The impact of diabatic heating over the Tibetan plateau uponnortheastern Asia arid region*Tomonori Sato1, Fujio Kimura2, 3
1: Japan Science and Technology Agency, Now at University of Tsukuba
2: Life and Environmental Sciences, University of Tsukuba
3: Frontier Research Center for Global Change)* Tomonori Sato, Life and Environmental Sciences, University of Tsukuba, Tsukuba, 305-8572, Japane-mail: sato@suiri.tsukuba.ac.jp
Abstract
Formation mechanism of arid climate over northern China is investigated by a series of numerical experiments. On thatregion summertime precipitation is strongly reduced by a regional scale subsidence. Sensitivity experiment without
condensation process is conducted in order to clarify the physical mechanism for the generation of the subsidence.
Simulated vertical velocity shows the descent at middle troposphere over the arid region. On the other hand ascending
motion appears immediate above the Tibetan plateau because the planetary boundary layer acts as an elevated heat source
over the plateau. However, only gentle vertical motion is left in the case when diabatic processes is turned off in the model.
That implies mechanically forced subsidence by the terrain, such as mountain wave, is not enough to cause the subsidence.
To the contrary, thermal effect of the plateau is fairly larger since the subsidence over arid region is enhanced when diabatic
heating is artificially given only over the plateau. Northward propagation of the stationary Rossby wave is inferred to cause
the robust subsidence over the arid region in northern China. The stationary Rossby wave tends to propagate northward
because subtropical westerly jet is being situated north of the plateau in boreal summer. Therefore, the stationary descending
motion prevails over northeastern Asia, which consequently reduces the precipitation around arid region. We also confirmedthat the stationary Rossby wave is likely to propagate not only northward but also southward of the plateau when zonal
mean of January is assumed as a boundary condition. Spatial relationship between the plateau and the westerly jet
undergoes transition period from spring to summer. The relationship could widely affect the seasonal evolution among
Asian region since the spatial distribution of the subsidence will be changed simultaneously around the plateau.
Keyword: arid climate, northeast Asia, regional climate model

1. Introduction
Broadly spread arid climate covers northern China,
for example the Taklimakan and Gobi desert. They can be
regarded as part of subtropical arid climate in Eurasiancontinent extending from Arabian Peninsula, Central Asia,
northern China, and Mongolia. Subtropical anticyclones
related with the subsidence branches of Hadley circulation
generally extend up to 30 degrees in both hemispheres.
Thus, its remarkably higher latitude is one of the majorinteresting issue involving with Eurasian continental
desert.
Previous studies with Global Circulation Models(GCMs) have shown that more precipitation is suppliedeven in these continental deserts if any orography is absent(Manabe and Terpstra, 1974); that is, the distance from theocean is not a sufficient reason for the continental interior
desert. Otherwise, the influences of orography should be
more important and need to be discussed for the case of
Taklimakan and Gobi desert because they are situatedalong northern periphery of the Tibetan plateau.
Broccoli and Manabe (1992) have stated that thelarge scale orography contributes to the winter dryness bythe results of a GCM experiment. They have suggested thatthe plateau causes a stationary ridge of subtropical westerly
over northeastern Asia, which makes synoptic disturbancesless active. During summer, however, heating contrasts
over the Asian monsoon region widely modify theatmospheric circulation, and forms quite different systemfrom that in wintertime. During summer, large amount ofprecipitation occurs over the Bay of Bengal, southeast Asia,and ITCZ regions. Also heavy precipitation is observed in
the regions of eastern China, Korea, and Japan. On theother hand, arid climate is found in central Asia around theCaspian Sea, where summer subtropical anticyclone isremotely enhanced by a huge amount of diabatic heating insouth and southeast Asia (Rodwell and Hoskins, 1996).
Moderate precipitation is observed in Mongolia and Far
East Siberia. More than half of annual precipitation occurs
during summertime in these regions. However, arid region
is locally present corresponding to Taklimakan and Gobi
desert. We aim to clarify the formation mechanism of thisarid climate by series of numerical experiments using aregional climate model (RCM).

2.

Numerical experiments

The RCMs are often used for the process studies ofregional climate system (e.g. Yoshikane and Kimura, 2003).
The advantages of RCM are the higher spatial resolution
and an ability to carry out the idealized sensitivity studies.
The modified version of RAMS (regional atmosphericmodeling system), TERC-RAMS (Sato and Kimura, 2004)
is adopted in this study. Horizontal resolution of the modelis 150 km, and the model domain covers almost whole Asia
and Siberia. The model atmosphere is divided into 30
layers with the finest resolution of 110 m at lowest level,
stretching up to 900 m in upper layers. Precipitation iscalculated using both microphysics and cumulus
parameterizations. In a zonal mean experiment, referred toas ZM, lateral boundary conditions are nudged by a global
zonal mean values during July of 2000 calculated using thereanalysis data. Since boundary variables are stationary
during the numerical integration, we can neglect thesynoptic disturbances passing through into the model
boundary. According to this, the circulation induced bysurface inhomogeneity, i.e., orography and ocean-landcontrast, and zonal flow itself becomes the most dominantfactor controlling a climate system in the ZM experiment.
Condensation process plays an important role on
atmospheric circulation during warm season, because it
widely modifies the energy balance as a result of change inwater phase. The sensitivity of the latent heat release is
examined in an experiment (referred to as NoC) by turning
off the condensation process in the model without anyother changes from the ZM experiment.
The Tibetan plateau may still act as an elevated heatsource even if no condensation occurs in the NoC
experiment, because the plateau is directly heated by thesolar radiation. Another sensitivity experiment (referred toas NoR), which is same as NoC but excludes the radiation
process, is carried out in order to simulate only
mechanically forced flow passing over and aroundmountain like Rossby waves and mountain waves. Allsensitivity experiments are carried out during 30 days, andthe averaged values during only last 10 days are analyzed.

3 Result
3.1
ZM experiment
Figure 1 shows 10-days averaged precipitation andwater vapor flux by the ZM experiment. The features ofrainfall distribution are simulated well, and aridity innorthern China appears even in this sensitivity experiment.
Rainfall amount around northeastern Asia arid region isless than 10 mm/month. The simulated rainfall distribution
indicates that the interaction between zonal mean
components and the surface boundary conditions, such as
topography, is essential for the formation of the arid are in
northern China.
Figure 2 illustrates the vertical velocity (shaded) andhorizontal wind at the level of 400 hPa. Velocity of thewesterly has a maximum around 43N over eastern Asia.
Distinct descending motion is simulated over central Asia
being correspond to the aridity around the Caspian Sea.
Another robust descent motion stronger than 1.0 cm sec-1
appears on just north of the plateau extending from 81E to98E. In comparison with precipitation of this run (Fig. 1), it
is apparent that each dry region completely corresponds tothe subsidence at this level. On the other hand, upwardvelocity is strong around Southeast Asia, Kunlun Mountain,
and northeast China. These regions are almost equal to theregion of heavy rainfall. In next section, we continue toinvestigate how the descending motion is formed over thearid region, since it is apparent that the subsidence strongly
reduces the precipitation in this area.

3.2

Figure 1: Average precipitation (mm/month) andvertically integrated water vapor transport (kg/m/sec) in
ZM experiment.

Figure 2: Averaged vertical motion (cm/sec) and wind(m/sec) at 400 hPa in ZM experiment.

NoC and NoR experiments

The vertical motion and horizontal wind at 400 hPa in
the NoC experiment is shown in Fig. 3. Ascending motion
is not so evident over southeast and south Asia regions
since convection process is turned off in this sensitivity
experiment. Figure 3, however, shows that the steadyupward motion is generated over the Tibetan plateau incase without condensation process. A robust descending
motion, which is greater than 2 cm sec-1 in magnitude, also
appears over the desert region. Since the level of 400 hPa
almost corresponds to the mean height of the planetary
boundary layer over the plateau. The upward

Figure 3: Same with Fig.2 but for NoC experiment.

motion should be mainly driven by the sensible heat flux
caused by the radiative heating on the plateau surface. Thismeans the heating process which may induce thesubsidence over the arid region is not always necessary to
be condensation heating associated with cumulus
convection.
Figure 4 illustrates the vertical motion and horizontal
wind at 400 hPa in the NoR experiment. Only very gentle
upward and downward motions are recognized over west
and east sides of the plateau, respectively. Nevertheless, theamplitudes of the vertical motions are about ten times
smaller than those simulated by the run with the radiative
heating (Fig. 3). Therefore, the mechanical influence isexpected to be quite small for the vertical motion during
summer, when subtropical westerlies are relatively weak,
while it contributes largely in winter dryness (Broccoli andManabe, 1992). Solar radiative heating, instead, plays animportant role on the intensification of both upward anddownward motion around the plateau. The relationshipbetween the positions of heat source and the downward
motion will be discussed in next section.

4 Discussion
4.1Elevated heat source
Since Tibetan plateau is elevated about 4500 m above
sea level, its mean planetary boundary layer may extend tothe middle troposphere. Solar heating on such an elevatedsurface should widely affect the atmospheric circulation.

Figure 4: Same with Fig.2 but for NoR experiment.

Figure 5: Same with Fig.2 but for TbQ experiment.

We now conduct a TbQ experiment in which artificial
heat source is given on NoR experiment. The heat source isplaced only over the Tibetan plateau where ground altitude
exceeds 3000 m ASL. Although vertically integrated
diabatic heating rate depends on ground altitude, it isapproximately 160 W m-2 over the Plateau. The heat sourceis distributed between ground surface and about 6500 m
ASL. In Fig.5 vertical motion and horizontal wind at 400hPa are shown. Artificial heating causes upward motion
over the plateau, while downward motion prevails over thenorthern side of the plateau. The downward velocity is
about -0.3 cm sec-1. Subsidence in Great Indian Desert isalso seen. The horizontal distribution of vertical motion isclose to that in the ZM or NoC experiment (see also Fig.2,Fig.3). Figure 5 indicates that the heating over the plateau
does cause the subsidence which reduces the precipitation
over the desert region.
The subsidence tends to be stronger when lower layer
is heated strongly in case that the total energy of theheating is fixed. This indicates that the sensible heating
associated with daytime mixed layer could be more
efficient to cause the subsidence than the condensation
heating by the cloud physics in upper layer.

4.2

Response to the heat source

The amount of diabatic heating estimated fromreanalysis data is larger in tropical regions, around the Bayof Bengal and Southeast Asia, than that over Tibetan
plateau. We, therefore, attempted another experiment byplacing the heat source over South Asia instead of theplateau. The experiment shows that the prevailingsubsidence appears only over Central Asia (not shown),
which is consistent with previous studies (Broccoli andManabe, 1992; Rodwell and Hoskins, 1996). This meansthat the huge amount of diabatic heating over south andsoutheast Asia region does not directly affect thesubsidence over desert region in northern China.
Figure 6 shows the global zonal mean of zonal windvelocity at 200 hPa.. In upper troposphere subtropicalwesterly jet retreats poleward during summer. It runs
completely north of the Tibetan plateau from June to
September when diabatic heating over the plateau becomesmuch stronger. As mentioned by Hoskins (1996), themid-latitude subsidence is likely to be accelerated
according to the interaction between Rossby wave andsubtropical westerly jet.
We performed very simplified experiments using
TERC-RAMS, in which topography is assumed to be flatand that surface is covered by land. The rectangular shaped
heat source is added around 35N,100E. Zonal mean fieldsof July, January and April are calculated fromNCEP/NCAR reanalysis to use as a boundary condition ofnumerical experiments in order to investigate thedifference of atmospheric response to the heat source.
Figure 7 illustrate the typical propagation pattern simulatedby the simplified experiments. In Fig. 7b, robust
subsidence is formed in west and north of the heat source,Figure 6: Global zonal mean of zonal wind velocity (m/sec)

Figure 7: Vertical wind velocity (m/sec) and wind vectors at 500 hPa responded by rectangular heat source.
Zonal mean of (a) January, (b) July, (c) April is used for lateral boundary condition.
which shows that the diabatic heating over the plateau willcause subsidence over northern China during summer. The
result is consistent with our sensitivity experiments shownin Fig. 3 and Fig. 4, although the results are so clear due tomany assumptions for simplicity. On the other hand,propagation to the south is obvious during winter (Fig. 7a)because subtropical jet situates in south of the heat source.
This means that the descending motion may prevail in
winter in the vicinity of the plateau over south andsoutheast Asia. Before middle summer, the subtropical jet
shifts poleward from south to north of the Tibetan plateau.
Thus, the response directions to the heat source extend
both south and north of the plateau. These results indicatethat the direction of the Rossby wave propagation strongly
depends on the meridional location of westerly jet. Since
the location of the westerly jet varies with a season, thehorizontal distributions of the subsidence become quitedifferent from spring to summer, which could affect widely
the seasonal evolution of the Asia and Indian monsoon.

5Summary

The reason of summertime aridity over northeastern
Asia is investigated by RCM experiments. Over desert area,
robust descending motion, which reduces the precipitation,
prevails in the control experiment with diabatic heatingprocess. However, the descending motion tends to be very
weak when diabatic process is turned off in the model.
Solar radiative heating on the Tibetan plateau leads to ageneration of Rossby wave. The stationary Rossby wave
tends to propagate northward because subtropical westerly
jet is being situated north of the plateau in boreal summer.

Therefore, the stationary descending motion prevails overnortheastern Asia, which consequently reduces theprecipitation around arid region. Diabatic heating overtropical region does not affect directly over the desert since
the position of subtropical westerly is not favorable fornorthward propagation of Rossby wave.

Acknowledgement
This work has been supported by a CREST project ofJST (Japan Science and Technology Agency)

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