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Updated 24 January 2006

The Global Water Cycle
USGCRP Fiscal Year 2003 Accomplishments

 

USGCRP
Program Elements

Atmospheric Composition

Ecosystems

Global Carbon Cycle

Decision-Support Resources Development and Related Research on Human Contributions and Responses

Climate Variability and Change

Global
Water Cycle

Cloud feedback effects:

A combination of improved models and better measurement technology is closing the gap between observed and modeled quantification of radiative fluxes in the atmosphere (flows of incoming solar radiation and outgoing radiation from the Earth through the atmosphere). These new results are helping to improve the radiative transfer calculations in climate models being developed for the start of the next international assessment by the IPCC.

The radiative transfer components of climate models account for how water vapor, other gases, and cloud droplets scatter and absorb solar radiation and absorb and reradiate infrared radiation. Studies in the mid-1990s indicated that clouds absorbed roughly 40 percent more sunlight than calculated by climate models, suggesting that the inaccuracy of model calculations of radiation absorption by clouds was undermining the ability of the models to simulate climate correctly. However, new model calculations of cloud absorption, using state-of-the-art radiative transfer models, closely match recently analyzed cloud observation data from DOE's Atmospheric Radiation Measurement Enhanced Shortwave Experiment-II, which was conducted at a site in Oklahoma. This data set was unique in its redundant measurements and high quality control. In addition, studies have shown that, by modifying models to account for radiation absorbed by microscopic aerosol particles such as dust and soot, the gap between models and observations can be narrowed even further.

Multi-scale simulation of cloud effects on climate:

The global circulation of the atmosphere has been simulated using a radically new kind of mathematical model that simulates cloud processes directly. A high-resolution cloud model is run "inside" the lower-resolution global model to create a "multi-scale modeling framework" (MMF). Results from the MMF show major improvements, relative to earlier models, for the simulation of many kinds of weather and climate systems, including the most powerful cloud systems in the tropics. Particularly encouraging is the improved representation of the diurnal cycle of precipitation. This modeling tool is one of several important research components being applied to achieve a better quantitative understanding of climate feedbacks related to atmospheric convection and hydrologic and cloud processes. The ever-increasing availability of new computational advances as well as high-quality observational data from field programs is making such advances possible. (See Figure 11)

Figure 11: Modeling the diurnal cycle of precipitation.

Modeling the diurnal cycle of precipitation. The mean June-July-August local solar time of non-drizzle precipitation frequency maximum has been simulated with the standard Community Atmosphere Model (CAM) (upper panel); super-parameterization CAM (middle panel); and from observational data set by Dai (bottom panel). Non-drizzle precipitation was defined as producing mean precipitation rate in excess of 1 mm per day over a 3-hour interval.

Source: From Khairoutdinov et al. 2004, J. Atmos. Sci., submitted.

Water vapor measurement:

Because water vapor is by far the most abundant of the greenhouse gases, accurate water vapor measurements are essential for understanding atmospheric processes and representing and evaluating them in regional and global climate models. Instruments and observational protocols are needed to measure water vapor accurately. Researchers have succeeded in reducing measurement uncertainties in water vapor concentrations from greater than 25% to less than 3% using ground-based instrumentation, such as Raman lidar and microwave radiometer. (See Figure 12)

Figure 12: Water vapor measurement using Raman lidar.

Water vapor measurement using Raman lidar. Laser technology captures continuous, vertical distributions of water vapor over the Atmospheric Radiation Measurement’s Southern Great Plains Site for the 29 November - 2 December 2002 time frame. The figure represents three measurements that are important for climate studies: (a) ratio of water vapor to dry air (CART Raman Lidar Mixing Ratio Data); (b) relative humidity (CART Raman Lidar RH Data); and (c) the total atmospheric water vapor contained in a vertical column of unit crosssectional area extending from the surface to the top of the atmosphere (precipitable water vapor).

Credit: David Turner, University of Wisconsin.

ICESat launched:

The Ice, Clouds, and Land Elevation Satellite (ICESat) was successfully launched in January 2003. This Earth Observing System mission, covering the Arctic, the Antarctic, continental high elevations, and the oceans, measures water cycle variables, including ice sheet mass balance, cloud and aerosol heights, and land topography and vegetation characteristics. ICESat provides, primarily, land-ice and sea-ice altimetry products, with cloud/aerosol Lidar and land/vegetation altimetry as secondary products. ICESat will provide multi-year elevation data needed to determine ice sheet mass balance around the globe, in addition to the polar-specific coverage over the Greenland and Antarctic ice sheets. IceSAT observations, together with those of the Terra and Aqua satellites, will more accurately quantify the changes in the Greenland Ice sheet and the interannual changes of Arctic ice. These regions, identified by models to be highly sensitive to climate warming, already show signs of a strong climate change signal with a shrinking of perennial ice-covered regions.

Figure 13: The water cycle with respect to polar ice and sea level.

The water cycle with respect to polar ice and sea level. Future changes in ice sheet mass balance will be a complex function of accumulation and melting as well as dynamic ice sheet behavior. Sea level response is still not well understood. ICESat observations will help answer important questions about trends that affect the ice mass balance and sea level change.

Credit: NASA.

Enhanced sea ice observations:

Data from the EOS Aqua satellite are providing the research community with sea ice data products at a higher spatial resolution and a greater spectral range than previously possible. To make these data products a useful research tool, a comprehensive sea ice validation program is currently underway. An Arctic field campaign with the NASA P3 aircraft was completed in March 2003. Enhanced calibration of satellite microwave sensors will permit a more accurate monitoring of sea ice variability and provide data for the validation of coupled models that require improved sea ice component models in order to better understand and predict polar responses to global climate change.

Water cycle observation from space:

NASA’s Gravity Recovery and Climate Experiment (GRACE) satellite, successfully launched in March 2002 to measure both the static and time-variable components of the Earth's gravity field, has delivered (in mid-2003) the first global analysis of data showing the distribution of gravity variations around the world. Due to an uneven distribution of mass inside the Earth, the Earth’s gravity field is not uniform. The gravity variations that GRACE will study include: changes due to surface and deep currents in the ocean; runoff and groundwater storage on land masses; exchanges between ice sheets or glaciers and the oceans; and variations of mass within the Earth.

Future data analysis and applications based on measurements from GRACE will provide information on changes in the extent and volume of water stored in continental water bodies (large reservoirs, lakes, and groundwater), as well as other physical changes, such as movement of warm water zones in the Pacific Ocean (El Niño) and shifting tectonic plates. Simulations of GRACE observations show that the time variations in the water budget of the Mississippi River basin, for example, are well-captured. Remote sensing of changes in water storage has potential applications to monitoring and management of regional water supplies, as well as national and international resource assessment and planning activities.

Water cycle-carbon cycle interactions:

A number of recent studies demonstrated the intimate links between the water cycle, the carbon cycle, and climate. A combination of data analysis and model integrations and diagnostics suggests that approximately 60% of the increase in terrestrial carbon sequestration in North America may be attributable to increases in rainfall over the North American continent. Previously, most of the increased carbon sequestration in the Northern Hemisphere was thought to be due to a combination of increases in temperature and the direct fertilization effect of increased atmospheric CO2 concentration. This study’s results indicate that changes in precipitation may be at least as important.

A recent study of chemical weathering, using a combination of precipitation, streamflow, and alkalinity measurements from USGS, showed that carbon export, in the form of alkalinity, has increased along with levels of streamflow and precipitation in the Mississippi River basin. Chemical weathering converts CO2 into dissolved bicarbonate or carbonate that is then transported by rivers to the ocean. River transport of alkalinity from land to ocean is a major source of oceanic alkalinity and thus is a regulator of the carbonate saturation state of the oceans. This has implications for the global carbon cycle and the function of oceans as carbon sinks.

A third study, by a multi-disciplinary group from government agencies, universities, and the private sector, analyzed the fires associated with the 1997-1998 El Niño drought conditions using satellite observations, together with output from biogeochemical and atmospheric chemistry transport models and observed carbon concentrations at flask stations. The study found that during the 1997-1998 El Niño event, fire emissions of carbon increased significantly (2.1 +/- 0.8 petagrams of carbon, or 66 +/- 24% of the carbon-dioxide growth rate anomaly). The study suggests that the variability and intensity of the water and energy cycle on interannual timescales may be among the most critical factors regulating carbon budgets. For example, when conditions support fires, regions that have long served as carbon sinks may suddenly become carbon sources.

Modeling the global water and energy cycles and their regional components:

The Global Energy and Water Cycle Experiment (GEWEX) Continental-scale International Project successfully completed Water and Energy Budget studies for the Mississippi River Basin. Results indicate that the water and energy budgets over the Mississippi River Basin can be closed to within 15%. Water and energy budgets account for the amount of water and energy entering a region, how they are partitioned among their various components (for example, evaporation, runoff, groundwater), and the amount leaving the region. To close a budget, it is necessary to understand the processes that control inflow, partitioning, and outflow, and to have sufficient data on key variables. In addition to establishing benchmarks for future modeling studies, data and results of these studies will be used in initializing and validating regional climate models.

Prediction of warm season rain:

In order to better understand the processes influencing warm season rain in the southwestern United States, process and modeling studies have been carried out that explore the role of the North American Monsoon system in the water budget stores over the region. These studies relied upon fine-resolution precipitation data products and improved representations of land processes in models during the monsoon period, and examined the effects of model resolution on the simulation of summer mountain-region precipitation processes. New process understanding that results from these studies will improve simulation and monthly-to-seasonal prediction of the monsoon and regional water resources.

Evaluation of water cycle prediction products for decision support:

A joint NOAA/NASA project on improving water demand analysis and prediction for U.S. Bureau of Reclamation water managers is designed to improve estimates of evapotranspiration (loss of water from the soil) in New Mexico. The project uses satellite remote sensing, radar, and surface-based observations, and numerical forecasts and surface modeling, to integrate Land Data Assimilation System information into water operations decision support systems, and displays decision data on the Web. Bureau of Reclamation water managers, water conservancy districts, and farmers may access the information daily to help them conserve the state’s extremely limited water resources.


 

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