GC51B-01
Water Resource Changes In Future Climate Change Scenarios In The Puget Sound, Washington
The Puget Sound drainage basin is home to over three quarters of the population of the State of Washington. Like most of the western U.S., the Puget Sound basin has experienced about one degree C of warming over the last century, and projections are for much larger increases to occur over coming decades. Changes to date have resulted in documented shifts in the accumulation and ablation of snow in the basin's headwater streams, especially at intermediate elevation, and these changes have resulted in documented shifts toward earlier peak runoff during the spring melt season. Such changes have important effects on municipal and industrial water supplies in the region, as well as for environmental uses of the basin's rivers, including protection and enhancement of fisheries. We report on initial results of an ongoing study of the implications of IPCC AR4 climate scenarios to the hydrology and managed water resources of the basin, with particular emphasis on changes in runoff timing, including low flow magnitudes, and performance of the reservoir systems that provide water supply for the major municipalities in the basin. Our projections are based on statistical downscaling of IPCC AR4 scenarios from a suite of 21 IPCC AR4 GCMs for which 100-year transient simulations for emissions scenarios A2B1 and B2 have been archived. A regional hydrological model – the Distributed Hydrology-Soil- Vegetation model (DHSVM) is applied at 150 m spatial resolution, forced by subdaily downscaled GCM output at 1/16 degree grid nodes, which act as "pseudo stations" to force DHSVM. The DHSVM streamflow output at selected stream control points is used to force reservoir models of the major municipal water systems in the basin. In addition to changes in hydrologic attributes and reservoir system performance, which are evaluated for three time periods during the 2001-2100 period, we evaluate the range of predicted changes in total freshwater flux to Puget Sound, and compare predicted changes over the past century associated with observed climate and land cover change with projected climate change over the next century. http://hydro.washington.edu/~lancuo/
GC51B-02
Development of a Statistical Downscaling Model for Projecting Monthly Rainfall over East Asia from a General Circulation Model Output
The objective of this paper is to develop a statistical downscaling model based on established links between large scale variables from a General Circulation Model (GCM) and regional rainfall to project climate change in the 21st century over East Asia. An SVD (singular value decomposition)-based regression model is developed to downscale the GCM data. In order to establish the link between monthly regional precipitation and large scale variables (mean sea level pressure, total precipitation, 10m wind speed, 2m temperature), the IS92a scenario data of ECHAM4/OPCY3 and observation data from the Climate Research Unit from 1901-1990 are used. In the 20th century, their connections have remained unchanged over time for all seasons. The continuity of these relationships to the next century is supported by two different techniques. The first technique, a hypothesis proposed here, expects that the similarity in the main SVD modes of predictors in the model and Empirical Orthogonal Functions of predictors throughout the 20th and 21st century will make the models technically acceptable. The second method assumes that the consistency in between the climate change signals of precipitation in 2090s with respect to 1990s derived by the downscaling model and GCM increases the acceptability of future applications. The applied two techniques support the future application of established relationships in all seasons. By the end of the 21st century, annual rainfall over East Asia is projected to increase by up to 20% over northern China and the Indochina peninsula and decrease by 10% over coast of southern China.
GC51B-03
Utility of daily vs. monthly large-scale climate data: an intercomparison of two statistical downscaling methods
Downscaling of climate model data is essential to most impact analysis. We compare two methods of statistical downscaling to produce continuous, gridded time series of precipitation and surface air temperature at a 1/8- degree (approximately 140km2 per grid cell) resolution over the western U.S. We use NCEP/NCAR Reanalysis data from 1950-1999 as a surrogate General Circulation Model (GCM). The two methods included are constructed analogues (CA) and a bias correction and spatial downscaling (BCSD), both of which have been shown to be skillful in different settings, and BCSD has been used extensively in hydrologic impact analysis. Both methods use the coarse scale Reanalysis fields of precipitation and temperature as predictors of the corresponding fine scale fields. CA downscales daily large-scale data directly and BCSD downscales monthly data, with a random resampling technique to generate daily values. The methods produce comparable skill in producing downscaled, gridded fields of precipitation and temperatures at a monthly and seasonal level. For daily precipitation, both methods exhibit some skill in reproducing both observed wet and dry extremes and the difference between the methods is not significant, reflecting the general low skill in daily precipitation variability in the reanalysis data. For low temperature extremes, the CA method produces greater downscaling skill than BCSD for fall and winter seasons. For high temperature extremes, CA demonstrates higher skill than BCSD in summer. We find that the choice of most appropriate downscaling technique depends on the variables, seasons, and regions of interest, on the availability of daily data, and whether the day to day correspondence of weather from the GCM needs to be reproduced for some applications. The ability to produce skillful downscaled daily data depends primarily on the ability of the climate model to show daily skill.
GC51B-04
Variability of basin-scale terrestrial water storage from a novel application of the water budget equation: the Amazon and the Mississippi
In an approach termed the P-E-R (or simply PER) method, we apply the basin water budget equation to diagnose the long-term variability of the total terrestrial water storage (TWS). The key input variables are observed precipitation (P) and runoff (R), and estimated evaporation (E). Unlike typical offline land-surface model estimate where only atmospheric variables are used as input, the direct use of observed runoff in the PER method imposes an important constraint on the diagnosed TWS. Although there lack basin-scale observations of evaporation, the tendency of E to have significantly less variability than the difference between precipitation and runoff (P-R) minimizes the uncertainties originating from estimated evaporation. Compared to the more traditional method using atmospheric moisture convergence (MC) minus R (MCR method), the use of observed precipitation in PER method is expected to lead to general improvement, especially in regions atmospheric radiosonde data are too sparse to constrain the atmospheric model analyzed MC such as in the remote tropics. TWS was diagnosed using the PER method for the Amazon (1970-2006) and the Mississippi Basin (1928-2006), and compared with MCR method, land-surface model and reanalyses, and NASA's GRACE satellite gravity data. The seasonal cycle of diagnosed TWS over the Amazon is about 300 mm. The interannual TWS variability in these two basins are 100-200 mm, but multi-dacadal changes can be as large as 600-800 mm. Major droughts such as the Dust Bowl period had large impact with water storage depleted by 500 mm over a decade. Within the short period 2003-2006 when GRACE data was available, PER and GRACE show good agreement both for seasonal cycle and interannual variability, providing potential to cross-validate each other. In contrast, land-surface model results are significantly smaller than PER and GRACE, especially towards longer timescales. While we currently lack independent means to verify these long-term changes, simple error analysis using 3 precipitation datasets and 3 evaporation estimates suggest that the multi-decadal amplitude can be uncertain up to a factor of 2, while the agreement is high on interannual timescales. The large TWS variability implies the remarkable capacity of land-surface in storing and taking up water that may be under-represented in models. The results also suggest the existence of water storage memories on multi-year time scales, significantly longer than typically assumed seasonal timescales associated with surface soil moisture.
GC51B-05
Global Precipitation Reanalysis and Reconstruction Based on Satellite and In Situ Data
Model simulation of global precipitation changes over time resulting from changes in greenhouse gas concentrations are difficult to verify due to the difficulty in creating a long-term homogenous time series from the varied observations and estimates available. In this paper we describe progress toward a global precipitation analysis and reconstruction that can be used to constrain available climate model simulations. Precipitation estimates derived from passive microwave satellite observations are combined with ERA-40 reanalysis precipitation to produce a spatially complete monthly precipitation analysis for 1992-2002. The resulting analysis provides consistent global precipitation fields for this period, without problems associated with changing data input types in time or across land-sea boundaries. Over oceanic regions this reanalysis is similar to the GPCP analysis, which relies heavily on the same microwave-based oceanic estimates. Using this new monthly 1992- 2002 analysis, spatial covariance modes are computed for precipitation anomalies. These covariance modes are used to reconstruct precipitation over an extended historical period. For each month, the historical anomaly reconstruction is a weighted sum of the set of modes. Weights for the set of modes are computed by fitting the available gauge data to the set of modes such that the mean-squared error of the fit is minimized. Cross- validation tests indicate reconstruction skill over the oceans between approximately 30°S and 60°N, with little skill poleward of this region. Skill is almost constant since 1950, and it is only slightly reduced in the first half of the 20th century. Additional testing showed that including ship data along with gauge data can not greatly improve the skill in these reconstructions.
GC51B-06
Assessing the Impact of Land Use and Land Cover Change on Global Water Resources
Land use and land cover changes (LULCC) significantly modify the hydrological regime of the watersheds, affecting water resources and environment from regional to global scale. This study seeks to advance and integrate water and energy cycle observation, scientific understanding, and human impacts to assess future water availability. To achieve the research objective, we integrate and interpret past and current space based and in situ observations into a global hydrologic model (GHM). GHM is developed with enhanced spatial and temporal resolution, physical complexity, hydrologic theory and processes to quantify the impact of LULCC on physical variables: surface runoff, subsurface flow, groundwater, infiltration, ET, soil moisture, etc. Coupled with the common land model (CLM), a 3-dimensional volume averaged soil-moisture transport (VAST) model is expanded to incorporate the lateral flow and subgrid heterogeneity. The model consists of 11 soil-hydrology layers to predict lateral as well as vertical moisture flux transport based on Richard's equations. The primary surface boundary conditions (SBCs) include surface elevation and its derivatives, land cover category, sand and clay fraction profiles, bedrock depth and fractional vegetation cover. A consistent global GIS-based dataset is constructed for the SBCs of the model from existing observational datasets comprising of various resolutions, map projections and data formats. Global ECMWF data at 6-hour time steps for the period 1971 through 2000 is processed to get the forcing data which includes incoming longwave and shortwave radiation, precipitation, air temperature, pressure, wind components, boundary layer height and specific humidity. Land use land cover data, generated using IPCC scenarios for every 10 years from 2000 to 2100 is used for future assessment on water resources. Alterations due to LULCC on surface water balance components: ET, groundwater recharge and runoff are then addressed in the study. Land use change disrupts the hydrological cycle through increasing the water yield at some places leading to floods while diminishing, or even eliminating the low flow at other places.
GC51B-07
Analysis of Global Atmospheric and Terrestrial Hydrological Budget Using Merged Satellite Data Sets
One of the main objectives in NASA Energy and Water Cycle Study (NEWS) is to assess our water cycle observational capabilities and promote the development of an experimental global observation system. As part of the integrating auspice of NEWS, efforts have been made to compile the state-of-the-art satellite-based data sets for the global atmospheric and terrestrial hydrological budget analyses. The NEWS Water-cycle integration and Analysis (WIA) has focused its initial efforts on combining precipitation, evaporation, total precipitable water change, and terrestrial water storage changes to evaluate their consistency in global scale water budgets, assess their spatial and temporal variations, and develop research and analysis toward improved observational capabilities. In this study, our global scale water budget analyses consider the more recently developed satellite- based products, which are limited in time (i.e. span less than a decade) and space (most cover from 50S to 50N). Wherever possible, rigorous estimates of sampling error/uncertainty for all water-cycle variables are provided for a more robust quantification of the consistency in these budget terms. The preliminary results indicate that there exist notable systematic differences in the monthly water export time series, which is mainly attributed to the use of various precipitation data sets. Nevertheless, all precipitation data sets convey a consistent depiction of overall evaporation excess for the 50S to 50N region, implying a net export of water vapor to higher latitudes. As far as global annual mean precipitation rates are concerned, most of the discrepancy stems from differences over the ocean. We also compare these budget and residual estimates to reanalyses products (e.g. NCEP-NCAR, NASA, and ECMWF) and coupled GCM simulations from the IPCC AR4 archive. Through such cross-comparison exercises, we highlight consistencies and discrepancies between model estimates and satellite observations to not only increase confidence in these products, but also to provide insights on the regions where the continued evaluation, future model improvement, in-situ networks, field campaigns, and (potential) experimental satellite missions should emphasize.
GC51B-08
Statistical Downscaling of Rainfall for Romania From six European GCMs for Present Day and Future Climate
Circulation Weather Types calculated from ERA40 SLP fields are correlated to rainfall for selected Romanian stations in the lower Danube catchment. The western, central, and eastern parts of the area show differing correlations between rainfall and CWTs in the observations. For all all regions and most CWTs, precipitation amount per rain day is larger in summer while occurrence frequency of rain days per CWT is larger in winter. Rain amount and frequency show high positive (negative) correlation with cyclonic (anti-cyclonic) days. In the western region rain amounts are highest for SE CWT, associated with synoptic disturbances originating from the central Mediterranean. In the central and eastern region N to E CWTs provide the highest rain amounts, associated with low pressure over the black sea and the eastern Mediterranean. SW to NW CWTs are negatively correlated with rain in the eastern part of the area due to diffluence south of the Carpathians. In the scope of the EU-Project ENSEMBLES, CWTs are also calculated using six European GCMs (BCC, NERSC, Norway; CNRM-CM3, CNRM, France; EGMAM, FU-Berlin, Germany; ECHAM5/MPI-OM1, MPI-M, Germany; HadGEM1, Hadley-Centre, UK; IPSL-CM4, Institute Pierre Simone Laplace, France). Comparison of the occurrence frequency of CWTs for present-day simulations to the ERA40 results shows a positive bias of W CWT in Romania, associated with a too strong northern polar low in all models. Additionally an overestimation of cyclonic and an underestimation of anti-cyclonic days is found in the models. This feature is consistent with a general tendency of GCMs to underestimate blocking situations. The annual cycle of CWTs for Romania is displayed in the different models in varying quality: while ECHAM5/MPI-OM shows an annual cycle close to observations, some of the other models are not suited to represent the annual cycle correctly. All models show an increase of anti-cyclonic days combined with a decrease of cyclonic days for the SRES A1B scenario when compared to the respective present-day simulation. For the other CWTs the signals are either not significant or differ between the participating models. With respect to future rainfall in the lower Danube basin a drying tendency is expected due to the shift from cyclonic to anti-cyclonic CWT occurrences.