Simulation of Southeast Asia Rainfall using RegCM3 and Problems
Kieu Thi Xin, Le Duc & Ho Thi Minh Ha, Vietnam National University of Hanoi.
Abstract
The Indochina Peninsula (IP) lies under main flow parth of the indian monsoonal westerlies. Whose moistureis vital for this region, at the same time causes heavy rainfall leading to severe flooding for IP and adjacent countries,
makes it extremly important to improve seasonal rainfall prediction in genereal. In this paper, as the first step, theRegional climate model of the thirth generation (RegCM3) has been experimentelly used in seasonal simulation ofSoutheast Asia Rainfall with using reanalysis of NCEP, ECMWF and ERA-40 as lateral boundary and initialconditions for driving the RegCM3. The later has a 56 km horizontal resolution, 18 vertical levels with the model
domain of 100S – 300N, 700E – 1300E is chosen. The model is intergrated during two rain seasons of 1996 and 1997 as
typical wet and dry year, with different convection schemes, in oder to determine the optimal model conditions for theSoutheast Asia region where rainfall causes most by deep convection originated from tropical oceans. The preliminary
results have shown that the precipitation was better simulated over IP using ERA-40 reanalysis data than using NCEP
or ECMWF reanalysis.
RegCM3 reproduce some main features of observed average temperature, precipitation and local circulations.,
but underestimates temperature by ~2 0C degrees and the simulated horizontal and vertical transport is normaly too
active for Southeast Asia region.
When ERA-40 reanalysis as initial and lateral and boundary conditions was used the simulated rainfall field
was more closer to analysis using Grell convection scheme with Arakawa- Shubert closure assumption (Grell-AS) andZeng Ocean Flux Model, howerver not only rainfall but also temperature are underestimated. Our funding show that
all current convection schemes in RegCM3 are not enough good working for the Southeast Asian region. So we haveinserted the convective parameterization scheme of Tiedtke (1989) in RegCM3 leading to improving considerably
simulated temperature field and rainfall distribution over Southeast Asian region.
Keyword: Rainfall, convection, seasonal simulation.

1. Introduction
The rain season in IP normaly started at theonset of the southwest monsoon, begin of May, andcloses at the ende of Desember. The moisture of theindian monsoonal westerlies is vital for IP, at the sametime leading to severe flooding causing a lot ofdamage to the crops and people for the IP.
The heavy precipitation regimes ofIPcharacterised by combined impact of quite different
weather systems such as monsoon trough, Intertropical
Convergence Zone (ITCZ), northeast and southwest
monsoon in combination with the ITCZ and also toTCs originating mostly from South China Sea region
and is currently quite poorly predicted. It is extremly
important for IP to improve the seasonal precipitation
prediction. Based on advantages of regional climate
models possible to be applied to seasonal prediction
the RegCM3 is used to simulate seasonal climate
regime of the Southeast Asia region with concentrationon the IP for understanding the ability of theRegCM3 to capture the main circulation features ofthe region and the pattern of temperature andprecipitation over IP and Southeast Asia region.

2. Experiment design
2.1. Regional climate model RegCM3
RegCM3 is the latest version of ICTP,
originated from NCAR-RegCM. It contains numerous
options for different applications.

•••••Dynamics of RegCM3 based on MM4/5 hydrostatical, lateral boundary conditions
relaxation.
Physics in RegCM3 : Radiation scheme(Kiehl et al, 1996); BATS land surface model(Dickinson et al, 1993); Non-local planetary
boundary layer scheme (Holtslag et al, 1990);
Different convective precipitation schemes[Kuo-Anthes scheme (Anthes, 1977), Grell
scheme (Grell 1993) with Fritsch & Chappell
closure assumption (FC-80) or Arakawa&Schubert closure assumption (AS-80)].
BATS1E vs. Zeng Ocean Flux Model (Zenget al, 1998).
SUBEX large-scale precipitation scheme (Palet al, 2000).
In this study we are trying to use theconvective parameterization scheme ofTiedtke (1989) instead of Grell scheme with
Arakawa-Shubert closure assumption (GrellAS) .

2.2. Model configuration and Data
The model configuration (domain, resolution,and the relation between them…) is an important issue
for regional climate modeling (Seth & Giorgi 1998,Giorgi (2004)), but it is duficult to determine asuitable model configuration for this region because ofthe local typical orography such as the mountain
Hymalya on the northwest of model domain... The
experiments were performed with domain size andlocation so that main large-scale forcings as ITCZ,

Fig. 2: Simulated temperature of June 1996 by RegCM3
with NCEP reanalysis (top), with ERA40 reanalysis (middle)
to compare with CRU-analysis (bottom).

monsoons, TCs…to be included. The results show thatthe model solution is sensivitive to domain size andlocation. In this study we use the model configurationof 56 km horizontal resolution, 18 vertical levels overthe domain of 100S –300N, 700E – 1300E. NCEP, ERA40 reanalysis fields are used to provide initial andboundary conditions for the atmosphere and sea surface
temperature (SST) fields.

3. Preliminary Results
Fig. 1: Simulated Rainfall of June 1996, by RegCM3 with

3.1. Case study A

NCEP reanalysis (top), ERA40 reanalysis (second) tocompare with CMAP analysis (third) & CRU -analysis(bottom).

Three different reanalysis were used as lateralboundary and initial conditions for driving theFig.3: Simulated Rainfall of June 1996, by RegCM3 with
ERA40 : using Grell - AS & Zeng in BATS (top), using
Tiedtke scheme & Zeng in BATS (second) to compare with
CMAP analysis (bottom).

RegCM3.
Fig.1 demonstrate simulated days means ofrainfall of June 1996 by RegCM3 using NCEP &ERA40 reanalysis.
Two first pictures of Fig.1 show that ERA40 provide
more moisture for this region. Howewer, in both cases
RegCM3 did not sumulate raifall area of Northeast ofVietnam and of South China coast and over ocean
around IP.
By comparison of simulated temperature ofJune 1996 of RegCM3 with CRU-Analysis in Fig.2 itis found that temperature over large area of modell
domain is underestimatedreanalysis.

1 ~ 20 Cwith bothFig.4: Simulated temperature of June 1996, by RegCM3
with ERA40 reanalysis: using Grell-AS & Zeng in BATS
(top) , Tiedtke scheme & Zeng in BATS (second) tocompare with CRU analysis (bottom).

3.2. Case study B
From numerical weather forecasting using
HRM for the Southeast Asian region we found that theconvective parameterization scheme of Tiedtke (1989)
is well working for the Southeast Asian region. In thiscase we performed an experiment using the convective
scheme of Tiedtke (1989) instead of Grell -AS scheme
in RegCM3 using ERA40 as initial and boundary
conditions, but because of limited computer power at
our university we used a smaller domain of 100S –300N, 800E – 1300E for this experiment.
Fig.3 presents the simulated days means ofprecipitation of June 1996 using Grell-AS scheme (firstpicture) andTiedtke scheme (second picture). By

comparison with CMAP analysis (bottom in Fig.3) we
see that major featutes in precipitation were much
more reasonably captured by RegCM3 using Tiedtke
scheme than using Grell-AS scheme. These features
include the rain bands from Southeast Phillipines toSoutheast China and the rain bands from southwest
area of model domain to the northeast coast of Bengal
gulf, where there is normaly strongest rainfall of theworld. The precipitation mount is overestimated inthose rain bands but underestimated over ocean areas
around of IP, where observations are very poor.
Considering temperature field in Fig.4 we canfind that using Tiedtke convection scheme in RegCM3
leading to a considerable improvement in simulatingtemterature of 2 metter hight over the surface. While
RegCM3 using Grell-AS scheme underestimates
temperature nearly 1 − 20 C (first picture in Fig.4), thesimulated temperature by RegCM3 using Tiedtke
convection scheme (the second picture in Fig.4)
scheme is more closed to CRU analysis over land and

20 C higher than one with Grell-AS convection
scheme for the most areas of the model domain (thefirst picture in Fig.4) , what show the important role ofthe convection parameterization for tropical regions.
In both experiments there is always problems
in simulated fields on the northwest boundary nearlyto Hymalya area.
The runs for 1997 year show nearly the similar results.

Conclusions••••

Our experiments have shown that the nested
regional climate model RegCM3-ERA40
using Tiedtke convection parameterization
scheme is capable in simulating major
seasonal rainbelts over Southeast Asian region
and promising to predict seasonal climate.
To progress the seasonal prediction by
RegCM3 it is very important to estimate asuitable model configuration fo this region.
The improvement of analysis data by given
observations is an important issue for climate
modeling of Southeast Asian region.
It is needed to perform more experiments.

Acknowledgements : We would like to express oursincere thanks to Prof. Fillipo Giorgi and his
collaborators for their assistances and supply of data.

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