The Role of Orography and Soil Moisture in Hydrological Transitions Associatedwith Monsoon Onset in Southeast Asia

*Koji Dairaku1(1: Disaster Prevention Research Group, National Research Institute for Earth Science and Disaster Prevention)* National Research Institute for Earth Science and Disaster Prevention, Japan, 3-1, Tennodai, Tsukuba Ibaraki,
Japan. e-mail: dairaku@bosai.go.jp
Abstract
The first transitions into the Asian Summer Monsoon (ASM) occur between late April and early May over inland Indochina,
before any transitions occur along the coast. This study used a regional climate model to elucidate the influence of
orography and ground wetness on subcontinental-scale hydrological processes. The model reproduced many elements of theonset of the Southeast Asia Monsoon (SEAM) associated with land surface conditions, including the early and abrupt onset
observed when mountain effects and relatively dry soil conditions were combined in the model simulations. The nonlinear
effects of mountains and ground wetness, combined with realistic increases in precipitation, can modify the hydrologicalcycle through changes in the surface energy budget. A positive feedback between soil moisture and precipitation increases
the moisture source for further precipitation in the first transition period.
Keyword: Monsoon onset, Southeast Asia, Hydrological Processes, Regional Climate Model.

1. Introduction
The Asian monsoon is associated with droughts andfloods, which affect billions of people and the economies
of dozens of countries in Southeast Asia. Monsoon
predictability is therefore a fundamental concern for theregion (Charney and Shukla, 1981). The first transition of
the Asian Summer Monsoon (ASM) is of great importancefor agricultural practices and may foreshadow subsequentmonsoon evolution. A more thorough understanding of
ASM dynamics is a matter of extreme significance.
ASM onset begins between late April and early May
over inland Indochina (Thailand, for example). This isearlier than ASM onset over coastal regions. By the middleof May, a mature ASM circulation establishes itself.
Subsequently, land-locked convection over southern
Thailand and northern Borneo abruptly advances
northward, expanding over the South China Sea (SCS)(Matsumoto, 1997; Lau et al., 1998; Zhang et al., 2002).
Previous studies have shown a lack of precise
knowledge of how land surface conditions affect the onsetof the Southeast Asia Monsoon (SEAM). This study used aregional climate model to produce a better understandingof the influence of orography and ground wetness on the
sub-continental scale hydrological processes during theearly onset of the SEAM.

2. Model and Experiments
This study uses a regional climate model to analyze theinfluence of land surface conditions on sub-continental
scale hydrological processes. Special attention was given
to the early transition of the ASM. The model used was athree-dimensional,
non-hydrostatic
compressible
dynamic-equations model (RAMS: Regional Atmospheric
Modeling System) (Pielke et al., 1992). Cloud activity wasparameterized with a simplified Kuo convective scheme(Molinari, 1985) and a cloud microphysics scheme. A
two-stream radiation scheme (Harrington et al., 1999)
parameterized radiation. The soil and vegetation model
used was LEAF-2 (Walko et al., 2000). The model used
topography with 30” longitude-latitude resolution(GTOPO30) and land-use data that were estimated fromsatellite data (AVHRR) by USGS. Sea surface temperatures
were derived from OISST Ver.2 (Reynolds et al., 2002),
with weekly updates, and were temporally interpolated
from weekly data during the experiments. The
GAME-reanalysis data with 0.5°×0.5° horizontal
resolution and four times daily temporal resolution(Yamazaki et al., 2001), which is objective analysis dataand includes observations from the Global Energy andWater Cycle Experiment (GEWEX), Asian Monsoon
Experiment Intensive Observing Period (GAME-IOP) from
April to October in 1998, provided initial and boundary
conditions. The model fields were nudged to the objective
analysis data at the 5 outermost lateral boundary grids andabove the tropopause. Horizontal model grid spacing was
20 km. The model domain was 200 by 160 grid points(4000 by 3200 km) and there were 35 vertical levels up to
23.4 km (Figure 1). Level thickness in the vertical ranges
was from 100 m at the bottom to 1000 m at the top of thedomain.
The present study consisted of three different model runs.
In each run, the model was integrated for about 2 months,
from 1 April to 25 May 1998, a period that included theinitiation of the summer monsoon over Southeast Asia. The
control run (hereafter CTL) included realistic mountains
and horizontally uniform initial soil moisture that was
relatively dry (the degree of saturation was 0.35). A secondrun (NOMNT) included no mountains but was otherwise
similar to CTL. The third run (WET) included mountains
and extremely wet soil (the degree of saturation was 1.0).
The results from the three runs clarify the roles oforography and ground wetness on ASM onset.

3. Role of mountains and soil moisture in SEAM
3.1.

onset
Hydrological Transition

Simulated precipitation was compared to daily Global
Precipitation Climatology Project (GPCP) data (Huffman
et al., 2001). Simulated results and observation wereaveraged over land in the region from 10°N-20ºN and from

180

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g/Kg

W/m^2

120
90
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Fig. 1: Simulated region. Solid rectangle indicates theregion used for averaging simulated results andobservation from 10° N-20º N and 95º E-100º E.

W/m^2

120
90
17
60
15

30
15

Obs(GPCP)
CTL
NOMNT
WET

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150

9

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g/Kg

W/m^2

mm/day

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0

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60

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0
4/1

4/11

4/21

5/1

5/11

5/21

Fig. 2: Evolution of observed and simulated
precipitation averaged over land, between 10° N-20º N
and 95º E-100º E, from 1 Apr to 25 May 1998. The solidline represents daily precipitation from the GPCP.
Simulated precipitation from CTL, NOMNT, and WET
runs are represented by the dotted line, solid line withcircle, and dotted line with triangle, respectively.
95ºE-100ºE (Fig. 1). Figure 2 shows that a relatively large
rainfall event (7.5 mm/day) occurred in mid-April, and thatprecipitation increased abruptly in mid-May. The CTL
experiment captured these events, although CTL
overestimated precipitation between 5-10 May. The
NOMNT experiment did not reproduce the abrupt increasein precipitation during mid-May, although NOMNT did
reproduce the event in April. Precipitation in the WET
experiment was excessive before monsoon onset. However,the WET experiment did reproduce the heavy precipitation
during mid-May. Some discrepancies between the

13
4/1

4/11
4/21
Sensible Heat

5/1
5/11
5/21
Latent Heat
Soil MoistureFig. 3: Evolution of sensible heat flux, latent heat flux,
and soil moisture averaged over land, between 10° N-20º
N and 95º E-100º E, from 1 Apr to 25 May 1998 forCTL (top), NOMNT(middle), and WET(bottom)
experiments.
experiments and observation can be attributed to the errors
during mid-May. Some discrepancies between theexperiments and observation can be attributed to the errors
produced by the internal circulations developed in themodel physics and by the uncertainties present in
observational data sets, which can be high especially overmountainous regions (Xie and Arkin, 1997; Dairaku et al.,2004).
Figures 3 shows the evolution of area-averaged sensible
heat, latent heat, and the vapor mixing ratio, which is inequilibrium with the soil and directly affects the surface
energy budget (hereafter referred to as soil moisture) for
each experiment.

3.2.

Precipitation and soil moisture feedback in
the first transition period of SEAM

Soil moisture is a source of water vapor that can greatly
modify the strength of monsoon onset and influence theinduced land-ocean thermal contrasts. Figures 4 depict, foreach model run, relationships between soil moisture,
evapotranspiration, and precipitation over land from 1
April to 25 May. The top panel in Figures 4 shows that in
CTL, soil moisture and precipitation have a strong positive
correlation (increment of 0.91; coefficient of determination
is 0.61). NOMNT and WET also show statisticallysignificant positive correlations (1% significance level) but
show relatively vague relationships, as compared to therelationships in CTL. In addition, evapotranspiration andsoil moisture have strong positive correlations in CTL (Thebottom panel of Figures 4), but no correlation in NOMNTor WET.
The evolution of equivalent potential temperature
indicates that strong convective instability is present during
pre-onset for CTL, which coincides with the precipitationin mid-April and with an abrupt increase in precipitation inearly May. Comparable strong convective instability occurs
in NOMNT. Convective instability is relatively weak,particularly during pre-onset, in WET (not shown).

4. Conclusions and discussions
Even though the experiments were forced by realistic
lateral boundary conditions, the model used in this study
reproduced the early evolution of the SEAM when themodel was subject only to imposed land surface conditions,and explained the influence on abrupt monsoon onset ofmountains and relatively dry soils. That is, the three

12

CTL
NOMNT
WET

10

mm/day

8
6
4
2
0
6

CTL
NOMNT
WET

5
4

mm/dayThe top panel in Figures 3 shows an abrupt decrease of
sensible heat and an increase in latent heat flux and soil
moisture for the CTL case. These changes coincide withincreased precipitation and decreased surface temperaturein the first transition period. These changes of sensible heat
flux, latent heat flux, soil moisture and surface temperaturein the CTL experiment are consistent with the observations
reported by Toda et al. (2002). Although pre-onset
precipitation slightly exceeds observed precipitation, an
abrupt onset and large variation are present in the CTL
case.
The middle panel in Figures 3 shows slight decreases insensible and latent heat and a moderate increase in soil
moisture, associated with weak precipitation and a slightdecrease in surface temperature during onset.
The bottom panel in Figures 3 shows nearly constantsmall sensible heat values, considerable soil moisture, andslight decreases in latent heat flux. Because precipitation
during pre-onset for the WET case was plentiful, relative
variations in the period were smaller, as compared to the
CTL case. However, the averaged amount of precipitation
during the onset period was closest to observations in the
WET case. Surface temperature was much lower than thatof CTL in the pre-onset period and was higher in the onset
period than in the pre-onset period.

3
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1
0
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16

18

20

22

g/Kg

Fig. 4: The area averaged precipitation as a function ofsoil moisture (top) and the area averaged
evapotranspiration as a function of soil moisture(bottom) for the CTL, NOMNT, and WET experiments(filled circle, circle, triangle, respectively). Data
averaged over land, between 10° N-20º N and 95º
E-100º E, from 1 Apr to 25 May 1998.
experiments showed that mountains and antecedent land
surface processes strongly influence the first, abrupt
transition into the SEAM. Figure 5 shows schematic
diagram of the mechanism of SEAM onset.
Relatively dry soil moisture facilitates greater sensible
heating over land and intensifies the land-ocean thermal
contrast (e.g., Meehl, 1994; Kawamura et al., 2002).
During pre-onset, wet soil moisture causes pre-onset
precipitation that weakens monsoon circulations by
reducing the land-ocean thermal contrast. In addition,
evaporative cooling chills the land surface and reduces
convective instability.
Mountains maintain and intensify a thermal low in one
preferred region mechanically and thermodynamically(Hahn and Manabe, 1975). Upward motion, which is
associated with the thermal low and the monsoon
circulation in the lower troposphere, is weak if mountains
are not present (not shown). Orographically induced moist
convection
enhancesmonsooncirculations
andconvergence induced by the thermal low. There is apositive feedback between soil moisture and precipitation:

precipitation in the transition period from dry season to wet
season increases surface moisture, which acts as a source
for more precipitation.
During 1998, the global climate system evolved from astate in which the warmest sea surface temperature
anomalies were over the eastern equatorial Pacific Ocean
in 1997/98. Global circulations were influenced by a warm
(El Niño) phase of the ENSO cycle (Anyamba et al., 2002).
Global precipitation data (GPCP) suggest that 1998 wasrelatively dry in the region. How the global circulation
associated with ENSO influenced the early transition into
the SEAM, however, was not clearly identified. The fuller
study of the influence of the annual and decadal global
climate variability associated with ENSO on the SEAM
onset lies outside the scope of this paper. Future
investigations will explore the robustness of the present
results, which provide evidence of the importance of
mountains and internal feedbacks (involving soil moisture)in the initiation of the Asian summer monsoon.
This study suggests that nonlinear effects, including
mountains and ground wetness, combine with increases inprecipitation to modify the hydrological cycle. Suchmodifications are achieved through changes in the surface
energy budget, and the modifications allow a positive
feedback between soil moisture and precipitation byincreasing the moisture source for further precipitation.

Fig. 5: Schematic diagram indicates dynamical,
thermo-dynamical and hydrological processes induce
abrupt SEAM onset.

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