Welcome ICWG

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2017-08-14 The second Workshop of the CGMS International Cloud Working Group (ICWG-2) will be organised by Vanderbildt University 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.

2016-01-04 The second announcement for the 1st Workshop of the ICWG (ICWG-1) is now available online as pdf. The abstract deadline is 15 February 2016.
On-line registration and abstract submission for the Workshop are possible through: http://www-loa.univ-lille1.fr/workshops/ICWG2016/

2015-05-05 The first announcement for the 1st Workshop of the ICWG (ICWG-1) is now available online as pdf.

2015-01-02 The 1st Workshop of the ICWG (ICWG-1), previous referred to as CREW, will be held from 17-20 May 2016 in Lille, France, Europe.

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). 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.