ative for most of the year and only positive during midsummer in the Arctic. The global net .... the ice station of the Surface Heat Budget of the Arc- tic Ocean ...
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第 22 卷 第 4 期 2 0 0 0 年 12 月
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JOU RNAL OF GL ACIOLOGY AND GEOCRYOL OGY
V ol. 22 N o. 4 Dec. , 2 0 0 0
Article ID: 1000-0240( 2000) 04-0384-07
Characteristics of Surface Radiative Fluxes and CloudRadiative Forcing with a Focus on the Arctic ( 北极地区地面辐射量和云辐射强迫特征) LIU Feng- jing( 刘凤景) 1, 2 , SU N Jun-ying( 孙俊英) 3 , ZHAN G T ing- jun( 张廷军) 4, CHENG Guo- dong( 程国栋) 2 ( 1. D ep ar t ment of G eography and Insti t ute of A rcti c and A lpi ne Research , Uni v ersi ty of Colorado, Bou lder , C O 80309, USA ; 2. St ate K ey L aboratory of Frozen Soil Engi neeri ng , CA REERI , CAS , L anz hou G ansu 730000, Chi na ; 3. L abor atory of I ce Core and Col d Regi ons E nv ir onmen t , CA REERI , CA S , L anz hou Gansu 730000, China ; 4. Cooper ati ve Insti t ute f or R esearch i n Env ir on me ntal S ciences, U niv ersity of Col orado, Bould er , CO 80309, USA )
Abstract: In this paper t he character istics o f sur face radiativ e flux es and cloud- radiative forcing are review ed w ith a focus o n the Arctic. T hree aspects are addressed, including ( i) changes in radiation flux over the global surface; ( ii) character istics of surface flux es in the A rct ic; and ( iii) character istics o f cloud-r adiative for cing in t he Arctic. T he clouds not only sig nificantly reduce the peak summer radiativ e heat ing of the surface but also r educe the wintertime radiativ e cooling at t he surface in higher latitudes. T he dow nw ard longw ave flux es dominates the incident radiativ e fluxes in the A rctic dur ing most of the y ear. I ncoming shortwave flux es are neglig ible during late fall, w inter and ear ly spring, and even during the midsummer the incoming shor twave fluxes are only slig htly greater than the downward lo ngw ave flux es. T he total net surface radiativ e flux is negative for most of the y ear and only positive during midsummer in the A rct ic. T he g lobal net cloud- radiative forcing is negative, but the cloud- radiative for cing is positive in the Arctic, showing a warming effect , ex cept for a short period in mid- summer. P ositive cloud- radiative forcing in the A rct ic is attributed to the presence of snow and ice with high albedo and the absence of solar r adiation dur ing the polar nig ht. Key words: surface radiative fluxes; cloud- r adiative forcing ; review; Arctic CLC number: P422. 3 Document code: A
1 INT RODU CT ION
- based Eart h [ LL ] Radiat ion Budget Ex periment ( ERBE ) show t hat t he annual mean albedo of t he
T he g lobal cloud distribution is recognized as
nort hern and sout hern hemispheres is nearly t he
forming a m ajor component of t he Earth s clim at e in it s influence on bot h t he energy and moist ure ex-
sam e . T his lack of hemispherical asymmetry in t he
changes in t he eart h - at mosphere system. Clouds
albedo reveals the dominant influence of clouds over
cover approx imately 50% of the surface area of the Earth, and account for about t wo- thirds of t he mean
surf ace ef fects in det ermining t he hemispherical mean albedo, since wit hout t he cloud ef fects the sig nificant
planetary albedo
1
. Observ at ions from t he sat ellit e
2
diff erences in t he surf ace feat ures bet ween the tw o hemispheres w ould have introduced large differences
Recei ved date: 1999- 12-27; Modi fied date: 2000-08- 14 Biography: LIU Feng-jing ( 1964~ ) , male, born in N angong of H ebei Province, graduat ed from Lanzhou U niversit y ( B. S . ) and Lanzhou Inst-i tut e of Glaciology and G eocryology ( M . S . ) , wit h research int erest s in w at er resources and hydrology. E - mail: Fengjing@ col orado. edu
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刘凤景等: 北 极地区地面辐射量和云辐射强迫特征
385
in t he hem ispherical albedo. Overall, clouds appear to
tudes, w ith a sect ion describing the eff ect of clouds
cool t he earth - at mosphere syst em according to
on the surf ace radiat ion budget
ERBE observation. T he global mean cooling varied from 14 to 21 W m - 2 bet ween April 1985 and Jan-
recent st udies w ill be present ed here in order t o update Crane and Barry s review , including ( i) changes
uary 1986. Hemispherical, the long wave and short-
in radiat ion f lux over the global surface; ( ii) charac-
w ave cloud f orcing nearly cancel each ot her in the
terist ics of surface f luxes in t he Arct ic; and ( iii) char-
w int er hemisphere, w hile in the summer t he negative shortw ave cloud f orcing is significantly lower than the
acterist ics of cloud- radiat ive forcing in the Arct ic.
posit ive longw ave cloud forcing in magnitudes, pro-
2 CHANGES IN RADIAT ION FL UX OVER T H E GLOBAL SU RFACE
ducing a strong cooling. T hus, clouds signif icantly reduce t he seasonal chang es in t he net radiative heating of the planet
3, 4
.
T he physical processes occurring in the Arctic climate system are in many ways unique compared w it h other regions of t he g lobe, part icularly due to the presence of t he highly reflecting snow and ice, the absence of solar radiat ion for a long port ion of the year, low temperatures and w ater vapor amount s, and presence of t emperat ure inversions 1 . Clouds di rectly af fect surface t em perature and surf ace albe5
do , and also indirectly affect t he st abilit y of t he atmospheric boundary layers and thus t he surface sensi
1
. M ajor results f rom
By using global observat ions of the propert ies of clouds, t he at mosphere and t he surf ace for m any years, Rossow and Zhang calculated global shortw ave and longwave fluxes at the t op of t he at mosphere and at t he surface at a resolut ion of 280 km and 3 hours for every t hird month f rom April 1985 to January 1989 15 . T he results show t hat t he annual mean net radiat ive balance at the surface is heating everyw here ex cept over t he high Antarctic Plateau ( F ig . 1) . T he ba-l ance results from a large shortw ave heating t hat isopposed by a small long wave cooling.
ble and latent flux es 6 . Calculat ions using a one- di mensional t hermodynamic model of sea ice suggest that t he pack ice could t otally melt during summert ime as a result of increasing cloud fraction 7 and al so by increasing the optical dept h of the wint ert ime 8
low er tropospheric ice cryst al clouds . In the Arct ic, t he cloud - radiation feedback is inext ricably linked w it h the snow
ice albedo feedback. It has
been hypot hesized that chang es in surface albedo associated w ith changes in snow and ice cover as a result of tem perature changes provides a signif icant positive feedback on climate change 9 . Over t he Arctic, accurate det ermination of the surface radiat ion fluxes is of part icular import ance due to t he sensit ivit y of snow and ice to the surface radiat ion fluxes
5, 6, 9, 10
net LW ( dashed) , and total net ( dashed- doted line) radiat ive flux es at the surface 15
. Impact of clouds on surface
fluxes and cloud- radiative forcing in t he Arctic have been st udied by many invest ig ators, for example, recent ones including Curry et al . 12
Fig . 1 A nnual, zonal mean values of net SW ( solid) ,
13
11
, Curry and E14
F ig. 2a shows t he annual, January and July zonal mean net radiat ion at surface 15 . T he large seasonal changes are caused almost ent irely by changing t he
bert , Zhang et al . , and T say et al . . Crane and Barry have exhaust ively review ed t he influ-
net shortw ave radiat ion associat ed wit h solar zenith angle variat ion. Not e t hat t he Arctic undergoes weak
ence of clouds on clim at e wit h a focus on high lati
net radiative heat ing at t he surf ace in summer but
386
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22 卷
t hat the surface of t he Ant arct ic Plat eau st ill shows a small net radiat ive cooling even in summer. Comparing F igures 2a and 2b shows t hat clouds not only signif icantly reduce the peak summer radiat ive heat ing of t he surf ace but also shif t t he polew ard edge of t he peak heat ing zone equatorw ard by about 30 of lat itude. At t he same time, clouds reduce t he w intertime radiat ive cooling at t he surface at higher lat itudes and shift the equatorw ard edge of t he cooling zone t oward t he pole by about 10 of lat it ude. T he patt ern of zonal mean cloud flux chang e show s t he major cloud zones and their seasonal shift s more clearly ( Fig. 2c) . T he cloud change of net radiation is primarily a short w ave flux change result ing in a decrease of net radiation relat ive t o clear condit ions in summer, but t he w eak longw ave eff ect s play a more important role in the high lat it udes of the w inter hemisphere, causing an increase of net radiat ion relat ive to clear condit ions in w inter ( F ig . 2c) . Ramanat han et al . found t hat in April 1985 t he short wave radiation cloud f orcing peaked in midlat-i tudes, unlike the longw ave forcing w hich peaked over t he t ropics 16 . In t he t ropics, large negative values w ere observed in t he tropical monsoon and deep convect ive reg ions. T he shortw ave forcing was also large nort h of 30 N in t he Atlantic and P acif ic. In t hese regions, the reduct ion of absorbed solar radiat ion as a result of clouds exceeded 100 W m - 2 . T able 1 cont ains some global cloud forcing estimates f rom ERBE data. When clouds are present , t he at mospheric column radiat es less t herm al energy and reflects more solar radiat ive flux es int o space than it w ould clear .
if
t he
skies
w ere
T he preliminary analyses of the dat a for f our
Table 1 Comparison of global cloud f orcing estimates using ERBE data ( W m - 2) A pril 1985
F ig. 2 Annual ( thin solid lines) , Januar y ( dashed lines) , and July ( thick solid lines) zonal mean v alues of ( a) fullsky net r adiative flux es, ( b) clear sky net fluxes, and ( c) cloud changes of net fluxes at the surface as a function of latitude 15
16, 17
July 1985 O ctober 1985 January 1986
Longw ave cloud f orcing
31. 3
30. 0
32. 0
30. 6
Short wave cloud f orcing
- 44. 5
- 46. 4
- 49. 4
- 51. 9
N et cloud f orcing
- 13. 2
- 16. 4
- 17. 4
- 21. 3
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刘凤景等: 北 极地区地面辐射量和云辐射强迫特征
387
mont hs show n in T able 1 conf irm t hat clouds have a
short wave fluxes are only slightly g reat er than t he
net cooling ef fect on t he global climat e.
dow nw ard longw ave flux es. Net shortw ave and long-
3 CHARACT ERIST ICS OF SU RFACE RADIAT IVE FL UXES IN T HE ARCT IC
w ave surf ace fluxes are g iven in F ig. 4. It is seen t hat t he t otal net radiat ive flux is negat ive for most of t he year and only posit ive during m idsummer.
T he most comprehensive in sit u measurem ent s of the surface radiat ion balance over t he Arct ic are the Russian measurements made f rom drif t ing ice st at ions 18~ 20 and t he most recent measurements from the ice st at ion of t he Surface Heat Budget of the Arct ic Ocean ( SH EBA)
21
. T he early observat ions f rom 22
Russia were assessed and int erpreted by F letcher . F let cher inf erred that values report ed by M ashunova
18
are the most com plete and reliable ( F ig. 3) .
F ig. 4
Annual cycle of modeled net shortw av e
and long wave flux es at the surface 12
T he modeled incoming shortw ave fluxes show ex cellent agreement w it h t he M ashunova values 18 , w it h t he modeled dow nw ard longw ave flux es not agreeing quite well w it h M ashunova s values ( Fig. 3) . T he discrepancies betw een t he model and Marshunova s w intertime longw ave v alues presumably ref lect Fig. 3
Annual cycle of the modeled inco ming shor twave
and downward longw ave fluxes at the surface
12
. Dots and
diamo nds corr espond to LW and SW v alues, r espectiv ely, as determined by M ashunova
18
. ( T he day starts on
Januar y 1; the follow ing figur es ar e the same as this one)
diff erences in the assumed phase of the condensate and the cont ribution of clear- sky ice crystal precip-i t at ion. T he summertime discrepancies in surf ace longw ave flux possibly ref lect dif ferences in cloud height , cloud- base temperature, and/ or cloud emissivit y. Com parison of t he sat ellit e
derived surf ace
By using available data on cloud f ract ion and
f lux es using ISCCP clouds w ith t he aforementioned
cloud microphysical propert ies, Curry and Ebert derived the annual cycles of surface temperature, sur-
Curry and Ebert analysis are show n in F ig. 5 and F ig. 6, w here t he surface radiat ive fluxes were deter-
f ace albedo, and cloud f ract ion and cloud opt ical prop-
mined follow ing Rossow and Zhang
erties, and furt her determined t he surf ace f luxes by
t here is agreement to w it hin 10 ~ 20 W m -
coupling the radiat ive calculations t o a one - dimensional t hermodynamic sea- ice model ( for 80 N) 12 .
April, the incoming shortw ave fluxes of t he ISCCP values in July are about 40 W m - 2 hig her than
As show n in F ig . 3, the dow nw ard longw ave f lux es
t hat f ound by Curry and Ebert , primarily because of
dominat es t he incident radiat ive f luxes in t he Arctic during most of t he year. Incoming shortw ave f lux es
t he much low er sum mert ime cloud cover in the ISC-
15
. Although 2
in
w inter and early
CP analysis ( F ig. 5) . How ever, t he higher surf ace albedo in the ISCCP analysis produces almost t he
spring, andeven during the midsummer the incoming
sam e net short w ave f luxes at t he surf ace as found by
are negli gible during late fall,
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Curry and Ebert.
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22 卷
T he agreement of downw ard
long wave flux es at surf ace in July is w it hin about 1 0 - 2
冻
despit e the larg e dif ferences in cloud cover
because t he clouds included in the ISCCP analysis, al t houg h they have hig her tops t han the clouds t hat are missed, have about the same base heig ht s in t hese
4 CHARACT ERIST ICS OF CLOUD- RADIAT IVE FORCING IN TH E ARCT IC Ramanat han et al . def ined t he cloud longw ave ( C LW ) and short w ave ( C SW ) radiat ive f orcing to be
calculations ( F ig . 6) . In wint ert ime, t he ISCCP
C LW = F ( A c ) - F( 0)
analysis overest imates t he downward longw ave f lux es relat ive to Curry and Ebert because the clouds have
C SW = Q ( A c) - Q( 0)
much higher optical thickness. How ever, in t he ISC-
Where A c is t he cloud fract ion and Q and F are t he short wave and longw ave flux es, respect ively ( note
CP analysis t he surf ace temperatures, w hich are w armer t han t hat of Curry and Ebert , produce larger
t hat here the radiat ive f luxes are defined to be posit ive
outgoing long wave fluxes so t hat t he net surface f luxes in bot h Curry and Ebert Z hang
15
12
agree t o w ithin 10 W m
and Rossow and - 2
16
dow nw ard) . T he values of cloud f orcing are negat ive for cooling and posit ive for w arming. T he net cloud forcing, C , is t he sum
.
C = C LW + C SW Using t he aforement ioned model of t he at mosphere and sea ice syst em, Curry and Ebert evaluat ed t he cloud radiat ive forcing at t he surf ace ( F ig. 12
7) . Strong seasonal variat ions w ere determined for surf ace C SW , w it h lit t le variation in C LW , resulting in a strong annual variation in C . Values of surface C are positive except for tw o weeks in midsummer. Over t he course of the y ear, clouds have a net w arming eff ect on t he surface in the Arct ic; this is in conFig . 5 A nnual cycle of incoming surface shortw ave r adiative flux for 80 N. Curry and Ebert Rosso w and Zhang
15
12
analysis ( heav y solid) .
analysis based on ISCCP for four
t rast to low er lat it udes, w here clouds have a net coo-l ing effect . T his difference arises due to absence of solar radiat ion during t he polar
months in 1985 ( thin solid) ; smoot hed annual cycle fit to the four- month ISCCP analysis ( dashed)
Fig. 7
Annual cycle of mo deled cloud forcing at the surface 12
Fig . 6 A nnual cycle of downward surface longw av e radiative flux for 80 N. Curry and Ebert Rossow and Zhang
15
12
analysis ( heav y solid) .
analysis basedon ISCCP fo r four
months in 1985 ( thin solid ) ; smoot hed annual cycle fit to the four- month ISCCP analysis ( dashed)
night and due to the hig h surface albedo of the sea ice. It is noted here t hat C SW and t hus C are strongly sensit ive to t he value of surf ace albedo. By using the data from the Arct ic Stratus Cloud
4期
刘凤景等: 北 极地区地面辐射量和云辐射强迫特征
389
Table 2 Shortwave, longwave and net cloud forcing over dif ferent geotypes determined from AVHRR data ( W m
- 2
)
Ice/
O cean/
O cean/
Lan d/
Snow
Land
Ice
Snow
- 108. 5
- 35. 3
- 115. 6
- 80. 5
- 69. 1
19. 7
33. 1
9. 6
25. 3
12. 6
15. 2
- 100. 7
- 75. 4
- 25. 7
- 90. 3
- 67. 9
- 53. 9
O cean
Land
SW cloud f orcing
- 120. 4
LW cloud forcing N et cloud f orcing
Ex periment collect ed on June 20 and 28 in 1980,
over land and ocean have the larg est mag nitudes, and
T say et al . provided tabulat ing results of fluxes for solar and near - inf rared radiat ion w it h various com-
over ice the mag nitude is the smallest. T he largest
ponents in the at mospheric profiles
14
. T he net
longw ave forcing is over land, w here temperatures are w armest , and t he sm allest forcing is over ice. T he
cloud- radiat ive forcing at surface, aft er recalculat ing, are 278 W m - 2 for June 20 and 373 W m - 2 for
net cloud forcing is neg at ive for all geot ypes and may
June 28, show ing a cooling effect . It is f urther in-
is based on midsummer data.
f erred t hat a clear sky condit ion results in more avail able dow nward f lux for snow m elt than does a cloudy
5
be explained for snow and ice because the calculat ion
CONCLU SIONS
sky condit ion in June. Zhang et al . investigated the impact of clouds on surface radiative fluxes and snowmelt in t he Arct ic and subarct ic f rom March to M ay and concluded t hat during t he period of
Globally, t he annual mean net radiat ive balance at the surface is heating everywhere ex cept over t he high Antarct ic plat eau. T he clouds not only signif-i
snowmelt in the Arct ic and subarct ic clouds t rend to
cant ly reduce the peak summer radiative heating of
at tenuate incoming short w ave radiat ion less t han clouds w hich enhance dow nw ard longw ave radiat ion,
t he surf ace but also reduce the w intertime radiat ive
resulting
on
w ard longw ave fluxes dominat e t he incident radiat ive
snowmelt . T he discrepancy betw een t hem arises ow ing to diff erence in surface albedo. T he albedo
f lux es in t he Arct ic during most of t he year. Incom-
used by Zhang et al . ( > 0. 8) is much higher t han
w inter and early spring, and even during the m id-
that used by T say et al . ( 0. 57) as the data of Zhang
summer the incoming short w ave f luxes are only
et al . is at least one and half months earlier t han the dat a of T say et al . Actually , both analyses are con-
slig ht ly greater t han t he downward longw ave f luxes. T he tot al net surface radiative flux is negat ive for
in
a
w arming
ef fect
of
cloud
13
sistent w it h Curry and Ebert
12
, w hose data shows
cooling at the surface at higher lat it udes. T he dow n-
ing shortw ave fluxes are negligible during lat e f all,
most of the year and only positive during midsummer
that over the course of the year the net cloud forcing at the surf ace is posit ive ex cept f or tw o weeks in mid-
in the Arct ic.
sum mer.
t ive forcing of bot h shortw ave and longw ave, w ith
T he net cloud- radiative forcing varies over diff erent geot ypes at t he surface because each geotype has
negative values f or shortw ave radiation and posit ive
it s unique albedo. L i et al . studied t he scene identifi
net cloud- radiat ive forcing is negative, the cloud-
cat ion and it s impact on cloud radiative forcing using
radiat ive forcing is posit ive in the Arctic, show ing a
AVHRR ( Advanced Very H igh Resolut ion Radiomet er) data 23 . T able 2 list s short wave, longw ave and
w arming effect , except for a short period in midsummer. Positive cloud- radiat ive forcing in the Arc-
net cloud radiat ive forcing over land, ocean, snow
t ic is att ributed t o t he presence of snow and ice w ith
andice for 4 days in July 1985 in the region nort h of 67. 5 N. As expect ed, shortw ave cloud forcing
high albedo and t he absence of solar radiat ion during
Clouds have a signif icant impact on surf ace radia-
values for long wave radiat ion. Alt hough the global
t he polar night .
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Acknowledgments:
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2
3
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actions [ J] . Journal of G eophysical Research, 1993, 98: 10 085 ~ 10 109. 8
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12
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22 卷
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土
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23
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