Introduction

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.