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Evaluation of seasonal forecasts over the northeast of Brazil using the RegCM3
Rubinei Dorneles Machado 1 and Rosmeri Porfírio da Rocha 1
1 University of São Paulo, São Paulo, Brazil; rmvip@usp.br
1. Introduction
Several works discussed the limitations of AGCMs
(Atmospheric General Circulation Models) for climate
forecasting due to the low horizontal resolution typically
used in these models (Giorgi and Mearns, 1999). The
interest in regional climate models (RCMs) is associated
with the more appropriated physical parameterizations and
higher spatial resolution that they can utilizes providing
better representation of the sub-grid processes and thus
reducing the errors found in AGCMs (Sen et al. 2004).
Over northeastern Brazil Misra (2006) shows that the COLA
AGCM did not present any ability to predict February-
March-April rainfall anomalies during the years considered
as normal, i.e. without influence of the large-scale
phenomena like El-Niño/La-Niña. A common systematic
error in some AGCMs is the overestimation of the austral
summer precipitation over the northeast Brazil (Cavalcanti
et al., 2002). Climate studies using RCMs are becoming
more frequent and important as they can reduce the
systematic errors of AGCMs in some regions (Seth et al.,
2007). The RCMs have been used not only for hindcasts of
past climate, but also for seasonal climate predictions (Chou
et al., 2001) with the aim of also capture the regional aspects
of the climate. Considering the interest in the application of
the RegCM3 (Regional Climate Model version 3; Pal et al.,
2007) for seasonal forecasting, Cuadra and da Rocha (2007)
studied the sensitivity of the simulations on the southeastern
South America to the specification of SST (sea surface
temperature). They show that the persisted SST affects little
the simulation of austral summer anomalies of precipitation
and air temperature over the continental parts of southsoutheast
Brazil. For this work, the goal is to investigate the
performance of seasonal forecasting over northeast Brazil
using the RegCM3 nested in the CPTEC/COLA (Center for
Weather Forecasting and Climate Studies/Center for Ocean-
Land-Atmosphere Studies) AGCM.
2. Methodology and Data Set
The RegCM3 is a primitive equation model, compressible,
and in the sigma-pressure vertical coordinate. A recent
description is given in Pal et al. (2007). In this study the
RegCM3 was integrated in the domain of Figure 1 using 60
km of horizontal resolution, 18 vertical levels, and Grell
convective scheme with the Fritsch-Chappell closure. The
RegCM3 forecasts used the initial and boundary conditions
of the CPTEC/COLA AGCM, which is described by
Cavalcanti et al. (2002), and over the Oceans the persisted
SST was specified. The 27 forecasts analyzed were initiated
at 00 UTC of day 16 of each month. The first 14-15 days of
integrations were considered as spin-up and the quarter of
validation correspond to the average of the following
three months. This average is referred as seasonal
forecasts.
For validation of precipitation we used the rainfall data
from CPC (Climate Prediction Center) analysis that has
horizontal resolution of 1 ° x 1 ° latitude by longitude (Silva
et al., 2007). The air temperature was compared with the
NCEP re-analysis (Kalnay et al. 1996) that is in a Gaussian
grid with about 1.875 o of horizontal resolution.
To perform an objective analysis on the subdomain
Northeast (NDE) (Figure 1) were calculated quarter
averages of seasonal climate forecasts, the linear
correlation coefficient and the index of efficiency of Nash
and Sutcliffe (1970) of time series. This index indicates the
skill of RegCM3 forecasts regarding the average of the
observations.
Figure 1. Forecast domain and topography (shaded with
scale at right) and the NDE subdomain (red box) used to
evaluate the RegCM3 forecasts.
3. Results
The Figure 2a shows over the NDE the area average
seasonal rainfall provided by CPC analysis,
CPTEC/COLA and RegCM3. It is important to note the
lack of ASO/2006 values due to the post-processing
problems. Figure 2a shows the overestimation of seasonal
rainfall by the CPTEC/COLA model in all quarters, except
in FMA/2006. Apparently there is a displacement of the
rainy season from FMA of the CPC to MAM in
CPTEC/COLA forecasting. According to Oyama (2006)
and our seasonal maps (Figure not shown) errors in the
positioning of ITCZ (Intertropical Convergence Zone) and
SACZ (South Atlantic Convergence Zone) found in low
horizontal resolutions AGCMs, including CPTEC/COLA,
can justify the wet bias in the northeast Brazil. In contrast,
Figure 2a indicates that RegCM3 produces a superior
forecasting of seasonal precipitation during all the 27
quarters. This improvement could be due to higher
horizontal resolution of the RegCM3 that also improves
the ITCZ localization.
The area average air temperature over NDE (Figure 2b)
presents small annual amplitude with maximum and
minimum values during the dry and wet seasons,
respectively. Both RegCM3 and CPTEC/COLA models
underestimates the air temperature during all seasons
(Figure 2b). However, Figure 2b shows that inter-seasonal
air temperature variability is well reproduced by RegCM3,
while CPTEC/COLA is out of phase regarding the
analysis as well as it occurs for rainfall (Figure 2a). This is
due to the control of the rainfall over the air temperature.
In the NDE area, located near the equator, the solar
radiation is almost constant throughout year. During the
rainy season there is an increase of cloudiness that reduces
the net solar radiation and the air temperature. The
opposite behavior is obtained during the dry periods.