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Observation and simulation with RegCM3 for the 2005-2006 rainy season
over the southeast of Brazil
Maria Elisa Siqueira Silva and Diogo Ladvocat Negrão Couto
Department of Geography, University of São Paulo, Brazil, mariaelisa.siqueirasilva@gmail.com
1. Introduction
Vegetation at central-southeast of Brazil and Amazonian
region has great potential to provide changes, both due to the
increase of dioxide carbon concentration in atmosphere and
by the rapid use of land (Ferniside, 2005), replacing natural
vegetation by crops or deforested areas. Salazar et al. (2007)
verified the relationship between climate and geographical
distribution of vegetation over South America, dealing with
biogeographic modeling. They defined spatial distributions
of vegetation based on 15 climatic models for two emissions
sceneries (A2 and B1) from IPCC (Moss et al., 2008),
considering the dioxide carbon increase in the atmosphere
for many future decades (2020-2029; 2050-2059; 2090-
2099). All models indicated different levels of aridization,
mainly over central areas of South America. Besides the
development of studies focused on climatic simulations,
there are initiatives in order to going on with biosphereatmosphere
observation over different types of coverage, as
is the case of the project Biosphere-Atmosphere Interaction
Phase II: Cerrados and Changing the Landuse (Rocha,
2004), from which is possible to obtain surface energy
fluxes data, since 2002, over sugar cane and Eucalyptus
crops and, Cerrado (savannah-like vegetation).
Considering locally observed data (those observed in the
context of the cited project) and simulation results, the
objective in this study is to estimate, through the use of
RegCM3, the rainy impact over the central-southeast of
Brazil due to modification of vegetation type accordingly to
Salazar et al. (2007), for the period comprised between 2020
and 2029. In this period, the reduction observed in tropical
forest is about 3%. The dryness of vegetation is taken in
account, as provided by Salazar et al. (2007), as
consequence of the increase of CO 2 concentration in the
atmosphere.
2. Data and Model
The simulations were generated with the regional model
RegCM3 for the major part of South America considering a
domain centered in 22S and 55W, and covering an area of
160x120 grid points with resolution of 50 km. RegCM3 is
originated from MM4, as documented by Giorgi et al.
(1993). Soil-plant-atmosphere interaction processes are
described by BATS, providing calculation of momentum,
heat and water vapor turbulent changes between surface and
atmosphere and solving prognostic equations for each grid
point. In this study, deep cumulus convection is
parameterized after Grell (1993). Reanalysis I (Kalnay et al.,
1996) is used as initial and boundary conditions.
To prevent dryness of atmosphere due to low
evapotranspiration, some adjustments were made in the
model for tropical forest vegetation type. The rate of root
between the two upper soil layers was modified to 0.40,
instead of 0.80, providing more water to be extracted by
roots from deeper soil layers. The depths of soil and root
layer were also increased to 4.5 and 3 m, respectively.
The model was run from July to December of 2005. Results
show the last three month of simulation by the model (Oct-
Dec) (period defined by the beginning of local rainy season)
while the first three months (Jul-Sep) were cut off to
prevent considering spin up problem.
Two different experiments were run: CTR, control
experiment and SAVAN, considering the aridization
proposed by Salazar et al. (2007). Observed data are
compared to those simulated for verification.
3. Results
As shown in Figure 1, it is possible to see the good
representation of averaged rain (Oct-Dec, 2005) by
RegCM3 when compared to CMAP data (not shown).
South Atlantic Convergence Zone (SACZ) is very well
positioned.
The comparison between simulated and local observed
data, for the north of São Paulo State (as mentioned
earlier) shows diurnal evolution of minimum and
maximum air temperature near surface, sensible and latent
heat fluxes (Figure 2). Local observed temperature shows
greater diurnal temperature amplitude in comparison to
that simulated by the model (Figure 2a). Diurnal
variability appears to be very well simulated in
comparison to observation. Although latent and sensible
heat fluxes appear to be well simulated, in terms of
absolute values, latent heat flux is overestimated for the
whole period (Figure b,c). The increase of latent flux
during the simulated period is accompanied by increase in
precipitation (figure not shown).
The simulation results considering vegetation change over
the southeast of Brazil, replacing evergreen shrub and crop
areas by deciduous shrub, show the diminishing of
precipitation over São Paulo state (Figure 3). Blue line
contours in Figure 3 show negative impact (experiment
minus control run).
(a)
Figure 1. (a) Precipitation simulated (RegCM3) for
South America, 2005 OCT-DEC.