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== News ==
== News ==
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'''2014-06-17''' The 1st workshop of the CGMS-ICWG (previously referred to as Cloud Retrieval Evaluation Workshops) is tentatively planned to be held in 2016 in Lille, France.
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'''2022-04-22''' The planning of ICWG-3 has once again been halted. Due to current uncertainties regarding conditions for international travel (both due to the continued pandemic and ongoing and escalating political/military conflicts), it was decided to not initiate a formal registration process for ICWG-3. Another Intersessional meeting is likely during the autumn but the official ICWG-3 meeting is postponed. More information on this will come in the near future.  
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'''2014-05-25''' The Cloud Retrieval Evaluation Working Group has been endorsed as permanent International Clouds Working Group of the Coordination Group for Meteorological Satellites (CGMS-ICWG) at CGMS’s 42nd meeting on 19-23 May 2014 (Guangzhou, China).  
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'''2022-02-07''' The final version of the report from the intersessional meeting 29-30 September 2021 has been uploaded. You can access it here [[Media:Report_ICWG-3_Interim_Meeting_29-30_September_2021.pdf|'''as pdf''']].  
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'''2014-03-18''' CREW 4 took place in Grainau in March 2014. 69 scientist from Europe, America, Asia and Africa participated the workshop. The  
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'''2021-12-03''' A virtual intersessional ICWG meeting was arranged 29-30 September 2021. The status of the work in Topical sub-groups was discussed and two new groups (Lightning applications and ISCCP-NG) were introduced. The next physical meeting, '''ICWG-3''', is now planned for '''26-30 September 2022''' in '''Darmstadt, Germany'''.
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[[CREW-4_Oral_presentations|'''oral''']] and [[CREW-4_Poster_presentations|'''poster''']] presentations are available on the "Meetings" page of this website.
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'''2014-03-10''' The [[SEVIRI_Inter-comparison_and_Validation_Software_Package| '''inter-comparison and validation software of the SEVIRI inter-comparison group''']] is available to the CREW participants.
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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.
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'''2014-03-05''' In order to support the discussion of the working groups, a [[Media:Handout_CREW4.pdf| '''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.
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'''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.
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'''2014-01-15''' The [http://www.atmos-meas-tech-discuss.net/7/401/2014/amtd-7-401-2014.pdf '''first publication using the CREW database'''] was submitted to AMT. The publications documents the inter-comparison and validation of the SEVIRI cloud top height retrievals.<br>
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'''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:
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<br> '''https://www.eventsforce.net/icwg-3'''. <br> It can also be downloaded here [[Media:1st_announcement_icwg-3_workshop_2020.pdf|'''as pdf''']].
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== What is CREW? ==
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'''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.
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CREW (Cloud Retrieval Evaluation Workshop) is a research activity to evaluate the strengths and weaknesses of the most important algorithms that retrieve cloud property from passive imager instruments onboard both polar and geostationary satellites (SEVIRI, AVHRR, and MODIS). The CREW working group members operate the most advanced cloud retrieval algorithms. Their level-2 data products have been collected in the '''[[CREW Data Set|CREW Common Database]]''' for 5 "golden" days, and have been inter-compared and validated against observations from the A-train satellite constellation (CALIPSO, CLOUDSAT, and AMSR). The results of these inter-comparison and validation activities are being discussed regularly at the bi-annual Eumetsat Cloud Retrieval Evaluation Workshops that have been held since 2006. The first CREW took place in Norrköping, Sweden from 17 - 19 May 2006, the second CREW in Locarno, Switzerland from 3 - 5 February 2009, and the third CREW in Madison, Wisconsin, USA from 15 -18 November 2011. The Fourth CREW took place from 4-7 March 2014, and will be held in Gainau, Germany, Europe. More details can be found on the '''[[Meetings|Meetings]]''' page. Please note that some document can only be accessed after registration, to register please [mailto:%75%6c%72%69%63%68%2e%68%61%6d%61%6e%6e%40%6d%65%74%65%6f%73%77%69%73%73%2e%63%68;%6a%65%72%6f%6d%65%2e%72%69%65%64%69%40%75%6e%69%76%2d%6c%69%6c%6c%651%2e%66%72 email us] for registration instructions).
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== Why? – The science ==
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'''2018-10-29''' The final program for the 2nd Workshop of the ICWG (ICWG-2) is now available online [[Media:Program_ICWG-2_Final.pdf|'''as pdf''']]. 
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'''2018-09-22'''  The third announcement for the 2nd Workshop of the ICWG (ICWG-2) is now available online [[Media:Announcement_III_ICWG-2_Workshop-2018.pdf| 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: <br> '''http://cimss.ssec.wisc.edu/icwg/index.html''' <br>
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'''2018-05-08'''  The second announcement for the 2nd Workshop of the ICWG (ICWG-2) is now available online [[Media:Announcement_II_ICWG-2_Workshop_2018.pdf| as pdf]]. '''The abstract deadline is 31 August 2018'''. <br>
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On-line registration and abstract submission for the Workshop are possible through:<br> '''http://www.ssec.wisc.edu/register/index.php?action=view_form&mid=883355&fid=88335501
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''' <br>
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'''2017-12-01''' The first announcement for the 2nd Workshop of the ICWG (ICWG-2) is now available online [[Media:Announcement_I_ICWG-2_Workshop_2018.pdf| as pdf]]. <br>
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'''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.<br>
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'''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.<br>
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'''2014-09-09''' The paper [http://www.atmos-meas-tech.net/7/2839/2014/amt-7-2839-2014.html '''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.<br>
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'''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).
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'''2014-03-10''' The [[SEVIRI_Inter-comparison_and_Validation_Software_Package| '''inter-comparison and validation software of the SEVIRI inter-comparison group''']] is available to the CREW participants.
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== 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).
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.
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.
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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.
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.
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== Who? - The members ==
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== The ICWG ==
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The CREW project lives on the participation of the cloud remote sensing community. First the participants of the CREWs contribute to the progress by presenting their latest results at the workshops. CREW-1 was organised by the Swedish Meteorological and Hydrological Institute in 2006 (23 participants), CREW-2 by Meteo Swiss in 2009 (44 participants), and the third by the University of Wisconsin in 2011 (71 participants). Second more than 15 groups provide data sets to the common CREW data base being the primary data source for the CREW inter-comparison and validation activities.
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<center>
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'''Funded by''' <br>
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[[Image:Eumetsat2.jpg|180x50px|link=http://www.eumetsat.int/|Eumetsat]]
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<br>
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'''Workshops and Fellowship organised by''' <br>
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[[Image:KNMI.jpg|50x45px|link=http://www.knmi.nl/index_en.html|KNMI, Host of the Eumetsat Fellowship]]  &nbsp;&nbsp;&nbsp; [[Image:SMHI.jpg | link=http://www.smhi.se/en |60x55px | Swedish Meteorological and Hydrological Institute, Organiser of CREW-1]]&nbsp; [[Image:MeteoSwiss.jpg |link=http://www.meteoswiss.admin.ch/web/en/weather.html |50x45px |MeteoSwiss, Organiser of CREW-2]] &nbsp;[[Image:UniWisconsin.jpg|57x53px|link=http://www.ssec.wisc.edu/|University of Wisconsin, Organiser of CREW-3]] &nbsp;&nbsp;&nbsp;  [[Image:DWD.jpg|50x45px|link=http://dwd.de/|Deutscher Wetterdienst, Organiser of CREW-4]] <br>
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<br>
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[[Participants|'''List of Participating Institutions (Data Providers)''']]<br>
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[[Image:KNMI.jpg|50x45px|link=http://www.knmi.nl/index_en.html|KNMI]]  &nbsp;
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[[Image:UniWisconsin.jpg|57x53px|link=http://www.ssec.wisc.edu/|University of Wisconsin]]
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[[Image:DWD.jpg|50x45px|link=http://dwd.de/|Deutscher Wetterdienst]]&nbsp;
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[[Image:DLR.jpg|50x45px|link=http://dlr.de|German Aerospace Center]]&nbsp;
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<!-- [[Image:IfT.jpg|50x45px|Leibniz Institute for Tropospheric Research]]&nbsp;-->
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[[Image:BIRA.jpg|50x45px|Belgian Institute for Space Aeronomy]]&nbsp;
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[[Image:MeteoFrance.jpg|70x45px|MeteoFrance]]&nbsp;
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[[Image:MetOffice.jpg|50x45px|UK MetOffice]]&nbsp;
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<!-- [[Image:ECMWF.jpg|50x45px|ECMWF]]&nbsp;-->
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<!-- [[Image:MeteoSwiss.jpg |link=http://www.meteoswiss.admin.ch/web/en/weather.html |50x45px |MeteoSwiss]] &nbsp;-->
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[[Image:NOAA.jpg|48x44px|National Oceanic and Atmospheric Administration]]&nbsp;
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[[Image:Nasa.jpg|50x45px|National Aeronautics and Space Administration]]&nbsp;
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[[Image:UniLille1.jpg|70x45px|University of Lille 1]]&nbsp;&nbsp; <br>
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<!--[[Image:UniEdinburgh.jpg|50x45px|University of Edinburgh]]&nbsp;-->
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<!--[[Image:TUDelft.jpg|50x45px|Technical University of Delft]]&nbsp;-->
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[[Image:RAL.jpg|110x45px|Rutherford Appleton Laboratory]] &nbsp;
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<!--[[Image:VITO.jpg|50x45px|Flemish Institue for Technological Research]] &nbsp;-->
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<!--[[Image:UniJerusalem.jpg|50x45px|The Hebrew University of Jerusalem]] &nbsp;-->
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<!--[[Image:CUNewYork.jpg|50x45px|City University of New York ]] &nbsp;-->
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[[Image:FUBerlin.jpg|50x45px|Free University of Berlin]]&nbsp;
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[[Image:SMHI.jpg | link=http://www.smhi.se/en |50x45px | Swedish Meteorological and Hydrological Institute]]&nbsp;
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[[Image:LMD.png|50x45px|Laboratoire de Météorologie Dynamique]]&nbsp;
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<!--[[Image:CSU.jpg|50x45px|Colorado State University]]&nbsp;-->
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<!--[[Image:CULeuven.jpg|50x45px|Katholic University of Leuven]]&nbsp;-->
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<!--[[Image:UniBremen.jpg|50x45px|University of Bremen]] &nbsp;-->
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[[Image:UniMarburg.jpg|70x45px|University of Marburg]]&nbsp;
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[[Image:Eumetsat2.jpg|140x50px|link=http://www.eumetsat.int/|Eumetsat]]
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</center>
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== How? - The approach ==  
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The International Clouds Working Group (ICWG) was established as a permanent Working Group within the  [http://www.cgms-info.org/ '''Coordination Group for Meteorological Satellites (CGMS)'''] at CGMS’s 42nd meeting on 19-23 May 2014, Guangzhou, China(for more details see the [[Media:ICWG_ToRs_2014.pdf| 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:
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a) to promote commonality in operational cloud retrieval algorithms;
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b) to address research questions identified by space agencies and WMO programs (e.g. GEWEX);
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c) to promote assessments of cloud retrievals;
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d) to foster adoption of standards, for example validation procedures and data formats;
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e) to contribute to the definition of new space borne observation capabilities for cloud retrievals and validation;
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f) to support and stimulate training;
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g) to enhance the communication in this field and develop international partnerships.
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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 '''[[Evaluation_Dataset_for_Passive_Imager_Retrievals|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|Meetings]]''' page. Please note that some document can only be accessed after registration, to register please [mailto:%75%6c%72%69%63%68%2e%68%61%6d%61%6e%6e%40%6d%65%74%65%6f%73%77%69%73%73%2e%63%68;%6a%65%72%6f%6d%65%2e%72%69%65%64%69%40%75%6e%69%76%2d%6c%69%6c%6c%651%2e%66%72 email us] for registration instructions).
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== The Members ==
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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.
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== 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.
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.
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== For whom? – The users ==
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== The International Cooperations ==  
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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. <br>
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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.
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The ICWG is a [http://www.cgms-info.org/index_.php/cgms/page?cat=ABOUT&page=Working+Groups 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 [http://www.gewex.org]. The communication with these working groups is established through nominated focal points. The ICWG also maintains close contacts with the [http://www.esa-cloud-cci.org/ ESA Climate Change Initiative-Clouds] that aims generate a climatological cloud data set from AVHRR heritage channels.
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== ICWG management and interaction with CGMS Working Group II ==
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== Cooperations ==
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ICWG activities are managed through two co-chairs. Current (December 2021) co-chairs are Karl-Göran Karlsson (SMHI) and Andrew Heidinger (NOAA). Reporting to CGMS (Working Group II) is made by a rapporteur which currently is vacant. New co-chairs and rapporteurs will be appointed at the next ICWG-3 workshop.
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We cooperate with the [http://climserv.ipsl.polytechnique.fr/gewexca/ 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  [http://www.esa-cloud-cci.org/ ESA Climate Change Initiative], that aims generate a climatological cloud data set from AVHRR heritage channels. <!--Probably CREW will become a working group of the [http://www.cgms-info.org/ Coordination Group for Meteorological Satellites].-->
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<center>
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[http://climserv.ipsl.polytechnique.fr/gewexca/ GEWEX cloud assessment]<br>
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[[Image:GEWEX-cloud-assessment.png|316x760px|GEWEX cloud assessment|link=http://climserv.ipsl.polytechnique.fr/gewexca/]]<br><br>
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<!--[http://www.cgms-info.org/ Coordination Group for Meteorological Satellites]<br>
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[[Image:CGMS_Final.png|316x760px|CGMS]]<br><br>-->
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[http://www.esa-cloud-cci.org/ ESA Clouds Climate Change Initiative (ESA Cloud CCI)]<br>
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[[Image:ESA-CCI-clouds.png|316x760px|ESA CCI clouds|link=http://www.esa-cloud-cci.org/]]<br>
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</center>
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Latest revision as of 13:23, 22 April 2022

Contents

News

2022-04-22 The planning of ICWG-3 has once again been halted. Due to current uncertainties regarding conditions for international travel (both due to the continued pandemic and ongoing and escalating political/military conflicts), it was decided to not initiate a formal registration process for ICWG-3. Another Intersessional meeting is likely during the autumn but the official ICWG-3 meeting is postponed. More information on this will come in the near future.

2022-02-07 The final version of the report from the intersessional meeting 29-30 September 2021 has been uploaded. You can access it here as pdf.

2021-12-03 A virtual intersessional ICWG meeting was arranged 29-30 September 2021. The status of the work in Topical sub-groups was discussed and two new groups (Lightning applications and ISCCP-NG) were introduced. The next physical meeting, ICWG-3, is now planned for 26-30 September 2022 in Darmstadt, Germany.

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:
https://www.eventsforce.net/icwg-3.
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:
http://cimss.ssec.wisc.edu/icwg/index.html

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:
http://www.ssec.wisc.edu/register/index.php?action=view_form&mid=883355&fid=88335501

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 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(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.

ICWG management and interaction with CGMS Working Group II

ICWG activities are managed through two co-chairs. Current (December 2021) co-chairs are Karl-Göran Karlsson (SMHI) and Andrew Heidinger (NOAA). Reporting to CGMS (Working Group II) is made by a rapporteur which currently is vacant. New co-chairs and rapporteurs will be appointed at the next ICWG-3 workshop.

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