第 21卷第 12 期2006 年 12月

地球科学进展 ADVANCES IN EARTH SCIENCE

Vol.21　 No.12 Dec.，2006

文章编号：1001-8166 （2006 ）12-1224-13

Energy and W ater Cycle over th e Tib eta n Pla tea u： Surfa ce Energy Bala nce and Turbule nt Heat Flu xes �

SU Zhong- bo 1， ZHANG Tin g

2， MA Yao- m in g

3，J IA Li

4， W EN J un

5

（1. International Institute for Geo- Inform ati on Science and Earth Observation ITC Enschede 7500 AA， the Netherlands ；2. Science ＆ Technology Departm ent ， Tian jin Municipal Engineering Bureau ， Tian jin 300022 ， China ；3. Institute of Tibetan Plateau Research ， CAS ， Bei jing 100085 ， China ；4. Alterra Green World Research ，

Wageningen University and Research Centre Wa genigen 6700 AA， the Netherlands ；5. Cold and Arid Regions Environm entaland Engi neering Research Institute ， CAS ， Lanzhou 730000 ， China ）

Abstract ： This contribution presents an overview and an outlook ofstudies on energy and water cycle ov er

the Tibetan plateau with focuses on the estim a tion ofenergy balance term s and turbulenthea tfluxes. On

the basis ofthe surface energy balance calcul ations ， we show thatthe phenom ena ofthe energy im bala nce

exist in GAME / Tibetexperim entdata ， although the explanations for the reasons are debated now and not

resolved yet. W e found thatthe derived latent heatflux ism uch higherthan the m easurem ents . However ， the corrected- m easurem ents ， which are calculated according to the hypothe sis of the energy balance ， com pare very well with the estim ation of SEBS. On this basis itis concluded thatthe deviatio n is caused

by the energy im balance ofground m easurem ent s in GAME / Tibetexperim entarea. The latenthe atfluxes

were likely under- observed.

Key w ords ： Energy and water cycle ； The Tibetan plateau ； Energy balance ； Turbulentfluxes

CLC num ber ： P339　　　 Docum entcode ：A

1　 Introductio n The Tibetan plateau is a significant heat and m o-

m entum source for the atm osphere ， its energy and wa-

ter cycles play an im portantrole in the Asian M onsoon

system ， which in turn is a m a jor com ponentofboth the

energy and water cycles of the global clim ate s ystem .

The Tibetan plateau contains the world ＇ s high est eleva-

tion relieffeatures. Much ofthem exceed an al titude of

4 000 m sl ， som e reaching into the m id- troposphere.

Due to its topographic characteristics ， the plateau land

surface absorbs a large am ount of solar radiat ion ， and

undergoes dram atic seasonal changes of surfa ce heat

and water fluxes ［1］. Land surface processes on the Ti-

betan plateau are m ultifold and com plex. Due t o the

highly com plex terrain ， wintertim e conditions are char-

acterized by irregular snow cover with extens ive areas

offrozen ground holding large quantities ofm oisture in

the surface layers. Seasonal freezing and m el ting

processes and their spatial distribution lea d to tim e-

space variations ofsurface wetness and to var iations of

the surface heat balance. Such variations hav e pro-

found im plications for follow- up m onsoon beh avior and

globalclim ate processes ［2，3］. In short ， the land cover

in Tibetan plateau shows spatialand tem poral variabili-

ty， which will affect the distributions of sensib le and

� 　 Receive date ：2006 -10-17.

　 Biography ： SU Zhong- bo . E - m ail ： b＿ Su ＠ itc. nl

latentheatfluxes. Hence ， itis very im portantto inves-

tigate the interactions between the land surf ace and at-

m osphere over the Tibetan plateau so that we ca n un-

derstand the com plete energy and water cycles and

their effects on the Asian Monsoon system ， and further

on the global atm ospheric circulations.

The atm ospheric turbulentfluxes （ evapotranspira-

tion when latentheatflux is expressed in wate rdepth ） at the land surface have long been recognized a s the

m ost im portantprocesses in the determ inatio n ofthe ex-

changes of energy and m ass am ong hydrosphere ， at-

m osphere and biosphere ［4，5］. Conventional techniques

thatem ploy pointm easurem ents to estim ate th e com po-

nents ofenergy balance are representative on ly oflocal

scales and cannotbe extended to large areas be cause of

the heterogeneity ofland surfaces and the dyn am ic na-

ture ofheattransferprocesses. Rem ote sensi ng is prob-

ably the only technique thatcan provide repre sentative

m easurem ents ofseveralrelevantphysicalpa ram eters at

scales from a point to a continent. Techniques using

rem ote sensing inform ation to estim ate atm os pheric tur-

bulentfluxes are therefore essential when de aling with

processes thatcannotbe represented by point m easure-

m ents only.

The Surface Energy Balance System （ SEBS ） pro-

posed by Su ［6］ is a validated algorithm am ong others to

estim ate atm ospheric turbulent fluxes and ev aporative

fraction using satellite earth observation d ata ， in com -

bination with m eteorological inform ation of Planetary

Boundary Layer in plain areas. The purpose of t his

study is to clarify whether SEBS is suitable to estim ate

the energy participation directly on the Tibe tan plat-

eau.

2　 Energy bala nce te rm s and tu rbule nt heat flu xes

2.1　 Surface energy balance term

Neglecting advection and heatstorage ， the surface

energy balance is com m only written as ：R

n ＝G0 ＋H ＋ λE （1）

　　 W here Rn is the net radiation ，G

0 is the soilheat

flux ，H is the turbulent sensible heat flux ， and λE is

the turbulent latentheatflux.

The equation to calculate the netradiation is given

by：R

n ＝（1－α）·R sw d ＋ε·R

lw d －ε·σ·T 40（2）

　　 W here α is the albedo ，R sw d the downward solar

radiation ，R lw d the downward long wave radiation ，ε the

em issivity of the surface ，σ the Stefan- Bolzm ann con-

stant ， and T0 the surface tem perature.

The equation to calculate soilheatflux is par am e-

terised as ：G 0 ＝R n·［Γc ＋（1－ fc）·（Γs－Γc）］（3）

　　 In which we assum e the ratio of soil heat flux to

net radiation Γc ＝0.05 for a full vegetation canopy ［7］

and Γs ＝0.315 for bare soil ［8］. An interpolation is

then perform ed between these lim it ing cases u sing the

fractionalcanopy coverage ，fc.

In order to derive the sensible and latent heat

flux ， use is m ade of sim ilarity theory. In Atm ospher ic

Surface Layer （ ASL ）， the sim ilarity relationships for

the profiles ofthe m ean wind speed ，u， and the m ean

tem perature θ0 -θα， usually can be written in integral

form as

u ＝u*

kln z － d 0

z0

( )

m

－Ψm

z － d 0( )

L ＋Ψm

z0m( )[ ]L 　（4）

θ0 －θα ＝H

ku* ρC p

ln z － d

0

z0

( )

h－Ψh

z － d 0

( )L ＋Ψh

z0 h

( )[ ]L

（5）

　　 W here z is the height above the surface ，u* ＝

（τ0/ρ）1 /2 is the friction velocity ，τ0 is the surface

shear stress ，ρ is the density of air ， k ＝0.4 is von

Karm an ＇ s constant ，d0 is the zero plane displacem ent

height ，z0 m isthe roughness heightform om entum trans-

fer ，θ0 is the potentialtem perature atthe surface ，θα is

the potentialair tem perature atheight z，z0h is the sca-

lar roughness heightforheattransfer ，ψm and ψh

are the

stability correction functions for m om entum and sensi-

ble heat transfer respectively ，L is the Obukhov length

defined as ：

L ＝－ρC pu

3* θv

kgH （6）

　　 W here g is the acceleration due to gravity ，θv is

the potential virtualtem perature near the su rface.

For field m easurem ents perform ed ata heighto f a

few m eters above ground ， clearly since the surface flux

are related to surface variables and variable s in the at-

m ospheric surface layer ， all calculations use the Mo-

5221 第 12 期　　 SU Zhong- bo ， etal ： Energy and W ater Cycle over the Tibetan Platea u： Surface Energy Balance and Turbulent Heat Flu xes

nin- Obukhov Sim ilarity （ MOS ） functions given by

Brutsaert ［9］. By replacing the MOS stability functions

with the Bulk Atm ospheric Boundary Layer （ ABL ） Sim ilarity （ BAS ） functions proposed by Brutsaert ［9］， the system of Eqns.（4-6 ） relates surface fluxes to sur-

face variables and the m ixed layer atm ospheri c varia-

bles. The criterion proposed by Brutsaert ［9］ is used to

determ ine if MOS or BAS scaling is appropriate for a

given situation.

2.2　 An extended m odel for determ ination of th e

roughness length for heat transfer

In the above derivations ， the aerodynam ic and

therm al dynam ic roughness param eters need to be

known. W hen near surface wind speed and vegeta tion

param eters （ heightand leafarea index ） are available ， the within- canopy turbulence m odelproposed by Mass-

m an ［10］ can be used to estim ate the aerodynam ic pa-

ram eters ，d0， the displacem ent height ， and ，z0 m， the

roughness heightfor m om entum . This m odel has been

shown by Su ， et al.［11］ to produce reliable estim ates of

the aerodynam ic param eters. If only the heigh t of the

vegetation is available ， the relationships proposed by

Brutsaert ［12］ m ay be used. Ifa detailed land use classi-

fication is available ， the tabulated values of W ierin-

ga［13，14］ can be used. However ， since aerodynam ic pa-

ram eters depend also on wind speed and directi on as

wellas the surface characteristics ［15］， the lattertwo ap-

proaches should be used only when the first m et hod

cannot be used due to lack ofdata.

The scalar roughness heightfor heattransfer ，z0h，

which changes with surface characteristics ， atm ospher-

ic flow and therm al dynam ic state of the surfac e， can

be derived from the roughness m odel for heat tr ansfer

proposed by Su ， et al.［11］. However ， their m odel re-

quires a functional form to describe the verti cal struc-

ture of the vegetation canopy to calculate the within

canopy wind speed profile extinction coeffic ient ，nec.

In this study ，nec， is form ulated as a function of the

cum ulative leafdrag area atthe canopy top ，

nec ＝

Cd· LAI

2u2* /u（h）2 （7）

　　 W here C d is the drag coefficientofthe foliage ele-

m ents assum ed to take the value of 0.2 ， LAI is the

one- sided leaf area index defined for the tota l area ，

u（h） is the horizontal wind speed at the canopy top.

The scalar roughness heightfor heat transfer ，z0 h， can

be derived from

z0 h ＝z0 m / exp （kB－1） （8）

　　 W here B－ 1 is the inverse Stanton num ber ， a di-

m ensionless heat transfer coefficient. The m odel pro-

posed by Su et al .［11］ and Su ［6］ is used to estim ate

kB－ 1：

kB－ 1 ＝kC

d

4C t

u*

u（h）（1－ene c/2）

f2c ＋

2fcfsk·u

*/u（h）·z

0 m/h

C *t

＋k B － 1s f2s （9）

　　 W here fc is the fractionalcanopy coverage and f

sis

its com plim ent. Cd is the drag coefficientofthe foliage

elem ents assum ed to take the value of 0.2. Ct is the

heattransfer coefficient of the leaf. For m os tcanopies

and environm ental conditions ，C t is bounded as 0.05 N

FC

tF0.075 N（N is num ber of sides of a leaf to par-

ticipate in heat exchange ），u（h） is the horizontal

wind speed atthe canopy top. The heattransfer coeffi-

cientofthe soilis given by C *t ＝ Pr -2 /3 Re-1 /2

* ， where Pr

is the Prandtlnum ber （0.71 ， Massm an 1999 ） and the

roughness Reynolds num ber Re * ＝h

su

*/v， with h

s the

roughness heightofthe soil. The kinem atic vi scosity of

the air is given by v ＝1. 327 ×10 -5（p0 /p）（T /

T 0）1.81 ［16］， with p and T the am bient pressure and

tem perature and p0 ＝101.3 k Pa and T

0 ＝273.15 K.

Physically and geom etrically ， the first term of Eqn.

（9） follows the full canopy only m odel of Choudhur y

and Monteith ［17］， the third term is thatof Brutsaert ［12］

for a bare soilsurface. A quadratic weighting based on

the fractional canopy coverage is used to acco m m odate

any situation between the full vegetation and bare soil

conditions. For bare soilsurface kB－1s

is calculated ac-

cording to Brutsaert ［12］

kB－1s ＝2.46（Re*）

1 /4－ ln［7.4］ （10）2.3　 A new form ulation for determ ination ofev ap-

orative fraction on the basis of energy bal-

ance in lim iting cases

To determ ine the evaporative fraction （ to be de-

fined below ）， use is m ade ofenergy balance consider-

ations in lim iting cases. Under the dry- lim it ， the latent

heat （ orthe evaporation ） becom es zero due to the lim -

6221 　　　　　　　　　　　　　　　　　地球科学进展　　　　　　　　　　　　　　　　　　　　第 21卷

itation ofsoilm oisture so that the sensible h eat flux is

atits m axim um value. From Eqn.（1）， itfollows ，

λE dry ＝R n－ G 0－ H dr y ≡ 0，or

H dr y ＝R

n－ G0 （11）

　　 Under the wet- lim it ， where the evaporation takes

place atpotentialrate ，λE w et ，（ i. e. the evaporation is

lim ited only by the energy available under the given

surface and atm ospheric conditions ）， the sensible heat

flux takes its m inim um value ，H w et ， i. e.

λE wet ＝R n－ G 0－ H w et ≡ 0，or

H w et ＝R

n－G0－λE w et （12）

　　 The relative evaporation then can be evalua ted as

Λr ＝λE

λE w et ＝1－

λE w et －λE

λE w et （13）

　　 Substitution of Eqns.（1），（11） and （12） in

Eqn.（13） and after som e algebra ：

Λr ＝1－ H － H

w et H

dry － H w et

（14）

　　 The actual sensible heat flux H defined by Eqn.

（5） is constrained in the range setby the sensible heat

flux atthe wetlim it H wet ， and the sensible heat flux at

the dry lim it H dr y ·H dry isgiven by Eqn.（11） and H w et

can be derived by com bination of Eqn.（12） and a

com bination equation sim ilar to the Penm an- M onteith

com bination equation ［18］. Menenti ［19］ showed that ， when the resistance term s are grouped into the bulk in-

ternal （ or surface ， or stom atal ） and external （ aerody-

nam ic ） resistances ， the com bination equation can be

written in the following form

λE ＝Δ·re·（R n－ G 0）＋ρC p·（e sa t － e）

re·（γ＋Δ）＋γ·r

i　（15）

　　 W here e and e sat

are actual vapour pressure and

saturated vapour respectively ；γ is the psychom etric

constant ， and Δ is the rate of change of saturation

vapour pressure with tem perature （ i. e.5es（T）/

5T）；ri

is the bulk surface internal resistance and re is the ex-

ternalor aerodynam ic resistance. In the abov e equation

it is assum ed that the roughness length for hea t and

vapour transferare the sam e ［12］. The Penm an- Monteith

equation is strictly valid only for vegetated canopy ， whereas the definition by m eans （15） is also valid for

soil surface with properly defined bulk inter nal resist-

ance.

At the wet- lim it ， the internal resistance ri≡0 by

definition. Using this property in Eqn. （15） and

changing the subscripts correspondingly to r eflect the

wet- lim it condition ， the sensible heat flux at the wet-

lim it is obtained as ：

H w et ＝（Rn－G

0）－ρC prew·

es－e[ ]γ

/ 1 ＋Δ( )γ

（16）

　　 The externalresistance depends also on the Obuk-

hov length ，L， which in turn is a function of the fric-

tion velocity and sensible heatflux （ Eqns. 4-6 ）. W ith

the friction velocity and the Obukhov length d eterm ined

by the num erical procedure described previou sly ， the

external resistance can be determ ined from Eq n.（5）as：

re ＝

1ku

*

ln z － d 0

z0

( )

h

－Ψh

z- d 0

L

( )

d

＋Ψh

z0hL

( )[ ]d

　（17）

　　 Sim ilarly ， the external resistance at the wet- lim it

can be derived as ：

rew ＝

1ku

*

ln z － d

0

z0

( )

h－Ψh

z － d 0

L

( )

w＋Ψh

z0h

L

( )[ ]w

（18）

　　 The wet lim it stability length can be determ ined

as：

Lw ＝

ρu3*

kg·0.61·（R n-G 0）/λ（19）

The evaporative fraction is finally given by ：

Λ＝ λER n－ G ＝

Λr·λE wet

R n－G （20）

　　 By inverting Eqn.（20）， the actual latent heat

flux λE can be obtained.

Eqns.（1-20 ） constitute the form ulation of SEBS ； its validation using four different data sets over the

com plex Tibetan plateau is the sub ject of the f ollowing

sections after a briefdescription of the data used.

3　 Data and m ate ria ls Hum an life ， agriculture ， econom ics ， and the en-

tire ecosystem ofthe Asian region seriously d epend up-

on the m onsoon clim ate and its predictabili ty . More

than 60％ ofthe Earth ＇ s population lives under the in-

fluence of this m onsoon clim ate. As a part of Gl obal

Energy and W ater Cycle Experim ent ， the GEW EX Asi-

an Monsoon Experim ent （ GAME ）［2 0］ was conducted to

understand the role ofthe Asian m onsoon in the global

energy and water cycle and to im prove the sim ul ation

7221 第 12 期　　 SU Zhong- bo ， etal ： Energy and W ater Cycle over the Tibetan Platea u： Surface Energy Balance and Turbulent Heat Flu xes

and seasonal prediction of Asian m onsoon patt erns and

regional water resources. To clarify the role s ofthe in-

teractions between the land surface and the at m osphere

over the Tibetan Plateau in the Asian m onsoon s ystem ， two experim ents using different scales were i m plem en-

ted in corporation with Chinese TIPEX （ TIbetan plat-

eau Experim entof Atm ospheric Sciences ） as follows ：

� A Plateau- scale experim ent （80 ～100° E ，27 ～

37° N ） using the north- south and east- west networks of

one- dim ensional observational stations. Sy stem s

（ Fig.1 ， Plate Ⅰ） installed include the special radio-

sondes ， the AW S ＇ s equipped with soil tem perature-

m oisture m easuring capability ， the PBL towers ， and

the precipitation sam pling system s includin g those fori-

sotope studies.

� A Meso- scale experim ent （91 ～92.5° E ，30 ～

33° N ， the Nu jiang basin ） in the central plateau with

two orthree- dim ensionalintensive observat ion. The ob-

serving system s including a three- dim ension al Doppler

radar ， m obile radio- controlled aerosondes ， the radio-

sonde network ， and the AW S network as shown in

Fig.2 （ Plate Ⅱ）. There are two spatialscales inherent

to this basin. The catchm ents area ofthe overa llbasin

is approxim ately 105 km 2； this is the larger basin

scale. A sm allerbasin ， about 103 km 2， is also em bed-

ded in the larger one. The characteristics of f rozen

ground vary overa wide range ， from continuousperm a-

frost in the north to seasonal perm afrost in th e south.

The distribution of land surface wetness is di rectly af-

fected by the perm afrost distribution.

The field data used in this thesis ， including the

surface tem perature and other m eteorology in form ation ， are collected during GAME- Tibet experim ent （ May to

Septem ber ） in 1998. The study area （ Fig.3 ， Plate Ⅶ） consists of Meso- Scale and a parts of Plateau- scale ex-

perim ent in 1998 ， which is located between 90° E and

95° E and 29° N up to 36° N ， with a total area about

322 300 km 2. It includes two radiosonde stations （ Lha-

sa and Linzi ）， three heat flux sites （ Anduo ， Naqu and

NPAM ） and three Autom ated W eather Stations （ AW -

SD66 ， AWSD110 and Tuotuohe ） in this area.

3.1　 Heat flux stations

3.1.1　 Anduo- PBL

Anduo PBL （ Lat.32.241° N ， Lon. 91.625° E and

Elev.4 700 m ） locates at the central Tibetan plateau.

The surface is essentially flat and open ， and partially

covered by very shortgrasses in the m onsoon se ason.

System atic m easurem ents were held at Anduo in

GAME / Tibet intensive observing period from M ay to

Septem ber 1998. The subsurface m easurem ents com -

prise ：（1） soilm oisture θs at six depths （4，20，60，

100 ，160 and 258 cm ） m easured by TDR system （ Tri-

m e MUX ）；（2） soil tem perature Ts at twelve depths

（4，5，10，20，40，60，80，100 ，130 ，160 ，200 and

279 cm ） by therm om eters （ Pt-100 ）；（3） soilheatflux

G at 10 and 20 cm depths by heat plate （ EKO MF-

81）. The m easurem entsin the atm ospheric surface lay-

erinclude ：（4） downward shortwave radiation R s↓w and

upward short- wave radiation R↑sw by EKO MS-801 ，

downward longwave radiation R ↓lw and upward long-

wave radiation R ↑lw by Eppley PIR ， and skin radiative

tem perature derived from R ↓lw and R↑

lw with surface em -

issivity ε＝0.98 suggested by the observers ；（5） sen-

sible heatflux H obs ， latentheatflux LE

obs and m om en-

tum flux τ ob s at 2.85 m levelabove the ground by a fast

response system consisting of a 3- D sonic anem o- ther-

m om eter （ Kai jo DA -300 ） and an infrared open- path

hygrom eter （ Kai jo AH -300 ）. In the post- processing of

turbulence data ， various corrections were m ade ， inclu-

ding cross wind and hum idity effects on tem per ature ， dynam ic calibration and correction due to the low fre-

quency instability for hum idity ， and W ebb et al .［21］ correction for fluxes. Tanaka et al. ［22］ indicated

thatthe sensor ofthe infrared hygrom eter did not work

wellduring and several hours after precipita tion ；（6） wind U

a by aerobane ， tem perature T

a by Pt-100 ， and

water vapour qa by electric capacitance ；（7） precipita-

tion by tipping buckets.

3.1.2　 NPAM （ M S3478 ）- North portable autom a-

ted m esonet

The surface of NPAM （ Lat. 31.926° N ， Lon.

91.716° E and Elev. 5 063 m ） is essentially flat earth

ham m ock. Surface m eteorological / hydrologi cal obser-

vation is carried out during GAME- Tibet IOP fo r the

understanding of surface- atm osphere direct interaction

from May to Septem ber 1998. PAM III （ FLUX - PAM ） from NCAR （ National Center for Atm ospheric Re-

search ， USA ） was used. Surface eddy fluxes of m o-

8221 　　　　　　　　　　　　　　　　　地球科学进展　　　　　　　　　　　　　　　　　　　　第 21卷

m entum ， sensible heatand latentheatare m easured in

real- tim e as wellas norm alm eteorological / h ydrological

param eters. Diurnal and seasonal variations of surface

processes are evaluated. System atic m easure m ents in-

clude （1） soil tem perature Ts at six depths （4，20，

60，100 ，160 and 260 cm ） m easured by STP-1

（ REBS ）；（2） soilm oisture θs atthe sam e depths （4，20，60，100 ，160 and 260 cm ） m easured by TRUME-

MUX6 （ IMKO Micro.）；（3） soil heat flux G at 1 cm

depth below the ground by heatplate （ REBSHFT-3 ）.

The m easurem ents in the atm ospheric surface l ayer in-

clude ：（4） downward short- wave radiation R ↓sw and up-

ward short- wave radiation R↑sw by kipp- Zonnen Pyrano-

m eter CM -11 Radiation ， downward long- wave radiation

R ↓lw and upward long- wave radiation R↑

lw by Eppley

PIR ， and Surface radiative tem perature by Everest

400. 4 G - Radiation therm om eter ；（5） sensible heat

flux H obs

by GILL SAT- R3 A and corrected forwater va-

por flux ， latent heat flux LE obs

by GILL SAT- R3 A ＋

Bandpass TRH （ Vaisala 50 Y ）；（6） wind Ua by Pro-

peller- vane Anem om eter Model-09101 ， and tem pera-

ture Ta， relative hum idity RH ， specific hum idity q by

Vaisala 50 Y . In the data processing ， net radiation is

found to be m issing from 4 com ponent m easurem e nts

（ due to m issing upward solar radiation ） in the periods

06：00 /7 J uly 08 ：30 /19 J uly ，00：30/5 August 24 ：00 /

6 August ，00：00 /9 August end of IOP. Net radiation

is replaced by a sim ple netradiom eter （ REBSQ7 ） da-

ta during this period （ They are roughly consistentdur-

ing the IOP ）.

4　 Results and discussio ns Before validating of SEBS energy balance com p o-

nents ， the investigation ofground m easurem ents for the

energy balance term s is conducted.

4.1　 Consistency of ground m easurem ents

In this study ， the datasets of two heat flux obser-

vation sites （ Anduo and NPAM ） are used to validate

SEBS estim ation. Data used are the ground m eas ure-

m ents of the net radiation ， sensible heat flux ， latent

heatflux and soil heat flux at the different so il depths

below the ground from J une to August in 1998. Th e

surface soilheat flux can be replaced by the m e asure-

m ents ofthe soil heat flux at 1 cm depth under gr ound

at NPAM ， because the m easurem entis close to the sur-

face ， and it will be used to validate SEBS estim ation

directly in this case. However ， only m easurem ents of

the soil heat flux at 10 cm and 20 cm depths under

ground are provided at Anduo during GAME / Tibe t，so

it is needs to be noticed that lots of heat stora ge takes

place between the land surface and m easurem en t

depth. The observation- derived value ， which is derived

from observationsat 20 cm depths underthe gro und ，is

used to evaluate energy budget Eqn.（1）. Itis calcu-

lated as follows.

G 0 ＝λs 5T s

5z ｜z＝0 （21）

　　 The tem perature gradient at the soil surfac e in

Eq.（21） is derived with therm al diffusion equa-

tion ［23］：

5ρscsT

s

5t ＝5

G

5z （22）

ρscs ＝4.18 ×106［0.3（1－θ sat ）＋θs］（23）

λs ＝0.418 ［2.2（1－θ sat ）＋2.3θs］

0.75 ＋0.65θ sat －0.4θs（24）

　　 W here Ts（K） is soiltem perature ，ρscs（J/（kg·

m 3）） is soil heat capacity ，λs（W /（m ·K）） is ther-

m alconductivity ， soilporosity θ sat （m 3 /m 3） is approxi-

m ately equal to the m axim um water content ， and θs

（m 3 /m 3） is volum etric water content ， which was ob-

served in the fieldwork.

Itis shown （ Fig.3 ， Plate Ⅶ） that derivation of

surface soilheatflux is greater than soilhea tflux at 20

cm depth during daytim e. In contrast ， the surface soil

heat flux is sm aller than that of 20 cm depth bel ow

ground at night. The observation- derived val ue has a

lag in com parison with those m easured at 20 cm d epth

below ground ， although their phases are alm ost coinci-

dent. Itindicates lots ofheatcan be really st ored under

the surface and it will take tim e for transport ation of

soil heatfrom surface to deep soillayers unde r ground.

The lag oftim e is about two hours. Moreover ， surface

soil heat flux is greatly influenced by the sur face tem -

perature and tim e. The sam e results are obtain ed by

the ground m easurem ents in J une and J uly at And uo in

1998 （ see Fig.4 ）.

The lack of energy budget closure is particula rly

noticeable in the Tibetan plateau during Asia n sum m er

9221 第 12 期　　 SU Zhong- bo ， etal ： Energy and W ater Cycle over the Tibetan Platea u： Surface Energy Balance and Turbulent Heat Flu xes

m onsoon ， based on the observations of the GAME - Ti-

bet and the TIPEX pro jects 1998 （ Figs. 5-7 ）. As showing in Fig.7 ， the average m axim um residualener-

gy R net －（G 0 ＋H ＋ λE） is approxim ately equal to 200

W / m 2 at NPAM in August 1998 ， and the average m ax- im um residual energy has exceeded 200 W / m 2 at An-

duo in August 1998. The sam e results are obtain ed by

the ground m easurem ents in J une and J uly at And uo

and NPAM in 1998 （ Figs.6-7 ）.

Yang et al .［2 3］ indicated thatthe daily average in-

com ing and outgoing energy fluxes at Naqu FX fo r 15 clear day from J uly and Septem ber and MS3478 of

GAME- Tibet ， where the residualenergy R n et －（G 0 ＋H

＋λE） often exceeded 1 /3 of net radiation. Sim ilar large residualwas also found atother sites on the Tibet

plateau ［24，25］. Ishikawa et al.［24］ pointed out that the

im balance problem m ainly occurred in the afte rnoon

and the residualenergy was greater than those reported

Fig.4 　 The diurnaltrends of both observation - derived soil heatflux and soil heatflux at 20 cm

depths below the ground at Anduo in （a） J une and （b） J uly 1998

Fig.5　 The diurnal trend of the energy residue at NPAM （a） and Anduo （b） in J une of 1998

The solid curves are the average diurnaltrend s of the energy residue

Fig.6　 The diurnal trend of the energy residue at NPAM （a） and Anduo （b） in J uly of 1998

The solid curves are the average diurnaltrend s of the energy residue

0321 　　　　　　　　　　　　　　　　　地球科学进展　　　　　　　　　　　　　　　　　　　　第 21 卷

Fig.7　 The diurnal trend of the energy residue at NPAM （a） and Anduo （b） in August of 1998

The solid curves are the average diurnaltrend s of the energy residue

in the FIFE and the BOREAS. Miyazaki et al.［26］ sug-

gested thatthe am ountofresidualenergy in th e plateau

was larger in the m onsoon period than in the pre - m on- soon period. One reason of the energy im balanc e can

be explained by topographic heterogeneity ， canopy en-

ergy storage or insufficientfetch. Consider ing the m elt-

ing process of frozen soil water ， Tanaka et al.［22］

pointed outthatthe soilheatflux m ightbe und er- m eas-

ured. W ang et al.［2 7］ suggested that the m easurem ent

error of latent heat flux m ight greatly contri bute to the energy closure problem at Naqu site of GAME / Ti bet.

The sam e resultwas obtained by Yang et al.［23］ at An-

duo ； however ， Kim et al.［2 8］ contested that netradia-

tion flux and latentheatflux were least erron eous while soilheat flux and sensible heat flux were m ost error-

prone for the sam e site. These opposite conclu sions are

resulted from different evaluation m ethods a nd incom - plete observations. On the other hand ， som e research-

ers w onder whether the im balance is physicall y related

to heattransport by m esoscale convection ， because the convective activities are so strong over the p lateau in

the sum m er season ［1］. Forexam ple ， Kim et al.［29］ sug-

gested considering the effect of som e local an d advec-

tion term s ， butquantitative estim ation of these term s is stilldiff icultbased on observations.

In order to clarify this problem ， we analysed the

behaviours of the evaporative fraction by inv erting Eqns.（1） and （20）.

ef1 ＝λE

R n－ G 0（25）

ef2 ＝λE

H ＋λE（26）

　　 Fig.8 shows the relationship between two ev apora-

tive fractions by using ground datasets at And uo in Au-

gust 1998. Obviously ， the evaporative fraction ef1 is sm aller than ef2 because the m easurem entofnet radia-

tion is greater than sum ofthe soilheatflux an d turbu-

lent heatflux m easurem ents. It also shows tha t the en-

ergy is not in equilibrium in the GAME / Tibet ex peri- m entareas. The sam e results can be obtained us ing the

ground m easurem ents of other m onths and other obser-

vation sites. In general ， the phenom ena of the energy im balance exist in GAME / Tibet experim ent acc ording

to the analysis ofthe ground datasets ， although the ex-

planations ofenergy im balance are debated. W hen val-

idation is m ade using such m easurem ent ， one should keep the im balance fact ofenergy budget in m in d.

Fig.8　 Com parison of tw o M easured efl （ LE / R n－G o） ground m easurem ents at Anduo in Augustof 1998

4.2　 Validation of SEBS com ponents

Due to lack of m easurem ent term s ， som e param e- ters in SEBS are estim ated by actual land surfa ce and

1321 第 12 期　　 SU Zhong- bo ， etal ： Energy and W ater Cycle over the Tibetan Platea u： Surface Energy Balance and Turbulent Heat Flu xes

vegetation distribution in GAME / Tibetexper im entarea

in this case. For exam ple ， canopy totalleafarea index

LAI and the fractional foliage coverage fc， are assum ed

to take the value of 0.15. The surface em issivi ty for

vegetation εv and bare soil εg

are 0.985 and 0.95 ， re-

spectively. The surface albedo is obtained by ratio of

upward solar radiation and downward solar rad iation.

Table 1　 Statist ics of SEBS estim ated versus o bserved heat

flux of datasets at Anduo in J une of 1998 （ M AD ： M ean

Absolute Deviation ； RM SE ： Root M ean Squared Error ）

S tatis tics （ est im ated v. m easured ）

　 R n G 0 H λE

（ MAD ）（ W / m 2） 26.30 47.73 27.73 68.69

（ RMSE ）（ W / m 2） 31.46 55.79 40.69 83.54

Num ber ofdata points used 389

Table 2　 Statist ics of SEBS estim ated versus o bserved heat

flux of datasets at Anduo in J uly of 1998 （ M AD ： M ean ab-

solute deviation ； RM SE ： Root M ean Squared Error ）

S tatis tics （ est im ated v. m easured ）

　 R n G 0 H λE

（ MAD ）（ W / m 2） 28.21 42.92 18.87 67.14

（ RMSE ）（ W / m 2） 32.62 53.94 26.32 84.43

Num ber ofdata points used 610

Table 3　 Statist ics of SEBS estim ated versus o bserved heat

flux of datasets at Anduo in August in 1998 （ M AD ： M ean

Absolute Deviation ； RM SE ： Root M ean Squared Error ）

S tatis tics （ est im ated v. m easured ）

　 R n G 0 H λE

（ MAD ）（ W / m 2） 28.28 47.44 22.25 66.02

（ RMSE ）（ W / m 2） 29.71 52.60 28.34 76.58

Num ber ofdata points used 622

The four energy balance term s predicted versu s

the m easured values and their diurnal behavio urs are

also shown in Figs. 9 ～21 （ Figs 10 、12、14 ～21 ， Plate

Ⅱ～Ⅵ）. Table 1 ～3 show the predicted versus ob-

served heat fluxes at Anduo in J une ，J uly and August

in 1998. Both MAD （ Mean Absolute Deviation ） and

RMSE （ Root Mean Squared Error ） ofthe predicted net

radiation and sensible heat flux are generall y less than

30 W / m 2. However ， bigger biases exist between esti-

m ations of SEBSand m easurem ents forsurface s oilheat

flux and latentheatflux with the estim ates of SEBSlar-

ger than m easurem ents. Sim ilar results are ob tained by

the ground m easurem ents in J une ，J uly and August at

Anduo and NPAM in 1998.

At the Anduo site in August 1998 ， surface soil

heatflux of SEBS estim ated is larger than thos e of ob-

servation- derived during daytim e. In contra st， it is

sm aller than that of observation- derived at n ight. The

m axim um estim ation （229 W / m 2） of SEBS is also

greater than that of observation- derived （90 W / m 2）.

Meanwhile ， the phase of observation- derived has a lag

about two hours in com parison with those of SEB Sesti-

m ates. Sim ilar conclusion can be obtained for the two

observation sites in J une and J uly in 1998. Iti ndicates

thatsurface soil heat flux is obviously relat ed with the

surface tem perature and the tim e of downward h eat

transportation. Hence ， the surface tem perature is an

im portantfactor forparam eterisation ofsur face soilheat

flux. However ， Eqn.（3） is derived from hom ogenous

land surface in the plain regions and the surfa ce tem -

perature is negligible. It cannotbe suitable to estim ate

surface heatflux in the GAME / Tibetexperim en tareas.

Estim ates oflatentheatflux are m uch higher t han

the m easurem ents ， buttheir phases coincide very well.

Itshows that the diurnal trends of latent heat flux of

SEBS estim ates are com parable well with those m eas-

ured. The basis of SEBSis thatthe energy com po nents

m ust be in balance. However ， as m entioned above ， the residualenergy R

net －（G0 ＋H ＋ λE） often excee-

ded 1 /3 ofnetradiation in GAME / Tibetexperim entar-

eas. To investigate the deviation oflatenthe atflux ， the

corrected- m easurem ents can be calculated by （R n－G 0）·ef2 according to the assum ption of the energy

balance （ Eqn. 1 ）. The results show that the correc-

ted- m easurem ents com pare very well with SEBS esti-

m ates in Fig.21. Italso certifies that the m et hodology

to derive latent heat flux in SEBS is reliable. Hence ， the m ain bias between SEBSestim ation and thos e m eas-

ured is generated by the surface energy im bala nce of

ground m easurem ents rather than the method of SEBS

estim ates. W e concluded therefore that the le aten heat

fluxes were underobserved atthese studied si tes and are

the m ain cause ofthe energy im balance. The pre lim ina-

ry resutls of this study were obtained in Zhang ［30］ and

the findings is aslo consistent with the resul ts obtained

for the Anduo site alone by Yang ， et al ［31］.

2321 　　　　　　　　　　　　　　　　　地球科学进展　　　　　　　　　　　　　　　　　　　　第 21卷

Fig. 9　 SEBS estim ated versus m easured surfac e energy balance term s at Anduo in J une of 1998

Fig. 11　 SEBS estim ated versus m easured surfa ce energy balance term s at Anduo in J uly of 1998

3321 第 12 期　　 SU Zhong- bo ， etal ： Energy and W ater Cycle over the Tibetan Platea u： Surface Energy Balance and Turbulent Heat Flu xes

Fig.13　 SEBS estim ated versus m easured surfa ce energy balance term s at Anduo in August of 19 98

　　 According to the assum ption ofthe energy ba lance

（ Eqn. 1 ）.

5　 Conclu sio ns

� The surface soilheatflux is largerthan soilh eat

flux atdeeper layer in the soil ， so a lot ofheat storage

takes place between the land surface and the m e asure-

m ent depth in het ground and is non- negligible in sur- face energy balance calculation using observ ation taken

under the surface. A lag of the average diurnal trend

between the surface and deeper layer in the soi l is

caused by downward heattransportation.

� The current param eterisation of the surface s oil

heat flux is derived from hom ogenous land surf ace in

the plain areas and the surface tem perature is neglected

in Eqn.（3）. Itis notvalid to estim ate the surface soil

heat flux directly in GAME / Tibet experim ent a rea.

Hence ， for the surface heatflux ， a new param eterisati-

on is needed ， in which the surface tem perature should be considered.

� The phenom ena ofthe energy im balance exist in

GAME / Tibet experim ent data ， although the explana-

tions of the energy im balance have been debate d and

notresolved yet. The residual energy R n et －（G 0 ＋H ＋

λE） often exceeded 1 /3 of net radiation ， the average

m axim um residualenergy is approxim ately 200 W / m 2 in

August 1998.

� Based on the calculated sataticstics ， the SEBS

predicted net radiation and sensible heat flu x com pare

very wellwith those ofground m easured ones.

� Estim ated latent heat fluxes are m uch higher

than the m easurem ents. However ， the corrected- m eas-

urem ents ， which are calculated according to the hy-

pothesis of the energy balance ， com pare with the esti-

m ation of SEBSvery well. Itclarifies thatthe deviation

is caused by the energy im balance of ground m ea sure-

m ents in GAME / Tibet experim ent area rather th an the

estim ation of SEBS. Since other term s estim at ed with

SEBS com pare very well with observed ones ， the latent

heatfluxes were likely under- observed.

References ：

［1］　 Ye D ， Gao Y X . The Meteorology of the Qinghai- Xizang （ Tibet ）

Plateau ［M］. Bei j ing ： Science Press ，1979 ：278.

4321 　　　　　　　　　　　　　　　　　地球科学进展　　　　　　　　　　　　　　　　　　　　第 21 卷

［2］　 Barnett TP ， Den il L ， Sch lese U ， etal . The effectof Eurasian snow

cover on regionaland global clim ate variati o ns［J］.J ournal of At-

m ospheric Scien ces ，1989 ，46：661-685.

［3］　 Vernekar A D ， Zhou J， Shukla J. The effectof Eurasian snow cover

on the Ind ian m onsoon ［J］. J ournalof C lim a te ，1995 ：8：248-266.

［4］　 Su Z ， Menenti M ， eds. Mesoscale Cli m ate Hydrology ： The Contri-

buti on of the New Observing System s ［R］. Report USP-2 ，99-05 ，

Pub lications ofthe Nati onal Rem ote Sensing B oard （ BCRS ），1999.

［5］　 Su Z ，J acobs C ， eds. Advanced Earth Observation- Land Surfac e

C lim ate ［R］. Report USP-2 ，01-02 ， Pub lications of the Nati onal

Rem ote Sensing Board （ BCRS ），2001.

［6］　 Su Z. The surface energy balance system （ SEBS ） for es tim ati on of

turbulentheat fluxes ［J］. Hydrology and Earth Syst em Sci ences ，2002 ，6（1）：85-99.

［7］　 Monteit h J L. Principles of Envi ronm ental Ph ysics ［M］. London ： Edward Arnold Press ，1973 ：241.

［8］　 Kus tas W P ， Daugh try C S T. Esti m at ion of the soil heat fl ux /net

radiat ion rat io f rom spectral data ［J］. Agricult ural and Forestogy

Met eorology ，1989 ，49：205-223.

［9］　 Bru tsaert W . Aspects ofbu lk atm ospheric bou ndary layer s im il ari ty

under free- convecti ve condi tions ［J］. Revi ew of Geophy ，1999 ，37：

439-451.

［10］　 Massm an W J. An analyticalone- d im ensionalm odelofm om entum

transfer by vegetati on of arbit rary structur e［J］. Boundary- Layer

Met eorology ，1997 ，83：407-421.

［11］　 Su Z ， Schm ugge T ， Kustas W P ， et al . An evaluation of two

m odels for estim ati on of the roughness height for heat t ransfer be-

tween the land surface and the atm osphere ［J］.J ournal of Applied

Met eorology ，2001 ，40：1 933-1 951.

［12］　 Brutsaert W . Evaporat ion into the Atm ospher e［M］. Holl and Dor-

drecht ： D . Reidel Publ ishing Com pany ，1982 ：299.

［13］　 W ieringa J. Roughness- dependent geographi cal interpolati on of

surface w ind speed averages ［J］. Quarterl y J ournalof Royal Mete-

orological Soci ety ，1986 ，112 ：867-889.

［14］　 W ieringa J. Representat ive roughness param eters forhom ogeneous

terrain ［J］. Boundary- Layer Met eorology ，1993 ，63：323-363.

［15］　 Bosveld F C. Exchange processes between a con i ferous forestand

the atm osphere ［D］. W agen ingen Agricult ural Universit y ，1999 ：181.

［16］　 Massm an W J. Molecular diff usi vi ties of Hg va por in air ，O 2 and

N 2 near STP and the kinem at ic vi scosi ty and the th erm ald iff usivi-

ty of air near STP ［J］. Atm ospheric Environm ent ，1999 ，33：453-

457.

［17］　 Choudhury B J， Monteith J L. A four layer m odel for the heat

budgetofhom ogeneous land surfaces ［J］. Quart erly J ournal of the

Royal Meteorolog ical Socie ty ，1988 ，114 ：373-398.

［18］　 Montei th J L. Evaporati on and envi ronm ent ［J］. Sym posia of the

Societ y Experim ental Biolology ，1965 ，19：205-234.

［19］　 Menenti M . Physical Aspects ofand Determ ina tion of Evaporati on

in Deserts App lying Rem ote Sensing Technique s［R］. Report 10

（ special issue ）， Inst itu te for Land and W ater Managem ent Re-

search （ ICW ）， The Netherlands ，1984.

［20］　 Ma Y M . Es tim ati on ofsurface heat fluxes usin g satel lit e and m e-

teorologicalobservat ions ［D］. W ageningen Un iversi ty ，2006.

［21］　 W ebb E K ， Pearm an G I ， Leuning R . Correcti on offl ux m easure-

m ent fordensi ty eff ectsdue to heatand water v apour transfer ＇ ［J］.

Quarterl y J ournal of Royal Met eorologi cal So ci ety ，1980 ，106 ：85-

100.

［22］　 Tanaka K ， Ishikawa H ， Hayashi T ， etal . Surface energy budgetat

am do on the Tibetan plateau using GAME / Tibet I OP98 data ［J］.

J ournal of Meteorolog ical Societ y of J apan ，2001 ，79（1B）：505-

517.

［23］　 Yang K ， Koike T ， Ishikawa H ， etal . A num ericalinvest igation of

observed energy im balance ata sit e of GAME- Ti bet ［J］. Boundary

Layer Met eorology ，2002.

［24］　 Ishikaw a H ， Hayashi T ， Tanaka T. Sum m ary and the Prel im i nary

Results of PBL Observat ion ［C］∥ Proceedings of the 1 st Interna-

ti onal W orkshop on GAME- Tibet. Xi ＇ an ， China ，11-13 ，J anuary

1999 ：69-72.

［25］　 Bian L ， Gao Z ， Xu X ， etal . Measurem ents of Turbulence Trans-

fer in the Near- surface Layer over the Southea stern Tibetan plateau

［J］. Boundary- Layer Me teorol ，2002 ，102 ：281-300.

［26］　 Miyazaki S ， Tsukam oto O ， Kaihotsu I ， et al. The energy im bal-

ances observed in Tibetan plateau and Mongoli an plateau ［C］//

The 2 nd Sessi on of International W orkshop on T IPEX- GAME / Ti-

bet. Kunm ing ， China ，20-22 ，J uly 2000 ：1-3.

［27］　 W ang J M ， Kim J， Liou Y ， et al. Energy balance analys is and

one- dim ens ionalsim ulati on of land surface p rocesses in a short-

grass sit e of cen tral Tibetan plateau ［C］ //Proceed ing of the 1 st

International W orkshop on GAME- Tibet. Xi ＇ an ， China ，11-13 ，J anuary ，1999 ：73-76.

［28］　 Kim J， Hong J， Hong SY ， etal. On m easuring and m odeling sur-

face energy parti tioning in a Tibetan Prai rie during GAME- IOP

1998 ［C］// Proceedings of the Fift h Internat ional S tu dy Confer-

ence on GEW EX in As ia and GAME. Volum e I. Nagoya ，J apan ，3-5 ， October ，2001 ：19-25.

［29］　 Kim J， Hong J， Gao Z ， et al . Can we close the surface energy

budget in the Tibetan Plateau ［C］∥ The 2 nd Sess ion of Interna-

ti onal W orkshop on TIPEX - GAME / Tibet. 20-22 ， J uly 2000 ，

Kunm ing ， China ，13-16.

［30］　 Zhang T. Validat ion of SEBSover Large Areas o f Tibet Us ing Sat-

ell ite and Field Data from Tibet Experim ent ［R］. Cen tre for Geo-

Inform ati on ， W ageningen University ，2003.

［31］　 Yang K ， Koike T ， Ishikawa H ， et al . Analys is of the surface en-

ergy budgetat a s ite of GAME / Tibetusing a sing le- source m odel

［J］. J ournal of Meteorological Soci ety of J apan ，2004 ，82（1）：

131-153.

5321 第 12 期　　 SU Zhong- bo ， etal ： Energy and W ater Cycle over the Tibetan Platea u： Surface Energy Balance and Turbulent Heat Flu xes

青藏高原地区能量水分循环：地表能量平衡和湍流热通量

苏中波1，张　廷

2，马耀明

3，贾　立

4，文　军

5

（1. International Institute for Geo- Inform ati on Science and Earth Observation ITC Enschede 7500 AA， the Netherlands ；2.天津市政工程局科学技术部，天津　300022 ；3.中国科学院青藏高原研究所，

北京　100085 ；4. Alterra Green World Research ， Wageningen University and Research Centre

Wagenigen 6700 AA， the Netherlands ；5.中国科学院寒区旱区环境与工程研究所，甘肃　兰州　730000 ）

摘　要：文章给出了青藏高原能量水分循环研究的概况和总结，着重估计了能量平衡各分项和湍流

热通量等。在能量平衡的计算基础上，尽管能量不平衡的原因解释仍有争论并且没有解决，但我们

揭示了 GAME / Tibet 试验观测资料中能量不平衡现象。我们发现估算的潜热通量比实际观测的要

高许多。然而，根据能量平衡假设的计算结果和 SEBS 的估算一致性很好。在此基础上可以归纳

出差异主要由 GAME / Tibet 试验观测资料中能量不平衡引起，潜热通量的实际观测可能偏小。

关　键　词：能量水分循环；青藏高原地区；能量平衡；通流通量

6321 　　　　　　　　　　　　　　　　　地球科学进展　　　　　　　　　　　　　　　　　　　　第 21卷