Objective
1.Introduction
The effects of human activities on weather and climate are receiving increased recognition by the scientific community and decision makers. The behaviour of the atmosphere and its links with other components of the Earth's system is closely related to the hydrological cycle. Successful prediction of weather and simulation of climate require a thorough understanding and an adequate monitoring of various aspects of this cycle, including the dynamic, radiative and chemical processes involved.
Microwave radiometry is particularly well suited to providing information to help the relevant research and monitoring activities. Microwave radiances sensed by instruments outside the atmosphere (i.e. on satellites) and within the atmosphere (i.e. on the surface, on aircraft, etc.) are sensitive to several important parameters of the hydrological cycles:
-in the troposphere: temperature, water vapour, cloud liquid water content and ice and precipitation,
-at the surface: sea-surface temperature and wind, sea-ice, snow cover, vegetation and soil moisture, and
-in the middle atmosphere: temperature, water vapour and atmospheric constituents affecting the concentrations of water vapour (and ozone).
Microwave instruments have been flown on satellites for many years. Sensors operating now include the Microwave Sounding Unit (MSU) on the NOAA satellites, the Special Sensor Microwave Imagery (SSM/I) on the DMSP satellites and the microwave component of the Along Track Scanning Radiometer (ATSR/M) on ERS/1. Research activities to use these data have been undertaken at several institutes in Europe. Many have led to fruitful results and some applications are already operational. However, the true potential of these data is very far from being fully exploited. Also, data from several new instruments will become available in the next few years, including: the Advanced Microwave Sounding Unit (AMSU), the Medium-resolution Imaging Microwave Radiometer (MIMR) and various limb sounding instruments. Extensive efforts are required to develop powerful data analysis procedures for existing and new instruments. For all these activities support is sought through the COST mechanism.
2.Objectives of the Action
The objectives of the Action are various and cover the different aspects of the scientific problems outlined above. The main aims are to improve the application of microwave radiometry to the understanding and monitoring of:
-the hydrological cycle, and
-tropospheric/stratospheric exchange.
This will include research to understand and to model the atmospheric processes involved in these phenomena. These scientific objectives will be achieved through developments in the following areas:
-improved models of the interaction of microwave radiation with the Earth's atmosphere and surface,
-improved retrieval, analysis and assimilation techniques, through which atmospheric and surface parameters are estimated from the data,
-verification and validation studies, through which the accuracy and characteristics of the data analysis techniques may be assessed,
-improved measurement facilities and techniques, including ground-based, aircraft-borne and space-based systems.
The Action will support the above activities through sharing of information on state-of-the-art methods and facilities, and through exchange of data and software where appropriate.
3.Content of the Action
The Action will be divided into 4 projects. Tasks under each project are listed in the table below.
3.1Development of radiative transfer models
To make effective use of microwave radiance measurements of the atmosphere-surface a good radiative transfer model (RTM) is a prerequisite for calculating radiances of each radiometer in use or under development (MSU, AMSU, MIMR, future limb sounders, etc.). With the launch of AMSU now imminent a good forward model for the AMSU channels is urgently required if we are to make the best use of this new data source. RTMs in the microwave region are still under development, and specific elements of the RTMs which are in need of improvement are:
-surface emissivities (i.e. sea, land, ice, snow)
-clouds (water and ice) and precipitation
-gaseous absorption (e.g; oxygen line mixing, water vapour continuum)
-refraction (limb sounding).
There are two different kinds of RTMs which have to be developed further: accurate, but slow, physical models including all the details of the physics, and fast parametric models which have to be used for real-time applications which require many thousands of RTM calculations without a major loss of accuracy. Both kinds of models need to be comprehensively tested, compared with others, validated by independent measurements, improved where necessary and distributed to interested Meteorological Services and Research Institutes in Europe. The work of the microwave group of the ITRA (Intercomparison of Transmittance and Radiance Algorithms) campaign which is carrying out a comparison of microwave RTMs will be a key element in the comparison of physical models. Groundbased and airborne microwave radiometer data can provide useful validation data for these models.
The development of microwave RTMs, model comparisons and validation and distribution to European Meteorological Services will be promoted and coordinated by this COST Action.
3.2Retrieval analysis and assimilation techniques
To exploit the wealth of information from present and planned data, particularly from satellites, it will be necessary to develop powerful systems for analysing the data. Atmospheric and surface parameters may be estimated from the radiometric data using several techniques: the term "retrieval" is usually used to describe methods in which a parameter (or a vertical profile of parameters) is estimated at a
single location, whereas the term "analysis" usually implies the estimation of 2- or 3-dimensional fields. "Data assimilation" describes analysis methods in which a forecast model is used to provide the prior or "background" information for the estimation process. Therefore, techniques in these three areas are highly interrelated.
The estimation theory underlying optimal, physical-statistical methods applicable to such retrieval/analysis problems is well established, and so theoretical work to derive new methods is not needed. However much work is still required on the details of the implementation of such methods for different data types (e.g. nadir and limb sounding data over a wide range of frequencies) and for a range of applications. Also there is scope for continuing work on simplified (sub-optimal) schemes. Although these may often provide adequately accurate results at much lower computational cost, validation campaigns have shown that, often, their applicability is limited and further improvements are necessary.
From measurements at microwave frequencies, information may be obtained on the 3-dimensional fields of temperature and water vapour (troposphere and middle atmosphere), on cloud liquid water and cloud ice, and on precipitation. Information on sea-surface wind speed, sea-ice and other surface parameters may also be obtained. In the stratosphere, unique information (mainly from limb sounding measurements) is available on several constituents important in ozone chemistry. Improvements in retrieval/analysis procedures are required in all these areas. As part of this work, improved understanding and modelling of the associated radiative transfer problems (as discussed above) will be necessary.
Several European groups are already active in these areas, but existing efforts are far from exploiting the full potential of even the currently-available data. The COST Action will provide a forum for exchange of information and for promoting new collaborative initiatives on appropriate retrieval/analysis techniques.
3.3Assessment of the methods
Beyond the comparison of retrieval and analysis methods, the performances of these methods, when applied to satellite, airborne and groundbased radiometers, must be rigorously assessed:
-to determine their accuracy (systematic and random errors),
-to learn about their performances in various weather situations.
The assessment can be carried out in different ways, which can support each other:
-taking into account the instrument characteristics (calibration performances, antenna pattern), results of the methods to be investigated must be compared with independent measurements and/or analyses, carefully selected to verify their adequacy to microwave data;
-results of the above methods, applied to the same spaceborne instrument, must be also checked in terms of the spatial coherence of the obtained fields, and sensitivity to variations of the relevant atmospheric parameters (humidity, temperature, wind, clouds).
For both aspects, the selection of case studies is an important point. Field campaigns about atmospheric processes over the oceans could be opportunities, as they provide good description of phenomena at small and meso-scales, with the larger scale environment given by increased meteorological network assimilated into meteorological models.
NWP models could also be used for comparison of products retrieved by different methods and for assimilation studies, because both can give interesting information on the problems with the satellite products and the models.
3.4Measurement facilities and techniques
Measurements from the ground, from aircraft or balloons and other satellites are needed to test and validate radiative transfer models before they can be used for developing satellite data analysis schemes, and parameter extraction methods. Such "ground truth" is also of extreme importance for the validation of space based measurements. Intensive field campaigns (e.g. EMAC of MACSI) will help to obtain observations from controlled test sites against which satellite data can be compared.
Comprehensive prelaunch tests are required to characterize individual instruments, in particular their radiometric calibration, spectral response, field of view and pointing all as a function of scan angle.
For these reasons facilities available for this purpose should be identified and their use for these activities as detailed above be promoted as part of this COST Action.
For future developments theoretical studies should ask for specific measurements in order to ensure progress in the development of new microwave instruments. Based on both theory and existing measurements, new measuring techniques and instruments for future projects might be recommended as part of the Action.
Project 1
Project 2
Project 3
Project 4
Development of radiative transfer models (RTM)
Retrieval, analysis and assimilation techniques
Assessment of the methods
Measurement facilities and techniques
-Development of improved accurate physical RTMs
-Development of improved fast parametric models
-Improvement of RTM elements i.e.
-surface emissivities (i.e. sea, land, ice, snow)
-water and ice clouds precipitation
-gaseous absorption
-refraction
-Model comparison and validation
-Model distribution to European Meteorological Services
-Development and improvement of retrieval/analysis methods for different data-types (e.g. nadir and limb sounding at different frequencies)
-Development and improvement of retrieval/analysis methods for different applications (e.g. temperature, cloud liquid water)
-Development and improvement of simplified retrieval/analysis schemes
-Promotion of exchange on information for retrieval/analysis techniques
-Promotion of new collaborative initiatives on retrieval/analysis techniques
-Intercomparison of different retrieval and analysis methods
-Comparison of model results with independent measurements and analysis
-Comparison of model results for different atmospheric situations
-Checking of special coherence of obtained fields
-Promotion of field campaigns over the ocean for verification purposes
-Identification of facilities capable of getting ground truth data for validation purposes and promotion of their use for this activity
-Identification of facilities which can carry out prelaunch tests of microwave instruments and promotion of their use for this activity
-Promoting the understanding of microwave radiometry techniques
-Carrying out of theoretical studies for recommendations of new measuring techniques and instruments
4.Timetable
The proposed length of the Action is four years. All projects will continue in parallel during this period.
The first year will be devoted to inventories and firmer specifications in the different projects. During the second, the third year and the first half of the fourth year, the specific tasks of each project are carried out. The last half year of the Action will be used to summarize the results of the projects and to produce a Final Report.
At least one major workshop would be arranged during the first half of the Action and another one during the last year of the Action.
5.Organization Management and Responsibilities
A Management Committee would be set up following the appropriate number of Signatories to the Action Memorandum of Understanding. A chairman would be elected and instructed to draft an outline Action plan based upon the items specified in Section 3 (The content of the Action). This plan would be approved by the Action Management Committee and submitted to the Technical Committee for Meteorology.
The Action would include cooperation between European meteorological organizations and research institutes.
The Action will consist of four projects. The Management Committee will elect coordinators for each project. The scope and content of the projects will be reviewed and if necessary amended by the Management Committee during the first year, in the light of the inventories of existing methods and the current interests of participation in the Action.
The Action Management Committee would report annually to the COST Senior Officials through the Technical Committee for Meteorology, but would provide short verbal or written reports to each meeting of the Technical Committee. The Management Committee will meet at leas twice per year.
Two workshops including all projects are foreseen during this Action. The projects may have their specific workshops as seen appropriate. A detailed Final Report would be written based on a series of technical reports produced by projects throughout the period of the Action.
6.Economic Dimension of the Action
As the Action will involve several meteorological institutes in most countries, a broad participation is expected, probably varying from 2 to 4 man years per country. Assuming the involvement of 10 countries, the estimate of annual scientific personnel cost is roughly ECU 2,2 million (30-40 man years). Annual overhead costs are estimated at ECU 0,3 million. Thus, the total cost of the four-year Action is approximately ECU 10 million.
Current status
The Action has been divided into 4 projects :
Project 1 : Development of radiative transfer models.
To make effective use of microwave radiance measurements of the atmosphere surface a good radiative transfer model (RTM) is a prerequisite for calculating radiances of each radiometer in use or under development (MSU, AMSU, MIMR, future limb sounders, etc.). The content of this project will be the :
Development of improved accurate physical RTMs ;
Development of improved fast parametric models ;
Improvement of RTM elements i.e.
surface emissivities (i.e. sea, land, ice, snow) ;
water and ice clouds, precipitation ;
gaseous absorption ;
refraction.
Model comparison and validation ;
Model distribution to European Meteorological Services and Research Institutes.
Project 2 : Retrieval, analysis and assimilation techniques.
To exploit the wealth of information from present and planned data, particularly from satellites, it will be necessary to develop powerful systems for analysing the data, using retrieval, analysis and data assimilation techniques. Main tasks will be the :
Development and improvement of retrieval/analysis methods for different data types (e.g. nadir and limb sounding at different frequencies)
Development and improvement of retrieval/analysis methods for different applications (e.g. temperature, cloud liquid water)
Development and improvement of simplified retrieval/analysis schemes
Promotion of exchange on information for retrieval/analysis techniques
Promotion of new collaborative initiatives on retrie-val/analysis techniques
The final output will be an assessment of the individual methods, likely applications, limitations, potential for improvement and further research, and error assessment.
Project 3 : Assessment of the methods.
The performances of retrieval and analysis methods, when applied to satellite, airborne and groundbased radiometers, must be rigorously assessed with a view to determining their accuracy and learning about their performances in various weather situations. Project tasks will be the :
Intercomparison of various retrieval and analysis methods
Comparison of model results with independent measurements and analysis
Comparison of model results for different atmospheric situations
Checking of special coherence of obtained fields
Promotion of field campaigns over the ocean for verification purposes
Part of the assessment process will be diagnostic and should iteratively lead to improvement or suggestions for further development of the techniques proposed in project 2.
The output from project 3 is an error assessment and characterisation of the limitations of the techniques proposed in project 2.
Project 4 : Measurement facilities and techniques.
Measurements from the ground, from aircraft or balloons and other satellites are needed to test and validate radiative transfer models before they can be used for developing satellite data analysis schemes, and parameter extraction methods. Such "ground truth" is also of extreme importance for the validation of space-based measurements. Intensive field campaigns (e.g. EMAC or MACSI) will help to obtain observations from controlled test sites against which satellite data can be compared. Tasks of this project will consist of :
Identifying facilities capable of providing independent data for validation purposes and promotion of their use for this activity
Identifying facilities which can carry out pre-launch tests of microwave instruments and promotion of their use for this activity
Carrying out of theoretical studies for recommendations of new measuring
The MC is preparing the workplan of the Action, to be adopted early in 1997.
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Belgium