Domain choice in an experimental nested modeling prediction system for South America |
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Authors: | S A Rauscher A Seth J-H Qian S J Camargo |
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Institution: | (1) Physics of Weather and Climate Group, The Abdus Salam International Centre for Theoretical Physics, Trieste, Italy;(2) Department of Geography, University of Connecticut, Storrs, CT, USA;(3) IRI, Columbia University, Palisades, NY, USA |
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Abstract: | Summary The purposes of this paper are to evaluate the new version of the regional model, RegCM3, over South America for two test
seasons, and to select a domain for use in an experimental nested prediction system, which incorporates RegCM3 and the European
Community-Hamburg (ECHAM) general circulation model (GCM). To evaluate RegCM3, control experiments were completed with RegCM3
driven by both the NCEP/NCAR Reanalysis (NNRP) and ECHAM, using a small control domain (D-CTRL) and integration periods of
January–March 1983 (El Ni?o) and January–March 1985 (La Ni?a). The new version of the regional model captures the primary
circulation and rainfall differences between the two years over tropical and subtropical South America. Both the NNRP-driven
and ECHAM-driven RegCM3 improve the simulation of the Atlantic intertropical convergence zone (ITCZ) compared to the GCM.
However, there are some simulation errors. Irrespective of the driving fields, weak northeasterlies associated with reduced
precipitation are observed over the Amazon. The simulation of the South Atlantic convergence zone is poor due to errors in
the boundary condition forcing which appear to be amplified by the regional model.
To select a domain for use in an experimental prediction system, sensitivity tests were performed for three domains, each
of which includes important regional features and processes of the climate system. The domain sensitivity experiments were
designed to determine how domain size and the location of the GCM boundary forcing affect the regional circulation, moisture
transport, and rainfall in two years with different large scale conditions. First, the control domain was extended southward
to include the exit region of the Andes low level jet (D-LLJ), then eastward to include the South Atlantic subtropical high
(D-ATL), and finally westward to include the subsidence region of the South Pacific subtropical high and to permit the regional
model more freedom to respond to the increased resolution of the Andes Mountains (D-PAC). In order to quantify differences
between the domain experiments, measures of bias, root mean square error, and the spatial correlation pattern were calculated
between the model results and the observed data for the seasonal average fields. The results show the GCM driving fields have
remarkable control over the RegCM3 simulations. Although no single domain clearly outperforms the others in both seasons,
the control domain, D-CTRL, compares most favorably with observations. Over the ITCZ region, the simulations were improved
by including a large portion of the South Atlantic subtropical high (D-ATL). The methodology presented here provides a quantitative
basis for evaluating domain choice in future studies. |
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