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2014-10-15 As part of our participation in the international CREW (Cloud Retrieval Evaluation Workshop) product inter-comparison and assessment effort, a new SEVIRI cloud product has been developed in the framework of a joint project between scientists and data experts from NASA, NOAA, LOA, and ICARE. This new dataset, called SEV06-CLD, is derived from the MSG/SEVIRI instrument using an algorithm that replicates the retrieval core of the Collection 6 MODIS Level-2 cloud product (MOD06_L2 and MYD06_L2) augmented with NOAA GOES-R cloud height product. The project team is pleased to announce the public release of the SEVIRI SEV06-CLD beta product at ICARE for evaluation purposes.

2014-09-14 The results of the CREW inter-comparison and validation of cloud top heights retrievals were publishedin the Atmospheric Measurement Techniques.

2014-06-17 The 1st workshop of the ICWG (previously referred to as Cloud Retrieval Evaluation Workshops) is tentatively planned to be held in 2016 in Lille, France.

2014-05-25 The CREW has been endorsed as International Clouds Working Group (ICWG) within the Coordination Group for Meteorological Satellites (CGMS) at CGMS’s 42nd meeting on 19-23 May 2014 (Guangzhou, China).

2014-03-18 CREW 4 took place in Grainau in March 2014. 69 scientist from Europe, America, Asia and Africa participated the workshop. The oral and poster presentations are available on the "Meetings" page of this website.

2014-03-10 The inter-comparison and validation software of the SEVIRI inter-comparison is available to the CREW participants.

2014-03-05 In order to support the discussion of the working groups, a CREW 4 handout with recent results from the calibration, inter-comparison and validation and aggregation methods research has been published. Special thanks to all voluntarily contributing authors.

What is the ICWG?

The International Clouds Working Group (ICWG) was established as a permanent Working Group within the Coordination Group for Meteorological Satellites (CGMS) at CGMS’s 42nd meeting on 19-23 May 2014 (Guangzhou, China). The ICWG is co-sponsored by CGMS and the World Meteorological Organization (WMO). It provides a forum for operational and research users of cloud parameter retrievals that aims to enhance knowledge on quantitative cloud parameter retrievals from state-of-the-art algorithms and identify shortcomings that need focused attention as a community. Continual improvement in the global description of cloud properties optimizes these algorithms for near-term (now-casting), short to medium term (weather forecasting), and long-term (regional and climatological analyses) applications, as well as for potential improvements in the cloud and convection parameterizations adopted in weather and climate models. The objectives of the ICWG include: a) to promote commonality in operational cloud retrieval algorithms; b) to address research questions identified by space agencies and WMO programs (e.g. GEWEX); c) to promote assessments of cloud retrievals; d) to foster adoption of standards, for example validation procedures and data formats; e) to contribute to the definition of new space borne observation capabilities for cloud retrievals and validation; f) to support and stimulate training; g) to enhance the communication in this field and develop international partnerships.

The ICWG's predecessor was the Cloud Retrieval Evaluation Working Group that has been organizing Cloud Retrieval Evaluation Workshops (CREWs) since 2006. CREW-1 took place in Norrköping, Sweden from 17 - 19 May 2006, CREW-2 in Locarno, Switzerland from 3 - 5 February 2009, CREW-3 in Madison, Wisconsin, USA from 15 -18 November 2011, and CREW-4 in Grainau, Germany from 4-7 March 2014. These CREWs focused on the evaluation of strengths and weaknesses in the most important algorithms that retrieve cloud properties from passive imager instruments onboard both polar and geostationary satellites (SEVIRI, AVHRR, and MODIS). At the CREWs level-2 cloud parameter retrievals from different providers, for 5 golden days collected in the CREW Common Database, have been inter-compared and validated against observations from the A-train satellite constellation (CALIPSO, CLOUDSAT, and AMSR). More details on the CREW meetings can be found on the Meetings page. Please note that some document can only be accessed after registration, to register please email us for registration instructions).

Why? – The science

Clouds cover about 70% of the Earth’s surface. They appears in various forms as marine stratocumulus, deep convective clouds in the tropics, frontal systems and many more. On the one hand cloud clouds have a global extent. On the other hand cloud formation is based on micro physics. Cloud droplets and ice crystal sizes are usually in the micro meter range. Temperature, humidity, aerosols and turbulence are critical parameters for cloud growth. The clouds may be described by their macro physical properties (like cloud phase, water content, cloud top and base height) or their optical properties (like cloud optical thickness and effective radius). Cloud influence in several ways atmospheric physics. Clouds play a dominant role in the hydrological cycle of our planet. By condensations water vapor is removed from the air. Droplets and ice crystals are transported within clouds by convection, are horizontally advected, and precipitation is formed. The phase change of water release latent energy altering small-scale convective or turbulent circulations and the vertical heating profile. Furthermore clouds modify the radiative transfer through the atmosphere. Clouds reflect a part of the sunlight, this effect cools the earth. They also reduce the amount of thermal radiation emitted to space thus warming the earth. The net effect depends on the cloud properties. For low and middle clouds the cooling effect dominates, while high clouds tend to have a warming effect. Due to their large impact on the energy and water cycle, clouds have to be well represented in weather and climate models. In order to validate these models accurate observations are needed. Where ground based measurements provide information of high accuracy at certain places, satellite observations provide continuous observation of the atmospheric state over the whole globe being indispensable for the validation of global models.

Who? - The members

The ICWG provides a forum for operational and research users of cloud parameter retrievals. The number of participants to the CREWs has been increasing since the first workshop in 2006. CREW-1 (hosted by the Swedish Meteorological and Hydrological Institute) was attended by 23 participants, CREW-2 (hosted by Meteo Swiss) by 44 participants, CREW-3 (hosted by University of Wisconsin) by 71 participants, and CREW-4 (hosted by Deutscher Wetterdienst) by 69 participants. Meanwhile more than 15 groups provided data sets to the common CREW data base being the primary data source for the inter-comparison and validation activities of the working group.

Co-sponsored by
Workshops and Fellowship organised by
KNMI, Host of the Eumetsat Fellowship     Swedish Meteorological and Hydrological Institute, Organiser of CREW-1  MeteoSwiss, Organiser of CREW-2  University of Wisconsin, Organiser of CREW-3     Deutscher Wetterdienst, Organiser of CREW-4

List of Participating Institutions (Data Providers)
KNMI   University of Wisconsin Deutscher Wetterdienst  German Aerospace Center  Belgian Institute for Space Aeronomy  MeteoFrance  UK MetOffice  National Oceanic and Atmospheric Administration  National Aeronautics and Space Administration  University of Lille 1  
Rutherford Appleton Laboratory   Free University of Berlin  Swedish Meteorological and Hydrological Institute  Laboratoire de Météorologie Dynamique  University of Marburg  Eumetsat

How? - The approach

Currently 13 groups provide data sets of retrieved cloud physical properties, i.e., Cloud mask, cloud fraction, cloud top temperature, cloud top pressure, cloud top height, cloud optical depth, effective radius, cloud water path, cloud type, and convective signature. The data is inter-compared for five days, that were most suitable for validation purposes.

For whom? – The users

Clouds probably will alter their coverage and physical properties in a changing climate. Due to the complexity of cloud microphysical processes especially with aerosols and their radiative interactions cloud representation in global circulation models (GCM) remains a challenging task. Using different techniques several GCM groups obtained widely varying results for the climate feedback of clouds, where even the sign remains unsure. This uncertainty is cited as one of the key factors explaining the spread in model simulations of future climate for a given emission scenario. A long history of cloud observations now runs parallel to that of model development. The comparison of observation and model results at daily, seasonal and inter-annual scale therefore is a necessary way to constrain models.
Apart from the validation of climate models, satellite observations of clouds provide the data base for a variety of applications, too. They are directly used for the short term monitoring and forecasts of irradiance for solar energy. Cloud observations are used to determine the irradiance in the ultra violet wavelength range having an effect on human health, the biological environment, and photochemical reactions. Finally the retrieved cloud properties are used to judge convective conditions in the atmosphere for aviation and estimations of the precipitation.


We cooperate with the GEWEX - Cloud Assessment project that rather compares climatologies, whereas CREW is intended for investigating the details of the retrieval algorithms. We are also in contact with the cloud project of the ESA Climate Change Initiative, that aims generate a climatological cloud data set from AVHRR heritage channels.

GEWEX cloud assessment
GEWEX cloud assessment

ESA Clouds Climate Change Initiative (ESA Cloud CCI)
ESA CCI clouds