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The Global Water Cycle
Additional Past Accomplishments
The following are selected highlights of recent research supported by CCSP participating agencies (as reported in the fiscal year 2009 edition of the annual report, Our Changing Planet). These research results address the strategic research questions on the global water cycle identified in the CCSP Strategic Plan.
Interagency Cloud and Land Surface Interaction Field Experiment. Improved understanding of the water/energy cycle is a key factor in reducing uncertainty in climate prediction. Parameters such as regional scale soil moisture and key processes involving the interactions between cloud formation and the moisture availability of land surfaces are characteristic of needed improvements. The development of continental cumulus convection is strongly modulated by land surface conditions, while influencing the land surface through rain-induced changes in soil moisture and photosynthesis. To improve understanding of cloud properties, the direct and indirect effect of aerosols on cloud formation processes, and interactions between clouds and the land surface, the first of a series of interagency CLASIC field studies was conducted in June 2007 (see <science.arm.gov/clasic>. The region surrounding DOE’s Southern Great Plains (SGP) site in Oklahoma was chosen for the field experiment due to its extensive surface-based instrumented facilities. In addition, three “supersites” were also heavily instrumented to obtain ground-based measurements to link observed carbon and moisture fluxes to atmospheric structure. Several instrumented research aircraft were provided by the CCSP agencies involved. Collaborations were established between CLASIC and the North American Carbon Program’s Mid-Continent Intensive (MCI) study, recognizing the strong synergy between measurements in SGP and the northern MCI locations, particularly because of air masses flowing from south to north and the influence of the land surface on atmospheric concentrations of aerosols, gases, and other constituents (see Figure 8).
Improved Methodology to Validate Remotely Sensed Soil Moisture Products.1 A novel methodology has been developed to validate the added value of remotely sensed soil moisture products [e.g., from AMSR-E, TRMM microwave imager (TMI)] using a Kalman filter-based strategy that does not require the availability of ground-based soil moisture measurements, and one that can be applied anywhere relatively high-quality rain gauge observations are available (e.g., the contiguous United States). The validation of global remote sensing products is typically based on the use of test bed sites in data-rich areas to characterize retrieval accuracy and value in higher order applications. However, the ability to validate space-borne soil moisture products against ground-based observations is currently limited by difficulties in maintaining soil moisture instrument networks and upscaling sparse point-scale observations of highly variable soil moisture fields to space-borne footprint scales (10-30 km). The new approach provides a quantitative measure of soil moisture with an accuracy linked to that of currently available global rainfall products by means of a simple water balance model. Results indicate that even retrievals from non-optimal X-band frequency sensors over heavily vegetated areas significantly enhanced the quality of soil moisture predictions derived from a simple water balance model and the space-borne precipitation data set. Overall, this study represents an important benchmark that remotely sensed soil moisture products must improve upon in order to contribute value to global land surface modeling applications. The presence of detectable skill at X-band frequencies bodes well for future space-borne missions based on lower frequency L-band measurements better suited for soil moisture measurements and penetration through dense vegetation canopies.
Influence of Land Cover and Soil Moisture on Heat Fluxes.2 Analysis of aircraft, surface-flux tower, and radar wind profiler data on six fair weather days with southerly winds and near-clear skies from the Cooperative Atmosphere-Surface Exchange Study (CASES-97) and the International H2O Project (IHOP-2002) shows that land-use patterns have a strong influence on the horizontal distribution of sensible and latent heat fluxes (H and LE) over southeastern Kansas. Combined with Land Surface Model (LSM) runs, the data suggest that soil moisture influences the relative magnitude of H and LE horizontal variation. In both field programs, H maxima occurred over dormant/sparse vegetation, with a minimum over green vegetation. To a lesser degree, LE maxima occurred over green vegetation, with a minimum over dormant/sparse vegetation. Small day-to-day differences in flux distribution occurred due to the effect of wind direction and speed and surface buoyancy fluxes at the scale of the surface heterogeneity as well as statistical uncertainty. The soil moisture and length of time after rainfall affect the amplitude and coherence of the LE and H horizontal patterns. Terrain could also modulate the horizontal variability in fluxes in this region.
Impact of Desert Dust Radiative Forcing on Sahel Precipitation.3 A recent investigation considered the role of radiative forcing by dust particles in the Sahelian drought, which occurred over the last 3 decades of the 20th century. The study compared atmospheric general circulation model simulations with meteorological and hydrological measurements. In comparison to previous studies, dust particles that are less absorptive of solar radiation and more emissive at long wavelengths were used in the present study. Cooling of the atmosphere due to dust radiative forcing was found to play an important role in reducing the precipitation over North Africa. The newly modeled circulation responses to this cooling over North Africa provide better agreement with the observations made in dry years in the Sahel region. The results are important because they show that the direct radiative forcing by dust has played a role in the observed droughts in the Sahel comparable to the roles played by sea surface temperatures and vegetation, which have been extensively studied. These results also provide a mechanism whereby drought in the Sahel region can cause increased dust, which then feeds back to cause a further precipitation reduction.
Climate Variability and Fluctuations in Daily Precipitation over the United States.4 Fluctuations in the frequency and intensity of daily precipitation over the United States during the period 1948 to 2004 were identified and linked to leading sources of interannual and interdecadal climate variability. The El Niño-Southern Oscillation (ENSO) phenomenon was implicated in interannual fluctuations while the Pacific Decadal Oscillation (PDO) and the Arctic Oscillation (AO) were linked to recent interdecadal fluctuations. For the conterminous United States as a whole, there have been increases in the annual frequency of occurrence of wet days and heavy precipitation days and in the mean daily and annual total precipitation over the past several decades, though these changes have not been uniform. The study explored the possibility of significant natural forcing of these interdecadal variations in precipitation, and found that the PDO is associated with these fluctuations over the western and southern United States, while the AO is also associated with them but to a much lesser extent over the southeastern United States. Because the interdecadal fluctuations are linked to changes in the global-scale circulation and sea surface temperatures associated with the PDO, the results imply that a significant portion of the skill of climate models in anticipating fluctuations in daily precipitation statistics over the United States will arise from an ability to forecast the temporal and spatial variability of the interdecadal shifts in tropical precipitation and in the associated teleconnection patterns into the mid-latitudes.
Consensus U.S. Drought Monitor. The National Drought Mitigation Center (NDMC) has developed an “integrated” Drought Monitor—a synthesis of multiple indices, outlooks, and new accounts that represents a consensus of Federal and academic scientists (see <drought.unl.edu/dm/monitor.html>). The experimental drought monitor product shown in Figure 9, to be refined over time, has improved techniques that are found to better reflect the needs of decisionmakers and others who use the information. The Drought Monitor integrates information from a range of data on rainfall, snowpack, streamflow, and other water supply indicators into a comprehensible picture. Drought measures used include Percent of Normal; Standardized Precipitation Index; Palmer Drought Severity Index; Crop Moisture Index; Surface Water Supply Index; Reclamation Drought Index; and Deciles, among others. With an emphasis on preparation and risk management, rather than crisis management, NDMC helps people and institutions develop and implement measures to reduce vulnerabilities to drought. The NDMC works in partnership with several of the CCSP agencies. These activities also contribute to the National Integrated Drought Information System (NIDIS).
Regional Climate Model for North and South America.5,6 Proper evaluation of climate change at regional scales is crucial for society planning to mitigate the impact of global climate change. A study using the National Centers for Environmental Prediction Eta regional climate model indicates that under proper choices of model domain size, imposed lateral boundary conditions, and horizontal resolution, the regional model is capable of producing better regional features, such as precipitation and the low-level jet than the general circulation model that provides the lateral boundary conditions to the regional model.
Impact of Vegetation and Soil Moisture Feedback on Precipitation.7,8 Accurate seasonal climate predictions of precipitation are critical for agriculture, water management and planning, and for the mitigation of natural hazards. Both large-scale oceanic forcing and local land surface conditions are important factors in determining precipitation over the United States. Past seasonal predictions have primarily relied on sea surface temperature due to its slow variation. Research over the past decade has produced abundant evidence that positive feedback between soil moisture and precipitation over most of the United States promotes the persistence of seasonal hydrological conditions. The time scale of this persistence, or the memory length of the land-atmosphere system, can be as long as 3 months in late spring and summer, suggesting that the slowly varying soil moisture can potentially serve as a good predictor for seasonal climate. To use soil moisture as a “predictor,” numerical model-based seasonal prediction requires accurate soil moisture initialization and the realistic simulation of important processes involved in soil moisture-precipitation coupling. One of these processes is the seasonal vegetation feedback. At the seasonal time scale, vegetation responds to concurrent and cumulative hydrometeorological conditions and feeds back to further influence the hydro-meteorological processes. This feedback has largely been neglected in the past as most models prescribe, instead of predict, the seasonal course of vegetation. In a recent study, a predictive phenology scheme (which predicts the seasonal variation of vegetation) was incorporated into a coupled land-atmosphere model, and the impact of soil moisture anomalies on subsequent precipitation examined. Vegetation feedback was found to enhance the impact of wet soil moisture anomalies on subsequent precipitation over most of the Mississippi River Basin. The contribution from soil moisture-induced vegetation feedback was found to be as important as the contribution from the initial soil moisture anomalies. This finding shows the importance of including predictive phenology schemes in seasonal prediction models.
A High-Resolution Meteorological Distribution Model for Atmospheric, Hydrologic, and Ecologic Applications.9 A snow evolution modeling system (SnowModel) was used to simulate seasonal snow evolution across three 30-km by 30-km domains enveloping the Cold Land Processes Field Experiment (CLPX) meso-cell study areas in Colorado. Simulations were performed using a 30-m grid increment and spanned the snow accumulation season for this region (1 October 2002 through 1 April 2003). Meteorological forcing was provided by 27 meteorological stations and 75 atmospheric analysis grid points distributed across the model simulation domains using a micrometeorological distribution model (MicroMet). The simulations included a data assimilation sub-model (SnowAssim) that adjusted snow water equivalent (SWE) toward a collection of ground-based and airborne SWE observations. Simulated SWE distributions displayed considerably more spatial heterogeneity compared with observations alone, and the simulated distribution patterns closely fit understanding of snow evolution processes and observed snow depths.
Land Surface-Atmosphere Interactions studied by Comparing Simulated vs. Observed Fluxes and Feedbacks.10 Land atmosphere interactions in the Weather Research and Forecasting (WRF) model and the Noah Land Surface Model (Noah LSM) were analyzed by comparing simulated fluxes and feedbacks to in situ and remotely sensed observations. Vegetation cover, vegetation water content, and land surface temperature data acquired from remotely sensed platforms are strongly correlated in semiarid regions, such as the North American Monsoon Region, compared to more humid regions. The assimilation of these data, including their covariance, into the Noah LSM improved the simulation of soil moisture and other land surface fields. However, the latent heat flux to the atmosphere, and therefore likelihood of precipitation, were found to be overestimated due to the parameterization of vegetation physiology.
New Water Stress Index to Assess the Impacts of Environmental Change on Water Availability and Use in the United States.11 Watershed water stress is caused by both water availability (i.e., lack of supply) and use (i.e., demand), both of which are influenced by ecosystem conditions and humans. This study developed a water stress index that integrates both natural and anthropogenic effects on water availability and use. Future availability and use scenarios were modeled via changes in climate, land management, land use/land cover, and population. Results suggest that population growth will greatly increase water use in metropolitan areas, but overall, changes in population will have little impact on total water demand over the next 40 years. In contrast, changes in air temperature and precipitation will likely affect regional water availability significantly in coming decades.
Will Thunderstorms be Stronger in a Warmer Climate?12 How thunderstorm intensity will change with global climate change is an interesting and particularly important question. While there is some evidence that the intensity of hurricanes will increase, predictions for the nature of thunderstorms that are not part of hurricanes is lacking. Using a global climate model with a new parameterization of vertical velocities (or updrafts) in thunderstorms, Atmospheric Radiation Measurement (ARM) researchers examined how the intensity of thunderstorms varies from region to region over ocean and land and how the vertical velocity will increase in a warmer climate. Their results indicate that a simple estimate of the upward velocity of thunderstorm updrafts in a global climate model reproduces observed land-ocean differences in thunderstorm intensity. Under a climate change scenario, updrafts strengthen by about 1 m s-1 in the lightning-producing regions of continental thunderstorms, primarily due to an upward shift in the freezing level. For the western United States, drying in the warmer climate reduces the frequency of thunderstorms that initiate forest fires, but the strongest storms occur 26% more often. For the central-eastern United States, stronger updrafts combined with weaker change of winds with height (or wind shear) suggest little change in severe storm occurrence with global climate change, but the most severe storms occur more often.
Rock Glaciers as Hydrologic Refugia in a Warming World.13 Rock glaciers are widespread but little studied landforms in semi-arid mountain ranges of the world. Rock mantling insulates ice from solar heating, creating a lag in response to climate relative to typical glaciers and winter snowpacks. A classification system has been developed and used to survey over 400 features in canyons of the Sierra Nevada, California. It was found that these features are undocumented sources of mountain water, and provide wetland refugia for mountain biodiversity. As snowpacks diminish in the future, they will likely gain in local importance.
Additional Past Accomplishments:
GLOBAL WATER CYCLE CHAPTER REFERENCES
2) Lemone, M.A., F. Chen, J.G. Alfieri, M. Tewari, B. Geerts, Q. Miao, R.L. Grossman, and R.L. Coulter, 2007: Influence of land cover and soil moisture on the horizontal distribution of sensible and latent heat fluxes in Southeast Kansas during IHOP-2002 and CASES-97. Journal of Hydrometeorology, 8, 68-87.
3) Yoshioka, M., N.M. Mahowald, A.J. Conley, W.D. Collins, D.F. Fillmore, C.S. Zender, and D.B. Coleman, 2007: Impact of desert dust radiative forcing on Sahel precipitation: Relative importance of dust compared to sea surface temperature variations, vegetation changes, and greenhouse gas warming. Journal of Climate, 20, 1445-1467.
4) Higgins, R.W., V.B.S. Silva, W. Shi, and J. Larson, 2007: Relationship between climate variability and fluctuations in daily precipitation over the United States. Journal of Climate, 20, 3561-3679.
5) Xue, Y., R. Vasic, Z. Janjic, F. Mesinger, and K.E. Mitchell, 2007: Assessment of dynamic downscaling of the continental U.S. regional climate using the Eta/SSiB Regional Climate Model. Journal of Climate, 20, 4172-4193.
6) De Sales, F., and Y. Xue, 2006: Investigation of seasonal prediction of the South American regional climate using the nested model system. Journal of Geophysical Research, 111, D20107, doi:10.1029/2005JD006989.
8) Kim, Y.J. and G. L. Wang, 2007b: Impact of vegetation feedback on the response of precipitation to antecedent soil moisture anomalies over North America. Journal of Hydrometeorology, 8(3), 534-550.
11) McNulty, S.G., G. Sun, E. Cohen, J. Moore-Myers, and D. Wear, 2007: Change in the southern U.S. water demand and supply over the next forty years. In: Wetland and Water Resource Modeling and Assessment: a Watershed Perspective [Jin, W. (ed.)]. CRC Press, Taylor & Francis Group, 312 pp.
13) Millar, C.I., and R.D. Westfall, 2008: Rock glaciers and related periglacial landforms in the Sierra Nevada, CA, USA: inventory, distribution, and climatic relationships. Quaternary International, corrected proof, doi:10.1016/j.quaint.2007.06.004.