JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 105, NO. Dll,
PAGES 14,873-14,890, JUNE 16, 2000
Impact of an improved longwaveradiation model, RRTM, on the energy budget and thermodynamic properties of the NCAR community climate model, CCM3 Michael J. Iacono, Eli J. Mlawer, and ShepardA. Clough Atmosphericand EnvironmentalResearch,Inc., Cambridge,Massachusetts
Jean-JacquesMorcrette European Center for Medium Range Weather Forecasts,Reading, England
Abstract. The effect of introducinga new longwaveradiation parameterization,RRTM, on the energybudget and thermodynamicpropertiesof the National Center for AtmosphericResearch(NCAR) communityclimate model (CCM3) is described.RRTM is a rapid and accurate,correlatedk, radiative transfer model that has been developedfor the AtmosphericRadiationMeasurement(ARM) programto addressthe ARM objective of improvingradiation modelsin GCMs. Among the important featuresof RRTM are its connectionto radiation measurementsthrough comparisonto the extensivelyvalidated line-by-lineradiativetransfermodel (LBLRTM) and its use of an improvedand validated water vapor continuummodel. Comparisonsbetween RRTM and the CCM3 longwave (LW) parameterizationhave been performedfor singleatmosphericprofilesusingthe CCM3 column radiation model and for two 5-year simulationsusingthe full CCM3 climate model. RRTM producesa significantenhancementof LW absorptionlargely due to its more physicaland spectrallyextensivewater vapor continuummodel relative to the current CCM3 water continuumtreatment. This reducesthe clear sky, outgoinglongwave
radiationoverthetropicsby 6-9 W m-2. Downward LW surface fluxesareincreased by 8-15 W m-2 at highlatitudes andotherdryregions. Thesechanges considerably improve known flux biases in CCM3 and other GCMs. At low and midlatitudes, RRTM
enhances
LW radiativecooling in theuppertroposphere by 0.2-0.4K d-• andreduces coolingin the lower troposphereby 0.2-0.5 K d- • . The enhancementof downward surfaceflux
contributesto increasinglower troposphericand surfacetemperaturesby 1-4 K, especially at high latitudes,which partly compensatesdocumented,CCM3 cold temperature biasesin these regions.Experimentswere performed with the weather prediction model of the EuropeanCenter for Medium Range Weather Forecasts(ECMWF), which showthat RRTM also impactstemperature on timescalesrelevant to forecastingapplications. RRTM is competitivewith the CCM3 LW model in computationalexpenseat 30 layers and with the ECMWF LW model at 60 layers, and it would be relatively faster at higher vertical
1.
resolution.
tween
Introduction
A primaryobjectiveof the AtmosphericRadiation Measurement (ARM) programis to developand test improvedradiation codesfor general circulationmodels (GCMs) used for climateresearchand prediction.A recentproductof this effort is the radiative transfer model RRTM [Mlawer et al., 1997]. Developedfor both the longwaveand the shortwave,RRTM utilizes the correlated-k method for radiative transfer [e.g., Goody et al., 1989; Lacis and Oinas, 1991]. This approach providesa significantreductionin computationalexpenseover the highlyaccurateline-by-lineradiativetransfermodel (LBLRTM) [Cloughet al., 1992 (hereinafterreferred to as CIM); Cloughand Iacono, 1995] on which RRTM is based.By obtaining the absorptioncoefficientsrequired for RRTM from the data-validated LBLRTM,
a direct link is established be-
Copyright2000 by the American GeophysicalUnion. Paper number 2000JD900091. 0148-0227/00/2000JD900091509.00
ARM
radiation
measurements
and a radiation
model
that is sufficientlyfast to operate within a GCM. This study incorporatesthe longwaveversion of RRTM into the NCAR CCM3 climate model to establishthe feasibility of its use within a GCM and to determine its impact on the simulated climate system.Additional experimentshave been performed in collaborationwith the European Center for Medium-Range Weather Forecasts(ECMWF) in which the impact of RRTM on short-termweather forecastsis examined[Morcretteet al., 1998]. Several recent programs have compared GCM longwave (LW) radiationmodelsand haveshownthem to producesubstantial disagreement. The Intercomparison of Radiation Codesin Climate Models (ICRCCM) examinedover 30 radiation modelsfor a variety of atmosphericprofilesand discov-
ereda significant rangeof 30-70 W m-2 amongthemodelsin clearand cloudyskyLW fluxes[Ellingsonet al., 1991].Over 30 GCMs contributedto phase 1 of the AtmosphericModel IntercomparisonProject (AMIP) in which 10-year simulations
14,873
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RADIATION
GCM IMPACTS
were performedusingprescribed,monthlyvarying,seasurface region,the self-continuumtreatment in CKD producesa simtemperatures[Gates,1992].Ten modelsfrom this grouphave ilar result to RSB, thoughthere are important spectraldifferbeenshownto produceclear-skyoutgoinglongwavefluxesthat ences. CIM demonstratedthe importance of the foreignvaryby 15 W m-2 overa widerangeof oceantemperaturesbroadenedwater continuumabsorption(due to water-N2and [Duvelet al., 1997]. An analysisof LW and shortwavecloud water-O2collisions)in the 200-600 cm-• spectralregion radiativeforcingfrom thesesimulationsshowsintermodelvari- (which is absentfrom RSB), especiallyin the upper tropoationsof 20-60 W m-2 [Cesset al., 1997].For comparison,sphere.The validity of the CKD foreign continuumhas also
moreaccurateline-by-line modelsagreeto within1 W m-2
beenestablished in the1600cm-• spectral region[Tobinetal.,
and thusprovidean effectivefoundationon whichto build the next generationof GCM longwavemodels.This is the motivation for the developmentof RRTM and its applicationto
both
GCM
simulations.
Observationalprogramsare continuallyimprovingour ability to diagnosesomeof the GCM longwavemodeldeficiencies. Satellite measurementsfrom the Earth Radiation Budget Experiment(ERBE) [Barkstrom et al., 1989]are widelyusedfor comparisonto top of the atmosphere(TOA) fluxes from GCMs. For example,the AMIP modelcomparisons of Duvelet al. [1997]showedthat many of the LW modelstestedoverestimate the clear-skyoutgoingLW radiation(OLR) by asmuch
as10W m-2 relativeto ERBE. A recentcomparison between CCM3 and ERBE showedthis GCM to overestimateOLR by
asmuchas5-10 W m-2 in sometropicalregions[Kiehlet al., 1998b]. However, while the spectrallyaveragedERBE data providevaluableobservations of the globalTOA energybalance,they do not provideimportantinformationon the partitioningof the outgoingenergyacrossthe LW spectrumcaused by the variousatmosphericphysicalprocesses. Thus they cannot be usedto evaluatefully a LW model'sability to correctly representtheseprocesses. Future satellite-based instruments (e.g., AIRS, IASI) will provide this capabilitywith spectral radiation measurementsfrom space.This informationis currently availablefor selectedinfrared and microwavechannels from the High ResolutionInfrared RadiationSounder(HIRS) and Microwave SoundingUnit (MSU) measurements.New studiesare using these data to validate GCM LW radiation modelsaspart of the secondphaseof AMIP (G. Stephensand J. Bates,personalcommunication,1999). Spectralradiancemeasurements at the surfacesuchasthose from the ARM cloud and radiation test bed (CART) sites [Stokesand Schwartz,1994]or the SurfaceHeat Budgetof the
Arctic Ocean(SHEBA) Ice Station[MoritzandPerovich,1996] are playingan importantrole in highlightingcurrentradiation model deficiencies.In particular, LBLRTM radianceshave been extensively validatedagainstobservations from the ARM CART sitesand from Antarctica [Waldenet al., 1998]. In addition, SHEBA data have made key contributionsto the ongoing validation of the water vapor continuum model of Cloughet al. [1989 (hereinafterreferred to as CKD)] for low water atmospheres[Tobin et al., 1999]. Additional measurements of downwardlongwaveradiation at the surface,especiallyover polar areas,haveshownthat manyGCM radiation modelsgreatlyunderestimatethisquantity[Garrattet al., 1998; Pinto et al., 1997]. One of the most important remaining differencesamong line-by-linemodelsand the broadbandLW radiation models usedin GCMs is the treatment of the water vapor continuum absorption.A number of GCM radiation codesstill use the continuumformulationof Robertset al. [1976 (hereinafterreferred to as RSB)]. This model only includesthe effect of the self-broadenedwater continuum(due to collisionsbetween
1996].The CKD continuummodelis an integralcomponentof LBLRTM
and RRTM
and continues
to be validated
againstsurface spectralradiance measurements.The important differencesbetweenthe CKD and the RSB water vapor continuua,which significantlycontribute to the LW model differencespresentedin thisstudy,are discussed in more detail in CIM. In addition to the presentwork, other studieshave demonstratedthe importance of including the CKD water continuumabsorptionin climatemodels[e.g.,Schwarzkopfand Ramaswamy,1999]. In section 2, RRTM and the CCM3 longwavemodel are describedin greater detail, and the modificationsmade to RRTM to prepare it for implementationinto the GCM are discussed. Single-columncomparisons of the radiationmodels are presentedin section3 for two clear-skyprofilesand several cloudy-skycases.The 5-year CCM3 climate simulationsusing both LW
models
are described
in section 4. Both
the initial
radiative forcing effect of introducingRRTM into CCM3 and its impacton the energybudgetand thermodynamic properties of the climatemodel are presented.Section5 containsa summary of the applicationof RRTM into the ECMWF weather predictionmodelto demonstratethe useand effectof RRTM on short-termGCM simulations.The paper concludeswith a discussion of the work presented.
2.
Model Descriptions
2.1.
RRTM
Since line-by-lineradiative transfer modelsare inappropriate for use in GCMs due to their lengthy executiontime, a radiationcodethat effectivelyreproducesthe fluxesand cooling ratesof a line-by-linemodel, RRTM [Mlaweret al., 1997], hasbeen developedfor the ARM program.Although RRTM uses the correlated-k method for radiative transfer, the line-
by-line approachplayed a significantrole in its development throughthe utilizationof the comprehensive LBLRTM model [CIM; Cloughand Iacono, 1995]. LBLRTM is used both to computethe absorptioncoefficientsused to generatethe k distributionsneeded by RRTM and to evaluate the RRTM calculationsof fluxesand coolingrates.It is of criticalimportancein this contextthat LBLRTM continuesto be extensively validatedagainsthigh-resolution,spectralmeasurementsfrom aircraftand the surface(CIM). Its overallaccuracyis within 2 W m-2 of observations, whichalsoreflectscurrentlimitations in the accuratespecification of atmospheric state(S. A. Clough et al., manuscriptin preparation,2000). This providesan evaluationof the rapid modelthat is traceablethroughthe comparison of LBLRTM
with observation.
The standard version of RRTM
was created as a stand-alone
LW radiation model and uses 16 spectralbands that were carefullyselectedto highlightimportantmolecularabsorption features.The essenceof the k-distributiontechniqueinvolves reorderingthe LBLRTM-generated absorptioncoefficientsas watermolecules) in the800-1200cm-• windowregionwhere a functionof wavenumber,k(v), into ascendingorderfor each its radiative impact is largest. Integrated over this window spectralband. From this a smoothlyincreasingrepresentation
IACONO
ET AL.: VALIDATED
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of absorptioncoefficientis producedas a functionof cumulative probability,k(#), for eachlayer,whichcan be integrated with a relatively small set of points. RRTM uses 16 subintervals, or # points, in each spectralband to calculateradiance from the k (9) functions.Thisproducesa total of 256 radiative transfer operationsacrossthe LW spectral region (0-3000
cm-•). Gaseousabsorption is includedfor H20, CO2, 03,
RADIATION
GCM IMPACTS
14,875
continuumincludesonly the self-continuumabsorptionand is limited to the 8-12/•m window region. CCM3_LW usesthe RSB self-continuumand scalesthe resultingabsorptioncoefficients to represent the foreign continuum in this spectral region. Nevertheless,an important remaining difference between the CKD and the RSB water vapor continuua is the effect of foreign broadening absorptionin the energetically
CH4, N20, 02, N2, CFC-11, CFC-12, CFC-22, and CCL 4. In important200-600 cm-• spectralregion,whichsignificantly addition, the water vapor continuumis treated following the contributesto the resultsof this study.CCM3_LW includesa approachof Cloughet al. [1989]whichincorporatesboth self- broadbandemissivityformulationfor absorptionby liquid and broadening(collisionsbetweenwater molecules)and foreign ice clouds. The shortwave radiation model in CCM3, which broadening(collisionsof water with N 2 and 02 molecules) parameterizesabsorptionand scatteringwith a •5-Eddington acrossthe full LW spectrum.Surface-basedspectralradiance scheme, has been used without modification. In CCM3 the observationsare regularly usedto validate and to update this standard interval at which the longwave absorptivitiesand water vapor continuum [e.g., Tobin et al., 1999]. Of critical emissivitiesare computedfrom the current atmosphericstate importanceis the proper partitioningof the local line contri- is 12 hours. They are reused every model hour to calculate bution and the continuum contribution to the k distributions longwavefluxesand coolingrates from updated temperatures suchthat their combinationprovidesthe correctresult. RRTM and cloud amounts.For the presentanalysisthe absorptivities also has the capabilityof includingspectralsurfaceemissivity and emissivitiesare updated every three hours to more comand reflectancein eachband. Finally, severaloptionsare avail- pletely representthe diurnal cycle. able for LW cloud liquid water and ice absorptionin RRTM [Mlawerand Clough,1997], includingthe broadbandeffective 2.3. RRTM CCM3 Implementation
emissivitymethodusedin the NCAR CCM (see section2.2) and multispectralapproachesfollowingthe work of Ebert and Curry [1992] and Hu and Stamnes[1993]. The k-distributiontechnique,the consequenterrors introducedby this approximatetreatment of radiative transfer,and comparisonsbetween RRTM and LBLRTM calculationsare all discussed in greaterdetailby Mlaweret al. [1997].RRTM is able to reproduce,for a wide rangeof atmosphericconditions,
A number
of modifications
have been made
to RRTM
to
enhanceits computationalefficiencyfor its applicationto general circulationmodels.Sincemany of the spectralbandscontain minimal absorptionor very little energy,fewer than 16 9 pointscan be used to determinethe flux adequatelyin these bands. A significantdecreasein computational expensehas been attained by reducingthe number of 9 points to 140 from the standard
RRTM
total of 256. Each band was treated
indi-
LBLRTM netfluxwithin1 W m 2 at anyaltitude,coolingrate viduallyto determinethe minimumnumberof 9 pointsneeded in the troposphere andlowerstratosphere within0.07K d • to retain the desired level of accuracy.The revisednumber of
and upper stratosphcric cooling rate within 0.75 Kd-•.RRT•
subintervalsvariesfrom 14 or 16 •7pointsin the four important
providesa substantialreductionin computationalexpenserelative to LBLRTM that results from reducing the radiative
watervaporandcarbondioxidebandsfrom250to 820cm • to as few as 2 9 points in the three high wavenumberbandsfrom
transferoperations from f-10•' to 256,sincethe detailedspec- 2250to 3000cm-•. The effecton overallaccuracy is lessthan tral information is not required. A benefit of the k-distribution 0.5W m-: for netfluxand0.1K d-• for cooling rate,whilethe technique over many GCM LW radiation models is its near linear scaling of computational expensewith the number of model layers.SomeLW models,suchasthat in CCM3, scaleas the squareof the number of layersand would prove prohibitivelyexpensiveat highverticalresolution.Finally, a shortwave versionof RRTM has also been developed,though it has not been incorporatedinto this study.The longwave,stand-alone
computationalexpenseis reduced by about 40%. Further efficiency improvementswere achieved by simplifying specific elementsof the RRTM algorithm.For example,an exponential is requiredto computetransmittancefrom optical depth in the standardRRTM. For the GCM versionof RRTM (hereinafter referred to as RRTMG) this expensehas been trans-
version
initializationroutinewhere a 5000-pointtransmittancelook-up table is computed.This providedan additional10% reduction in costwith negligibleeffect on accuracy.The flux integration in RRTMG is accomplished with a diffusivityanglefor consistency with CCM3_LW. While faster than the more precise three-angle integration in the standard RRTM, this single-
of RRTM
is available
from
the AER
www.aer.com.
2.2.
NCAR Community Climate Model
Web
site at
ferred
from
the radiative
transfer
section of the model
to an
For our analyseswe used the NCAR CCM3 general circulation model versionCCM3.6.6 [Kiehl et al., 1996, 1998]. This modelwas selectedbecauseit is in wide useby the community, angleapproach introduces anerrorof theorderof 1 W m-: in has successfully simulatedclimate parameters,has a modular clouds. To provide further timing reduction on vectorizing format that facilitatesmodification,and is adaptableto several computers,RRTMG has been receded to enhanceits vectorcomputer platforms.The standardCCM3 format includes18 izability. Finally, the standardRRTM usesa Pade approximaverticallayersand a T42 horizontalresolution(-2.8 x 2.8ø), tion to representthe Planck function variation with optical which has not been changedfor this study.The CCM3 long- depth in a layer similar to the method of CIM. RRTMG wave parameterization (hereinafter CCM3_LW) utilizes a incorporatesan accuracyimprovementby applyinga look-up broadband, nonisothermalabsorptivityformulation with six table for this Planck function transition that is based on the overlappingspectral intervals for H20, CO2, and 03. Also "linear in r" approximation(CIM). includedare the effectsof the H20 continuumof Robertset al. Additional modificationsprovide enhancementsrelative to [1976], absorptionfrom the trace gasesCH4, N20 , CFC-11, CCM3 LW. First, RRTMG functions over a full additional and CFC-12, and the radiative properties of the two minor layer relative to the CCM layering.This layer extendsfrom the CO2 bandsat 9.4 and 10.4/•m. The original RSB water vapor CCM model top pressureat 2.9 mbar to the top of the atmo-
14,876
8.)
IACONO
ET AL.' VALIDATED
Fin
LONGWAVE
BNDin Agas
Anld
,.BNDout
Fout b) ,
Fin ,
,
BNDin
RADIATION
GCM IMPACTS
and emissionby the clouditself. In effect,the cloudis present only as a thin band at the outgoingboundaryof the layer and the radianceemitted by the cloud is not attenuatedby the gas in that layer. For a contiguousgroup of multilayercloudsthe sourceradiancedue to a cloud in a layer is not attenuatedby gas absorptionin any layer within this group of clouds.This radianceis, however,attenuatedby cloud absorptionin these layers.Also, becausethe cloud is treated as a thin band along the outgoinglayer boundaryits emitranceis derived from the Planck function at the layer boundary. For a cloud that is assumedto fill a layer,a morephysicallyappropriatechoicefor calculating the cloud emitrance is to use a Planck function,
Bgas+cld, withaneffective radiativetemperature dependent on the combinedgasand cloud optical depth. Figure 1 illustrates theseissuesfor downwellingflux througha layer with a cloud fraction of 1. In Figure l a the flux exiting the bottom of the layer as computedin CCM3_LW can be expressedas
cid
BNDout
Fout
Fout= F•nTgasTcl d4-BgasAgasTcld 4-BcldAcl d
(3)
where the first right-handterm is the flux enteringthe top of Figure 1. Diagram of the cloud and gaseousoverlapabsorp- the layer,Fin, attenuated by the gaseous transmittance Tgas tion within a layer as formulatedin the (a) CCM3 longwave and the cloud transmittancercld. The secondterm represents radiationmodel and (b) RRTMG longwavemodel. the cloud attenuation of the radiation emitted by gas in the sphere to provide values for both the downwardflux at 2.9 mbar and the outgoinglongwaveradiation. Mixing ratios in this extra layer are identical to those in the CCM top layer exceptthe ozone mixingratio, which is reducedby a factor of 0.6. The temperature of the extra layer is fixed to be 18 K higherthan the CCM top model layer. Both changesare based on the U.S. standardatmosphereat this level. For consistency with CCM3_LW, surfaceemissivityis handledidenticallyto its applicationin the CCM3 land surfacemodel(i.e., it is generally one with a few non-onevaluesin specificareas).Aerosol absorptioneffectsin the longwavehave not been included. Cloud radiative effectsare modeledwithin RRTM to reproducetheir contributionto the coolingrate profilesas closelyas possibleto their effects in CCM3_LW. Our rationale is to examine
the feedback
effects
due to the more
accurate
clear-
skyradiativetransferwithout significantchangesin the cloud treatment.However, the treatment of longwavecloud absorption hasbeen revisedto provide a more physicallyappropriate solutionthan that used in CCM3_LW. This improvementutilizes the ability of the RRTM k-distributionapproachto treat uniformlythe combinedgaseousand cloudabsorptionwithin a layer. At present, CCM3_LW usesthe "effective emissivity" method for cloud absorption[Kiehl et al., 1996] in which an effectivecloud fraction is defined as the productof the cloud emissivityand the computedcloud fraction Ac,
A; = ec•0A•.
(1)
The cloud emissivityis given as
/3cld = 1 - e-D•
Z o
Z
Z
Z
Z
0 Z
Z
Z
Z ø
0
0
0
0
u,J
0
a:: x.
o
•' 03 (/> 03 03 00
•5
IACONO ET AL.: VALIDATED
LONGWAVE
same way as the single-columncomparisonsin section 3 and represent the global, initial forcing impact of introducing RRTMG into the climatemodel. For this study,the interval at which LW gaseousabsorption was computed was reduced from the standardCCM3 interval of every 12 model hours to every three model hours to better simulatethe diurnal cycle. LW fluxes and cooling rates were then calculated at every model hour with updatedtemperatureand cloud information. A 5-year simulationwas performed using monthly averaged, climatologicalsea surfacetemperatures(SSTs)with all radiative and dynamicalfields stored as daily averages.The simulation was precededby a 4-month adjustmentperiod during which the radiative feedbackwas gradually transferred from CCM3_LW to RRTMG over the first three monthsby applying a cosinetransitionfunctionto the coolingrates and downward
surface
fluxes from
each LW
model.
The
results
on the CCM3
climate
model
GCM IMPACTS
FIRIMG-CCM3
RRTMG-CCM3
II
10001 ,,,•,,,, ,,t
,, ,l•l,,,,•
....
I ....
-30-20 FLUX -10 DIFF. 0 (•/
I
0 (W 10m20 0 (• m20 30 -30-20-10 FLUXDIFF. 2) 30 -30-20-10 FLUXDIFF 2) 30
RRTMG-CCM3
of this
RRTMG-CCM3
•
...... up
/::
___ DOWN NET 2o0
•
400[/i,t
400 600
6OO
\
\\
800
200
20
400
•
40
I 60
eoo
I
,oo< • ....
-0 5
• ....
0.0
0.5
I
1000/,
1.0
-1.0
,, ,_L, -0.5
0.0
800
• ,,, 1000 0 5
1.0
COOLING RATEDIFF.(Kd'j)
-1.0
•,,,, •. -0.5
0 0
0 5
10
COOLING RATEDIFF.(Kd'*)
RRTMG-CCM3 o
/
RRTMG-CCM3
'f'
o ......
200
simulation.
CCM3 Sub-arcticWinter Longwave Fluxes
_\\
2OO
I
.
COOLING RATEDIFF.(Kd'1)
/i
t
400
-1.0
-\
RRTMG-CCM3
2oo
1000 •
o
14,881
o
test are presentedin section4.1. For the secondphase, a 5-year, climatologicalSST control simulation was performed using the original, unmodified CCM3 to providea basisfor comparisonto the first simulation. Since each experimentwas run with a different LW model, differencesin the radiative and dynamicalfields between the two simulationsinclude the changesin atmosphericstate producedby the two LW methods.Thesedifferences,presentedin section4.2, are more representativeof the long-term impact of RRTMG
RADIATION
Figure 4. Midlatitude summer flux and cooling rate differencesbetweenRRTMG and CCM3 longwavemodelsfor three cloudcases.Differencesare shownfor (a) flux and (b) cooling rate with a low cloud, (c) flux, and (d) coolingrate with a multilevelcloud, and (e) flux and (f) coolingrate with a high cloud. The line stylesused in Figures 4a, 4c, and 4e are identified in Figure 3a. Layers with cloud are marked with black barsto the right in eachfigure.All cloudshave an opticaldepth of 5 in eachlayer. Figures4a-4d are for a cloud fraction of 0.5 in each layer, and Figures 4e and 4f are for a cloud fraction of 1.0.
80O
1000
0
100
200
300
400
500
-30-20-10
FLUX(W m2)
b
10 20
30
RRTMG-CCM3
CCM3 Sub-arcticWinter LongwaveCoolingRate
o
0
FLUXDIFF.(W rn2)
20O
with observationsfrom the Earth Radiation Budget Experi-
ment (ERBE) [Kiehlet al., 1998b]and to ensurea TOA radiative balance.Also, the outgoinglongwaveradiation changes producedby RRTMG are of the sameorder or larger than the magnitudeof the changesintroducedby this tuning.Therefore sinceno attempt was made in the present studyto retune the climate model when running with RRTMG, direct compari-
200
•.• 400
400
m
600
600
800 [
1000 /
.....
0
1
I
2
....
I
....
3
I
4
,
,
800
,
1000
5
-1.0
COOLINGRATE(K d'1)
-0.5
0.0
0.5
1.0
CR DIFF.(K d'•)
Figure 3. Subarcticwinter upward,downward,and net fluxes (a) as computedby the CCM3 longwavemodel, with differencesbetween RRTMG and CCM3 shownto the right. Subarcticwinter coolingrate (b) as computedby the CCM3 longwave model, with differences between RRTMG
the right.
It shouldbe noted that the CCM3 global averagetop of thc
atmosphere(TOA) fluxeshavebeentunedby NCAR to agree
and CCM3 to
sonsto ERBE fluxesare not includedin this analysis.Changes to the CCM3 energy budget causedby RRTMG should be interpreted qualitatively in the context of the improvement attainedby RRTM as establishedthroughits validationpath to observedradiances.Kiehl et al. [1998b] give a thorough descriptionof the CCM3 energybudget, includingcomparisons of CCM3
4.1.
LW
fluxes to ERBE
data.
Initial Forcing Impacts
Differences in the annual average OLR and surface LW fluxes (RRTMG - CCM3_LW), which representthe initial forcing effect of RRTMG, are shown in Plate 1. Clear-sky
OLR (Plate la) is reduced by RRTMG most extensively
14,882
IACONO ET AL.: VALIDATED LONGWAVE RADIATION GCM IMPACTS
throughout thetropics, byasmuchas9 W m-2, decreasing to 4.2. Climate SimulationImpacts smallchanges overthepolarareas.Thegreatest OLR changes The RRTMG initialforcingfluxandcoolingratechanges
occur in the wettest latitudes, which is consistentwith the
described in section 4.1 alterthe atmospheric statethrough
significant enhancement of absorption from the CKD water feedbacks between theradiationandthemodeldynamics until
vapor continuumrelative to the RSB formulation. The annual
a new equilibrium state is reached.These feedbackeffectsare
mean,totalskyOLR changes (Platelb) aregenerally smaller, now considered by looking at differences between the since clouds reduce the LW model OLR differences as shown
RRTMG CCM3 simulation and the control CCM3 simulation.
in Figure4. Exceptions aretypically clearareas(e.g.,Sahara Plate 3 shows the annual mean TOA and surface LW flux Desert) or regionswhere low cloudsdominate,suchas the
differences between RRTMG
eastern midlatitude oceans. Such low clouds are below much of
their respective changes in atmospheric state.The colorscale
the enhanced absorption fromthe CKD foreigncontinuum in RRTMG. Severaltropicalregionsin Platelb (e.g.,westof CentralAmerica,centralAfrica,etc.)showanincrease in total skyOLR dueto RRTMG. Theseareareaswherehighclouds
and CCM3 LW that include
andthefieldsshown arethesameasin Plate1.Theclear-sky OLR difference in Plate 3a shows considerable decreasesin
OLR dueto RRTMG throughoutthe low andmidlatitudes of
4-12 W m-2, withsmallincreases at highlatitudes. Thetotal skyOLR difference in Plate3bcontains considerable regional improvement in the-LWcloudabsorption algorithm, discussedvariationwith tropicaldecreases of asmuchas 18 W m-2 and dominate in this simulation. For these clouds the RRTMG
in section2.3, producesan increasein OLR of severalW m-2
increases of 11W m-2. At thesurface, mostofthewidespread
asdemonstrated in Figure4e. Smallfluxdifferences at high increase in clear-sky downward fluxathighlatitudes of 8-15 W latitudes in Plate lb result from combinationsof clouds at all levels.
m-2 remains (Plate 3c),whilethetotalskyfluxchanges (Plate
3d) aregenerally smallwitha fewlargeregionaldifferences. In RRTMG hasa substantial impacton clear-sky netradiation general,the -LWfluxdifferences in Plate3 havegreaterreat the surface,shownasan annualaveragedifferencein Plate gionalvariabilitythanthe initialforcingimpactsin Plate 1
duetochanges in atmospheric andsurface temperature lc (netfluxisdefined asupward minusdownward radiation). largely Sincethe LW modelsarebeingcompared for identicalatmo- andto thespatial andtemporal redistribution of watervapor bythe LW modelchange. spheric states(including thesurface temperatures) in thiscon- andcloudcoverinduced As mentioned previously, the CCM3 globalmeanTOA entext, the upwardsurfaceradiationdifferenceswill be smalland most of the flux changesshownin Plate l c are due to an increase in the downward radiation toward the surface. The
ergybudgetwastunedto agreewithina few W m-2 of ERBE measurements.Despite this, regional differencesbetween CCM3 and ERBE remain, which can be inferred from the
largestincreases occurat highlatitudes,overdeserts,and in OLR comparisons in Figures15and16 of mountainous areaswheretheatmospheric watervaporcontent OLR andclear-sky in manytropicalareas,CCM3 is low.In theseregionsthe downward fluxis enhanced by Kiehletal. [1998b].Specifically, the OLR and clear-skyOLR by 5-10 W m-2 12-15W m-2. Thisconsiderable impactisconsistent withthe overestimates relative to ERBE. Overmuchof Antarctica, CCM3outgoing discussion in section3 of the fluxchanges for the SAW atmoflux is too low. The RRTMG OLR impacts shown in Plates3a sphere(Figure3a).As notedearlier,manyGCMsgreatlyunderestimate theclear-sky downward surface fluxin dryregions and3bhavethecorrectmagnitudeanddirectionto offsetthese Kiehlet al. [1998b]suggest a deficiency in by10-20W m-2. In particular, Pintoetat.[1997]showed that CCM3fluxbiases. CCM2 downward surface fluxes over the Arctic were too low
water vapor absorptionin CCM3 LW as one causefor the
in CCM3.Briegleb andBromwich [1998a] also by 20 W m-2. It is a significant resultof thisworkthatthe TOA fluxbiases suggest insufficient water vapor rotation band absorption in additional absorption provided bytheCKDwatervaporconCCM3 to explain a 10-20 W m -2 deficiency in-LW clear-sky tinuumaccounts for muchof thisdiscrepancy. Thetotalsky, surface fluxin polarlatitudes. Theresultpresented net fluxchange at the surface in Platel d shows consistencydownward here for RRTMG is strong evidence that improved absorption, withFigure4 in thatsurface fluxdifferences aregreatlyreducedin cloudyareas.Large differencesremain over land
providedby the CKD watervaporcontinuummodel,makesa
significant contribution to the accurate representation of LW areasthathavetheleastcloudiness in thissimulation. Clearly, radiativeprocessesof relevanceto a GCM. and the CKD water continuum have substantial imTheimpactof RRTMG ontheCCM3energy budgetcanbe pactson clear-sky LW fluxat boththetopof theatmosphere summarized withannualmean,zonalaverages of the energy andat thesurface thatwill significantly influence theenergy budgetcomponents. Figure 5 presents the zonal average OLR balance of the climate model. for totalsky(Figure5a), clearsky(Figure5b), andthe LW The primary mechanismby which a -LW radiationmodel cloudforcing(Figure5c) definedasthe clearskyminusthe impacts the atmospheric stateof a GCM is throughtheradi- totalskyOLR.RRTMGvalues (dashed lines)andCCM3_LW ativecoolingrate.The initialforcingeffectof RRTMG on the values (dottedlines)usethescaleto theleftin eachplot.The LW cooling rate of CCM3 is shown as a difference differences (solidlines)usethe scaleto the right.Bothclear (RRTMG - CCM3_-LW) in Plate2. Thisandall subsequentand total skyOLR are reducedby RRTMG at nearlyall RRTMG
figuresthat have pressureas the vertical axisuse the CCM
latitudesexceptfor the polarareaswheresmallincreases are
hybridsigma-pressure verticalcoordinate times1000[Kiehtet noted.RRTMG reduces cloudforcingin the tropicsandthe at., 1998a].RRTMG increases the coolingratefromthemid- 50o-60 ø latitudes, wherecloudiness is generally grehtest. Aldletroposphere to thelowerstratosphere, except forthetrop- thoughthe CCM3 shortwave (SW) modelwasusedin both icaltropopause. Peakcoolingenhancement of 0.4K d- • occurs simulations, the annualmean,zonalaverageTOA absorbed in theuppertropicaltroposphere. Coolingratesarereduced at SW fluxes,shownin Figure 6, are impactedby RRTMG all latitudes in thelowertroposphere withpeakvaluesaway throughthechanges in atmospheric state,primarilywatervafrom the polesof 0.4-0.5 K d- • porandcloudcover. Thetotalsky,TOASWfluxes (Figure6a)
IACONO
ET AL.' VALIDATED
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are increased by 3-4 W m-2, especially in the tropicsand midlatitudes.Clear-skyTOA SW fluxes(Figure 6b) are only negligiblyaltered, thus the SW cloud-forcingchange(Figure 6c) resemblesthe total skydifference. Zonal averagesurfaceLW net fluxes,shownin Figure 7, are alsoreducedat nearlyall latitudesby RRTMG. The decreases in total skyflux at the surface(Figure 7a) are largestin mid-
a 400 360
•
160
,,
120
of downward
shown. are affected
in a similar fashion to the TOA
SW fluxes
heat
fluxes
at the surface
due to RRTMG
a
50 •-.
280 i RRTMG
RRTMG -CCM3 40•
E
260t CCM3
//
200i!
lO,,ux
140 i 120
40N
20N
0
20S
40S
60S
80S
LATITUDE
b
'
.}-:?:V : .....
280 260
,;'? ..,,
so•-
ß ":4.
240......
40v•
"
•.,•
LONGWAVE
30 v.
20 øz
•,.
1Ou_
',,. .•
..•.•..,-
•.a..•.
=
o• -10
20S '40S '6;S '80S
LATITUDE
b
SurfaceNet LongwaveRadiation(ClearSky)
12o
30
lOO
20•
80
10 ø z
60
o•
40
-20
20
80N
60N
40N
20N
0
20S
40S
60S
80S
LATITUDE
c
Surface Longwave Cloud Forcing
1 oo
3o
80
20 •
•. E60 •X
40
..";" x'.. 10z 0 ...;" ''....... rr _0 ,,uJ
•" •"-•'.... v. '•"
u_
.9
+...
20
.:'
:
'
-lO•
o
-2o
80N
60N
40N
20N
0
20S
40S
60S
80S
LATITUDE
GCM IMPACTS
band of reduced land temperaturesshifts southwardduring DJF and northwardduringJJA relative to the annualmean in responseto the presenceof atmosphericwater vapor.An analysisby Bonan [1998] of surfaceair temperaturesgeneratedby the CCM3
.•..
80N 60N 40N 20N 0
RADIATION
Land Surface Model
umented
biases in CCM3.
As the primary greenhousegas in the troposphere,water vaporplaysa significantrole in regulatingthe energybudgetof the Earth's climate system.Its distributiongreatlyimpactsradiative fluxesthroughoutthe atmosphereboth as a direct gaseous absorber and in its liquid or ice form as clouds and precipitation.The impact of RRTMG on the annual mean, zonal average moisture in CCM3 is shown in Plate 7 as a percent difference.The most dramaticchangeis an extensive decreasein water vapor due to RRTMG at low latitudesin the lower to middle troposphereby 20-30%. This reductionpeaks at -1.1 g/kg over the tropicsnear 600 mbar and is larger than the CCM3 tropicalmoistbiasesseenin Figure 5 of Hack et al. [1998].Smallincreasesin moistureare noted at mostlatitudes closeto the surface,which slightlyimproveCCM3 dry biases. Clearly, the distributionof water vapor is closelytied to many dynamicalcomponentsof the climate model, suchas the circulation,boundarylayerprocesses, and convection.A full analysisof the RRTMG impact on the dynamicalcomponentsof CCM3 is beyondthe scopeof this work. However, as noted
a 2OO 18O 160
½
Latent
nental-wide
cold bias of 2-5
K over Antarctica
RRTMG - CCM3
_
CCM3
'-'
.•..•
•" \
ing occursprimarily in northern winter. At lower latitudesthe
/
i.u
'•'
20 Z
_
,,-½ 60 40 20 0
_=• -
•-
/
80N
60N
40N
b
½
'
x.
'
-:0 x
_-- D
.........................
20N
0 LATITUDE
20S
40S
60S
80S
Sensible Heat Flux
8O 70
40 •
60
30 •
E
5O o
½:40
-•
30
X
D
2O -2_ :
u_
10
20 z ..x,..,-
-10 -20
%.,.,
60N
40N
_
120
/
J ! ..•
_
_
80
k.
20S
40S
60S
80S
40 E
// "%
h. .[ ,,..,
',.•.
30w O Z
\
\
2o•
_
1.,.•...c.'
_
log
40 2o
x
Heat Flux
!
x• 60 u_
0 LATITUDE
!
• 100_
-.--
20N
Latent + Sensible
C 180 160 -
140
o
, 80N
½
lO Lu
"---"'
__=...>..•.• 0
mean. Warming is also seen acrossmuch of Eurasia, parts of
Antarctica in both summer and winter, while the Arctic warm-
•-.•
x• 80
North America, the Sahara Desert, and Australia. For some
a result of this feedback. Cooler regionsinclude most of the tropicalland areas,Scandinavia,and part of northernAsia. To illustrate the seasonalvariability of the surface temperature changes,we showthe correspondingdifferencesfor June,July, andAugust(JJA) in Plate 6b and for December,January,and February(DJF) in Plate 6c. Warmingis presentover muchof
E
30 •
.......
----- 100
in the annual
areas(e.g.,Antarctica,the northernSahara,andAustralia)the enhanced downward surface flux is compensatedby an increasein the upward surfaceflux due to the higher surface temperatures.Reduced net flux differencesin these regions relativeto the initial forcingchange(seePlateslc and 3c) are
Heat Flux
- RRTMG ___
140 _
• 120
ference(Plate 6a) showsthe regionaleffect of the enhanced greenhousewarming causedby RRTMG. (These resultsare based on CCM3 simulationsthat used identicallyprescribed climatologicalSSTs,and thustemperaturedifferencesare zero over the oceans.)Over the Arctic, Antarctic and the adjacent sea ice, RRTMG increasessurface temperature by 1-4 K. Briegleband Bromwich[1998b] note that CCM3 has a conti-
has cold
Greenlandin both DJF and JJA. The impact of RRTMG on surfacetemperatureeffectivelyaddressesmany of these doc-
Figure 7. Zonal average,annualmean (a) surfacenet longwaveradiation,(b) clear-skysurfacenet LW radiation,and (c) surfaceLW cloudforcingfor RRTMG and CCM3 LW models (scaleson the left) and for the RRTMG-CCM3 difference (scaleson the right).
level relative to both the ECMWF reanalysisand the NCAR/ NCEP reanalysistemperaturefields. Surfacetemperatureis alsoimpactedby the increaseddownward surfaceflux producedby RRTMG, and this is demonstratedin Plate 6. The annual mean, surfacetemperaturedif-
showed that CCM3
biases over Antarctica, Australia, the Sahara Desert, and
•. •'* ....
\
x
o D
o -20
80N
60N
40N
20N
0 LATITUDE
20S
40S
60S
80S
Figure 8. Zonal average,annualmean (a) latent heat flux, (b) sensibleheat flux, and (c) latent plus sensibleheat fluxes for RRTMG and the CCM3 LW model (scaleson the left), and for the RRTMG-CCM3 difference(scaleson the right).
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RRTMG- CCM3 LW CoolingRate Difference
200
400
600
1000
80N
40N
60N
20N
0 MIN:-3.18
20S
80S
60S
40S
MAX: 0.27
K d4
.0.•
.0.•
.{;.4
-0.• • '0
0.•
0.4
0.•
0.8
Plate 4. Zonal average,annual mean, coolingrate differencesbetweenRRTMG and CCM3 longwave modelfromtwo5-yearCCM3 simulations, includingfeedbackeffectsof eachLW modelon atmospheric state Units are in K d-l
RRTMG - CCM3 Temperature Difference
200
•
4OO
600 ',• 800
' ..
80N
.-
60N
., 40N
20N MiN:-6.47
, 0 MAX: 4.71
20S
-3
-2
-1
0
40S
60S
80S
GLOBAL:o0.5?
•
-4
J
'
1
;•
K
3
4
Plate 5. Annual mean, zonal averagetemperaturedifferencebetween RRTMG and CCM3 longwave radiationmodelsfrom two, 5-yearCCM3 simulations.Units are in K.
14,886
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RRTMG -CCM3 SurfaceTemperatureDifference,ANN
80NF-'
"'--'"---,-•-••
f_
•
•
r'"'•.,,
•
'?
•
•
•"--
-""-"• %•----, •
20S --- '• •'• 8os '-• r-
,
!
18( IW
120W
b
•-••'
,
:
'• •
I,
!
!
60W
0
60E
_••
-•-•.
•---
60N ..•."
ß
120E
18!
.._-
•'
•._
•
•
60S
--. -
L --'---'---• • •
180W
• •
120W
C
--
• -.•'.c• -
o•
80N
_.,2'"
RRTMG- CCM3SurfaceTemperature Difference, JJA
•ON•
80S •
--
60W
0
60E
120E
180E
RRTMG - CCM3 SurfaceTemperatureDifference,DJF
- .....
_"•:.z•.:":' ......
':.•.''"'."':
'•.•''"'..: ....:
60N 40N
20N 0
20S 40S
60S 80S
120W
180W
60W
0
60E
120E
180E K
-8
-6
-4
-2
0
2
4
6
8
Plate 6. Surface temperature differencebetween two CCM3 simulationsusingRRTMG and the CCM3 longwaveradiation model for (a) the annual mean, (b) the June, July, and August mean, and (c) the December, January,and February mean. Units are in K.
IACONO ET AL.: VALIDATED
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14,887
(RRTMG - CCM3)/CCM3 SpecificHumidityPercent Difference
200
4OO
6O0
8OO
lOOO
80N
60N
40N
20N
0
MAX:22,89
-40
-30
-20
MIN:-30.26
-10
20S
40S
60S
80S
GLOBAL:-2,10
0
10
20
30
40
Plate 7. Annual mean, zonal averagespecifichumidity differencebetweenRRTMG and CCM3 longwave radiation modelsfrom two, 5-year CCM3 simulations.Units are in percent.
earlier, the application of a data-validated,accurate radiative transfermodel suchas RRTMG to a GCM providesa foundation on which the dynamicalcomponentscan be more effectivelyanalyzedand improved.
5.
Application to Weather Forecast Models The
ECMWF
weather
forecast
model
has been
used
to
examine the effect of RRTMG on short-term temperature forecastsand seasonalsimulations[Morcretteet al., 1998].Two setsof 12 T213, 31-layerexperimentsstartingfrom the 15th of eachmonth betweenApril 15, 1996 and March 15, 1997, were performed. A compositeof the resultsfrom each month was computedfor each longwavemodel. The 10-day temperature forecast errors averagedover the Northern Hemisphere for four atmosphericlayers due to each LW model is shown in Figure 9. At 850 mbar the enhancedcontinuumabsorptionin RRTMG (solid line) warms the lower atmosphereand prevents the cooling seenover time in the ECMWF LW scheme (dashedline). The oppositeoccursat 500 mbar where the model produces too much cooling when running with RRTMG. This indicatesa deficiencyin some aspectof the ECMWF model, not in RRTMG. At higher levels, RRTMG has a largelypositiveimpact by coolingthe atmosphereand preventingwarm temperaturesfrom developing.In these experiments,RRTMG had relativelysmalleffectson the geopotential height and wind fields in the ECMWF model. Similar temperature impactsare evident in the zonal averagetemperaturefield computedby the ECMWF model.A pair of four-month simulationswere performed, and the results presentedin Plate 8 are averagesover the last three monthsof each simulationfor JJA 1987 and DJF 1987-1988. Tempera-
tures are shownas differencesfrom the ECMWF Reanalysis (ERA) temperaturefields for each season.Plates8a and 8b show differences for the ECMWF model running with RRTMG,
and Plates 8c and 8d show differences for the cur-
rent operational ECMWF model. For the JJA season, RRTMG slightlyenhancestemperaturesin the lower tropospherein the Northern Hemisphereand reducesthe difference from ERA. Cooling due to RRTMG in the upper troposphere and stratospherereducesthe excesswarminggeneratedby the ECMWF model, especiallyin the tropics.In the DJF season, RRTMG somewhatincreasesthe temperature errors near the surfaceat high latitudeswith little changein the tropics,while a reduction in temperature is seen in most latitudes in the upper troposphere and stratosphere.Figure 9 and Plate 8 demonstratethat the improved accuracyof RRTMG significantlyimpactsGCM temperaturesover time periodsof importance to short-term and medium-rangeweather forecasts.
6.
Conclusions
One objective of the ARM program is to support the improvementof GCM radiativetransfermodelsfor the benefit of climateresearchand weatherforecastingapplications.For this purpose,a radiation model, RRTM, hasbeen developedwhich providesthe capabilityof radiativetransferfor GCMs at a high level of accuracythat is establishedby comparisonto radiance measurements.The longwave version of RRTM has been adapted for and applied to the NCAR climate model and the ECMWF weather forecastmodel to examine the feasibility of its application to GCMs and to determine the impact of the improved accuracyof RRTMG on GCM simulations. We have shownthat RRTMG can have a substantialimpact
14,888
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z
z
z
z
z
z
I I 0
0
0
0
0
o
z
z
o
z
z o
LUI I I I
z
Ol
t
1 I l1 1 1 I I
z
z
t 0
0
0
0
0
0
0
0
(qtu) 3•n•-•3ud
o o
0 0
0 0
0 0
(qtu)3Ufl•31:ld
0 0
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14,889
ECMWF Northern Hemisphere Temperature Forecast Errors 50 mb
200 mb
0.6
0.6
0.4
0.4
,o o.2
,o 0.2
o.o
o.o
-0.2
-0.2 EC_LW
-0.4
-0.4
RRTMG
-0.6
0
1
2
3-- 4
5
6
7
8
9
0
0
1
2
3
Forecast Day 500
4
5
6
7
8
9
10
7
8
9
0
Forecast Day
mb
850 mb 0.6
ø6f ,o
0.2
,o
0.2
•
0.0
•
0.0 •
• -02' I
•- -0.2 x....
E
-04
-0.4
-0.6
-0.6
0
1
2
3
4
5
6
7
8
9
10
0
1
Forecast Day
2
3
4
5
6
Forecast Day
Figure 9. Ten-day temperatureforecasterrors for the Northern Hemisphere averagedover 12 months as computedby the ECMWF weatherforecastmodelusingthe ECMWF operationalLW model (EC_LW) and RRTMG.
Errors are shown for 50, 200, 500, and 850 mbar.
on the energybudgetof a GCM at both the surfaceand the top well as for the integrated quantities. This assuresthat the of the atmosphere.The CKD water vapor continuum model physicalprocessesassociatedwith variousatmosphericspecies includesthe important foreign-broadenedcontinuumabsorp- are properly accountedfor and the distribution of radiative tion in the 200-600 cm i spectralregionthat is not included energy is correctly modeled acrossthe LW spectrum. The in the RSB continuumformulation usedin many GCMs. The relevanceof applyinga spectralperspectiveto the analysisand resulting enhanced absorptionsignificantlyaffects LW fluxes improvementof GCMs hasbeen demonstratedin severalstudand cooling rates. Relative to current CCM3 energy budget ies [e.g., Slingoand Webb,1997]. Applying a radiation model quantities, clear-sky OLR is reducedby 6-12 W m-2 in trop- that attainsthis level of accuracyto a GCM providesa founical latitudes,while downwardsurfaceflux is increasedby 8-15 dation on whichthe dynamicalGCM componentscanbe more W m-2 at highlatitudes. Thesechanges aresufficient to com- successfullyevaluated. pensatefor knownerrorsin the energybudgetof CCM3 [Kiehl The enhanced radiative transfer capability attainable with et al., 1998b].It is a significantresultof thiswork that the CKD RRTMG and the CKD water vapor continuum has been continuum model can greatly improve flux calculations in shownto be extremelyrelevantto the verificationandimproveGCMs that use older continuum methods. ment of GCMs, but they have evenwider applications.Recent By altering the longwavefluxesRRTMG modifiesthe radi- studieshave identified and attempted to quantify the magniative coolingrate profiles,whichin turn impact the thermody- tude of a positiveclear-skybias in the ERBE global radiation namic
structure
of both
the NCAR
climate
model
and the
ECMWF weather forecast model. In CCM3, lower tropospheric and surface temperaturesare increasedby 1-4 K, especiallyat high latitudes,largely due to the enhancementof downward
surface flux. These
increases address known
defi-
cienciesin the CCM3 atmosphericand surfacetemperatures [Hack e! al., 1998]. In addition, a CCM3 warm bias in the tropical stratospherecentered at 30 mbar relative to both the NCAR/NCEP and the ECMWF Reanalysesis substantially compensatedby changesin the temperaturefield producedby RRTMG. Finally, water vapor amountsare generallyreduced throughoutthe middle tropospherein the tropics.The extent to whichthe RRTMG thermodynamicchangesimpact the full dynamicalstate of CCM3 is currentlyunder analysis. It should be emphasizedthat RRTMG provides accurate fluxesand coolingrates in each of its 16 LW spectralbandsas
measurements
relative
to both
GCMs
and radiosonde
data
[Collinsand Inamdar, 1995;Hoet al., 1998;Slingoet al., 1998]. This bias, which is believed to result from errors in the selec-
tion of clear-sky scenes, is estimatedto be about4 W m-2. It is of great importancethat thesestudiesbe conductedwith the most accurate and physicallysound radiative transfer algorithms available,suchas that usedby Slingoet al. [1998]. This
ismadeevidentbythegloballyaveraged 3-5 W m-2 impactof the CKD water vapor continuumon clear-skyTOA fluxes.In addition,improvedclear-skyradiation is an imperativestarting point from which to studythe more complexissuesof cloud absorptionand cloud overlap.Finally, a data-validatedmodel, which maintains a high level of accuracywithin each of its spectralbands,is highly suited to the important objectiveof validating GCM simulationswith current and future satellite spectralradiation measurements.
14,890
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Acknowledgments. The authorswould like to thank J. Kiehl and J. Kiehl, J. T., J. J. Hack, G. B. Bonan,B. A. Boville,B. P. Briegleb,D. L. Hack of NCAR for their assistanceand for many helpful discussions Williamson, and P. J. Rasch,Descriptionof the NCAR Community Climate Model (CCM3), NCAR Tech. Note, NCAR/TN-420+STR, during the course of this project. We also thank the reviewersfor commentsand suggestions that improved the paper. This work was 152 pp., Natl. Cent. for Atmos. Res., Boulder, Colo., 1996. Kiehl, J. T., J. J. Hack, G. B. Bonan, B. A. Boville, D. L. Williamson, supported by the Department of Energy under grant DE-FG0293ER61549 and by the National Science Foundation under grant and P. J. Rasch, The National Center for AtmosphericResearch ATM-9812184. CommunityClimate Model: CCM3, J. Clim., 11, 1131-1149, 1998a. Kiehl, J. T., J. J. Hack, and J. W. Hurrell, The energybudget of the NCAR CommunityClimate Model: CCM3, J. Clim., 11, 1151-1178, 1998b. References Lacis, A. A., and V. Oinas, A description of the correlated kBarkstrom, B., E. Harrison, G. Smith, R. Green, J. Kibler, R. Cess,and distributionmethodfor modelingnongraygaseousabsorption,therthe ERBE Science Team, Earth Radiation Budget Experiment mal emission,and multiple scatteringin vertically inhomogeneous (ERBE) archivalandApril 1985results,Bull.Am. Meteorol.Soc.,70, atmospheres,J. Geophys.Res., 96, 9027-9074, 1991. 1254-1262, 1989. Bonan,G. B., The land surfaceclimatologyof the NCAR Land Surface Liang, X.-Z., and W.-C. Wang, Cloud overlap effectson generalcirculation model climate simulations,J. Geophys.Res., 102, 11,039Model coupledto the NCAR CommunityClimate Model, J. Clim., 11, 1307-1326, 1998.
Briegleb, B. P., Delta-Eddington approximationfor solar radiation in the NCAR Community Climate Model, J. Geophys.Res., 97, 76037612, 1992.
Briegleb, B. P., and D. H. Bromwich,Polar radiation budgetsof the
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