On average, more than 60% of the Earth's surface is covered with clouds. Clouds strongly affect atmospheric radiative fluxes and heating rates. In producing precipitation, clouds form an essential link in the hydrological cycle. Despite the significance of clouds in the climate system, cloud induced processes are still not well understood. This is partly due to the complexity of inferring accurate and unambiguous information from in-situ and remote-sensed cloud measurements. As a consequence, the representation of clouds remains one of the greatest sources of uncertainty in present-day climate models. With the recent advance of powerful instruments like cloud radar and multi-channel microwave radiometer and the ongoing development of synergy algorithms to consistently interpret the measurements from different instruments the experimental outlook has improved considerably.

In this contribution we report on the first results of the recently initiated Cloud Liquid Water Network project (CLIWA-NET) which is operated under the umbrella of the continental-scale experiment BALTEX. Its main objectives are i) to implement a prototype of a European cloud observational network, ii) to objectively evaluate cloud related output of atmospheric models for weather forecast and climate prediction iii) to contribute to the BALTEX BRIDGE continental scale experiment

Within CLIWA-NET observations are taken during three campaigns. These campaigns are co-ordinated with the BALTEX BRIDGE Enhanced Observational Periods (EOP). The CLIWA-NET Network (CNN) consists of 12 stations, each equipped with a microwave radiometer and various other remote-sensing instruments. CNN was operated in August/September 2000 (CNN I) and in April/May 2001 (CNN II). In the third campaign, referred to as the BALTEX BRIDGE Cloud campaign (BBC), all available ground-based instruments are brought together in a 100x100 km2 region in the Netherlands. Coordinated from the central facility in Cabauw a comprehensive local-scale experiment is organized in cooperation with other projects including measurements with aircraft and tethered balloon. The BBC was conducted in August/September 2001.

Four European institutes are participating in the evaluation of model predicted cloud parameters: ECMWF with the global forecast model operated at a horizontal resolution of 55 km, DWD with the recently developed Lokal Modell (LM) operated in non-hydrostatic mode at a resolution of 7 km, the Rossby Center with a climate version of the limited area model HIRLAM, and KNMI with a regional version of the ECHAM4 GCM. The latter two models have are operated at the same resolution (18 km) and on the same domain of integration. They are both driven from the lateral boundaries by ECMWF analyses. In this contribution we discuss results of the model evaluation focussing on the liquid water path.

A full description of the project and the latest results can be found on the CLIWA-NET web site: www.knmi.nl/samenw/cliwa-net

The authors like to thank all members of the CLIWA-NET project for their continuous efforts in contributing to the project. Part of this work is sponsored by the EU under contract number EVK2CT-1999-00007.