanced absorption, the relation between albedo and optical depth in satellite retrievals, and the relation between dynamical exchange processes and the 4D cloud structure.

The measurement component of this project was executed together with the EU-project CLIWA-NET in the Baltex Bridge Campaign (BBC), which was held in the Netherlands around Cabauw in August and September 2001. It encompasses satellite and diverse ground based remote sensing measurements and simultaneous in situ measurements of the microphysical and radiative properties with three aeroplanes. In total 7 remote sensing stations, with a lidar ceilometer, an infrared-radiometer and a microwave radiometer, were located in a region of 100 by 100 km. Furthermore, 4D cloud fields have been measured at Cabauw by making scanning measurements with a microwave radiometer and a cloud radar.

In the radiation modelling part of the project, three dimensional radiative transfer models in all relevant frequencies, will be further improved and the results compared with the measured spectral radiative densities and fluxes at the ground and at cloud level. The models will be used to improve satellite retrievals and to derive parameterisations (including error estimates) for the radiative transfer modules of cloud resolving models and non-cloud resolving models (LM, HRM and REMO)

The project will be concluded by a dynamical modelling component in which the influence of inhomogeneous clouds on the vertical transport will be studied in the coupled system of surface, vegetation, boundary layer and free atmosphere.

In this presentation the BBC campaign will be introduced. However the focus will be on scanning measurements made with the cloud microwave radiometer MICCY and the cloud radar MIRACLE withing the BBC campaign and the analysis of this data to get information about the cloud structure. The microwave radiometer MICCY was specially build for cloud research, has a small antenna beam of less than 1 degree, 22 channels between 20 and 30 GHz, 50 and 60 GHz and at 90 Ghz. For these scanning measurements the temporal resolution was 0.1 second, allowing for relatively rapid scanning. Miracle is a 95 GHz cloud radar, which is ideal for cloud structure research with its very small antenna beam of 0.17 degree.

Which these two instruments simultaneous scans were made, azimuth scans at various elevation angles, elevation scans at various azimuth angles, and sector scans (azimuth scans with each scan a different elevation or visa versa). These scans will be analysed to get amongst others the auto-correlation function of the cloud fields, the scaling properties of clouds, and determine anisotrophies.