GEWEX and Other Global, Long-Term Datasets

Assessments of GEWEX Data Products

Discussions at the 2003 meetings of the GEWEX Radiation Panel (GRP) and its Working Group on Data Management and Analysis (WGDMA) led to undertaking a formal assessment, like the SPARC WAVAS, of all of the GRP/WGDMA global, long-term data products, namely clouds (ISCCP), precipitation (GPCP), radiation (both ERB and SRB) and aerosols (GACP). This idea is motivated by the current mature state of these data products (i.e., a lot of work has already been completed to evaluate their accuracies) and the variety of uses (and misuses) of these data products for scientific research and other applications, as well as the emphasis in the most recent IPCC report on quantifying uncertainties of climate sensitivity and IPCC plans to highlight the role of water in climate in their next report. Such assessments would provide specific information about the quality and suitability of these datasets for the diagnosis of the global energy and water cycle and the study of variations of the climate. If these projects are continued through the remainder of GEWEX Phase II, then this assessment would also set the stage for a proposed coordinated re-analysis of all of these data products.

There are many open questions about these data, both whether the observed variations are accurately portrayed (their quality) and whether the interpretation of the observed variations is correct (their completeness). The value of these datasets for many kinds of studies, particularly concerning the larger variations at shorter time scales (weather and process studies), has already been demonstrated but there are still questions associated with subtle problems in the retrieval procedures that produce inconsistent results when used together to diagnose the energy and water cycle. Moreover, when using these data products to study climate variations, the uncertainties associated with the heterogeneous nature of their time records, particularly associated with instrument calibration uncertainties, becomes crucial.

These assessments will focus on all those extensive data products that provide global (or nearly so), long-term (at least decadal) coverage, not just the GEWEX products; and will also exploit any and all other datasets (from surface-based and experimental satellite measurements) to understand the accuracy of these products. It is expected that these assessments will take about 2 years to complete.

Working groups have been formed to:

(1) obtain an complete overview of all available datasets describing the same basic quantities (clouds, precipitation, radiation, aerosols),

(2) develop a strategy for the assessment, particularly establishing a basis for estimating the range of uncertainties in such data on various time and space scales, and

(3) identify and solicit requirements from stakeholders.

• Clouds
• Precipitation
• Radiation
• Aerosols

A. Assessment Objectives

The overarching goal of the GEWEX global data analysis projects is to obtain observations of the elements of the global energy and water cycle with sufficient detail and accuracy to diagnose the causes of recent climate variations in terms of the energy and water exchanges among the main climate components (atmosphere, ocean, land, cryosphere, biosphere). A number of global, long-term data products have now been produced, primarily concerning clouds, precipitation, top-of-atmosphere and surface radiative fluxes and aerosols. These products have been produced separately, so there is a need to evolve to ever-more consistent analysis products to achieve the objective. To establish a foundation for better global energy and water cycle analysis, the GEWEX (and if possible other) data products need to be evaluated by comparison with each other and by investigating the causes of differences.

B. Requirements for Data Product Accuracy

Several general uses for these data products can be identified. The most general is for scientific analysis of the climate system and its variability in terms of the energy and water cycle, including the variations in exchanges of radiative and latent heat that drive the atmospheric and oceanic circulations. Long-term variations of these quantities help characterize the changing climate. The complexity of the climate system requires modeling studies as well as observational studies, so another general use is for the development and evaluation of climate models. However, the extension of forecasting range from weekly into seasonal ranges and beyond also requires use of the same datasets for the development and evaluation of forecasting models as well. The required accuracy of the data products for each of these uses depends on the space-time scales of the variability. A complete error assessment over the whole range of scales from weather to global-decadal is needed.

C. Assessment Task Priorities:

(1) Collect, document, characterize and provide error estimates for the main long-term data products and post at least the monthly mean datasets online for further evaluation and analysis.

(2) Compile a bibliography of research papers that evaluate these data products.

(3) Evaluate the monthly-mean, large-scale (regional) variability of the data products by comparing them with various other products and investigating causes for differences. Also compare with model variability.

(4) Evaluate systematic variations of the data products (diurnal, seasonal); compare with model variations.

(5) Evaluate the instantaneous accuracy of the data products.

D. Assessment Procedure

The assessment will be open to all contributions but the focus will be on the assessment of the GEWEX data products from ISCCP, GACP, GPCP and SRB, together with the ERB products (from NIMBUS-7 ERB, ERBE, SCARAB and CERES). The assessment of the GEWEX data products will be reported as a consensus of analysis results, but individual contributions on related datasets or subjects will also be included in the final report.

To carry out the broadest possible evaluation, Web site will be established on which will be posted: (1) statement of goals & requirements, (2) the detailed assessment plans, (3) all the monthly mean datasets to be compared/assessed (eventually with ftp access to full time resolution versions) and (4) tools for analysis of the data product differences. The monthly mean data will be in the form of maps as well as global and zonal monthly summaries.

The following listing indicates datasets Now Available from GEWEX and selected projects. The associated Web sites provide more detailed information about these data products and how to obtain them.

Reconstructions of the Radiative Forcings of Climate Change Used in the GISS
Climate Model Simulations

                Well-Mixed Anthropogenic Greenhouse Gases
                Stratospheric Aerosols
                Solar Irradiance
                Ozone
                Tropospheric Aerosols