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8
Improvement of long-term integrations by increasing RCM domain size
Sin Chan Chou, André Lyra, José Pesquero, Lincoln Alves, Gustavo Sueiro, Diego Chagas, José
Marengo and Vladimir Djurdjevic
National Institute for Space Research - INPE, Cachoeira Paulista, SP, Brasil, 12630-000, chou@cptec.inpe.br
1. Introduction
Global models are used to generate climate change
according to different future scenarios. However, these
models have rather coarse resolution. The nesting of a
Regional Climate Model (RCM) over the area of interest
allows the reduction of grid size which is more desirable for
impact studies. In addition, the climate generated from a
regional model is strongly dependent on the lateral boundary
conditions (LBC) and, consequently, on the domain size or
the position of the LBCs (Xue et al. 2007).
The objective of this work is to show the dependence of the
tropical South America climate from a RCM on the choice
of the position of the lateral boundary conditions.
2. The models
The driver model is the INGV-SXG coupled GCM (Gualdi
et al 2003a,b) which uses ECHAM4 (Roeckner, 1996) as
atmospheric component, OPA 8.2 (Madec et al, 1999) as
ocean model and LIM as the sea ice model (Fichefet e
Morales Maqueda, 1999). The atmospheric resolution is
about T106L19.
The RCM is the Eta Model (Black, 1994; Mesinger et al,
1988; Janjic, 1994) configured with 40-km horizontal
resolution and 38 vertical layers. The model has been used
for weather and seasonal forecasts over South America
(Chou et al, 2005) and was adapted to run long-term
integrations (Pesquero et al, 2009).
A continuous integration for the present climate period from
1961-1990 was carried out, with the lateral boundary
conditions provided at every 6 hours. The sea surface
temperature was taken from monthly values produced by the
AOGCM. The small domain was setup with 135x293x38
points, with the boundaries positioned about 15 degrees
outside the continent in the tropics, and the larger domain
with 201x333x38 points, spanned additional 20 degrees in
the east-west direction. In the second domain, the
Intertropical Convergence Zone can produced by the RCM.
2). The small domain nested run did not show this double
structure, however, the ITCZ rain was very weak, and
therefore rains over Amazon region and northern part of
Northeast Brazil were underestimated. The larger domain
run exhibited the ITCZ rain position more correctly and
the rain amounts over Amazonia and northern part of
Northeast of Brazil have increased. The precipitation in
subtropical areas and southern parts of the continent was
not clearly affected by the increase of the domain size.
Similar results were found in other seasons of the year
The large positive temperature bias occurred over
Amazonia and northern part of Northeast Brazil in the run
using small domain area. These errors may have been
caused by the reduced amount of precipitation. These
positive bias have reduced or changed sign in the larger
domain run. Again, negligible temperature effects
occurred in subtropical or higher latitudes.
The annual cycle of precipitation over Amazonia region
showed substantial improvement in the larger domain
setup. The amounts during rainy months were more
accurate and the peak occurred in the correct month.
These results are independent of LBC schemes, as it has
been shown by Veljovic (2009) that using another scheme
(Davies, 1976) large scale structure is retained.
Figure 1.
Domains of the Eta Model.
Figure 2. 1961-1990 DJF Eta precipitation
(mm/d): small (lhs) and large (rhs) domains.
4. Some Conclusions
The Eta model improved the large scale tropical
precipitation and temperatures by positioning the lateral
boundaries away from the region of interest. Negligible
effects could be noticed outside the tropics.
3. Results
During DJF, the austral summer, the major precipitation
areas over South America continent occur in the Amazon,
northern part of Northeast Brazil, central and southeastern
parts of Brazil. These areas are associated with the
Intertropical Convergence Zone (ITCZ), the South Atlantic
Convergence Zone (SACZ) and frontal passages. The driver
model produced the rain band associated to the ITCZ to the
south of the observed position; a secondary band was
positioned to the north suggesting a double structure (Figure
References
Chou, S.C., Bustamante, J., Gomes, J.L, Evaluation of Eta
Model seasonal precipitation forecasts over South
America. Nonlinear Processes in Geophysics, v.12,
pp.537 – 555, 2005.
Pesquero, J.F., Chou, S.C, Nobre, C.A, Marengo, J.A,
Climate downscaling over South America for 1961-
1970 using the Eta Model, Theoretical and Applied
Climatology, 2009 DOI: 10.1007/s00704-009-0123-z.