Heating processes over the Asiatic landmass
prior to the first transition of the summer monsoon
Hiroaki Ueda
Graduate School of Life and Environmental Sciences, University of Tsukuba,
Tsukuba, Ibaraki 305-8572, Japan. e-mail: hueda@sakura.cc.tsukuba.ac.jp
Abstract

Using the GAME (GEWEX Asian Monsoon Experiment) 4DDA upper-air data, the large-scale heat source (Q1) andmoisture sink (Q2) over the western and eastern Tibetan Plateau are examined for a 4-month period from 1 May to 31
August 1998. The computations were performed on the sigma-pressure hybrid coordinate, named η-coordinate,since the analysis area includes high-elevating mountains.
Over the western Plateau in May, there is a deep layer of heating occupying the whole troposphere with the
maximum value exceeding 3K day-1 around 400-600 hPa. The smaller magnitude of the apparent moisture sink is
confined in the lower troposphere 1 km above the ground surface. Vertically integrated heat sources of 103 Wm-2 over
the western Tibetan Plateau are accompanied by a moisture sink of about half that (60 Wm-2). These results indicatethat the latent heat release associated with the condensation plays an important role in the total heating besides thesensible heat supply from the land surface. Later in July, the moisture sink over the eastern Tibetan Plateau nearly
equals to the heat source indicating that the dominance of moist process associated with summertime monsoon rains.
The contrasting features of the heat source and moisture sink are closely related to the circulation fields.
Throughout May and June, we observe strong upward motion along with the western and southwestern slopes of thewestern Plateau, while there is salient subsidence motion over the eastern Plateau. The analyses of the static stability
and the lifting condensation level indicate that the release of latent heat relevant to the moist convection is a dominantfactor for the tropospheric heating after the monsoon onset. While the pre-monsoon period (May) is composed of boththe convective rainfall and the dry thermal convection.
Thus, the heating mechanism prior to the onset of the monsoon, especially over the western Tibetan Plateau, can becharacterized by the hybrid nature of “wet” processes due to condensation heating and “dry” processes associated withthe sensible heat flux from the elevated mountain surface.
Keyword: Tibetan Plateau, First transition, Q1,Q2, GAME reanalysis

1. Introduction
The Asian monsoon is a major component of the
global climate system. A number of observational
studies (Yeh et al. 1957; Staff Members Academia
Sinica 1958; Yeh and Gao 1979; Nitta 1983; Yanai
and Li. 1994 and many others) have recognized thatthe high Tibetan Plateau plays an important role in
the establishment and maintenance of the Asiansummer monsoon as an elevated heat source. Afterthe First GARP Global Experiment (FGGE), there
have been several observational studies of heatsources over the Tibetan Plateau and surrounding
areas. Nitta (1983) described the 100-day mean
vertical profiles of heat and moisture sources for thefour parts of the eastern Tibetan Plateau. He foundthat the contribution to the total heating by thesensible heat flux from the elevated surface is nearly
the same as that by the condensation heating due tolatent heat release.
Luo and Yanai (1984) determined the heat andmoisture budgets over the Tibetan Plateau using the
FGGE data for 1979. They showed the deep mixed

layer at 1200 UTC on the western Plateau andsuggested that dry thermal convection is a key
process which is responsible for the deep troposphericheating in the afternoon hours. They also identified
the principal components of the heat source after the
onset as the addition of condensation heating over theeastern Plateau.
The seasonal evolution of the monsoon circulation
exhibits distinct changes during its onset phase (Liand Yanai 1996; Ueda and Yasunari 1998; Wu andZhang 1998 and many others). He et al. (1987)
showed that the summer monsoon of 1979
commenced in two transition stages. The firsttransition seen in middle May is characterized by theeastward intrusion of low-level southwesterlies to thewest of 120°E. The second transition is the onset of
the Indian monsoon that usually occurs in early Juneto the west of 80°E. They inferred that these abruptchanges are responses to the differential heating
between the Asian continental land mass and theadjacent oceans. Yanai et al. (1992) extended the heatand moisture budget analysis of He et al. (1987) tothe warming process of the upper troposphere. They
revealed that, during the first transition period,
diabatic heating and warm horizontal advection plays
a primary role in the temperature increase over theeastern Tibetan Plateau. During the second transition
period, adiabatic warming due to large-scale
subsidence is a key process that induces atemperature increase over the western Tibetan Plateau,
Iran and Afghanistan region. Recently, the desert
formation over these regions is attributed to thedownward motion associated with Rossby-wave
pattern induced by remote diabatic heating in the
Asian monsoon region (Rodwell and Hoskins 1996).
The GEWEX Asian Monsoon Experiment(GAME) Intensive Observation Period (IOP) was
conducted from May to August 1998. One of the
purpose was to obtain high-quality four-dimensional
data assimilation (4DDA) for energy and water cycle
process of the Asian summer monsoon by means offour-times/day soundings of radiosonde observation
at more than 100 stations covering Southeast Asia,
the northern part of South Asia (India, Bangladesh,and Myanmar), the Tibetan Plateau, central Eastern
China around Huai-He River, the South China Sea,
Korea, and Southwest Japan (GAME International
Science Plan 1998).
Based on the above enhanced experimental upper
air data, as well as operational observation data, the
4DDA was conducted through collaboration between
the Meteorological Research Institute (MRI),
Numerical Prediction Division of the Japan
Meteorological Agency (JMA) and the Earth
Observation Research Center of National Space
Development Agency of Japan (NASDA/EORC).
The 4DDA products have hybrid vertical η
coordinates, which are identical to the conventional
sigma coordinates in the lower troposphere and nearly
equal to the pressure coordinate in the upper
troposphere. The benefit of this coordinate is that itprovides a good representation of the lower boundary
condition and reduction of finite difference errors of
the pressure gradient force at the higher levels (e.g.,
Kasahara 1974; Simmons and Burridge 1981). Since
the Asian monsoon region contains the high-altitude
Tibetan Plateau, we performed the heat and moisturebudget analysis on the vertical η coordinate.
This study attempts to determine quantitatively thehorizontal and vertical distributions of heat sources
and moisture sinks over and around the Tibetan
Plateau through budget computations of mass, heatand moisture with the GAME reanalysis data set
archived during the GAME IOP from 1 May to 31
August 1998. The other objective of the present work
is to reveal the seasonal evolution process of the heatsources and moisture sinks on a pentad time scale.
We compare the results of heat and moisture budget

analyses by using satellite-derived precipitation dataas an independent resource of the GAME reanalysis
product.

Fig. 1.
Orographic structure of the Tibetan Plateau. The solidlines denote elevation contours. The ground surface withelevations > 2000 (4000) m are shown by light (dark) shading.
Two areas enclosed by thick bold lines are used for budget
analysis. Open triangles over the western Tibetan Plateau denotenewly set up stations for four-times/day upper air observation
during the enhanced observation period. The other open triangles
correspond to stations where four times/day upper-air soundings
were conducted but not transmitted by the GTS on-line network.
Additional observations at 06 and 18 UTC (besides the routine
soundings at 00 and 12 UTC) are depicted by colored circles. The
operational stations usually make twice-daily observations, which
are shown by open circles.

2. Data and study method
The major data used in this study are the 18 and 24hour forecasted 4 DDA products by the GAMEreanalysis project on a 2.25°
2.25° grid for 1 May
to 31 August 1998. Temperature (T), heating rate oflong-wave radiation (QR), zonal and meridional windcomponents (u, v), and specific humidity (q) are
given on 30 η levels. Surface pressure (Ps) is alsoutilized to determine the vertical distribution of thepressure field.
The global spectral model, which was processedwith the JMA operational global forecast analysis
cycle model (GSM9912), has an equivalent grid
spacing of about 55 km (T213) horizontally and 30
η-levels (L30) vertically with a model top of 10 hPa.
The convection scheme used in the model is the
prognostic Arakawa-Schubert scheme (Kuma 2000).
The apparent heat source Q1 and apparent moisturesink Q2 (e.g. Nitta 1972; Yanai et al. 1973) arecalculated from the thermodynamic and moisturebudget equations in the η coordinates as follows:∂T
Rd dp ˙ ∂T Q1= −V ⋅ ∇T +−η+∂t
CpPk dt∂η Cp ,∂q∂q Q2
= −V ⋅ ∇q − η˙−∂t∂η Lc ,

where T is temperature; q, the mixing ratio of watervapor; v, the horizontal wind; Rd and Cp, the gas
constant and the specific heat at constantpressure of.
dry air; p, the pressure; and η vertical, ηvelocity and Lc, the latent heat condensation.
The pressure on half levels is
k max
Δp
dp
= V ⋅ ∇p− ∑ ∇ ⋅(V ΔP ) − (∇ ⋅ V ) kkk +1 2
l
l l
k 2 ,
dt
l=k

where k is the full level number and k+1/2 is the . halflevel number that increases with altitude.
η iscomputed from∂p∂p
k − 1 2 k max)=−− ∑ ∇(V Δp ) ,
k
−
1
2
l l
∂η
∂t
l= k
 ∂p

∂T
1
∂p
(η˙ ) =)(T
− T ) + (η˙ )(T − T)(η˙∂η2Δp  ∂η k − 1 2 k −1 k∂η k + 1 2 k k +1 k,(η˙

3. East-west contrasting features over the Plateau
The budget computations were made in the westernand eastern Tibetan Plateau enclosed by solid lines inFig. 1. The western part (30°-40°N, 65°-85°E)
denotes the area extending from the Plateau region to
the southern foot of the Plateau. The eastern Plateau(30°-40°N, 90°-110°E) has an average surface
elevation of about 3000 m. The distribution of fourtimes/day rawinsonde observations covering over the
Tibetan Plateau and its surrounding area during the
GAME-IOP are shown by three symbols. Open
triangles over the western Tibetan Plateau, enclosed
by a bold line, denote newly set up stations for thisexperiment. Enhanced observation were made at 06
and 18 UTC (Coordinated Universal Time) by using
the convectional (00 and 12 UTC) observation
network (colored circles). In addition to thesesoundings, we conducted 4DDA by use of the upperair data observed by the operational stations (opencircles).

Fig. 2.
Vertical-time sections of areal mean (a) apparent heatsource, (b) moisture sink for the western Plateau (30°-40°N, 65°85°E). Dark (Light) shaded regions indicate a heating rate greater(less) than +2 (-2) K day-1.

Figure 2 show the seasonal evolution of heat sources(upper panel) and moisture sinks (lower panel) over
the western Tibetan Plateau through the months of
May and August, 1998. During May, the premonsoon period, the maximum heating (exceeding
3K day-1) exists in the layer between the land surface
and 300 hPa. The smaller magnitude of the apparent
moisture sink is confined to 1 km above the near
mountain surface. This smallness of Q2 suggests that
another source of heating besides the latent heatrelease is needed to explain the total heating. Thus,
the contribution of the sensible heat flux, directlytransported by dry thermal convection, may be
important to the total heating. Between late June andmiddle July (except for early July), the heat source isrelatively smaller and the evaporation exceeds theprecipitation seen as negative Q2. Until August, thelarge positive Q1 recurs in the whole layer and isaccompanied by positive Q2 seen in the 2 km layer
between the mountain surface and 500 hPa. These
results indicate that the large heat source is be due toboth effects of the condensation heating by deep
cumulus convection and the sensible heat supply fromthe ground surface.
4. Conclusions and remarks
We have used GAME 4DDA products from the
GAME reanalysis joint project between MRI, JMA
and NASDA/EORC to study spatial and temporalvariations of heat sources and moisture sinks over andaround the Tibetan Plateau for the period from 1 May
to 31 August 1998. We have mainly focused on theheat and moisture budgets over the western andeastern portion of the Plateau, including the southernand eastern foot of the high elevating mountains, The
main findings of the present study may besummarized as follows:1) During the pre-monsoon period of May, the resultsof heat and moisture budget analyses indicate thatthe western Tibetan Plateau plays an important role
as one of center of the heat source in the firsttransition of the Asian summer monsoon, while theeastern Tibetan Plateau remains a heat sink in thesame period.
2) During July, corresponding to the mature phase of
the summer monsoon, there is a large heat source inthe eastern Plateau with amplitude about two times
those over the western Plateau in May. In contrast,
the heating over the western Plateau becomes
weaker compared to those in May.
3) Throughout May and June, we observe strong
upward motion along with the western andsouthwestern slopes of the western Plateau, while
the obvious subsidence motion is found over theeastern Plateau. This paired circulation becomes
weaker in July and August.
4) The condensation heating generated by convective
rainfall and the sensible heat supply from the grandsurface is nearly equal during May over the western
Plateau. A similar relationship, but relatively larger
contribution of latent heat release to the total
heating, can be recognizable over the eastern
Plateau in July.
5) The analyses of the static stability and the lifting
condensation level indicate that the total heating is
attributed to both the convective rainfall and the drythermal convection in the pre-monsoon period(May). On the other hand, the release of latent heat
relevant to the moist convection is a dominantfactor for the tropospheric heating after the
monsoon onset.
In conclusion, we have shown that the western
Plateau in the pre-monsoon period can becharacterized by the hybrid nature of “wet” processes
due to condensation heating and “dry” processes
associated with the dry thermal convection. Finally
we should note here that the analysis period of thepresent study corresponds to a peculiar year. From
May through August 1998, sea surface temperature
was higher than normal in the whole Indian Ocean,and the Asian summer monsoon was much modulated(Matsumoto et al. 1999; Shen and Kimoto 2001).
This is due to combined effects of termination of adipole mode in the equatorial Indian Ocean and the
La Niña phase in the Pacific Ocean (Ueda andMatsumoto 2000). The Asian monsoon is considered
to have large interannual variability that may be
responsible for the different magnitudes of budget
results among several studies. In this respect, theability to accurately observe the heat and moisturebalances in the Tibetan Plateau over a complete
annual cycle, as well as variability on longer
timescales, would significantly promotes progress ofthe monsoon prediction.
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