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2021-09-13 A virtual inter-sessional ICWG meeting is planned for 29-30 September 2021. The meeting will give status reports of the topical groups and restart the planning for ICWG-3. More information can be found at https://www.eventsforce.net/eumetsat/10/home.

2021-08-17 The path to ICWG-3 has been severely interrupted because of the covid-19 pandemic. We hope that ICWG-3 can be arranged sometime in 2022. In the mean time, we are planning several virtual workshops in the near future to prepare for ICWG-3. More details on this will be announced shortly.

2020-01-21 A shift of dates for ICWG-3 has been decided. New dates: 23 to 25 September 2020. The first announcement can be found at:
It can also be downloaded here as pdf.

2019-09-05 The third Workshop of the CGMS International Cloud Working Group (ICWG-3) will be hosted by EUMETSAT in Darmstadt, Germany from 24 to 28 August 2020. More information on workshop registration, abstract submission, and accommodations is under preparation and will be posted soon.

2018-10-29 The final program for the 2nd Workshop of the ICWG (ICWG-2) is now available online as pdf.

2018-09-22 The third announcement for the 2nd Workshop of the ICWG (ICWG-2) is now available online as pdf. The organising committee is reviewing the submitted abstracts. The authors of the abstracts will be notified whether their presentation is accepted as oral or poster presentation no later than the first week of October. Please note that participation of workshop is subject to a registration fee of $125 (when registered until 22 October 2018), and of $150 (when registered after 22 October 2018). More details on the workshop can be found at:

2018-05-08 The second announcement for the 2nd Workshop of the ICWG (ICWG-2) is now available online as pdf. The abstract deadline is 31 August 2018.
On-line registration and abstract submission for the Workshop are possible through:

2017-12-01 The first announcement for the 2nd Workshop of the ICWG (ICWG-2) is now available online as pdf.

2017-08-14 The second Workshop of the CGMS International Cloud Working Group (ICWG-2) will be organised by the Space Science and Engineering Center of the University of Wisconsin – Madison (SSEC) and will take place in Madison, USA from 29 October till 2 November 2018. More Information on: workshop registration, abstract submission, and general organization will be provided at the end of September 2017.

2014-12-27 The meeting summary of CREW 4 titled: Summary of the Fourth Cloud Retrieval Evaluation Workshop has been accepted for publication in BAMS.

2014-09-09 The paper Remote sensing of cloud top pressure/height from SEVIRI: analysis of ten current retrieval algorithms is published in Atmospheric Measurement Techniques. The inter-comparison and validation activities described in this publication were examined in the framework of ICWG. Ten algorithm groups participated this research effort.

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).

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

The Motivation

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.


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(for more details see the ICWG Terms of Reference). 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 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).

The Members

The ICWG a forum for operational and research users of cloud parameter retrievals. Since the first workshop in 2006 the workshops have been attended by an increasing number of participants . 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.

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.

The International Cooperations

The ICWG is a CGMS working group that maintains close relationships with other CGMS working groups serving the meteorological community, as well as with working groups from relevant World Weather Research Program (WWRP) and World Climate Research Programme (WCRP), and in particular with the Global Energy and Water Exchanges Project [1]. The communication with these working groups is established through nominated focal points. The ICWG also maintains close contacts with the ESA Climate Change Initiative-Clouds that aims generate a climatological cloud data set from AVHRR heritage channels.