GEWEX Continental-Scale International Project Research Articles

Latent Heat Flux and Canopy Conductance Based on Penman–Monteith, Priestley–Taylor Equation, and Bouchet’s Complementary Hypothesis

AbstractA novel method is presented to analytically resolve the terrestrial latent heat flux (λE) and conductances (boundary layer gB and surface gS) using net radiation (RN), ground heat flux (G), air temperature (Ta), and relative humidity (RH). This method consists of set of equations where the two unknown internal state variables (gB and gS) were expressed in terms of the known core variables, combining diffusion equations, the Penman–Monteith equation, the Priestley–Taylor equation, and Bouchet’s complementary hypothesis. Estimated λE is validated with the independent eddy covariance λE observations over Soil Moisture Experiment 2002 (SMEX-02); the Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project (GCIP) selected sites from FLUXNET and tropics eddy flux, representing four climate zones (tropics, subtropics, temperate, and cold); and multiple biomes. The authors find a RMSE of 23.8–54.6 W m−2 for hourly λE over SMEX-02 and GCIP and 23.8–29.0 W m−2 for monthly λE over the FLUXNET and tropics. Observational and modeled evidence in the reduction in annual evaporation (E) pattern on the order of 33% from 1999 to 2006 was found in central Amazonia. Retrieved gS responded to vapor pressure deficit, measured λE, and gross photosynthesis in a theoretically robust behavior. However, the current scheme [Penman–Monteith–Bouchet–Lhomme (PMBL)] showed some overestimation of λE in limited soil moisture regimes. PMBL provides similar results when compared with another Priestley–Taylor–based λE estimation approach [Priestley–Taylor–Jet Propulsion Laboratory (PT-JPL)] but with the advantage of having the conductances analytically recovered.

High‐resolution ensemble surface insolation estimates through assimilation of coarse‐scale retrievals into a simple physical model: 2. Ensemble implementation and Shortwave Radiation Budget data assimilation

A simple surface insolation model was used in conjunction with the Ensemble Kalman Filter (EnKF) to ultimately derive an ensemble of high‐resolution posterior insolation fields conditioned on the GEWEX Continental Scale International Project and GEWEX Americas Prediction Project (GCIP/GAPP) Shortwave Radiation Budget (SRB) product. When compared to ground‐based observations, the ensemble of prior estimates was shown to reasonably capture the space‐time variability and uncertainty in surface insolation except in cases where the Visible Infrared Solar‐Infrared Split Window Technique (VISST) product did not properly detect clouds. The EnKF was chosen to provide a systematic framework for merging the insolation information contained in the prior estimates with the well‐validated SRB product. When compared to independent ground‐based observations, the posterior ensemble mean estimate showed a significant reduction in error relative to the prior. The resulting posterior ensemble mean error statistics are of similar magnitude to other recent studies at similar spatial resolution. The reduction in error is highest in cloudy‐sky conditions (where prior uncertainty is largest) and lowest in clear‐sky conditions (where insolation is much easier to predict). A significant value added through application of the data assimilation scheme is a posterior estimate that provides an ensemble that captures the complex space‐, time‐, and state‐dependent uncertainty that would otherwise be very difficult to obtain using a more traditional forward modeling approach.

Comparison of GCIP and stage III radar-rainfall estimates over the Mississippi River Basin for 1997

Summary The worth of archival, long-term, high spatial and temporal resolution radar-rainfall estimates for hydrologic modeling has not yet been established. In particular, uncertainties remain regarding accuracy at the event time-scale, which is a modeling requirement for mass conservation. To explore this issue, an archival radar-rainfall precipitation dataset produced from the National Reflectivity Composite for the GEWEX Continental-Scale International Project (GCIP) and US National Weather Service, Weather Surveillance Radar 1988-D (WSR-88D) Stage III (SIII) data were compared with data from 982 National Climatic Data Center (NCDC) rain gages throughout the Mississippi River Basin. These two datasets are derived from the same radar-reflectivity observations but they are different. The GCIP dataset was developed from low resolution (5 dBZ bins) national radar mosaic imagery using a systematic procedure that employed one climatologically-derived reflectivity-rainfall relationship, while the SIII radar-rainfall estimates are produced using a processing algorithm developed by the US National Weather Service that employs near-real time adjustment and varied parameterizations in different River Forecast Centers around the Mississippi River Basin. Statistical analyses were performed to compare both radar-rainfall datasets with rain gage data on an event time-scale for the entire year of 1997. The investigation shows that the GCIP dataset, given a change in the equation used to calculate rainfall from reflectivity, has a lower root mean square difference for the majority of gages in four of the five river forecast centers that cover the Mississippi River Basin compared to SIII in 1997.

The multi‐institution North American Land Data Assimilation System (NLDAS): Utilizing multiple GCIP products and partners in a continental distributed hydrological modeling system

Results are presented from the multi‐institution partnership to develop a real‐time and retrospective North American Land Data Assimilation System (NLDAS). NLDAS consists of (1) four land models executing in parallel in uncoupled mode, (2) common hourly surface forcing, and (3) common streamflow routing: all using a 1/8° grid over the continental United States. The initiative is largely sponsored by the Global Energy and Water Cycle Experiment (GEWEX) Continental‐Scale International Project (GCIP). As the overview for nine NLDAS papers, this paper describes and evaluates the 3‐year NLDAS execution of 1 October 1996 to 30 September 1999, a period rich in observations for validation. The validation emphasizes (1) the land states, fluxes, and input forcing of four land models, (2) the application of new GCIP‐sponsored products, and (3) a multiscale approach. The validation includes (1) mesoscale observing networks of land surface forcing, fluxes, and states, (2) regional snowpack measurements, (3) daily streamflow measurements, and (4) satellite‐based retrievals of snow cover, land surface skin temperature (LST), and surface insolation. The results show substantial intermodel differences in surface evaporation and runoff (especially over nonsparse vegetation), soil moisture storage, snowpack, and LST. Owing to surprisingly large intermodel differences in aerodynamic conductance, intermodel differences in midday summer LST were unlike those expected from the intermodel differences in Bowen ratio. Last, anticipating future assimilation of LST, an NLDAS effort unique to this overview paper assesses geostationary‐satellite‐derived LST, determines the latter to be of good quality, and applies the latter to validate modeled LST.

Surface radiation budgets in support of the GEWEX Continental‐Scale International Project (GCIP) and the GEWEX Americas Prediction Project (GAPP), including the North American Land Data Assimilation System (NLDAS) project

In support of the World Climate Research Program GEWEX Continental‐Scale International Project (GCIP) and the GEWEX Americas Prediction Project (GAPP), real‐time estimates of shortwave radiative fluxes, both at the surface and at the top of the atmosphere, are being produced operationally by the National Oceanic and Atmospheric Administration (NOAA)/National Environmental Satellite Data and Information Service using observations from GOES images. The inference scheme has been developed at the Department of Meteorology, University of Maryland, and the atmospheric and surface model input parameters are produced and provided by the NOAA/National Centers for Environmental Prediction. The radiative fluxes are being evaluated on hourly, daily, and monthly timescales using observations at about 50 stations. The satellite estimates have been found to be within acceptable limits during snow‐free periods, but the difficulty in detecting clouds over snow affects the accuracy during the winter season. In what follows, this activity is discussed, and evaluation results of the derived fluxes against ground observations for time periods of 1–2 years are presented.

GCIP water and energy budget synthesis (WEBS)

As part of the World Climate Research Program's (WCRPs) Global Energy and Water‐Cycle Experiment (GEWEX) Continental‐scale International Project (GCIP), a preliminary water and energy budget synthesis (WEBS) was developed for the period 1996–1999 from the “best available” observations and models. Besides this summary paper, a companion CD‐ROM with more extensive discussion, figures, tables, and raw data is available to the interested researcher from the GEWEX project office, the GAPP project office, or the first author. An updated online version of the CD‐ROM is also available at http://ecpc.ucsd.edu/gcip/webs.htm/. Observations cannot adequately characterize or “close” budgets since too many fundamental processes are missing. Models that properly represent the many complicated atmospheric and near‐surface interactions are also required. This preliminary synthesis therefore included a representative global general circulation model, regional climate model, and a macroscale hydrologic model as well as a global reanalysis and a regional analysis. By the qualitative agreement among the models and available observations, it did appear that we now qualitatively understand water and energy budgets of the Mississippi River Basin. However, there is still much quantitative uncertainty. In that regard, there did appear to be a clear advantage to using a regional analysis over a global analysis or a regional simulation over a global simulation to describe the Mississippi River Basin water and energy budgets. There also appeared to be some advantage to using a macroscale hydrologic model for at least the surface water budgets.

Preface to special section: GEWEX Continental‐Scale International Project (GCIP)‐3

[1] The Global Energy and Water Cycle Experiment (GEWEX) started the GEWEX Continental-Scale International Project (GCIP) in 1995. The goals of GCIP were to understand the hydrology and water balance of the Mississippi River Basin. Results from GCIP have already been published in two previous special issues of the Journal of Geophysical Research-Atmospheres, in 1996 and 1999. GCIP held its grand finale conference in New Orleans, Louisiana, in May 2002. Participants at that conference along with the scientists funded through the GCIP program were invited to contribute papers to the final GCIP special section in JGR-Atmospheres. This special section, GCIP-3, presents the current state-of-the-art in our understanding of (1) water and energy budget studies, (2) warm season precipitation, (3) predictability and prediction systems, (4) land surface hydrology models, (5) coupled land-atmosphere models, (6) soil moisture, and (7) climate and water resources applications. The research areas cover observations, modeling, process studies, and water resources applications. [2] GCIP has now transitioned into the GEWEX America Prediction Project (GAPP). The mission of GAPP is to demonstrate skill in predicting changes in water resources on timescales up to seasonal and annual, as an integral part of the climate system. [3] The program manager for both GCIP and GAPP is Richard Lawford, of the Office of Global Programs, National Oceanic and Atmospheric Administration. The quality and quantity of science in the three JGR-Atmospheres special sections are due to his leadership and tireless efforts and those of his colleagues, including John Leese, Paul Houser, Eric Wood, Dennis Lettenmaier, Jin Huang, and Jared Entin. Jin Huang is the coordinator of the GCIP-3 special section. [4] The GCIP-3 special section will be printed in two parts. Part 1 is printed in this issue. Part 2 will be printed on 27 November 2003 in issue D22.

Data collection and management for Global Energy and Water Cycle Experiment (GEWEX) Continental‐Scale International Project (GCIP)

The Global Energy and Water Cycle Experiment (GEWEX) Continental‐Scale International Project (GCIP) was focused on the Mississippi River basin to take advantage of the existing meteorological and hydrological networks that were upgraded with new Doppler radars, wind profilers, and automatic weather stations together with an upgraded version of the Geostationary Operational Environmental Satellites (GOES) operated by the United States. The Mississippi River basin encompasses a wide range of climate, soil moisture conditions, vegetation types, and surface topography. The GCIP Science Plan identified the major phase of GCIP as the 5‐year Enhanced Observing Period (EOP), targeted for 1995 to 2000, during which original data sets would be assembled for the GCIP database. GCIP organized these data into in situ, model output and satellite remote sensing data. Data from three mesoscale models were included with the start dates ranging from May 1995 to April 1997. A number of GCIP Initial Data Sets (GIDS) were prepared, starting in 1993, to provide the data services support during the buildup period before the 5‐year EOP. The data sets were compiled for on‐line access by GCIP investigators and were also published on CD‐ROM. A number of special data sets, in addition to the EOP data sets, were compiled during the course of GCIP. Some of the most significant special data sets are summarized in this paper.

Archival precipitation data set for the Mississippi River Basin: development of a GIS-based data browser

To aid in modeling studies over the Mississippi River Basin, we have developed an archival precipitation data set for the GEWEX Continental-Scale International Project. The data set spans from 1996–2000, a 5-year continuous period of record. Inputs for the data set are the National Reflectivity Composite that we obtained in Hierarchical Data Format. The size of the input data is 40 GB. In order to significantly reduce the size of the data set, we have developed an efficient data format that can be read from a disc faster than the input Hierarchical Data Format. We have also developed browsing tools for users of the data set. The data browser allows for fast and efficient viewing of the precipitation data and was produced using a common Geographic Information System software. Peripheral data are also included in the browser as an aid to the user.

Geostationary satellite parameters for surface energy balance

Since January 1996, surface climate parameters for the continental US have been produced in real time at NOAA/NESDIS from GOES-8 observations, and have been made available to the NOAA/NWS National Centers for Environmental Prediction (NCEP), to the GEWEX Continental-Scale International Project (GCIP) community, as well as to the public. The products include radiative fluxes, such as surface and top of the atmosphere short-wave radiation, photosynthetically active radiation (PAR), cloud fraction, as well as surface temperature. The data can be used for evaluating corresponding parameters as produced from the NCEP (ETA) model or other meso-scale models, as inputs to hydrological models for computing evapotranspiration and snow-melt, for estimating net primary productivity, and other applications. As of July 2000, this product has become operational at NOAA/NESDIS, and it is distributed to the public within one day after being produced. In this paper provided will be background information about this product and identified will be issues that require further attention.

Diagnosing warm season precipitation over the GCIP region from a GCM and reanalysis

A 45‐year simulation using a global general circulation model (GCM), the National Center for Atmospheric Research (NCAR) Community Climate Model version 3 (CCM3), forced with observed sea surface temperatures (SST), and 39 years of global National Centers for Environmental Prediction (NCEP) reanalyses were analyzed to determine Mississippi River basin warm season (May, June, July or MJJ) wet and dry year composites in the water and energy budgets. Years that have increased MJJ soil moisture over the Global Energy and Water Cycle Experiment (GEWEX) Continental‐Scale International Project (GCIP) region also have high precipitation, lower surface temperature, decreased Bowen ratio, and reduced 500‐hPa geopotential height (essentially reduced MJJ ridging). The reverse is true for years that have reduced MJJ soil moisture. Wet years are also accompanied by a general increase in moisture transport from the Gulf of Mexico into the central United States. There is some indication (though weaker) that soil moisture may then affect precipitation and other quantities and be affected in turn by 500‐hPa geopotential heights. The correlations are somewhat low, however, demonstrating the difficulty in providing definitive physical links between the remote and local effects. Analysis of two individual years with an extreme wet event (1993) and an extreme dry event (1988) yields the same general relationships as with the wet and dry composites. The composites from this study are currently serving as the basis for a series of experiments aimed at determining the predictability of the land surface and remote SST on the Mississippi River basin and other large‐scale river basins.

A midterm report on the GEWEX Continental‐Scale International Project (GCIP)

The GEWEX Continental‐Scale International Project (GCIP) is being carried out in the Mississippi River Basin under the auspices of the World Climate Research Program and its Global Energy and Water Cycle Experiment (GEWEX). This paper provides a brief history of GCIP's development and its mission and objectives. The scientific achievements of GCIP are then reviewed against each of these objectives. The paper concludes with a brief discussion of options for the period beyond 2000 after the current phase of the project is completed.

Assessment of land‐surface energy budgets from regional and global models

The surface energy budgets estimated from the 0‐ to 12‐hour forecasts of three operational model suites and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) Reanalysis are analyzed at local to continental scales. The models are (1) the Eta model, (2) the Mesoscale Analysis and Prediction System (MAPS), and (3) the Global Environmental Multiscale (GEM) model. The first two are regional, while the third one is global with a variable grid with a resolution over North America that is equivalent to that of the regional models. This assessment of one summer month (August 1997) and one winter month (January 1998) has the purpose of estimating the reliability of the surface energy budgets within the context of the Global Energy and Water Cycle Experiment (GEWEX) Continental‐Scale International Project (GCIP) goals. Satellite estimates were used to evaluate the downward short wave radiation at the surface, while measurements from the southern Great Plains region were used to evaluate the model computed surface energy budget estimates. The results show that the surface short wave radiation biases of the models are widespread and of the order of 25–50 W m−2 and averaging over larger areas does not help reduce the differences. These biases are compensated by the other long and short wave radiation terms so that the resulting errors in the net radiation are smaller. During August 1997, continental east‐west gradients of latent heat flux and Bowen ratio were surprisingly dissimilar among models. Still, the Bowen ratio estimated from the Eta and GEM models was close to observations over the southern Great Plains region, while both the Reanalysis and MAPS had ratios that at least doubled the observed ones. In the case of MAPS a revised latent heat flux formulation was introduced in fall 1997, and, subsequently, for January 1998, estimates were closer to the other models' estimates. However, during January 1998 all models had difficulties reproducing the Bowen ratios from observations. Further, daily time series showed that models' estimates also tended to miss the amplitude of the day‐to‐day variability. It is conceivable that this may be the result of difficulties in parameterizing the total cloud cover, and, particularly, attenuation by clouds may still be insufficient.

The CERES/ARM/GEWEX Experiment (CAGEX) for the Retrieval of Radiative Fluxes with Satellite Data

Results from a temporally intensive, limited area, radiative transfer model experiment are on-line for investigating the vertical profile of shortwave and longwave radiative fluxes from the surface to the top of the atmosphere (TOA). The CERES/ARM/GEWEX Experiment (CAGEX) Version 1 provides a record of fluxes that have been computed with a radiative transfer code; the atmospheric sounding, aerosol, and satellite-retrieved cloud data on which the computations have been based; and surface-based measurements of radiative fluxes and cloud properties from ARM for comparison. The computed broadband fluxes at TOA show considerable scatter when compared with fluxes that are inferred empirically from narrowband operational satellite data. At the surface, LW fluxes computed with an alternate sounding dataset compare well with pyrgeometer measurements. In agreement with earlier work, the authors find that the calculated SW surface insolation is larger than the measurements for clear-sky and total-sky conditions. This experiment has been developed to test retrievals of radiative fluxes and the associated forcings by clouds, aerosols, surface properties, and water vapor. Collaboration is sought; the goal is to extend the domain of meteorological conditions for which such retrievals can be done accurately. CAGEX Version 1 covers April 1994. Subsequent versions will (a) at first span the same limited geographical area with data from October 1995, (b) then expand to cover a significant fraction of the GEWEX Continental-Scale International Project region for April 1996 through September 1996, and (c) eventually be used in a more advanced form to validate CERES.

Application of a macroscale hydrologic model to estimate the water balance of the Arkansas‐Red River Basin

An approach for estimation of the parameters of a macroscale land surface hydrology model is illustrated for the Global and Water Cycle Experiment (GEWEX) Continental Scale International Project (GCIP) large‐scale area southwest (LSA‐SW) which essentially comprises the Arkansas‐Red River basin. The macroscale land surface hydrology model parameters were estimated for 44 catchments within LSA‐SW with areas ranging from 180 to 7100 km2 using an automated search procedure. The catchment parameters were then linearly interpolated and overlaid on a one degree grid, which was used to represent the drainage network. The macroscale grid network model was run off‐line at a daily time step, forced by gridded station precipitation and potential evapotranspiration. The model‐generated long‐term mean streamflows were compared with observations (corrected for management effects such as reservoir storage and diversions) and were found to agree to within one percent for the Arkansas River and about two percent for the Red River. For both rivers, the model underestimates the seasonal peak streamflow in late spring, and overestimates the late summer and early fall minimum. Model‐derived evapotranspiration, spatially averaged over the entire Arkansas‐Red basin, was compared to evapotranspiration derived from an atmospheric moisture budget of the Arkansas‐Red River basin. On an average annual basis, for the period 1973–1986, the two agree to within one percent. The mean seasonal cycles for the two estimates agree quite closely from late winter to midsummer. However, the hydrologic model estimates less evapotranspiration in the fall, and more in midwinter, than the atmospheric budget.

The Global Energy and Water Cycle Experiment (GEWEX) Continental‐Scale International Project (GCIP): An overview

This paper provides a brief historical overview of the Global Energy and Water Cycle Experiment (GEWEX) Continental‐Scale International Project (GCIP), the basis for selecting the principal site for this experiment, the scientific objectives, and the strategic planning that has been brought to bear in preparing for the implementation of the project. It also provides a summary of the 25 papers published in this special issue, which collectively represent the launching point for research in the GEWEX's initiatives on continental‐scale hydrometeorology.

Climate variability, hydrology and water resources: do we communicate in the field? (HAPEX, FIFE, GEWEX, GCIP and what next?)

All the field “experiments” indicated in the sub-title above include the hydrological cycle in some or all of its aspects and thus require the use of “hydrological sciences”. Is hydrology actually included in them? This depends on the definition of hydrology used. The paper briefly describes the different definitions of “hydrology” in the context of the scientific objectives and concepts of these experiments and related scientific activities. It further evaluates the role actually assigned to hydrologists in past cross-disciplinary endeavours to conduct “field experiments” and goes on to propose adjustments to this role in the GEWEX Continental-scale International Project (GCIP). Related activities are analysed in some detail in this framework and proposals are presented for increased inter-disciplinary communication.

Intercomparison of land‐surface parameterizations launched

One of the crucial tasks for climatic and hydrological scientists over the next several years will be validating land surface process parameterizations used in climate models. There is not, necessarily, a unique set of parameters to be used. Different scientists will want to attempt to capture processes through various methods “for example, Avissar and Verstraete, 1990”. Validation of some aspects of the available (and proposed) schemes' performance is clearly required. It would also be valuable to compare the behavior of the existing schemes [for example, Dickinson et al., 1991; Henderson‐Sellers, 1992a].The WMO‐CAS Working Group on Numerical Experimentation (WGNE) and the Science Panel of the GEWEX Continental‐Scale International Project (GCIP) [for example, Chahine, 1992] have agreed to launch the joint WGNE/GCIP Project for Intercomparison of Land‐Surface Parameterization Schemes (PILPS). The principal goal of this project is to achieve greater understanding of the capabilities and potential applications of existing and new land‐surface schemes in atmospheric models. It is not anticipated that a single “best” scheme will emerge. Rather, the aim is to explore alternative models in ways compatible with their authors' or exploiters' goals and to increase understanding of the characteristics of these models in the scientific community.