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Evaluation of dynamical downscaling of the East Asian regional climate
using the HadGEM3-RA: Summer monsoon of 1997 and 1998
Johan Lee 1 , Suhee Park 1 , Wilfran Moufouma-Okia 2 , David Hassell 2 , Richard Jones 2 , Hyun-Suk Kang 1 ,
Young-Hwa Byun 1 , and Won-Tae Kwon 1
1 National Institute of Meteorological Research/Korea Meteorological Administration, Seoul, Republic of Korea,
Johan.lee@kma.go.kr, 2 Met Office Hadley Centre, Exeter, UK
1. Introduction
The HadGEM3-RA is a regional atmospheric model based
on the global atmospheric model of the Met Office Hadley
Centre (MOHC). Korea Meteorological Administration
(KMA) has a plan to adopt this model for dynamical
downscaling from seasonal to decadal scales over the East
Asian region according to the collaboration agreement
between MOHC and KMA.
As the first stage of this effort, the performance of the
HadGEM3-RA for the East Asian summer monsoon is
investigated. The East Asian summer monsoon is the most
prominent feature in the East Asian region climate and a
distinct component of the Asian summer monsoon system.
Therefore, it is important to examine the model’s capability
to simulate the East Asian summer monsoon before utilizing
the model for downscaling for longer time scales over the
East Asian region.
In this paper, we will show a preliminary evaluation of its
performance for the East Asian summer monsoon.
2. Experiment design
The model domain is the East Asian monsoon region
including Korean Peninsula, East China, Mongolia, and
Japan. Number of grid points are 190 (in west-east) by 158
(in north-south) and horizontal resolution is selected to be
0.22 degree, about 25 km. The buffer zone for lateral
boundary conditions is 8 grids for each direction.
Configuration of HadGEM3-RA for the East Asian region is
almost the same as that of the HadGEM3-A, except that the
dynamic settings are taken from the operational limited area
model. Dynamics core and physical packages are described
in Davies et al. (2005) and Martin et al. (2006), respectively.
Atmospheric initial conditions were obtained from the
European Centre for Medium-Range Weather Forecasts
(ECMWF) 40-year reanalysis (ERA40) data. Lateral
boundary conditions to drive HadGEM3-RA were derived
from the 6-hourly data. Sea surface temperature and sea ice
were prescribed by Atmospheric Model Inter-comparison
Project (AMIP) II data.
We selected two summer seasons to examine the model’s
capability to simulate the East Asian summer monsoon and
its interannual variation. The model integration periods are 3
months from 0000 UTC 1 June 1997 and 1998, respectively.
Typical characteristics of the East Asian summer are heavy
rainfall from June to July and heat waves from late July to
August. The two selected summers represented extreme
climatic events which were opposite extremes in terms of
precipitation. In summer of 1997, there were drought and
heat waves in North China. In the other hand, there were
abnormal heavy rains in summer of 1998 which had caused
severe flooding in the Yangtze River valley and northeast
China and record-breaking daily precipitations over the
Korean Peninsula. Therefore, the model’s capability to
capture interannual variability of the summer monsoon can
be evaluated by comparing the simulations of 1998 and
1997.
3. Results and discussions
Figure 1 shows observed and simulated precipitations for
summer of 1998. As mentioned above, East Asia
experienced severe heavy rainfall in summer of 1998. In
Fig. 1a, there are intense rainbands over central China
around the Yangtze River basin, the Korean Peninsula,
and Japan. On the contrary the major rainband is weaker
and shifted southward (not shown) in 1997. Therefore, in
the difference between 1998 and 1997, positive anomalies
occur in central China, Korea, and the northwest Pacific
Ocean, and a negative anomaly in south China and Taiwan
(Fig.1b). HadGEM3-RA reproduces well the observed
features of both summers in terms of the distribution of
seasonal precipitation. The pattern correlation coefficients
are 0.53 and 0.61 for 1998 and 1997, respectively, and the
model reproduces well the anomaly from 1998 to 1997
(Table 1). In Fig. 2b, the rainfall deficit over south China
and its surplus across central China and the Korean
Peninsula are distinct as in the observations. However, the
HadGEM3-RA tends to overestimate the intensity of
precipitations. Simulated precipitations of 1998 and 1997
are greater than observations and wet biases are 66.5% and
47.9%, respectively (Table 1).
Simulations of surface air temperature are better than
those of precipitation. The pattern correlations are 0.96 for
both summers (Table 1) with the distribution and
magnitude of surface air temperature very close to
observations (not shown). In addition, the features of 1998
and 1997 anomalies are well captured, e.g. the cold
anomaly in north China and warm anomalies in south
China and the north-eastern region of Asia. The simulated
surface air temperature is generally warmer than
observations with area averaged biases of 0.5 °C and 0.9
°C respectively (Table 1).
Table 1. Comparison of statistical values for
precipitation and surface air temperature of
observation and simulations. In case of biases of
precipitation, the ratio (%) of bias to observation is
presented in parentheses.
Observed
mean
Simulated
mean
Bias
Precipitation Temperature
1998 1997 1998 1997
5.27 4.35 21.0 21.4
8.78 6.44 21.5 22.3
3.51
(66.5)
2.09
(47.9)
0.5 0.9
RMSE 4.32 2.66 1.0 1.3
Pattern
Correlation
0.53 0.63 0.96 0.96