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Sensitivity of the simulated East Asian summer climatology to convective
schemes using the NCEP regional spectral model
Hyun-Suk Kang 1 , Song-You Hong 2 , and Won-Tae Kwon 1
1 Climate Research Laboratory, National Institute of Meteorological Research, Korea Meteorological Administration
2 Department of Atmospheric Sciences, Yonsei University. 1 Email: hyunsuk@kma.go.kr
1. Introduction
The representation of subgrid-scale precipitation process is
known as the cumulus parameterization. Several cumulus
parameterization schemes (CPSs) are available for regional
and global atmospheric models; however, they do not
explicitly resolve the convective clouds. Therefore, the CPS
has been considered as one of the most challenging and
uncertain aspects in numerical atmospheric modeling.
Different CPSs have distinct design history; in some cases,
these schemes have completely different conceptual underprintings.
In this context, the objective of this study is to
examine the sensitivity of the simulated East Asian summer
monsoon’s (EASM) climatology to the four CPSs within the
National Centers for Environmental Prediction (NCEP)
Regional Spectral Model (RSM) (Juang and Kanamitsu,
1994; Juang et al., 1997). The RSM is capable of
reproducing the EASM in 1987 and 1988 (Hong et al., 1999).
In addition, Hong and Choi (2006) investigated the
sensitivity of the 1997 and 1998 EASM to three CPSs with
the RSM; however, a study on the sensitivity of the longterm
climatology of the EASM to physical processes has
been rarely conducted.
2. Model and Experiments
The spectral representation of the RSM is a two-dimensional
cosine series for perturbations of pressure, divergence,
temperature, and mixing ratio, and a sine series for vorticity.
Linear computations such as horizontal diffusion and semiimplicit
adjustment are only considered as perturbations,
which eliminates the error due to the re-evaluation of linear
forcing from base fields by the regional model. The physical
processes in the RSM follow the package of Hong and
Leetmaa (1999).
The model grids consist of 109 (west-east) by 86 (northsouth)
grid lines at approximately 60 km horizontal
separation, and 28 sigma layers in vertical separation. Initial
conditions and large-scale forcing are obtained from the 6
hourly NCEP-DOE reanalysis (R-2) data (Kanamitsu et al.,
2002). Observed sea surface temperature (SST) is updated
daily from the optimal interpolation SST (OISST) weekly
dataset. The three-month-long simulations were conducted
starting on June the 1st for ten years from 1997 to 2006.
Four different CPSs - SAS, RAS, CCM, and KF2 schemes -
were used to study the sensitivity of the EASM to cumulus
effects. By and large, the four CPSs exhibit unique features.
The SAS scheme has been operational in the NCEP global
forecast system. Thus, its performance has been evaluated
for weather systems all over the world, whereas the RAS
outperforms the SAS in reproducing the climate signal in
response to the SST anomalies over the tropics. The CCM
and KF2 schemes have been successfully applied to climate
and mesoscale modeling studies, respectively, using the
NCAR CCM and MM5 models. Hence, it would be
interesting to clarify the characteristics of the four CPSs in
the simulation of summer monsoonal rainfall over East Asia.
3. Results
3.1 Large-scale Circulations
The 10-year averaged JJA precipitation of the GPCP data
represents monsoonal rain bands extending northeastward
from southern China to the southern part of the Korean
peninsula and Japan (Fig. 1a). In association with the
monsoonal rain band, the southerly and/or southwesterly
low-level jet (LLJ) transports significant moisture from
southern China and the South China Sea across Korea to
Japan, along the western periphery of a subtropical high
over the northwestern Pacific (Fig. 1b). The climatology
of the upper atmospheric circulation provides favorable
dynamic circulation for the monsoon precipitation in the
upper level with an anticyclonic (divergent) circulation
above the monsoonal front (Fig. 1c, d).
Figure 1. The analyzed 10-year JJA climatology averaged
over 1997-2006 for (a) precipitation (mm), (b) 850 hPa
winds and relative humidity (> 70%), (c) 500 hPa
geopotential height and temperature, and (d) 200 hPa
geopotential height and winds.
Overall, the RSM successfully reproduces the large-scale
patterns, regardless of the selected CPS. The PC
coefficients reveal relatively less skill in the simulation of
the lower-level fields. The LLJ transporting warm and
moist air from the south to the heavy rainfall region is one
of the principle components, which is captured by all
experiments, but with a certain systematic biases. A
common wet bias appears in northwestern part of the
model domain, and a dry bias in its south. Compared to
the results from other schemes, dryness is pronounced in
the CCM run, particularly, over southern China and the
ocean. In the CCM run the humidity bias is the largest, but
temperature bias is the smallest. The deviations in the 500
hPa geopotential height (GPH) also show similar error
patterns for SAS, RAS, and KF2 runs, which reveal a
decrease in height along a continental trough, indicating
an exaggerated trough to the west of the Korean peninsula.