Impact of an improved longwave radiation model, RRTM, on the energy budget and thermodynamic properties of the NCAR

Transcription

1 JOURNAL OF GEOPHYSCAL RESEARCH, VOL. 105, NO. Dll, PAGES 14,873-14,890, JUNE 16, 2000 mpact f an imprved lngwave radiatin mdel, RRTM, n the energy budget and thermdynamic prperties f the NCAR cmmunity climate mdel, CCM3 Michael J. acn, Eli J. Mlawer, and Shepard A. Clugh Atmspheric and Envirnmental Research, nc., Cambridge, Massachusetts Jean-Jacques Mrcrette Eurpean Center fr Medium Range Weather Frecasts, Reading, England Abstract. The effect f intrducing a new lngwave radiatin parameteriatin, RRTM, n the energy budget and thermdynamic prperties f the Natinal Center fr Atmspheric Research (NCAR) cmmunity climate mdel (CCM3) is described. RRTM is a rapid and accurate, crrelated k, radiative transfer mdel that has been develped fr the Atmspheric Radiatin Measurement (ARM) prgram t address the ARM bjective f imprving radiatin mdels in GCMs. Amng the imprtant features f RRTM are its cnnectin t radiatin measurements thrugh cmparisn t the extensively validated line-by-line radiative transfer mdel (LBLRTM) and its use f an imprved and validated water vapr cntinuum mdel. Cmparisns between RRTM and the CCM3 lngwave (LW) parameteriatin have been perfrmed fr single atmspheric prfiles using the CCM3 clumn radiatin mdel and fr tw 5-year simulatins using the full CCM3 climate mdel. RRTM prduces a significant enhancement f LW absrptin largely due t its mre physical and spectrally extensive water vapr cntinuum mdel relative t the current CCM3 water cntinuum treatment. This reduces the clear sky, utging lngwave radiatin ver the trpics by 6-9 W m -2. Dwnward LW surface fluxes are increased by 8-15 W m -2 at high latitudes and ther dry regins. These changes cnsiderably imprve knwn flux biases in CCM3 and ther GCMs. At lw and midlatitudes, RRTM enhances LW radiative cling in the upper trpsphere by K d- and reduces cling in the lwer trpsphere by K d-. The enhancement f dwnward surface flux cntributes t increasing lwer trpspheric and surface temperatures by 1-4 K, especially at high latitudes, which partly cmpensates dcumented, CCM3 cld temperature biases in these regins. Experiments were perfrmed with the weather predictin mdel f the Eurpean Center fr Medium Range Weather Frecasts (ECMWF), which shw that RRTM als impacts temperature n timescales relevant t frecasting applicatins. RRTM is cmpetitive with the CCM3 LW mdel in cmputatinal expense at 30 layers and with the ECMWF LW mdel at 60 layers, and it wuld be relatively faster at higher vertical reslutin. 1. ntrductin A primary bjective f the Atmspheric Radiatin Measurement (ARM) prgram is t develp and test imprved radiatin cdes fr general circulatin mdels (GCMs) used fr climate research and predictin. A recent prduct f this effrt is the radiative transfer mdel RRTM [Mlawer et al., 1997]. Develped fr bth the lngwave and the shrtwave, RRTM utilies the crrelated-k methd fr radiative transfer [e.g., Gdy et al., 1989; Lacis and Oinas, 1991]. This apprach prvides a significant reductin in cmputatinal expense ver the highly accurate line-by-line radiative transfer mdel (LBL- RTM) [Clugh et al., 1992 (hereinafter referred t as CM); Clugh and acn, 1995] n which RRTM is based. By btaining the absrptin cefficients required fr RRTM frm the data-validated LBLRTM, a direct link is established be- Cpyright 2000 by the American Gephysical Unin. Paper number 2000JD /00/2000JD tween ARM radiatin measurements and a radiatin mdel that is sufficiently fast t perate within a GCM. This study incrprates the lngwave versin f RRTM int the NCAR CCM3 climate mdel t establish the feasibility f its use within a GCM and t determine its impact n the simulated climate system. Additinal experiments have been perfrmed in cllabratin with the Eurpean Center fr Medium-Range Weather Frecasts (ECMWF) in which the impact f RRTM n shrt-term weather frecasts is examined [Mrcrette et al., 1998]. Several recent prgrams have cmpared GCM lngwave (LW) radiatin mdels and have shwn them t prduce substantial disagreement. The ntercmparisn f Radiatin Cdes in Climate Mdels (CRCCM) examined ver 30 radiatin mdels fr a variety f atmspheric prfiles and discvered a significant range f W m -2 amng the mdels in clear and cludy sky LW fluxes [Ellingsn et al., 1991]. Over 30 GCMs cntributed t phase 1 f the Atmspheric Mdel ntercmparisn Prject (AMP) in which 10-year simulatins 14,873

2 14,874 ACONO ET AL.: VALDATED LONGWAVE RADATON GCM MPACTS were perfrmed using prescribed, mnthly varying, sea surface temperatures [Gates, 1992]. Ten mdels frm this grup have been shwn t prduce clear-sky utging lngwave fluxes that regin, the self-cntinuum treatment in CKD prduces a similar result t RSB, thugh there are imprtant spectral differences. CM demnstrated the imprtance f the freignvary by 15 W m -2 ver a wide range f cean temperatures bradened water cntinuum absrptin (due t water-n2 and [Duvel et al., 1997]. An analysis f LW and shrtwave clud radiative frcing frm these simulatins shws intermdel variwater-o2 cllisins) in the cm - spectral regin (which is absent frm RSB), especially in the upper trpatins f W m -2 [Cess et al., 1997]. Fr cmparisn, sphere. The validity f the CKD freign cntinuum has als mre accurate line-by-line mdels agree t within 1 W m -2 and thus prvide an effective fundatin n which t build the been established in the 1600 cm - spectral regin [Tbin et al., 1996]. The CKD cntinuum mdel is an integral cmpnent f next generatin f GCM lngwave mdels. This is the mti- bth LBLRTM and RRTM and cntinues t be validated vatin fr the develpment f RRTM and its applicatin t against surface spectral radiance measurements. The impr- GCM simulatins. tant differences between the CKD and the RSB water vapr Observatinal prgrams are cntinually imprving ur abil- cntinuua, which significantly cntribute t the LW mdel ity t diagnse sme f the GCM lngwave mdel deficiencies. differences presented in this study, are discussed in mre detail Satellite measurements frm the Earth Radiatin Budget Experiment (ERBE) [Barkstrm et al., 1989] are widely used fr in CM. n additin t the present wrk, ther studies have demnstrated the imprtance f including the CKD water cmparisn t tp f the atmsphere (TOA) fluxes frm GCMs. Fr example, the AMP mdel cmparisns f Duvel et al. [1997] shwed that many f the LW mdels tested verestimate the clear-sky utging LW radiatin (OLR) by as much as 10 W m -2 relative t ERBE. A recent cmparisn between CCM3 and ERBE shwed this GCM t verestimate OLR by as much as 5-10 W m -2 in sme trpical regins [Kiehl et al., 1998b]. Hwever, while the spectrally averaged ERBE data prvide valuable bservatins f the glbal TOA energy balance, they d nt prvide imprtant infrmatin n the partitining f the utging energy acrss the LW spectrum caused by the varius atmspheric physical prcesses. Thus they cannt be used t evaluate fully a LW mdel's ability t crrectly represent these prcesses. Future satellite-based instruments (e.g., ARS, AS) will prvide this capability with spectral radiatin measurements frm space. This infrmatin is currently available fr selected infrared and micrwave channels frm the High Reslutin nfrared Radiatin Sunder (HRS) and Micrwave Sunding Unit (MSU) measurements. New studies are using these data t validate GCM LW radiatin mdels as part f the secnd phase f AMP (G. Stephens and J. Bates, persnal cmmunicatin, 1999). Spectral radiance measurements at the surface such as thse frm the ARM clud and radiatin test bed (CART) sites [Stkes and Schwart, 1994] r the Surface Heat Budget f the Arctic Ocean (SHEBA) ce Statin [Mrit and Pervich, 1996] are playing an imprtant rle in highlighting current radiatin mdel deficiencies. n particular, LBLRTM radiances have been extensively validated against bservatins frm the ARM CART sites and frm Antarctica [Walden et al., 1998]. n additin, SHEBA data have made key cntributins t the nging validatin f the water vapr cntinuum mdel f Clugh et al. [1989 (hereinafter referred t as CKD)] fr lw water atmspheres [Tbin et al., 1999]. Additinal measurements f dwnward lngwave radiatin at the surface, especially ver plar areas, have shwn that many GCM radiatin mdels greatly underestimate this quantity [Garratt et al., 1998; Pint et al., 1997]. One f the mst imprtant remaining differences amng line-by-line mdels and the bradband LW radiatin mdels used in GCMs is the treatment f the water vapr cntinuum absrptin. A number f GCM radiatin cdes still use the cntinuum frmulatin f Rberts et al. [1976(hereinafter re- ferred t as RSB)]. This mdel nly includes the effect f the self-bradened water cntinuum (due t cllisins between water mlecules) in the cm - windw regin where its radiative impact is largest. ntegrated ver this windw cntinuum absrptin in climate mdels [e.g., Schwarkpfand Ramaswamy, 1999]. n sectin 2, RRTM and the CCM3 lngwave mdel are described in greater detail, and the mdificatins made t RRTM t prepare it fr implementatin int the GCM are discussed. Single-clumn cmparisns f the radiatin mdels are presented in sectin 3 fr tw clear-sky prfiles and several cludy-sky cases. The 5-year CCM3 climate simulatins using bth LW mdels are described in sectin 4. Bth the initial radiative frcing effect f intrducing RRTM int CCM3 and its impact n the energy budget and thermdynamic prperties f the climate mdel are presented. Sectin 5 cntains a summary f the applicatin f RRTM int the ECMWF weather predictin mdel t demnstrate the use and effect f RRTM n shrt-term GCM simulatins. The paper cncludes with a discussin f the wrk presented. 2. Mdel Descriptins 2.1. RRTM Since line-by-line radiative transfer mdels are inapprpriate fr use in GCMs due t their lengthy executin time, a radiatin cde that effectively reprduces the fluxes and cling rates f a line-by-line mdel, RRTM [Mlawer et al., 1997], has been develped fr the ARM prgram. Althugh RRTM uses the crrelated-k methd fr radiative transfer, the lineby-line apprach played a significant rle in its develpment thrugh the utiliatin f the cmprehensive LBLRTM mdel [CM; Clugh and acn, 1995]. LBLRTM is used bth t cmpute the absrptin cefficients used t generate the k distributins needed by RRTM and t evaluate the RRTM calculatins f fluxes and cling rates. t is f critical imprtance in this cntext that LBLRTM cntinues t be extensively validated against high-reslutin, spectral measurements frm aircraft and the surface (CM). ts verall accuracy is within 2 W m -2 f bservatins, which als reflects current limitatins in the accurate specificatin f atmspheric state (S. A. Clugh et al., manuscript in preparatin, 2000). This prvides an evaluatin f the rapid mdel that is traceable thrugh the cmpar- isn f LBLRTM with bservatin. The standard versin f RRTM was created as a stand-alne LW radiatin mdel and uses 16 spectral bands that were carefully selected t highlight imprtant mlecular absrptin features. The essence f the k-distributin technique invlves rerdering the LBLRTM-generated absrptin cefficients as a functin f wavenumber, k(v), int ascending rder fr each spectral band. Frm this a smthly increasing representatin

3 ACONO ET AL.: VALDATED LONGWAVE RADATON GCM MPACTS 14,875 f absrptin cefficient is prduced as a functin f cumulative prbability, k(#), fr each layer, which can be integrated with a relatively small set f pints. RRTM uses 16 subintervals, r # pints, in each spectral band t calculate radiance frm the k (9) functins. This prduces a ttal f 256 radiative transfer peratins acrss the LW spectral regin ( cm- ). Gaseus absrptin is included fr H20, CO2, 03, CH4, N20, 02, N2, CFC-11, CFC-12, CFC-22, and CCL 4. n additin, the water vapr cntinuum is treated fllwing the apprach f Clugh et al. [1989] which incrprates bth selfbradening (cllisins between water mlecules) and freign cntinuum includes nly the self-cntinuum absrptin and is limited t the 8-12/ m windw regin. CCM3_LW uses the RSB self-cntinuum and scales the resulting absrptin cefficients t represent the freign cntinuum in this spectral regin. Nevertheless, an imprtant remaining difference between the CKD and the RSB water vapr cntinuua is the effect f freign bradening absrptin in the energetically imprtant cm - spectral regin, which significantly cntributes t the results f this study. CCM3_LW includes a bradband emissivity frmulatin fr absrptin by liquid and ice cluds. The shrtwave radiatin mdel in CCM3, which bradening (cllisins f water with N 2 and 02 mlecules) parameteries absrptin and scattering with a 5-Eddingtn acrss the full LW spectrum. Surface-based spectral radiance bservatins are regularly used t validate and t update this water vapr cntinuum [e.g., Tbin et al., 1999]. Of critical imprtance is the prper partitining f the lcal line cntrischeme, has been used withut mdificatin. n CCM3 the standard interval at which the lngwave absrptivities and emissivities are cmputed frm the current atmspheric state is 12 hurs. They are reused every mdel hur t calculate butin and the cntinuum cntributin t the k distributins lngwave fluxes and cling rates frm updated temperatures such that their cmbinatin prvides the crrect result. RRTM and clud amunts. Fr the present analysis the absrptivities als has the capability f including spectral surface emissivity and emissivities are updated every three hurs t mre cmand reflectance in each band. Finally, several ptins are avail- pletely represent the diurnal cycle. able fr LW clud liquid water and ice absrptin in RRTM [Mlawer and Clugh, 1997], including the bradband effective 2.3. RRTM CCM3 mplementatin emissivity methd used in the NCAR CCM (see sectin 2.2) A number f mdificatins have been made t RRTM t and multispectral appraches fllwing the wrk f Ebert and Curry [1992] and Hu and Stamnes [1993]. The k-distributin technique, the cnsequent errrs intrduced by this apprximate treatment f radiative transfer, and cmparisns between RRTM and LBLRTM calculatins are all discussed in greater detail by Mlawer et al. [1997]. RRTM is enhance its cmputatinal efficiency fr its applicatin t general circulatin mdels. Since many f the spectral bands cntain minimal absrptin r very little energy, fewer than 16 9 pints can be used t determine the flux adequately in these bands. A significant decrease in cmputatinal expense has been attained by reducing the number f 9 pints t 140 frm able t reprduce, fr a wide range f atmspheric cnditins, the standard RRTM ttal f 256. Each band was treated indi- LBLRTM net flux within 1 W m 2 at any altitude, cling rate vidually t determine the minimum number f 9 pints needed in the trpsphere and lwer stratsphere within 0.07 K d t retain the desired level f accuracy. The revised number f and upper stratsphcric cling rate within 0.75 K d-. RRT subintervals varies frm 14 r 16 7 pints in the fur imprtant prvides a substantial reductin in cmputatinal expense relative t LBLRTM that results frm reducing the radiative water vapr and carbn dixide bands frm 250 t 820 cm t as few as 2 9 pints in the three high wavenumber bands frm transfer peratins frm f-10 ' t 256, since the detailed spec t 3000 cm-. The effect n verall accuracy is less than tral infrmatin is nt required. A benefit f the k-distributin technique ver many GCM LW radiatin mdels is its near linear scaling f cmputatinal expense with the number f mdel layers. Sme LW mdels, such as that in CCM3, scale as the square f the number f layers and wuld prve prhibi- 0.5 W m-: fr net flux and 0.1 K d- fr cling rate, while the cmputatinal expense is reduced by abut 40%. Further efficiency imprvements were achieved by simplifying specific elements f the RRTM algrithm. Fr example, an expnential is required t cmpute transmittance frm ptical depth in tively expensive at high vertical reslutin. Finally, a shrtwave versin f RRTM has als been develped, thugh it has nt been incrprated int this study. The lngwave, stand-alne versin f RRTM is available frm the AER Web site at NCAR Cmmunity Climate Mdel Fr ur analyses we used the NCAR CCM3 general circulatin mdel versin CCM3.6.6 [Kiehl et al., 1996, 1998]. This mdel was selected because it is in wide use by the cmmunity, has successfully simulated climate parameters, has a mdular frmat that facilitates mdificatin, and is adaptable t several cmputer platfrms. The standard CCM3 frmat includes 18 vertical layers and a T42 hrintal reslutin (-2.8 x 2.8ø), which has nt been changed fr this study. The CCM3 lngwave parameteriatin (hereinafter CCM3_LW) utilies a bradband, nnisthermal absrptivity frmulatin with six verlapping spectral intervals fr H20, CO2, and 03. Als included are the effects f the H20 cntinuum f Rberts et al. [1976], absrptin frm the trace gases CH4, N20, CFC-11, and CFC-12, and the radiative prperties f the tw minr CO2 bands at 9.4 and 10.4/ m. The riginal RSB water vapr the standard RRTM. Fr the GCM versin f RRTM (hereinafter referred t as RRTMG) this expense has been transferred frm the radiative transfer sectin f the mdel t an initialiatin rutine where a 5000-pint transmittance lk-up table is cmputed. This prvided an additinal 10% reductin in cst with negligible effect n accuracy. The flux integratin in RRTMG is accmplished with a diffusivity angle fr cnsistency with CCM3_LW. While faster than the mre precise three-angle integratin in the standard RRTM, this singleangle apprach intrduces an errr f the rder f 1 W m-: in cluds. T prvide further timing reductin n vectriing cmputers, RRTMG has been receded t enhance its vectriability. Finally, the standard RRTM uses a Pade apprximatin t represent the Planck functin variatin with ptical depth in a layer similar t the methd f CM. RRTMG incrprates an accuracy imprvement by applying a lk-up table fr this Planck functin transitin that is based n the "linear in r" apprximatin (CM). Additinal mdificatins prvide enhancements relative t CCM3 LW. First, RRTMG functins ver a full additinal layer relative t the CCM layering. This layer extends frm the CCM mdel tp pressure at 2.9 mbar t the tp f the atm-

4 14,876 ACONO ET AL.' VALDATED LONGWAVE RADATON GCM MPACTS 8.) Fin BNDin Agas Fut Anld,. BNDut b), Fin,, BNDin Fut cid BNDut Figure 1. Diagram f the clud and gaseus verlap absrptin within a layer as frmulated in the (a) CCM3 lngwave radiatin mdel and (b) RRTMG lngwave mdel. sphere t prvide values fr bth the dwnward flux at 2.9 mbar and the utging lngwave radiatin. Mixing ratis in this extra layer are identical t thse in the CCM tp layer except the ne mixing rati, which is reduced by a factr f 0.6. The temperature f the extra layer is fixed t be 18 K higher than the CCM tp mdel layer. Bth changes are based n the U.S. standard atmsphere at this level. Fr cnsistency with CCM3_LW, surface emissivity is handled identically t its applicatin in the CCM3 land surface mdel (i.e., it is generally ne with a few nn-ne values in specific areas). Aersl absrptin effects in the lngwave have nt been included. Clud radiative effects are mdeled within RRTM t reprduce their cntributin t the cling rate prfiles as clsely as pssible t their effects in CCM3_LW. Our ratinale is t examine the feedback effects due t the mre accurate clear- and emissin by the clud itself. n effect, the clud is present nly as a thin band at the utging bundary f the layer and the radiance emitted by the clud is nt attenuated by the gas in that layer. Fr a cntiguus grup f multilayer cluds the surce radiance due t a clud in a layer is nt attenuated by gas absrptin in any layer within this grup f cluds. This radiance is, hwever, attenuated by clud absrptin in these layers. Als, because the clud is treated as a thin band alng the utging layer bundary its emitrance is derived frm the Planck functin at the layer bundary. Fr a clud that is assumed t fill a layer, a mre physically apprpriate chice fr calculating the clud emitrance is t use a Planck functin, Bgas+cld, with an effective radiative temperature dependent n the cmbined gas and clud ptical depth. Figure 1 illustrates these issues fr dwnwelling flux thrugh a layer with a clud fractin f 1. n Figure l a the flux exiting the bttm f the layer as cmputed in CCM3_LW can be expressed as Fut = F ntgastcl d 4- BgasAgasTcld 4- BcldAcl d (3) where the first right-hand term is the flux entering the tp f the layer, Fin, attenuated by the gaseus transmittance Tgas and the clud transmittance rcl d. The secnd term represents the clud attenuatin f the radiatin emitted by gas in the layer, where Bgas is the effective Planck functin fr the emitting gas and A gas ( = i -- Tgas ) is the gaseus absrptivity. Nte that in the third term, the clud emissin in the layer, which is the prduct f the effective Planck functin fr the clud, B and clud absrptivity, Acta (=% a = 1 - Tga) is nt attenuated by gas in the layer. Als, nte that B c a is cmputed at the lwer bundary temperature. The LW clud absrptin in RRTM prvides a mre physical slutin by making use f the k-distributin capability f effectively cmbining gaseus and clud absrptin within a layer. As in CCM3_LW, clud emissivity is assumed t be cmpletely gray, fllwing (2). Fr the example described abve, the cmparable expressin fr dwnward flux exiting a cludy layer as calculated by RRTM is illustrated in Figure lb and is given by Fur-- FinTgasTcld 4- Bgas+cldAgas+cld. (4) sky radiative transfer withut significant changes in the clud The attenuatin f the flux entering the tp f the layer is treatment. Hwever, the treatment f lngwave clud absrptreated identically as in (3). Hwever, in this frmulatin the tin has been revised t prvide a mre physically apprpriate cmbined gas and clud radiatin frm the layer, B gas+do, is slutin than that used in CCM3_LW. This imprvement utiattenuated in the layer by bth gas and clud, where A gas+c d = lies the ability f the RRTM k-distributin apprach t treat i -- TgasTc d. The impact f this change is mst prnunced fr unifrmly the cmbined gaseus and clud absrptin within a cluds that extend cntinuusly thrugh many layers and has a layer. At present, CCM3_LW uses the "effective emissivity" magnitude f as much as several W m -2. This is illustrated fr methd fr clud absrptin [Kiehl et al., 1996] in which an a few single-clumn, cludy-sky cases in sectin 3. effective clud fractin is defined as the prduct f the clud An imprtant cnsideratin in the calculatin f LW radiaemissivity and the cmputed clud fractin Ac, tin thrugh multiple cludy layers is the vertical relatinship, A; = ec 0A. (1) r clud verlap, amng the clud fractins in each layer. Many GCMs apply randm clud verlap in which cluds in The clud emissivity is given as adjacent layers are assumed t be unrelated. The CCM3_LW /3cld = 1 - e-d <abscwp mdel and RRTMG were develped using a randm clud, (2) verlap assumptin, and this is used in the CCM and ECMWF where D is the cnstant diffusivity factr f 1.66, <abs is the simulatins described in sectins 4 and 5. Hwever, several cmbined liquid and ice phase LW clud absrptin ceffi- recent studies [Jakb and Klein, 1999; Liang and Wang, 1997] cient, and CWP is the clud water path. have demnstrated the imprtance f the clud verlap treat- A cnsequence f the CCM3_LW frmulatin fr upward ment t GCM simulatins. Als, many f the GCMs cntriband dwnward flux within a cludy layer given by Kiehl et al. uting t AMP use a mixed clud verlap methd in place f [1996] is that the gaseus and clud absrptin in a layer are the simpler randm methd [Phillips, 1994]. Fr this reasn a treated sequentially. That is, the absrptin and emissin by maximum/randm clud verlap apprach has been develped gas within a clud are treated as ccurring prir t absrptin fr RRTMG t mre realistically represent the vertical rela-

5 ß ACONO ET AL.' VALDATED LONGWAVE RADATON GCM MPACTS 14,877 tinship f multilevel cluds fr situatins in which they verlap unifrmly (e.g., deep cnvectin), while maintaining a randm verlap cmputatin fr nnadjacent clud layers. The effect f this imprved clud verlap apprach n the applicatin f RRTMG t GCMs is currently being examined. n terms f cmputatinal expense, RRTMG is cmpetitive with the CCM3 LW mdel n vectriing cmputers and is substantially faster n scalar machines. On a vectriing platfrm, RRTMG is apprximately 4 times faster than CCM3_LW fr a cstly full radiatin time step (dne every 12 mdel hurs) fr 30 layers and is 5 times slwer fr a less cstly radiative transfer time step (dne every intervening mdel hur). Fr a full mdel day, RRTMG runs abut 7-8% slwer than CCM3_LW n a vectriing system (such as CRAY) and abut 40-50% faster n a scalar system (such as SG) at 30 layers. At higher vertical reslutin, RRTMG wuld gain significant advantage ver CCM3_LW due t the linear relatin between expense and the number f layers prduced by the k-distributin technique. Fr GCMs that cmplete a full radiatin calculatin mre than twice per day and have a similar expense t CCM3_LW, RRTMG wuld be even mre cmpetitive. Althugh it wuld be pssible t imprve the cmputatinal time by decreasing the number f g pints further, the resulting lss in accuracy wuld nt be acceptable frm ur perspective RRTM ECMWF mplementatin A versin f RRTMG has als been prepared fr use in the ECMWF weather frecast mdel that is substantially similar t the versin tested in the NCAR climate mdel. Additinal mdificatins fr ECMWF include implementing the capability fr spectral surface emissivity and reflectance and spectral clud absrptin prperties [Mreterret al., 1998]. The surface emissivity has been implemented using a windw (8-12 / m) and a nnwindw value, thugh the capability exists in RRTM t use a separate emissivity in each f the 16 spectral bands. Als fr each spectral interval, ice clud ptical prperties are based n Ebert and Curry [1992] and water clud ptical prperties are derived frm Fuquart [1987]. Finally, this versin f RRTM incrprates aersl absrptin by specifying apprpriate radiative characteristics fr fur different aersl types fr each spectral band. Results frm this mdel are shwn in sectin Clumn Radiatin Mdel Cmparisns Single-clumn cmparisns between RRTMG and CCM3_LW were accmplished with the NCAR Clumn Radiatin Mdel (CRM). This prvides a straightfrward cntext in which t establish the flux and cling rate differences between the tw radiatin mdels. The CRM is a stand-alne LW and SW radiatin package that is identical t the radiatin mdels used in CCM3. t has been mdified t run with either CCM3_LW r RRTMG. Fr these cmparisns, standard midlatitude summer (MLS) and subarctic winter (SAW) atmspheric prfiles were prepared fr the same 18 layers used in CCM3. Figure 2a shws the upward, dwnward, and net fluxes fr the clear-sky MLS atmsphere as cmputed by the CCM3 LW mdel n the left. The flux differences between RRTMG and CCM3_LW are shwn t the right. Fr this atmsphere the upward LW flux is reduced by as much as 9 W m -2 frm the middle trpsphere t the tp f the atmsphere. The dwnward flux is increased thrughut the trp- a ' 400 tu CCM3 Mid-latitude Summer Lngwave Fluxes "... ' ' ' ' i... 'u ' ' ' DOWN \ \ \ \ ';'t NET \\ \\\\ '!,....?...,', FLUX (W m '2) CCM3 Mid-latitude Summer Lngwave Cling Rate b 2OO 400 6øø f 800 f COOLNG RATE (K d ' ) RRTMG-CCM3... i i... i FLUX DFF. (W m '2) 2 RRTMG-CCM CR DFF. (K d '1) Figure 2. Midlatitude summer upward, dwnward, and net fluxes (a) as cmputed by the CCM3 lngwave mdel, with differences between RRTMG and CCM3 shwn t the right. Midlatitude summer cling rate (b) as cmputed by the CCM3 lngwave mdel, with differences between RRTMG and CCM3 t the right. sphere by as much as 16 W m -2 at 650 mbar and by a mre mdest 4 W m -2 at the surface. Althugh the change in the LW algrithm causes sme f this difference, mst f this dramatic alteratin in LW flux is due t the enhanced absrptin f the CKD water vapr cntinuum mdel in RRTMG relative t the RSB cntinuum in CCM3_LW. The principal rle f the freign cntinuum in this cntext is t increase absrptin between the mlecular lines, t decrease the pressure regime ver which the transitin frm atmspheric pacity t transparency ccurs, and thus t increase the flux divergence and cling rate. Clser t the surface the greater atmspheric pacity in the spectral regin in which freign bradening is significant, diminishes the impact f the cntinuum differences between CKD and RSB, and the flux divergence is reduced. Since cling rate (Figure 2b) is determined frm the divergence f the net flux acrss a layer, the net flux changes prduced by RRTMG significantly mdify the MLS clear-sky LW cling rate as seen t the right in Figure 2b. Cling rate is enhanced in the upper trpsphere by as much as 0.4 K d- and is als slightly enhanced in the lwer stratsphere. Because f the decrease in flux divergence in the lwer atmsphere, the cling rate is reduced here by as much as 0.5 K d-. Evidence fr the rle f the water cntin- uum in these cling rate changes can be seen in Figure 19 f CM. This plt presented cling rates fr the MLS atmsphere fr tw line-by-line calculatins that used the CKD and

6 14,878 ACONO ET AL.: VALDATED LONGWAVE RADATON GCM MPACTS Z Z c

7 ACONO ET AL.: VALDATED LONGWAVE RADATON GCM MPACTS 14,879 RRTMG - CCM3 LW Cling Rate Difference (nitial Frcing). 2OO Ll 4OO Ll ,, '- ',. ', -,,,,---0 MN:-2.44 MAX: 0.40 K d ' ø Plate 2. Znal average, annual mean, initial frcing cling rate difference between RRTMG and CCM3 lngwave mdels frm a 5-year CCM3 simulatin. Units are in K d -. RSB cntinuua. Cling rate differences, inferred frm these calculatins, are very similar t thse shwn in Figure 2b. A cmparisn f the clear-sky LW fluxes and cling rates frm each radiatin mdel fr the SAW atmsphere is shwn in Figure 3. n this drier atmsphere, RRTMG reduces the upward LW flux and OLR by nly 2-3 W m -2 (Figure 3a). Hwever, the enhancement f dwnward flux is as high as 14 W m -2 and is shifted tward the surface. The lwer water cntent f the SAW atmsphere makes it less paque near the surface than mister prfiles. The CKD freign cntinuum absrptin, which is absent frm the RSB cntinuum, peaks in the cm - spectral regin where the atmsphere is nearly paque in high water atmspheres. n the trpics, fr example, this extra cntinuum absrptin has nly a small impact n the dwnward surface flux due t the high pacity in this spectral band. n drier regins, hwever, the atmsphere is less paque frm 400 t 600 cm - and the additinal freign cntinuum absrptin in RRTMG strngly enhances the dwnward LW flux t the surface. SAW cling rate (Figure 3b) is slightly enhanced by RRTMG by as much as 0.2 K d - in much f the lwer stratsphere and trpsphere except near the surface. Differences between the RRTMG and CCM3 LW MLS fluxes and cling rates fr three clud cases are illustrated in Figure 4. The RRTMG imprvement in LW clud absrptin discussed in sectin 2.2 is included in these cmparisns. Flux and cling rate differences have the same frmat and scale as thse in Figures 2 and 3. Layers cntaining clud are marked with black bars t the right f each plt. All three cases use a clud ptical depth, % d, f 5 in each layer. Fr a lw clud with a clud fractin, A c, f 0.5 (Figures 4a and 4b) the verall similarity f the clud absrptin treatments reduces the dwnward flux difference within and belw the clud. Since the large clear-sky differences remain abve the clud, the vertical divergence f net flux near the tp f the clud is increased, which prduces a greater reductin in cling rate at that level than the clear sky case (see Figure 2b). A multilayer clud with a unifrm A c f 0.5 in each layer reduces the flux and cling rate differences at all levels (Figures 4c and 4d) relative t clear sky. The 2-4 W m -2 flux differences within the clud are primarily due t the revisin f the cmbined LW clud and gas absrptin described earlier. The impact n flux and cling rate f a high clud with an A f 1.0, shwn in Figures 4e and 4f, results in reversing the clear-sky OLR reductin due t RRTMG. The extent t which this clear-sky OLR reductin is ffset by high clud is heavily dependent n the clud fractin, ptical depth, and height. These cmparisns will help clarify the interpretatin f the RRTMG glbal flux impacts described in sectin Climate Mdel Simulatins An assessment f the effect f RRTMG n the NCAR climate mdel was accmplished in tw stages. First, a versin f CCM3 was develped that perfrmed tw LW calculatins, ne with RRTMG and ne with CCM3 LW. The RRTMG fluxes and cling rates prvided the LW radiative feedback fr the simulatin, while the CCM3_LW quantities were stred as diagnstic utput. This apprach allwed the tw LW mdels t be cmpared fr identical atmspheric states that varied ver a full range f glbal cnditins. Flux and cling rate differences between the LW mdels can be interpreted in the

8 14,880 ACONO ET AL.' VALDATED LONGWAVE RADATON GCM MPACTS tlj LU 'r E 5 --! i,.- D Z Z Z Z M c:> Z Z Z Z i u,j 0 a:: x. Z 0 Z Z Z ø 03 (/> '

9 ACONO ET AL.: VALDATED LONGWAVE RADATON GCM MPACTS 14,881 same way as the single-clumn cmparisns in sectin 3 and represent the glbal, initial frcing impact f intrducing RRTMG int the climate mdel. Fr this study, the interval at which LW gaseus absrptin was cmputed was reduced frm the standard CCM3 interval f every 12 mdel hurs t every three mdel hurs t better simulate the diurnal cycle. LW fluxes and cling rates were then calculated at every mdel hur with updated temperature and clud infrmatin. A 5-year simulatin was perfrmed using mnthly averaged, climatlgical sea surface temperatures (SSTs) with all radiative and dynamical fields stred as daily averages. The simulatin was preceded by a 4-mnth adjustment perid during which the radiative feedback was gradually transferred frm CCM3_LW t RRTMG ver the first three mnths by applying a csine transitin functin t the cling rates and dwnward surface fluxes frm each LW mdel. The results f this test are presented in sectin 4.1. Fr the secnd phase, a 5-year, climatlgical SST cntrl simulatin was perfrmed using the riginal, unmdified CCM3 t prvide a basis fr cmparisn t the first simulatin. Since each experiment was run with a different LW mdel, differences in the radiative and dynamical fields between the tw simulatins include the changes in atmspheric state prduced by the tw LW methds. These differences, presented in sectin 4.2, are mre representative f the lng-term impact f RRTMG n the CCM3 climate mdel simulatin. 2OO CCM3 Sub-arctic Winter Lngwave Fluxes -\ /i... up 20 _\ /:: DOWN \ / NET \ \\ RRTMG-CCM3 400[/i, t 6OO 80O 2 FRMG-CCM ,,,,,,,,,t,,,l l,,,, RRTMG-CCM e. 60 RRTMG-CCM3 RRTMG-CCM3,< /,,,,_L,,,, 1000,,,, FLUX -10 DFF. 0 ( / FLUX DFF. 0 (W 10 m 20 2) FLUX DFF 0 ( m 20 2) 30 COOLNG RATE DFF. (K d '1) RRTMG-CCM3 COOLNG RATE DFF. (K d 'j) RRTMG-CCM3 'f' t COOLNG RATE DFF. (K d '*) Figure 4. Midlatitude summer flux and cling rate differences between RRTMG and CCM3 lngwave mdels fr three clud cases. Differences are shwn fr (a) flux and (b) cling rate with a lw clud, (c) flux, and (d) cling rate with a multilevel clud, and (e) flux and (f) cling rate with a high clud. The line styles used in Figures 4a, 4c, and 4e are identified in Figure 3a. Layers with clud are marked with black bars t the right in each figure. All cluds have an ptical depth f 5 in each layer. Figures 4a-4d are fr a clud fractin f 0.5 in each layer, and Figures 4e and 4f are fr a clud fractin f 1.0. b FLUX (W m 2) CCM3 Sub-arctic Winter Lngwave Cling Rate 20O m [ / ,,, FLUX DFF. (W rn 2) RRTMG-CCM COOLNG RATE (K d '1) CR DFF. (K d ' ) Figure 3. Subarctic winter upward, dwnward, and net fluxes (a) as cmputed by the CCM3 lngwave mdel, with differences between RRTMG and CCM3 shwn t the right. Subarctic winter cling rate (b) as cmputed by the CCM3 lngwave mdel, with differences between RRTMG and CCM3 t the right. t shuld be nted that the CCM3 glbal average tp f thc atmsphere (TOA) fluxes have been tuned by NCAR t agree with bservatins frm the Earth Radiatin Budget Experiment (ERBE) [Kiehl et al., 1998b] and t ensure a TOA radiative balance. Als, the utging lngwave radiatin changes prduced by RRTMG are f the same rder r larger than the magnitude f the changes intrduced by this tuning. Therefre since n attempt was made in the present study t retune the climate mdel when running with RRTMG, direct cmparisns t ERBE fluxes are nt included in this analysis. Changes t the CCM3 energy budget caused by RRTMG shuld be interpreted qualitatively in the cntext f the imprvement attained by RRTM as established thrugh its validatin path t bserved radiances. Kiehl et al. [1998b] give a thrugh descriptin f the CCM3 energy budget, including cmparisns f CCM3 LW fluxes t ERBE data nitial Frcing mpacts Differences in the annual average OLR and surface LW fluxes (RRTMG - CCM3_LW), which represent the initial frcing effect f RRTMG, are shwn in Plate 1. Clear-sky OLR (Plate la) is reduced by RRTMG mst extensively

10 14,882 ACONO ET AL.: VALDATED LONGWAVE RADATON GCM MPACTS thrughut the trpics, by as much as 9 W m -2, decreasing t 4.2. Climate Simulatin mpacts small changes ver the plar areas. The greatest OLR changes The RRTMG initial frcing flux and cling rate changes ccur in the wettest latitudes, which is cnsistent with the described in sectin 4.1 alter the atmspheric state thrugh significant enhancement f absrptin frm the CKD water feedbacks between the radiatin and the mdel dynamics until vapr cntinuum relative t the RSB frmulatin. The annual a new equilibrium state is reached. These feedback effects are mean, ttal sky OLR changes (Plate lb) are generally smaller, nw cnsidered by lking at differences between the since cluds reduce the LW mdel OLR differences as shwn RRTMG CCM3 simulatin and the cntrl CCM3 simulatin. in Figure 4. Exceptins are typically clear areas (e.g., Sahara Plate 3 shws the annual mean TOA and surface LW flux Desert) r regins where lw cluds dminate, such as the differences between RRTMG and CCM3 LW that include eastern midlatitude ceans. Such lw cluds are belw much f their respective changes in atmspheric state. The clr scale the enhanced absrptin frm the CKD freign cntinuum and the field shwn are the same as in Plate 1. The clear-sky RRTMG. Several trpical regins in Plate lb (e.g., west f OLR difference in Plate 3a shws cnsiderable decreases in Central America, central Africa, etc.) shw an increase ttal OLR due t RRTMG thrughut the lw and midlatitudes f sky OLR due t RRTMG. These are areas where high cluds 4-12 W m -2, with small increases at high latitudes. The ttal dminate in this simulatin. Fr these cluds the RRTMG sky OLR difference in Plate 3b cntains cnsiderable reginal imprvement in the -LW clud absrptin algrithm, discussed variatin with trpical decreases f as much as 18 W m -2 and in sectin 2.3, prduces an increase in OLR f several W m -2 increases f 11 W m -2. At the surface, mst f the widespread as demnstrated in Figure 4e. Small flux differences at high increase clear-sky dwnward flux at high latitudes f 8-15 W latitudes in Plate lb result frm cmbinatins f cluds at all m -2 remains (Plate 3c), while the ttal sky flux changes (Plate levels. 3d) are generally small with a few large reginal differences. n RRTMG has a substantial impact n clear-sky net radiatin general, the -LW flux differences in Plate 3 have greatereat the surface, shwn as an annual average difference in Plate ginal variability than the initial frcing impacts in Plate 1 lc (net flux is defined as upward minus dwnward radiatin). largely due t changes in atmspheric and surface temperature Since the LW mdels are being cmpared fr identical atm- and t the spatial and tempral redistributin f water vapr spheric states (including the surface temperatures) in this cn- and clud cver induced by the LW mdel change. text, the upward surface radiatin differences will be small and As mentined previusly, the CCM3 glbal mean TOA enmst f the flux changes shwn in Plate l c are due t an ergy budget was tuned t agree within a few W m -2 f ERBE increase in the dwnward radiatin tward the surface. The measurements. Despite this, reginal differences between largest increases ccur at high latitudes, ver deserts, and in CCM3 and ERBE remain, which can be inferred frm the muntainus areas where the atmspheric water vapr cntent OLR and clear-sky OLR cmparisns in Figures 15 and 16 f is lw. n these regins the dwnward flux is enhanced by Kiehl et al. [1998b]. Specifically, in many trpical areas, CCM W m -2. This cnsiderable impact is cnsistent with the verestimates the OLR and clear-sky OLR by 5-10 W m -2 discussin in sectin 3 f the flux changes fr the SAW atm- relative t ERBE. Over much f Antarctica, CCM3 utging sphere (Figure 3a). As nted earlier, many GCMs greatly un- flux is t lw. The RRTMG OLR impactshwn in Plates 3a derestimate the clear-sky dwnward surface flux in dry regins and 3b have the crrect magnitude and directin t ffsethese by W m -2. n particular, Pint et at. [1997] shwed that CCM3 flux biases. Kiehl et al. [1998b] suggest a deficiency in CCM2 dwnward surface fluxes ver the Arctic were t lw water vapr absrptin in CCM3 LW as ne cause fr the by 20 W m -2. t is a significant result f this wrk that the TOA flux biases in CCM3. Briegleb and Brmwich [1998a] als additinal absrptin prvided by the CKD water vapr cn- suggest insufficient water vapr rtatin band absrptin in tinuum accunts fr much f this discrepancy. The ttal sky, CCM3 t explain a W m -2 deficiency in-lw clear-sky net flux change at the surface in Plate l d shws cnsistency dwnward surface flux in plar latitudes. The result presented with Figure 4 in that surface flux differences are greatly rehere fr RRTMG is strng evidence that imprved absrptin, prvided by the CKD water vapr cntinuum mdel, makes a duced in cludy areas. Large differences remain ver land significant cntributin t the accurate representatin f LW areas that have the least cludiness this simulatin. Clearly, radiative prcesses f relevance t a GCM. RRTMG and the CKD water cntinuum have substantial im- The impact f RRTMG n the CCM3 energy budget can be pacts n clear-sky LW flux at bth the tp f the atmsphere summaried with annual mean, nal averages f the energy and at the surface that will significantly influence the energy balance f the climate mdel. budget cmpnents. Figure 5 presents the nal average OLR fr ttal sky (Figure 5a), clear sky (Figure 5b), and the LW The primary mechanism by which a -LW radiatin mdel clud frcing (Figure 5c) defined as the clear sky minus the impacts the atmspheric state f a GCM is thrugh the radittal sky OLR. RRTMG values (dashed lines) and CCM3_LW ative cling rate. The initial frcing effect f RRTMG n the values (dtted lines) use the scale t the left in each plt. The LW cling rate f CCM3 is shwn as a difference differences (slid lines) use the scale t the right. Bth clear (RRTMG - CCM3_-LW) in Plate 2. This and all subsequent and ttal sky OLR are reduced by RRTMG at nearly all figures that have pressure as the vertical axis use the CCM latitudes except fr the plar areas where small increases are hybrid sigma-pressure vertical crdinate times 1000 [Kieht et nted. RRTMG reduces clud frcing in the trpics and the at., 1998a]. RRTMG increases the cling rate frm the mid ø latitudes, where cludiness is generally grehtest. Aldle trpsphere t the lwer stratsphere, except fr the trp- thugh the CCM3 shrtwave (SW) mdel was used in bth ical trppause. Peak cling enhancement f 0.4 K d- ccurs simulatins, the annual mean, nal average TOA absrbed in the upper trpical trpsphere. Cling rates are reduced at SW fluxes, shwn in Figure 6, are impacted by RRTMG all latitudes in the lwer trpsphere with peak values away thrugh the changes in atmspheric state, primarily water vafrm the ples f K d- pr and clud cver. The ttal sky, TOA SW fluxes (Figure 6a)

11 ß ß ß ACONO ET AL.' VALDATED LONGWAVE RADATON GCM MPACTS 14,883 are increased by 3-4 W m -2, especially in the trpics and midlatitudes. Clear-sky TOA SW fluxes (Figure 6b) are nly negligibly altered, thus the SW clud-frcing change (Figure 6c) resembles the ttal sky difference. Znal average surface LW net fluxes, shwn in Figure 7, are als reduced at nearly all latitudes by RRTMG. The decreases in ttal sky flux at the surface (Figure 7a) are largest in midlatitudes. Large differences f up t 11 W m -2 are seen in the clear sky, surface net fluxes pleward f 50 ø in each hemisphere (Figure 7b). Because f changes in atmspheric state, these differences result frm a cmbinatin f dwnward and upward surface flux changes but are dminated by the increased dwnward flux at high latitudes. Surface clud frcing (Figure 7c) is largely decreased at middle and high latitudes due t the large clear-sky effect. The surface SW fluxes, nt shwn. are affected in a similar fashin t the TOA SW fluxes in Figure 6. Finally, the respnse f the nal average latent and sensible heat fluxes at the surface due t RRTMG are shwn in Figure 8. n general, latent heat (Figure 8a) and sensible heat (Figure 8b) changes and their cmbined change (Figure 8c) are less than 5 W m -2 at all latitudes. The annual mean, nal average difference in LW radiative cling rate is shwn in Plate 4. This is in cntrast t the initial frcing cling rate impact in Plate 2. Althugh the patterns arc very similar between Plates 2 and 4, several dramatic changes are nticeablc. First, the decrease in cling rate frmerly seen near thc surface at all latitudes has shifted upward, especially in the trpics. Here the reduced cling ccurs as 300 [: a 280 i RRTMG 260 t CCM3 Outging Lngwave Radiatin / i! / / x i RRTMG - CCM E O 20 m lo,, u- b Outging Lngwave Radiatin (Clear Sky) '.}-:?:V :... s ,, ß ,;'? ":4. v "< [. d' <'. k 20 m Z L \ 10 m 140 D 120 F_ N 60N 40N 20N 0 20s 40s 60S 80S c TOA Lngwave Clud Frcing..p ) '..:,,.....*"-" '-. 0, x O m -10 Figure 5. Znal average, annual mean (a) utging lngwave radiatin, (b) clear-sky OLR, and (c) tp f the atmsphere lngwave clud frcing fr RRTMG and CCM3 LW mdels (scales n the lcft) and fr the RRTMG-CCM3 difference (scales n the right). a TOA Absrbed Shrtwave Radiatin RRTMG _ RRTMG - CCM3 320 ½ 280 E ,,".\ -! _ / \ x 160 / ø,% /.,, d-'- 280 E D x 160 J LL CCM3...,,.,--' "",.,,,.,..,.- "'.,/------,,,., b TOA Absrbed Shrtwave Radiatin (Clear Sky) ½ TOA Shrtwave Clud Frcing 50 E O 20 m 10,, 0 X 5O 40 3O LU 0 2O m loll 0 X do,, -J 0.= 'x\.\ / _.10rm ) E '7... [ [ Z : /.,-' ',-.,,,,..-:.<.7...! -20 '"" F '." ', r' ' q :D "' " "- 1' mm x J -1 O LL Figure 6. Znal average, annual mean (a) tp f the atmsphere absrbed shrtwave radiatin, (b) clcar-sky TOAabsrbed SW, and (c) TOA SW clud frcing fr RRTMG and CCM3 LW mdcls (scales n the lcft) and fr the RRTMG- CCM3 difference (scales n the right). high as 400 mbar. A small area f greater cling near the surface in the trpics has expanded t nearly all latitudcs and extends t 800 mbar ver the equatr. Secnd, thc area f greatest cling in the upper trpical trpsphere has been reduced in magnitude frm 0.4 t K d. Finally, all f the cling rate differences in the stratsphere and at the trpical trppause have been cmpensated, mstly by changes in atmspheric temperature. The differences bctwccn Plates 2 and 4 serve as evidence that the ir/itial frcing cling rate changes due t RRTMG are f sufficient magnitude t mdify the thermdynamic structure f the climate mdel. As mentined earlier, the LW radiative cling rate is a primary mechanism by which the radiatin parameteriatin affects the climate mdel dynamics thrugh its impact n the thermal structure. The extent t which the atmspheric temperatures are mdified by RRTMG is shwn in Plate 5 as an annual mean, nal average difference. Because f the enhancement f dwnward LW radiatin t the surface seen in Plate 3, RRTMG warms the lwer atmsphere at all latitudes, except near the equatr, with the greatest warming ccurring at high latitudes. This 1-2 K warming partly ffsets the dcumented 2-4 K CCM3 cld bias at high latitudes in the lwer trpsphere [Hack et al., 1998; Briegleb and Brmwich, 1998b]. Temperature is reduced as much as 1.5 K acrss much f the middle and upper trpsphere, which cntributes t the CCM3 upper trpspheric cld bias. The large reductin in temperature centered at 30 mbar f 5-6 K is f the right magnitude and lcatin t entirely cmpensate a CCM3 warm bias at this

12 _... 14,884 ACONO ET AL.: VALDATED LONGWAVE RADATON GCM MPACTS E loo 8 12 loo a Surface Net Lngwave Radiatin RRTMG RRTMG - CCM3 CCM3...'> <... /.,,..),.½- v......,- ß ' k.->.-2.,...,. ',,.. = 80N 60N 40N 20N 0 b.a... 20S '40S '6;S '80S Surface Net Lngwave Radiatin (Clear Sky) c Surface Lngwave Clud Frcing ø 1Ou_ ø E 60 " - '......;"..";" x'.. '' rr X 40 ' " v.. 9 _0,,uJ u_ +....:' : 20 ' -lo -2 Figure 7. Znal average, annual mean (a) surface net lngwave radiatin, (b) clear-sky surface net LW radiatin, and (c) surface LW clud frcing fr RRTMG and CCM3 LW mdels (scales n the left) and fr the RRTMG-CCM3 difference (scales n the right). level relative t bth the ECMWF reanalysis and the NCAR/ NCEP reanalysis temperature fields. Surface temperature is als impacted by the increased dwnward surface flux prduced by RRTMG, and this is demnstrated in Plate 6. The annual mean, surface temperature difference (Plate 6a) shws the reginal effect f the enhanced greenhuse warming caused by RRTMG. (These results are based n CCM3 simulatins that used identically prescribed climatlgical SSTs, and thus temperature differences are er ver the ceans.) Over the Arctic, Antarctic and the adjacent sea ice, RRTMG increases surface temperature by 1-4 K. Briegleb and Brmwich [1998b] nte that CCM3 has a cntinental-wide cld bias f 2-5 K ver Antarctica in the annual mean. Warming is als seen acrss much f Eurasia, parts f Nrth America, the Sahara Desert, and Australia. Fr sme areas (e.g., Antarctica, the nrthern Sahara, and Australia) the enhanced dwnward surface flux is cmpensated by an increase in the upward surface flux due t the higher surface temperatures. Reduced net flux differences in these regins relative t the initial frcing change (see Plates lc and 3c) are a result f this feedback. Cler regins include mst f the trpical land areas, Scandinavia, and part f nrthern Asia. T illustrate the seasnal variability f the surface temperature changes, we shw the crrespnding differences fr June, July, and August (JJA) in Plate 6b and fr December, January, and February (DJF) in Plate 6c. Warming is present ver much f Antarctica in bth summer and winter, while the Arctic warming ccurs primarily in nrthern winter. At lwer latitudes the -20 band f reduced land temperatures shifts suthward during DJF and nrthward during JJA relative t the annual mean in respnse t the presence f atmspheric water vapr. An analysis by Bnan [1998] f surface air temperatures generated by the CCM3 Land Surface Mdel shwed that CCM3 has cld biases ver Antarctica, Australia, the Sahara Desert, and Greenland in bth DJF and JJA. The impact f RRTMG n surface temperature effectively addresses many f these dcumented biases in CCM3. As the primary greenhuse gas in the trpsphere, water vapr plays a significant rle in regulating the energy budget f the Earth's climate system. ts distributin greatly impacts radiative fluxes thrughut the atmsphere bth as a direct gaseus absrber and in its liquid r ice frm as cluds and precipitatin. The impact f RRTMG n the annual mean, nal average misture in CCM3 is shwn in Plate 7 as a percent difference. The mst dramatic change is an extensive decrease in water vapr due t RRTMG at lw latitudes in the lwer t middle trpsphere by 20-30%. This reductin peaks at -1.1 g/kg ver the trpics near 600 mbar and is larger than the CCM3 trpical mist biases seen in Figure 5 f Hack et al. [1998]. Small increases in misture are nted at mst latitudes clse t the surface, which slightly imprve CCM3 dry biases. Clearly, the distributin f water vapr is clsely tied t many dynamical cmpnents f the climate mdel, such as the circulatin, bundary layer prcesses, and cnvectin. A full analysis f the RRTMG impact n the dynamical cmpnents f CCM3 is beynd the scpe f this wrk. Hwever, as nted 2OO 18O x 80,,-½ 60 ½ O O ½: X D 2O :..x,..,- "---"' u_ a Latent Heat Flux - _ RRTMG RRTMG - CCM3 E 160 _ 30 ½ 140 _ CCM3... '-' i.u ½ _ 140 _ 120 _ " \ / ' ' 20 Z _= - ' ' -:0 x - / x. _-- D b -2_ Sensible Heat Flux =...>... %.,.,, C 100_ x 60 1.,....c.' u_ '*... \ -20 Latent + Sensible Heat Flux / k. // "% J h..[!..,,.., ',..! \! \ 40 E lo Lu x 40 E 30w O Z 2 lg x D Figure 8. Znal average, annual mean (a) latent heat flux, (b) sensible heat flux, and (c) latent plus sensible heat fluxes fr RRTMG and the CCM3 LW mdel (scales n the left), and fr the RRTMG-CCM3 difference (scales n the right).

13 ACONO ET AL.: VALDATED LONGWAVE RADATON GCM MPACTS 14,885 RRTMG- CCM3 LW Cling Rate Difference N 60N 40N 20N 0 20S 40S MN:-3.18 MAX: {;.4-0. ' S S K d 4 Plate 4. Znal average, annual mean, cling rate differences between RRTMG and CCM3 lngwave mdel frm tw 5-year CCM3 simulatins, including feedback effects f each LW mdel n atmspheric state Units are in K d-l RRTMG - CCM3 Temperature Difference 200 4OO 600 ', ' ,, J MiN:-6.47 MAX: 4.71 GLOBAL:0.5? ' K ; 3 4 Plate 5. Annual mean, nal average temperature difference between RRTMG and CCM3 lngwave radiatin mdels frm tw, 5-year CCM3 simulatins. Units are in K.

14 14,886 ACONO ET AL.: VALDATED LONGWAVE RADATON GCM MPACTS a RRTMG -CCM3 Surface Temperature Difference, ANN 80N F-' "'--'"---,- - "-- -- f_ r'"'.,, '? -""-" % ----, _.,2'" : 20S --- ' ' ' 8s '- - ' r-,!,,!! ß 18( W 120W 60W 0 60E 120E 18! b RRTMG - CCM3 Surface Temperature Difference, JJA ON _ _- '._ -- 60N..." -. '.c S L 80S --'---' W 120W 60W 0 60E 120E 180E 80N 60N C RRTMG - CCM3 Surface Temperature Difference, DJF -... _" :..:":'... ':..''"'."': '. ''"'..:... : 40N 20N 0 20S 40S 60S 80S 180W 120W 60W 0 60E 120E 180E K Plate 6. Surface temperature difference between tw CCM3 simulatins using RRTMG and the CCM3 lngwave radiatin mdel fr (a) the annual mean, (b) the June, July, and August mean, and (c) the December, January, and February mean. Units are in K.

15 ACONO ET AL.: VALDATED LONGWAVE RADATON GCM MPACTS 14,887 (RRTMG - CCM3)/CCM3 Specific Humidity Percent Difference 200 4OO 6O0 8OO looo MAX:22,89 MN: GLOBAL:-2, Plate 7. Annual mean, nal average specific humidity difference between RRTMG and CCM3 lngwave radiatin mdels frm tw, 5-year CCM3 simulatins. Units are in percent. earlier, the applicatin f a data-validated, accurate radiative transfer mdel such as RRTMG t a GCM prvides a fundatin n which the dynamical cmpnents can be mre effectively analyed and imprved. 5. Applicatin t Weather Frecast Mdels The ECMWF weather frecast mdel has been used t examine the effect f RRTMG n shrt-term temperature frecasts and seasnal simulatins [Mrcrett et al., 1998]. Tw sets f 12 T213, 31-layer experiments starting frm the 15th f each mnth between April 15, 1996 and March 15, 1997, were perfrmed. A cmpsite f the results frm each mnth was cmputed fr each lngwave mdel. The 10-day temperature frecast errrs averaged ver the Nrthern Hemisphere fr fur atmspheric layers due t each LW mdel is shwn in Figure 9. At 850 mbar the enhanced cntinuum absrptin in RRTMG (slid line) warms the lwer atmsphere and prevents the cling seen ver time in the ECMWF LW scheme (dashed line). The ppsite ccurs at 500 mbar where the mdel prduces t much cling when running with RRTMG. This indicates a deficiency in sme aspect f the ECMWF mdel, nt in RRTMG. At higher levels, RRTMG has a largely psitive impact by cling the atmsphere and preventing warm temperatures frm develping. n these experiments, RRTMG had relatively small effects n the geptential height and wind fields in the ECMWF mdel. Similar temperature impacts are evident in the nal average temperature field cmputed by the ECMWF mdel. A pair f fur-mnth simulatins were perfrmed, and the results presented in Plate 8 are averages ver the last three mnths f each simulatin fr JJA 1987 and DJF Tempera- tures are shwn as differences frm the ECMWF Reanalysis (ERA) temperature fields fr each seasn. Plates 8a and 8b shw differences fr the ECMWF mdel running with RRTMG, and Plates 8c and 8d shw differences fr the current peratinal ECMWF mdel. Fr the JJA seasn, RRTMG slightly enhances temperatures in the lwer trpsphere in the Nrthern Hemisphere and reduces the difference frm ERA. Cling due t RRTMG in the upper trpsphere and stratsphere reduces the excess warming generated by the ECMWF mdel, especially in the trpics. n the DJF seasn, RRTMG smewhat increases the temperature errrs near the surface at high latitudes with little change in the trpics, while a reductin in temperature is seen in mst latitudes in the upper trpsphere and stratsphere. Figure 9 and Plate 8 demnstrate that the imprved accuracy f RRTMG significantly impacts GCM temperatures ver time perids f imprtance t shrt-term and medium-range weather frecasts. 6. Cnclusins One bjective f the ARM prgram is t supprt the imprvement f GCM radiative transfer mdels fr the benefit f climate research and weather frecasting applicatins. Fr this purpse, a radiatin mdel, RRTM, has been develped which prvides the capability f radiative transfer fr GCMs at a high level f accuracy that is established by cmparisn t radiance measurements. The lngwave versin f RRTM has been adapted fr and applied t the NCAR climate mdel and the ECMWF weather frecast mdel t examine the feasibility f its applicatin t GCMs and t determine the impact f the imprved accuracy f RRTMG n GCM simulatins. We have shwn that RRTMG can have a substantial impact

16 14,888 ACONO ET AL.: VALDATED LONGWAVE RADATON GCM MPACTS LU t l Ol t (qtu) 3 n - 3ud (qtu) 3Ufl 31:ld

17 ACONO ET AL.' VALDATED LONGWAVE RADATON GCM MPACTS 14, ECMWF Nrthern Hemisphere Temperature Frecast Errrs 50 mb 200 mb ,.2, EC_LW RRTMG ø6 f Frecast Day 500 mb Frecast Day 850 mb, E -02' -04, x Frecast Day Frecast Day Figure 9. Ten-day temperature frecast errrs fr the Nrthern Hemisphere averaged ver 12 mnths as cmputed by the ECMWF weather frecast mdel using the ECMWF peratinal LW mdel (EC_LW) and RRTMG. Errrs are shwn fr 50, 200, 500, and 850 mbar. n the energy budget f a GCM at bth the surface and the tp f the atmsphere. The CKD water vapr cntinuum mdel includes the imprtant freign-bradened cntinuum absrpwell as fr the integrated quantities. This assures that the physical prcesses assciated with varius atmspheric species are prperly accunted fr and the distributin f radiative tin in the cm i spectral regin that is nt included energy is crrectly mdeled acrss the LW spectrum. The in the RSB cntinuum frmulatin used in many GCMs. The resulting enhanced absrptin significantly affects LW fluxes and cling rates. Relative t current CCM3 energy budget quantities, clear-sky OLR is reduced by 6-12 W m -2 in trpical latitudes, while dwnward surface flux is increased by 8-15 W m -2 at high latitudes. These changes are sufficient t cmpensate fr knwn errrs in the energy budget f CCM3 [Kiehl et al., 1998b]. t is a significant result f this wrk that the CKD cntinuum mdel can greatly imprve flux calculatins in relevance f applying a spectral perspective t the analysis and imprvement f GCMs has been demnstrated in several studies [e.g., Sling and Webb, 1997]. Applying a radiatin mdel that attains this level f accuracy t a GCM prvides a fundatin n which the dynamical GCM cmpnents can be mre successfully evaluated. The enhanced radiative transfer capability attainable with RRTMG and the CKD water vapr cntinuum has been shwn t be extremely relevant t the verificatin and imprve- GCMs that use lder cntinuum methds. ment f GCMs, but they have even wider applicatins. Recent By altering the lngwave fluxes RRTMG mdifies the radiative cling rate prfiles, which in turn impact the thermdystudies have identified and attempted t quantify the magnitude f a psitive clear-sky bias in the ERBE glbal radiatin namic structure f bth the NCAR climate mdel and the measurements relative t bth GCMs and radisnde data ECMWF weather frecast mdel. n CCM3, lwer trpspheric and surface temperatures are increased by 1-4 K, especially at high latitudes, largely due t the enhancement f dwnward surface flux. These increases address knwn defi- ciencies in the CCM3 atmspheric and surface temperatures [Hack e! al., 1998]. n additin, a CCM3 warm bias in the trpical stratsphere centered at 30 mbar relative t bth the NCAR/NCEP and the ECMWF Reanalyses is substantially cmpensated by changes in the temperature field prduced by RRTMG. Finally, water vapr amunts are generally reduced thrughut the middle trpsphere in the trpics. The extent t which the RRTMG thermdynami changes impact the full dynamical state f CCM3 is currently under analysis. t shuld be emphasied that RRTMG prvides accurate fluxes and cling rates in each f its 16 LW spectral bands as [Cllins and namdar, 1995; Het al., 1998; Sling et al., 1998]. This bias, which is believed t result frm errrs in the selectin f clear-sky scenes, is estimated t be abut 4 W m -2. t is f great imprtance that these studies be cnducted with the mst accurate and physically sund radiative transfer algrithms available, such as that used by Sling et al. [1998]. This is made evident by the glbally averaged 3-5 W m -2 impact f the CKD water vapr cntinuum n clear-sky TOA fluxes. n additin, imprved clear-sky radiatin is an imperative starting pint frm which t study the mre cmplex issues f clud absrptin and clud verlap. Finally, a data-validated mdel, which maintains a high level f accuracy within each f its spectral bands, is highly suited t the imprtant bjective f validating GCM simulatins with current and future satellite spectral radiatin measurements.

18 14,890 ACONO ET AL.: VALDATED LONGWAVE RADATON GCM MPACTS Acknwledgments. The authrs wuld like t thank J. Kiehl and J. Kiehl, J. T., J. J. Hack, G. B. Bnan, B. A. Bville, B. P. Briegleb, D. L. Hack f NCAR fr their assistance and fr many helpful discussins Williamsn, and P. J. Rasch, Descriptin f the NCAR Cmmunity during the curse f this prject. We als thank the reviewers fr Climate Mdel (CCM3), NCAR Tech. Nte, NCAR/TN-420+STR, cmments and suggestins that imprved the paper. This wrk was 152 pp., Natl. Cent. fr Atms. Res., Bulder, Cl., supprted by the Department f Energy under grant DE-FG02- Kiehl, J. T., J. J. Hack, G. B. Bnan, B. A. Bville, D. L. Williamsn, 93ER61549 and by the Natinal Science Fundatin under grant and P. J. Rasch, The Natinal Center fr Atmspheric Research ATM Cmmunity Climate Mdel: CCM3, J. Clim., 11, , 1998a. Kiehl, J. T., J. J. Hack, and J. W. Hurrell, The energy budget f the NCAR Cmmunity Climate Mdel: CCM3, J. Clim., 11, , References 1998b. Barkstrm, B., E. 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Yan, Cmpar- Envirnmental Research, 840 Memrial Drive, Cambridge, MA isn f mdel-calculated and ERBE-retrieved clear sky utging (mike@aer.cm) J.-J. Mrcrette, Eurpean Center fr Medium-Range Weather lngwave radiatin, J. Gephys. Res., 103, 11,529-11,536, Frecasts, Shinfield Park, Reading, UK, RG2 9AX. Hu, Y. X., and K. Stamnes, An accurate parameteriatin f the radiative prperties f water cluds suitable fr use in climate mdels, J. Clim., 6, , Jakb, C., and S. A. Klein, The rle f vertically varying clud fractin in the parameteriatin f micrphysical prcesses in the ECMWF (Received September 3, 1999; revised January 18, 2000; mdel, Q. J. R. Meterl. Sc., 125, , accepted February 4, 2000.)