Global Energy and Water Cycle Experiment (GEWEX) Continental-Scale International Project A Review of Progress and Opportunities (1998) / Chapter Skim
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2 COUPLED LAND-ATMOSPHERE MODELS
2 COUPLED LAND-ATMOSPHERE MODELS
Pages 30-42
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From page 30... ...
However, small-scale precipitation patterns important to the surface water budget and to the occurrence of floods have not yet been adequately considered. Also, GCM vertical grids do not have the capability of describing smallscale vertical features in the planetary boundary layer that influence the transport of heat and moisture from an extremely heterogeneous land surface to the atmosphere, and accurate planetary boundary-layer parameterizations still have to be developed.
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From page 31... ...
Just as the early weather models increased their number of levels to deal with the complexity of the atmospheric vertical structure, so too have LSP modelers increased the number of subsurface levels. This increased number of levels now allows a fast upper soil moisture layer, as well as a root zone and subsurface storage region.
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From page 32... ...
32 Ct Cal ·_.
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From page 33... ...
Usually, this is considerably smaller than the smallest grid size in GCMs and even most regional models. Because of the effects of snow and soil moisture storage, hydrologic models have much longer memories than do atmospheric models.
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From page 34... ...
As indicated above, there is a distinction in that hydrologic prediction models focus on streamflow generation, whereas land surface parameterizations focus on surface energy fluxes, which in these models however also depend on variations in runoff. However, because both types model evapotranspiration, there may be an inherent inconsistency in approaches that attempt to use GCM or numerical weather prediction model output (e.g., precipitation)
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From page 35... ...
It remains to be seen whether LSPs can ever truly compete with the tuned hydrologic models used in river forecasting. For example, although it would be ideal if LSP parameters could be estimated from land surface physical characteristics alone, past experience indicates that direct relationships among model parameters and land surface characteristics are elusive.
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From page 36... ...
36 Ct Cal so so · ~ o Cal sit so Cal Ct Cal so Ct · ~ Ct o o o o sit = ~ Ct ~ .0 Cal ~ o sly ~ ~ o o o V: ¢
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From page 37... ...
Streamflow is now simulated in these models with an accuracy at least comparable to that achievable by the current generation of lumped conceptual models now used for operational purposes. Basically, GCIP and the other continental experiments have produced real innovations in the way regional to global models treat aspects of the land surface scheme, and the present segregation of hydrologic models and LSPs may soon be eliminated.
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From page 38... ...
PILPS 2c will also provide the first real test of the hydrologic performance of the current generation of LSPs as well as the first real test of macroscale hydrologic models. Other Model Improvements GCIP has done more than simply improve the representation of surface climate; it has also been instrumental in generating new and improved highresolution operational regional analysis models that better describe the atmospheric climate.
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From page 39... ...
Like the RFE, the Eta model now includes a prognostic cloud parameterization scheme along with an additional predictive variable, the cloud water-ice mixing ratio, in the model's prognostic equations to represent both liquid water and cloud ice at every model atmospheric level. The model-predicted cloud water-ice mixing ratio and relative humidity are then used to estimate the cloud fraction at each model grid point.
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From page 40... ...
Improve Methodologies to Disaggregate Regional Model Output to the Scales of Hydrologic Processes LSPs and hydrologic models have many parameters that are tuned to the scale of the model. For example, a GCM LSP would use different parameter values than an LSP designed to work with local historical station data; recently Gao et al.
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From page 41... ...
LSPs with routing schemes that allow direct comparison of predicted and observed streamflow for major tributaries of the Mississippi would provide a unique means to validate the hydrological component of climate predictions. Limited work has been done with macroscale hydrological models, which typically operate at spatial scales compatible with numerical weather prediction or climate models.
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From page 42... ...
Cloud formation and dissipation are important processes because of their effects on the atmospheric and surface energy balance. The patterns of cloud, precipitation, and surface temperatures, among properties, are generally quite inhomogeneous.
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