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Siegfried Schubert, David Gutzler, Hailan Wang, Aiguo Dai, Tom Delworth, Clara Deser, Kirsten Findell, Rong Fu, Wayne Higgins, Martin Hoerling, Ben Kirtman, Randal Koster, Arun Kumar, David Legler, Dennis Lettenmaier, Bradfield Lyon, Victor Magana, Kingtse Mo, Sumant Nigam, Philip Pegion, Adam Phillips, Roger Pulwarty, David Rind, Alfredo Ruiz-Barradas, Jae Schemm, Richard Seager, Ronald Stewart, Max Suarez, Jozef Syktus, Mingfang Ting, Chunzai Wang, Scott Weaver, and Ning Zeng

change on drought processes” ( Gutzler and Schubert 2007 ). The specific tasks of the working group were to 1) propose a working definition of drought and related model predictands of drought, 2) coordinate evaluations of existing relevant model simulations, 3) suggest new experiments (coupled and uncoupled) designed to address outstanding uncertainties in the nature of drought, 4) coordinate and encourage the analysis of observational datasets to reveal antecedent linkages of multiyear droughts, and

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Scott J. Weaver, Siegfried Schubert, and Hailan Wang

), NOAA/Geophysical Fluid Dynamics Laboratory (GFDL), Columbia University, and many other government and university scientists. These modeling centers have completed identical idealized SST-forced runs using their respective AGCMs. These idealized simulations provide a unique resource for assessing SST impacts on North American regional hydroclimate features, including the GPLLJ, and are utilized here. Additional analyses using observationally constrained reanalysis systems and Atmospheric Model

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Caio A. S. Coelho and Lisa Goddard

the DEMETER seasonal forecast model group incorporates the observed standardized precipitation anomaly composite (horizontal dashed line) for northern South America ( Fig. 7a ), southern Africa ( Fig. 7b ), and north Australia regions ( Fig. 7d ). The mean (black dot) and median (thick horizontal line) values for the DEMETER group over these three regions are also close to the observed standardized precipitation anomaly, indicating good performance in predicting the observed standardized

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Bradfield Lyon

) Coupled Model Intercomparison Project Phase 3 (CMIP3) multimodel dataset. The Twentieth-Century Climate in Coupled Model (20C3M) runs, which include forcing from observed aerosols and greenhouse gas concentrations are used for this purpose. The possible future behavior of drought and heat waves, considered separately and jointly, are then evaluated using the same CMIP3 models forced with the Special Report on Emissions Scenarios (SRES) A1B greenhouse gas scenario where CO 2 concentrations reach a

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M. Biasutti, A. H. Sobel, and Suzana J. Camargo

et al. (2008) , and Reichler and Kim (2008) . [Integrations are made available to the community by the Program for Climate Model Diagnosis and Intercomparison (PCMDI) through their Web site ( www-pcmdi.llnl.gov/ipcc/model_documentation/ipcc_model_documentation.php ), where a full description of the models can be found.] The performance of the CMIP3 dataset in simulating African climatology and variability has been described in previous studies ( Cook and Vizy 2006 ; Biasutti et al. 2008 ; Lau

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Rachel R. McCrary and David A. Randall

, our limited understanding of the complex climate system and the factors that influence precipitation variability makes assessing the accuracy of the CGCMs projections a challenge. One way to evaluate the performance of CGCMs is to compare their simulations of the variations in twentieth-century climate with observations. The ability of the models to realistically simulate the observed climate is one measure of their capability to project the future. In this context, we evaluate the ability of

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Randal D. Koster, Hailan Wang, Siegfried D. Schubert, Max J. Suarez, and Sarith Mahanama

above-mentioned connection between seasonally averaged evaporation and surface temperature, used the curve in Fig. 1 to interpret interannual variations in June–August (JJA) temperature averages in terms of the two evaporation regimes. Using both atmospheric general circulation model (AGCM) data and multidecadal precipitation and temperature observations, they showed that drier-than-average JJA conditions in regions characterized by soil moisture–controlled evaporation do indeed lead to positive

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