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Modeling of the Atlantic tropical cyclone activity with the RegCM3
Daniela Cruz-Pastrana 1 , Ernesto Caetano 1 and Rosmeri Porfírio da Rocha 2
1 National Autonomous University of Mexico, Mexico; 2 São Paulo University , São Paulo, Brazil;
daniela@atmosfera.unam.mx
1. Introduction
The North Atlantic basin (including North Atlantic Ocean,
Caribbean Sea and Gulf of Mexico) has a substantial
interdecadal and interannual variability which modulates
tropical cyclone activity over the region (Goldenberg et al.,
2001). Additionally this activity according some authors
(eg Emanuel, 2005, Mann and Emanuel, 2006, Trenberth
and Shea, 2006) has been enhanced due to global warming.
The dynamic simulations of tropical cyclones over Atlantic
basin permit to the study the controlling factors of the
variability and long-term trend. However, the typical
resolution global climate models are often unsuitable for
this kind of simulations. Knutson et al. (2007) used the
Geophysical Fluid Dynamics Laboratory (GFDL) Regional
Atmospheric Model (Pauluis and Garner, 2006) to simulate
successfully various aspects of tropical storms, obtaining a
high correlation between observed and simulated variability
and trends.
The Northern Hemisphere interannual variability cyclone
activity is assessed through simulations with the RegCM3
(Pal et al., 2007) and applying an algorithm for the
detection and tracking of cyclones based on the relative
vorticity and warm cores.
3. Results
The modeled tropical cyclone activity is higher in the
central part of the basin and the lower over the Gulf of
Mexico compared (Fig. 1a) with the best-track of National
Hurricane Center dataset (Fig.1b). (HURDAT;
http://www.aoml.noaa.gov/hrd/hurdat/) Additionally the
simulated cyclone trajectories are more erratic and lasting
less than those observed.
The simulated daily average cumulated rainfall (Fig.2b) for
August to October is underestimated for the region of the
InterTropical Convergence Zone (ITCZ) and in the
northern Gulf of Mexico while it is overestimated at the
basin central part compared with the CMAP (CPC Merged
Analysis of Precipitation) (Xie and Arkin, 1997) (Fig 2a).
a)
2. Methodology
RegCM3
The RegCM3 is a primitive equations model, compressible,
in sigma vertical coordinates (Pal et al., 2007). The
turbulent surface fluxes over the ocean are parameterized
using the Zeng scheme (Zeng et al., 1998) and the
convective scheme proposed by Grell (1993) with the
Fritsch Chappell cloud- model (Pal et al., 2007).
The simulation period was January 1982 to December 1991
for the domain from 110 °W to 10 °W and 6 °S to 36 °N.
The initial and boundary conditions are provided by the
NCEP-NCAR Reanalysis 1 (Kalnay et al., 1996) and the
sea surface temperature (SST) is the average weekly from
OISST (Optimum Interpolation SST) dataset with 1°x 1°
resolution (Reynolds et al., 2002). The model horizontal
resolution is 25 km with 18 sigma levels in the vertical,
with the top of model at 75hPa.
Hurricane tracking
The algorithm used for identification and tracking of
cyclones was developed by Sugahara (2000) and adapted
by Reboita (2008) for extratropical cyclones. The
numerical scheme uses a similar method proposed by
Sinclair (1994) through the identification of the minimum
(maximum) cyclonic relative vorticity. We performed an
adaptation to tropical cyclones taking the account the
consideration of Knutson et al. (2007) to identify warm
cores from vortices that correspond to hurricanes. Before
applying the tracking method, the zonal and meridional
wind at 850hPa and mean sea level pressure were
interpolated to a regular grid of 1° x 1°.
b)
Figure 1. Tropical cyclones trajectories for the 1982-
1991 period: (a) RegCM3 with the Sugahara
modified algorithm; (b) best-track from National
Hurricane Center dataset.