CGMS-41 EUM-WP-05
v2A, 27 June 2013
Prepared by EUMETSAT
Agenda Item: WGII/8
Discussed in WGII
CLOUD RETRIEVAL EVALUATION WORKING GROUP (CRE)
In response to CGMS action CGMS-40: WG II Action 40.26: To request the CRE-WG
to develop a proposal related to establishment of a new International Science WG,
involving all CGMS members and WMO, WCRP and GCOS, including the proposed
focus of the group, an account of ongoing activities in other fora (a “gap analysis”),
and financial implications, for discussion at CGMS-41.
Cloud parameter retrievals are used increasingly in data assimilation for Numerical Weather
Prediction (NWP), regional now-casting especially for severe weather forecasting, cloudchemistry modelling, and more. These developments have led to an increasing interest of
space agencies to make cloud detection and cloud parameter retrievals part of their
operational services. There is a need at CGMS space agencies for a mechanism that
facilitates access to and sharing of knowledge, commonality of approaches, requirements,
and training on cloud parameter retrievals. The international Clouds Working Group within
CGMS that is presented in this working paper aims to provide such a mechanism.
The proposed Working Group (WG) intends to provide a forum for space agencies to seek
coherent progress in science and applications and also to act as bridge between the space
agencies and the cloud research community. The WG plans to serve as a forum: i) to
exchanges and enhance the knowledge on state-of-art cloud parameter retrievals algorithms,
ii) to stimulate support to trainings on cloud parameters, and iii) to encourage space agencies
and the cloud research community to use and share commonality algorithms. The WG plans
to prepare recommendations to guide the direction of future research, for example on
observing severe weather events or on data assimilation, and to influence relevant programs
of WMO, WCRP, GCOS and the space agencies.
This working paper presents the response to the action assigned to the Cloud Retrieval
Evaluation Working Group (CRE-WG) at the 40th CGMS meeting (CGMS-40: WG II Action
40.26), asking CRE-WG to develop a proposal to establishment of a new International
Science WG. Besides this working paper, establishing an international Clouds Working Group
will be further discussed at the 4th Cloud Retrieval Evaluation Workshop in Grainau, Germany
from 4-7 March 2014. The members of CGMS are cordially invited to attend this workshop.
Action/Recommendation proposed:
The CRE-WG representatives and the members of its science advisory committee to
prepare the Terms of References for a Clouds Working group within CGMS, following
the recommendations made by the participants of the 4 th CREW workshop to be held
in Grainau, Germany from 4–7 March 2014, for endorsement at CGMS-42.
Page 1 of 7
CGMS-41 EUM-WP-05
v2A, 27 June 2013
1
INTRODUCTION
The Coordinated Group for Meteorological Satellites (CGMS), established in 1972,
provides an international forum for the exchange of technical information on
geostationary and polar orbiting meteorological satellite systems. Its members
comprise current and prospective developers and operators of meteorological
satellites, the World Meteorological Organisation (WMO), and space agencies
operating R&D satellites contributing to WMO programmes. CGMS focuses on the
coordination of long-term and sustainable satellite systems relevant to weather and
climate. Through its working groups, CGMS stimulates science development and the
improvement and utilisation of satellite products. Additionally it closely interacts with
WMO to respond to requirements from WMO and related programmes, such as the
Global Climate Observing System (GCOS).
At CGMS-40 (CGMS-40 EUM-WP-10) the Cloud Retrieval Evaluation Working Group
(CRE-WG) presented first ideas to establish a permanent International Clouds
Working Group within CGMS. Further to this presentation the following action was
defined:
CGMS-40: WG II Action 40.26
“To request the CRE-WG to develop a proposal related to establishment of a new
International Science WG, involving all CGMS members and WMO, WCRP and GCOS,
including the proposed focus of the group, an account of ongoing activities in other fora
(a “gap analysis”), and financial implications, for discussion at CGMS-41”.
In support to this action the CGMS secretariat invited the CGMS members, WMO,
WCRP, and GCOS to nominated representatives for the CRE-WG. This invitation
was received positively, and resulted in the establishment of the CRE-WG
representatives group that is given in Table 1. The newly nominated CRE-WG
representatives contributed actively to this working paper by discussing its content
during a Teleconference and providing written comments.
This working paper presents the response to action: CGMS-40: WG II Action 40.26.
This response was prepared by the CRE-WG co-chairs, the CRE-WG
representatives, and the members of the scientific advisory committee. The latter
was established to advice on the previously held Cloud Retrieval Evaluation
Workshops. Based on their discussions an analysis of the ongoing activities has
been made, the focus of the proposed international Clouds Working Group within
CGMS has been drafted, and an estimate of the financial implications has been
made. This working paper is intended to take the next step towards endorsement of
the CRE-WG as a permanent international Clouds Working Group within CGMS, i.e.,
the CRE-WG representatives and the members of its science advisory committee to
prepare the Terms of References for a Clouds Working group within CGMS, following
the recommendations made by the participants of the fourth CREW workshop to be
held in Grainau, Germany from 4–7 March 2014, for endorsement at CGMS-42.
Page 2 of 7
CGMS-41 EUM-WP-05
v2A, 27 June 2013
Name
Organisation
Country
Mr. Chung, Sung-Rae
National Meteorological Satellite Center, KMA
Korea
Ms. Takeuchi, Ayako
Meteorological Satellite Center (MSC)
Japan Meteorological Agency (JMA)
Japan
Dr. Puviarasan, N.
India Meteorological Department
India
Dr. Rublev, Alexei
Roshydromet
Russia
Dr. Bojinski, Stefan
WMO
Switzerland
Dr. Heidinger, Andrew
NOAA
USA
Dr. Roebeling, Rob
EUMETSAT
Germany
Dr. Feng, Lu
CMA
China
Table 1: Representatives for the CRE-WG
2
BACKGROUND
In 2006, EUMETSAT initiated the Cloud Retrieval Evaluation Workshops (CREWs) to
evaluate instantaneous (level-2) cloud parameter retrievals from a variety of passive
imager algorithms that are provided by different research groups, i.e., CM-SAF,
CNSR, DLR, EUMETSAT, FU Berlin, IFT, NASA Goddard Space Flight Center,
NASA Langley Research Center, NOAA/NESDIS/STAR, NWC-SAF, University of
Wisconsin-Madison, Univ. Lille, and Univ. Marburg. Since 2006, three Cloud
Retrieval Evaluation Workshops have been organised with increasing attendance in
each succeeding workshop. An important component of these workshops was the
comparison of passive imager cloud parameter retrievals by the various teams and
their validation against active sensor measurements. Through these workshops,
operational and scientific users of observations from passive imaging satellites have
exchanged information on their algorithms for the retrieval of cloud parameters from
these observations, and have discussed the results of the comparison and validation
efforts using level-2 cloud parameter retrievals from the different algorithms.
Scientists working actively in the field of cloud retrievals and atmospheric radiative
transfer vigorously contribute to the CREWs. The organisation of the CREWs is done
through the CRE-WG, which currently has two co-chairs who are supported by a
science advisory committee:
Rob Roebeling
Bryan Baum
(co-chair)
(co-chair)
Please note that the procedures on selecting the co-chairs and the members of the
science advisory committee will be revisited for the International Clouds Working
Group within CGMS that is proposed in this working paper.
Page 3 of 7
CGMS-41 EUM-WP-05
v2A, 27 June 2013
3
INTERNATIONAL CONTEXT OF THE WORKING GROUP
3.1
Ongoing cloud activities
This section gives an overview of ongoing activities in other international forums where
the technical and scientific aspects of cloud parameter products are discussed and/or
assessed.
GEWEX-CA
In 2005, the GEWEX Radiation Panel (GRP) initiated the GEWEX Cloud Assessment
(GEWEX-CA) to evaluate the decadal time-series of global level-3 cloud parameters
(gridded monthly products). The twelve data sets participating in the assessment
comprised multi-spectral imagers, multi-angle-multi-spectral imagers, infrared (IR)
sounders, and active spaceborne lidar. With regards to cloud fraction and cloud top
pressure, the results of the assessment show reasonable agreement between the data
sets on a diurnal, monthly and seasonal scale. However, over the long-term the data
sets reveal different trends and variations that indicate the need for further maturity and
communication between the providers. The GEWEX-CA final report lists several
sources of uncertainty causing these differences such as: i) the physical concept of the
retrieval algorithm, in particular the treatment of semi-transparent and broken cloud
fields as well as ice clouds, ii) the cloud mask, iii) the ancillary data, and iv) the gridding
of level-2 retrievals to level-3 products. The GEWEX-CA recommends further
improvement of the retrieval algorithms, harmonisation of the use of ancillary data, as
well as investigating and mitigating the effect of sampling uncertainties in level-3 cloud
climatologies. GEWEX is currently not planning for another cloud assessment.
However, if cloud retrievals continue to improve in line with recommendations of the
GEWEX-CA an assessment focused on the analysis of long-term trends in level-3 cloud
products may be considered in the time frame of about eight years.
Scientific conferences
There are several scientific conferences at which the latest status of research on cloud
parameter retrieval methods, radiative transfer in cloudy atmospheres, and the
validation of cloud parameters are presented and discussed. While the list is
incomplete, some examples are:
− American Meteorological Society (AMS) on Cloud Physics (quadrennial);
− International Radiation Symposium (IRS) conference (quadrennial);
− EUMETSAT Meteorological Satellite Conference (annual);
− Int. Symposium Atmospheric Radiation and Dynamics (ISARD) (biennial).
Scientific and operational projects
There are several projects with the goal of preparing cloud products for weather and
climate research. Operational forecasters have stated the need for improved cloud
products to help with forecasting severe weather events. The assessment and
validation of cloud parameter retrievals is part of some of these projects, some
examples are:
− International Satellite Cloud Climatology Project (ISCCP);
− AVHRR Pathfinder Atmospheres – Extended (PATMOS-X);
Page 4 of 7
CGMS-41 EUM-WP-05
v2A, 27 June 2013
− EUMETSAT Climate Monitoring SAF (CM-SAF);
− EUMETSAT Now casting SAF (NWC-SAF);
− ESA Cloud Climate Change Initiative (Cloud-CCI).
3.2
Gap analysis
Beside the forums mentioned above there are currently no other organised activities
regarding the retrieval of cloud parameters from spaceborne sensors. As mentioned
above, GEWEX-CA aimed at the assessment of long-term data records of level-3
cloud parameters. Currently there are no plans for a continuation of the GEWEX-CA.
On the other hand, scientific conferences exchange knowledge in a broader context,
among others on cloud parameter retrievals and cloud radiative transfer. The
International Clouds Working Group within CGMS that is proposed in this working
paper aims to fill the gap that is left after the completion of the GEWEX-CA. To
ensure a link with GEWEX, the proposed Clouds Working Group is invited to report
to the GEWEX panel on a biennial basis. Since the proposed working group
concentrate on research issues related to cloud parameter retrievals, its scientific
focus is much narrower than the focus of scientific conferences. By bringing together
a dedicated group of active cloud scientists, it is not only possible to exchange
information on the latest state of the research, but also to define research priorities
and to set up research collaborations that focus on well-defined issues.
4
FOCUS OF THE WORKING GROUP
4.1
Objectives
Cloud parameter retrievals are used increasingly in data assimilation for Numerical
Weather Prediction (NWP), regional now-casting especially for severe weather
forecasting, cloud-chemistry modelling, and more. The proposed Clouds Working
Group intends to serve as a forum for operational and scientific users to enhance
their knowledge on cloud parameter retrievals and corresponding error estimates
from state-of-the-art algorithms. The Clouds Working Group builds upon the expertise
of scientists involved in atmospheric radiative transfer and cloud parameter retrievals
from passive polar-orbiting and geostationary sensors. The Clouds Working Group
can facilitate the path towards optimising these algorithms for near real–time
applications (especially for severe weather events), data assimilation, and improving
the use of satellite cloud products by weather and climate modelling teams. The
Working Group plans to prepare recommendations to guide the direction of future
research and to influence relevant programs of WMO, WCRP, GCOS and the space
agencies.
The overarching objectives of the Working Group are:
1. to identify and address research questions on cloud parameter retrieval
algorithms;
2. to contribute to the assessment of differences between level-2 cloud parameter
retrievals and the causes for those differences;
Page 5 of 7
CGMS-41 EUM-WP-05
v2A, 27 June 2013
3. to contribute to the validation of both level-2 cloud parameter retrievals and their
error estimates;
4. to foster commonality for operational cloud retrievals/products among the different
CGMS satellite operators;
5. to support and stimulate training of the operational and scientific community;
6. to enhance the communication in this field and develop international partnerships;
In addressing these objectives and to underpin the value of cloud parameter
retrievals in nowcasting, NWP assimilation and climate and weather model analysis,
the Working Group proposes to promote the:









sharing of consensus approaches among algorithm developers;
use of common data bases and validation procedures;
use of common ancillary data;
use of forward simulators to analyse algorithm differences;
comparison of cloud parameter retrievals with observations from active sensors
(e.g. CALIPSO, CloudSat) and complementary sensors (e.g. MERIS, CERES,
GERB);
quantification of the sensitivities of level-2 cloud parameter retrievals to different
sources of error;
assessment of error estimates of level-2 cloud parameter retrievals through
comparison with active sensors and through comparison of global products
between teams;
investigation of new approaches to building a more flexible and useful level-3
global gridded product;
establishment and coordination of sub-working groups addressing specific
research topics.
4.2 Planned activities
To achieve its objectives, the Working Group needs a structure that provides:






Biennial workshops;
Training towards retrieving and interpreting cloud parameter products;
A forum to Initiate and support validation efforts;
Annual reports and, on an ad hoc basis, collaborative papers;
Opportunities to participate in joint research projects;
Outreach to users in the nowcasting, and NWP and climate modeling
communities.
Page 6 of 7
CGMS-41 EUM-WP-05
v2A, 27 June 2013
5
FINANCIAL IMPLICATIONS
The proposed Clouds Working Group plans to organize biennial scientific workshops
under the lead of the co-chairs. The third workshop was attended by about 75
participants from the operational and scientific user community, and lasted 4 days.
As noted earlier, attendance has increased with each workshop because they are
found to be effective forums for advancing international collaboration. Depending on
the number of participants and the length of the workshops the costs vary between
K€ 10 and K€ 16. These costs can be broken down as follows:



Venue plenary (1x) and breakout (2x) rooms (between 1000 and 1500 € per day);
Venue equipments (between 200 and 400 € per day);
Participants costs for coffee breaks and lunches (~20 € per person per day).
In addition, there may be some costs associated to the maintenance of the webpage
and administration of global data products for use by the cloud community.
6
MEMBERSHIP AND WORKING ARRANGEMENTS
The Clouds Working Group shall be chaired by two co-chairs to be elected by the
CRE-WG representatives and the members of the scientific advisory committee.
These co-chairs will be presented to CGMS at their annual meetings. The co-chairs
shall be responsible for ensuring that the scientific aims of the Clouds Working Group
are effectively organized and coordinated. The co-chairs shall be assisted by a
scientific advisory committee that will define, oversee, and evaluate the scientific and
technical activities of the working group. The members of the scientific advisory
committee shall be nominated by invitation.
The exchange and coordination between CGMS and the Clouds Working Group will
be realised through a rapporteur who will attend and report to the CGMS meetings.
The rapporteur shall participate in the meetings of the Clouds Working Group. The
two co-chairs serve as interface between the rapporteur and the scientific meetings
of the Working Group. The co-chairs shall compile a report on relevant activities for
the scheduled plenary meetings of the Working Group.
The Cloud Working Group shall maintain a website that provides information on the
Working Groups Terms of Reference, the biennial Workshops and Trainings, as well
as the outcomes of the work done within the sub-working groups. The website shall
be hosted by one of the CGMS space agencies.
Page 7 of 7