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Development of a regional ocean-atmosphere coupled model and its
performance in simulating the western North Pacific summer monsoon
Liwei Zou, Tianjun Zhou and Rucong Yu
LASG, Institute of Atmospheric Physics, Chinese Academy of Science, Beijing 100029, China.
zoulw@mail.iap.ac.cn
1. Abstract
A regional ocean-atmosphere coupled model is developed
with the aim to improve Asian monsoon simulations. It
consists of the Regional Climate model (CREM) developed
at the State Key Laboratory of Numerical Modeling for
Atmospherics Sciences and Geophysical Fluid Dynamics /
Institute of Atmospheric Physics (LASG/IAP) and the
revised Princeton Ocean Model (POM2000) developed by
Princeton University. The exchanges of coupling fields,
including sea surface temperature (SST), heat flux and wind
stress, are synchronized by Ocean Atmosphere Sea Ice Soil
3.0 (OASIS3.0) coupler developed at CERFACS (Toulouse,
France).
The performance of the coupled model in simulating the
Western North Pacific Summer Monsoon (WNPSM) during
the warm season (May-August) in 1998 is compared with a
stand-alone CREM simulation. The results show that the
rainfall and heat fluxes, especially the latent flux over
Western North Pacific (WNP), are significantly improved in
the coupled model. The coupled simulation improves the
spatial pattern of the rainfall and increases the intensity of
the rainfall over the WNP. Furthermore, the intra-seasonal
oscillation is better reproduced in coupled simulation.
However, the model overestimates the rainfall over the
western northern flank of the western Pacific subtropical
high and SST over the whole domain.
2. Model configuration and experiment design
The configuration of the coupled model is illustrated in
Fig.1.The atmospheric component of the coupled model is
CREM which is developed at LASG/IAP based on a
numerical forecast model. The CREM is a hydro-static,
primitive-equation, grid point model and has uneven 32
vertical levels in an eta (η) coordinate, with the model top at
10 hPa. The Arakawa E-grid is employed in CREM and the
spatial resolution is 37 km in current version. The
Biosphere-Atmosphere Transfer Scheme version 1e
(BATS1e) is used to compute the exchanges between the
land and atmosphere, and the modified Betts-Miller cumulus
parameterization scheme is applied to calculate the
convective rainfall (Shi et al., 2009)
The ocean component is the Princeton Ocean Model 2000
(POM2000). It is a three-dimensional, primitive equation
model using a sigma vertical coordinate and a free ocean
surface with embedded turbulence. The version used in this
study was improved by Qian (2000). The model employs a
horizontal resolution of 0.5°×0.5°, and there are 16 levels in
the vertical direction. A simple radiation method is adopted
as the open boundary condition.
The OASIS3.0 coupler is used to bridge the atmospheric
model (CREM) and ocean model (POM2000). The heat flux
and wind stress which are necessary to drive POM2000 are
derived from CREM, while the SST provided by POM2000
is used as the lower boundary of CREM.
The model domain covers the region of -5°S-40°N, 100°E-
160°E. The couple model is integrated from May 1 to Aug
31 in 1998 (Coupled Run). In order to facilitate comparison,
a stand-alone CREM simulation (Control Run) forced by
weekly OISST is also performed in the same period. The
daily GPCP rainfall data (resolution: 1°×1°) are used as
observational evidence for model assessment.
Figure 1. Schematic description of the coupled
model; initial condition and lateral boundary
condition of the CREM are obtained from NCEP
RA2. A simple radiation method is adopted to deal
with the open boundary problem of the regional
ocean model (POM2000). The CREM and
POM2000 are coupled sequentially with an interval
of 3hr.
3. Some results
Figure 2. The spatial distribution of the rainfall
(units: mm/day) over WNP area averaged during the
simulation period from (a) GPCP (b) Coupled Run
(c) Control Run