Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project: A Review of Progress and Opportunities (1998)

The Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project (GCIP), formulated in 1990 by the World Climate Research Programme, is a joint effort of atmospheric scientists and hydrologists to develop data sets, models, and a research framework to understand land-atmosphere interactions on climatic time scales (i.e., seasonal, annual) in the Mississippi River basin. The overall goal of the GCIP is to demonstrate skill in predicting changes in water resources on time scales up to seasonal and annual, as an integral part of the climate system.

The National Research Council's (NRC's) GEWEX Panel was asked to evaluate the proposed research strategy for GCIP and to suggest revisions to the overall program, particularly the U.S.-based effort. The panel began this study by assessing the objectives of GCIP in the context of its overall goal. Based on this review, the panel recommends the following objectives for GCIP:

1. Determine and explain the annual, interannual, and spatial variability of the water and energy cycles within the Mississippi River basin.

2. Develop and evaluate coupled hydrologic-atmospheric models at resolutions appropriate to large-scale continental basins.

3. Develop and evaluate atmospheric, land, and coupled data assimilation schemes that incorporate both remote and in situ observations.

4. Provide access to comprehensive in situ, remote sensing, and model output data sets for use in GCIP research and as a benchmark for future studies.

5. Improve the utility of hydrologic predictions for water resources management up to seasonal and interannual time scales.

These five objectives are modified slightly from GCIP's previous objectives (GCIP, 1993). The panel based this review on the modified objectives.

Substantial progress has been made toward GCIP's objective of characterizing the variability of water and energy cycling in the Mississippi basin. Further progress will require increased care in the estimation of such critical variables as precipitation and surface radiative fluxes. Improvements in the use of available measurements could markedly enhance the value of GCIP. Rigorous investigations of local and remote forcing of hydroclimatic anomalies are needed to satisfy the objective of explaining the observed variations of the water and energy cycles in the Mississippi River basin.

Some of the most advanced regional-scale models in the world are being developed by GCIP. Parameterizations in these models, including clouds, precipitation, and radiation require special attention, however. Large-scale hydrologic modules also have to be included in these models. Systematic comparisons, such as those being carried out by the Project for Intercomparison of Land-Surface Parameterization Schemes (PILPS) over parts of the Mississippi basin are important for understanding model biases. High-resolution modeling studies—for example, of how the land surface affects the Great Plains low-level jet—are also important because many important features of low-level jets are not easily observed with the current sparse observational network.

GCIP is developing a comprehensive set of regional analysis archives from three regional data assimilation systems (Eta, Mesoscale Analysis and Prediction System, Regional Finite Element). These regional archives are likely to be highly model dependent, however, and must be compared with each other as well as with global analyses and field observations. Surface products will be one of the most innovative and potentially most problematic regional analysis outputs. These surface products have to be compared to outputs from hydrologic models, especially Land Data Assimilation Systems (LDASs), which are just beginning and require special attention. Finally, given all the scientific developments that will occur in the development of analysis methods for these surface variables as well as atmospheric variables, including longwave radiation and precipitation observations, it is important that GCIP begin to plan for an eventual reanalysis of GCIP projects.

GCIP's data collection and management effort has done a commendable job of beginning the initial data acquisition and archiving. It now has to add a scientific users group to identify data sets of highest priority to GCIP researchers. The scientific users group should recommend methods for the efficient archiving and reprocessing of high-volume Next-Generation Weather Radar (NEXRAD) data for climatological precipitation estimates. Methods to archive and access

research data, especially remote sensing data sets created by individual GCIP scientists, also have to be developed.

GCIP's research focus on hydrology is primarily geared toward addressing atmosphere-land-surface interactions. The panel recognizes, however, that there are many other important hydrologic issues not covered under the purview of GCIP's research—such as surface and subsurface interactions—that are critical for an ''end-to-end" approach to the hydrology of the Mississippi River basin. Furthermore, although GCIP's primary goal has been identified as demonstrating skill in predicting changes in water resources on various time scales, GCIP is not directly involved in the development of any new water resources management models. It is anticipated that GCIP's observational and modeling efforts will enhance the "front-end" modeling tools used for water resources management purposes. At the present time, the panel believes that the increased involvement of some additional key agencies is needed. These agencies could address some of the hydrologic and water resources issues that are not the main focus of GCIP but are critical nonetheless to GCIP's overall goals. Fostering an interactive dialogue between GCIP and the water resources management community to formulate research priorities currently not addressed by GCIP is critical to the program's success. The panel recognizes that many of the crucial hydrologic research questions have been identified in the NRC report Opportunities in the Hydrologic Sciences (NRC, 1991) and that much of this research would be of great benefit to GCIP and any future large-scale hydrologic studies.

GCIP is the first international project to bring together the hydrologic and meteorological science communities for a common research goal. This cooperation has already been beneficial to both communities and is the basis for the initial success of GCIP. Thus, the panel's primary recommendation is that GCIP should stay the course and continue with the implementation of its scientific research plans. In addition, the panel recommends that GCIP focus its efforts in the following areas:

• Develop accurate quantitative precipitation estimates based on high-resolution weather radar observations.

• Develop improved large-scale estimates of soil moisture consistent with large-scale estimates of precipitation, evaporation, and runoff.

• Further improve the coupling between atmospheric and land surface hydrologic models.

• Develop and apply coupled land data assimilation systems.

• Prepare data archives to facilitate future reanalyses.

• Foster active dialogue between GCIP and the water management community.

From a wider national perspective, GCIP does not address hydroclimatic phenomena that are characteristic of the semiarid U.S. Southwest, a region where the availability of water is a critical resource issue as well as a challenging scientific problem. Applying the methodologies and technical facilities developed for GCIP to a study of the Colorado River basin and surrounding mountain regions is a challenge for the future.

GCIP accomplishments include the following: (1) the most comprehensive accounting to date of atmospheric and surface water and energy budgets on a continental scale; (2) the advancement of land surface parameterizations and atmospheric models incorporating hydrologic principles; (3) the development of new regional land surface data assimilation methods using new high-resolution precipitation observations; (4) the development of a comprehensive GCIP data set that will be a basic support for twenty-first century developments in remote sensing and data assimilation; and (5) a new paradigm for water resources management utilizing new operational weather prediction analyses and forecasts. When fully implemented, GCIP can be expected to strengthen our nation's capability for climate prediction and water resource management. It will provide a sound basis for hydroclimatological research at the beginning of the twenty-first century.