An interactive cirrus cloud radiative parameterization

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D8, PAGES , APRIL 27, 1999 An interactive cirrus clud radiative parameterizatin fr glbal climate mdels Everette Jseph and Wei-Chyung Wang Atmspheric Sciences Research Center, State University f New Yrk at Albany Abstract. An interactive cirrus clud radiative parameterizatin is develped fr glbal climate mdels frm recent bservatins and analytical results that mre accurately characterize cirrus clud ptical and micrphysical prperties. The radiative prperties are based n the assumptin that cirrus cluds are cmpsed f hexagnal crystals. Fr the infrared cmpnent, a new mass absrptin cefficient is parameterized t calculate emissivity, and fr the slar, single-scattering prperties frm an existing parameterizatin are mdified and emplyed. The slar and infrared ptical prperties are given as a functin f ice water cntent and effective particle size. Aircraft bservatins are used t parameterize the micrphysical prperties in terms f temperature, thus allwing the radiative prperties t interact with the lcal mdel climate. The interactive cirrus radiative parameterizatin is evaluated in mdel-t-bservatin cmparisns with a cmprehensive set f clud and radiatin measurements btained during the spring 1994 and fall 1995 Intensive Observatin Perids f the Atmspheric Radiatin Measurement prgram. It is shwn that the mdel with the new parameterizatin calculates realistic infrared radiatin and imprved slar radiatin incident at the surface. Specifically, biases in calculated slar direct and diffuse fluxes are reduced by 60 and 40%, respectively. Further, the shrtwave flux is shwn t be mre sensitive than the lngwave flux t variability in the ice water cntent and in the base and tp heights f bserved cluds replicated in mdel calculatins. The ptential effect f the new parameterizatin n climate simulatins is investigated in the cntext f initial radiative frcing. The new parameterizatin calculates a significantly different ice water path distributin frm an existing parameterizatin that has been used fr glbal climate change studies. Fr example, in the high latitudes f the summer hemisphere the new ice water path is larger by mre than 7.7 g m -2 (>100%), and in the trpics it can be smaller by as much as -3.5 g m -2 (-80%). These differences lead t an increased slar albed effect in the high latitudes f the summer hemisphere and a decreased greenhus effect in the trpics, bth f which cntribute t a smaller, 2.26 W m -2, glbal- and annual-mean frcing f the surface-trpsphere system. 1. Intrductin Cirrus cluds cver -20% f the glbe n an annual mean basis; cnsequently, they substantially impact the surfaceatmsphere radiatin budget. This budget determines the glbal circulatin pattern which in turn gverns the glbal climate [Liu, 1986]. Therefre realistic treatment f cirrus clud radiative prperties in climate mdels is f critical imprtance. In fact, general circulatin mdel (GCM) studies shw that present climate and natural r anthrpgeni cli- mate change simulatins are very sensitive t the treatment f cirrus radiative prperties. Fr example, Ramanathan et al. [1983] demnstrate that znal winds are sensitive t cirrus emissivity and further that nnblack cirrus emissivity causes the reprductin f bserved znal circulatin patterns. Lhmann and Reckner [1995] prvide evidence f cirrus effects Nw at Department f Physics and Astrnmy, Center fr the Study f Terrestrial and Extraterrestrial Atmspheres, Hward University, Washingtn, D.C. Cpyright 1999 by the American Gephysical Unin. Paper number 1999JD /99/1999JD n atmspheric circulatin thrugh a radiative-hydrlgicaldynamical feedback mechanism. Reckner et al. [1987] attribute the net glbal warming in their GCM equilibrium simulatin f dubled CO2 (2 x CO2) t a redistributin f vertical clud amunts in favr f cirrus cluds. Despite the climatic imprtance f cirrus cluds, develpment f realistic cirrus clud radiative parameterizatins fr climate mdels has been limited. The main reasns include the cmplexity f radiative and micrphysical prcesses that gvern cirrus clud interactin with the transprt f slar and infrared radiatin (IR), the difficulty f scaling these prcesses t the limited tempral and spatial reslutins f climate mdels, and the lack f bservatinal evidence [Liu, 1986; Stephens et al., 1990]. Mre adequate treatment f cirrus radiative prperties is increasingly pssible wing t bservatinal and analytical develpments. Heymsfield and Platt [1984] analyzed cirrus temperature and micrphysics data frm Heymsfield [1977] t shw that the shape f cirrus ice crystal size spectra and crystal habit change systematically with changes in the ambient clud temperature. Heymsfield et al. [1990] and Mc- Farquhar and Heymsfield [1996] have expanded upn this wrk with their analyses f in situ bservatins f cirrus micrphysical prperties in midlatitude and trpical cirrus cluds, re-

2 9502 JOSEPH AND WANG: CIRRUS RADIATIVE PARAMETERIZATION FOR GCM Cirrus Parameterizatin and fall 1995 Intensive Observatins Perids (IOPs) f the Atmspheric Radiatin Measurement Prgram (ARM) [Stkes and Schwartz, 1994]. Finally, initial radiative frcing experiments are cnducted t investigate the ptential effect f the cirrus parameterizatin n GCM simulatins. Clud Micrphysics Clud Optics m mm mm mm mm mm mm mm mm mm mm m mm mm mm m Infrared Radiative Mdel Slar 2. Interactive Cirrus Clud Radiative Parameterizatin In this sectin a brief descriptin is given f the interactive cirrus clud radiative parameterizatin, a schematic diagram f which is illustrated in Figure 1. The clud micrphysical prperties, ice water cntent (IWC), and effective particle size interact with the mdel by parameterizatin in terms f the mdel cludy grid layer temperature (see upper prtin f Figure 1). The ice cntent parameterizatin is develped frm a fit f mean bserved values f IWC and temperature reprted by Heymsfield and Platt [1984]; it is represented by the fllwing expnential relatinship IWC = y exp ( TøC) (1) Figure 1. Diagram f interactive cirrus clud radiative paramctcrizatin. The clud ice water cntent (IWC) and genwhere / = 0.4 g m -3 and = 0.1 deg -. This functin is eralized effective particle size (Dee) are parameterized in analgus t that given by Stephens et al. [1990]. Integratin f terms f the mdel layer clud temperature Tc. The ice water IWC ver the clud thickness results in the ice water path path (IWP) is the IWC integrated ver the clud depth. The (IWP). slar (r, ptical depths; t, single scattering albed; #, asym- The particle size parameterizatin derives frm the fllwmetry factr; and f, frward scattering factr) and infrared (k, ing definitin fr "generalized effective particle size" (Dee) mass absrptin cefficient; r, ptical depths; and e, emissiv- frmulated by Fu [1996] ity) ptical prperties are parameterized as a functin f IWP and Dee. The ptical prperties are passed t the radiative mdel t calculate the bulk radiative prperties f the layer. The dash line represents the interface between the cirrus pa- max DDLn (L) dl mln rameterizatin and the radiative mdel. = (2) DL + 4 D 2 n (L) dl spectively. Fu [1996] emplys the mst recent ray-tracing techwhere n(l) (m-3 /xm- ) is the crystal size distributin, D is niques t parameterize the single-scattering clud prperties the diameter f the basal plane f a hexagnal crystal, L is the fr 25 slar bands, using 28 ice crystal size distributins frm in length, and L mi n and L ma x are the minimum and maximum situ bservatins f midlatitude and trpical cirrus cluds. lengths f crystals in the distributin, respectively. This frmu- Chylek and Videen [1994] use anmalus diffractin apprxilatin is unique in that the denminatr and numeratr are matin t calculate mre accurate infrared radiative prperties prprtinal t surface area and vlume f a hexagnal crystal, fr cirrus particles assumed t be hexagnal clumns. Addirespectively. The eight bserved crystal size distributins given tinally, bservatinal evidence f the dependency f cirrus by Heymsfield and Platt [1984] are used t parameterize Dee in prperties n temperature and the need t ecnmize GCM terms f temperature. Each bserve distributin is discretized cmputatinal resurces have led a few studies t cnsider int five regins using relatinships f crystal length t diamparameterizatin f cirrus radiative prperties as a functin f eter that crrespnd t bservatins frm On [1969] andauer temperature [e.g., Platt and Harshvardhan, 1988; Ou and Liu, and Veal [1970]. Figure 2 shws the dependence f Dee n 1995]. temperature. The fit, included in the plt, results in the fl- The present study, relying upn the abve develpments, lwing plynmial relatinship prvides a temperature-based interactive cirrus clud radiative parameterizatin suitable fr GCMs. The shrtwave (SW) 3 cmpnent is mdified frm Fu [1996]. In the lngwave (LW), = a,r' (3) cirrus emissivity is parameterized emplying the results f t=0 Chylek and Videen [1994]. The LW treatment differs frm ther wrks in that it accunts fr the nnsphericity f cirrus where T is in degrees Celsius, and k , k = -2.47, ice crystals. Fr bth the SW and LW, cirrus ptical prperties k 2 = -0.11, and k The rt-mean-square (rms) depend n ice water cntent and effective particle size, which difference between the fit and calculated values f Dee is 4.85 are parameterized in terms f clud temperature [Heymsfield / m. The methdlgy fr parameterizing Dee is similar t Ou and Platt, 1984]. In additin, the present parameterizatin is and Liu [1995], but the present study uses the "generalized implemented in mdel-t-bservatin cmparisns t address effective particle size" definitin. Additinally, fllwing Fu, the validity f temperature-based cirrus radiative parameter- the size distributins are extraplated t 12.5 / m as these izatins. The measurements used cme frm the spring 1994 distributins are based n measurements frm ptical array mln

3 JOSEPH AND WANG: CIRRUS RADIATIVE PARAMETERIZATION FOR GCM 9503 prbes that have difficulty measuring small particles [Heymsfield and Platt, 1984]. Assuming that cirrus cluds are entirely cmpsed f hexagnal crystals has enabled the develpment f apprximatins that prvide ciri'us ptical prperties within particular SW and LW spectral regins. In the SW case, gemetric raytracing techniques can be applied at certain slar wavelengths t btain slutins fr cirrus SW single-scattering prperties. This apprximatin becmes increasingly accurate as the size parameter appraches infinity, since the apprximatin assumes that light cnsists f separate lcalized rays traveling in straight-line paths [Liu, 1980]. Fu [1996] emplys the mst recent ray-tracing techniques t parameterize single-scattering clud prperties fr 25 slar bands, using 28 ice crystal size distributins frm in situ bservatins f midlatitude and trpical cirrus cluds. This parameterizatin includes SW ptical depth, single-scattering albed, and asymmetry factr in terms f D ae and IWP (see lwer prtin f Figure 1). Ntable advantages f the imprved ray-tracing techniques include mre realistic absrptin in the near-infrared frm the use f I I I I I new slutins fr the imaginary index f refractin and accu racy extended t a size parameter as lw as 15. The present Temperature study emplys Fu's SW ptical parameterizatin, where md- Figure 2. Effective particle size (/ m) versus temperature ificatins are made fr seven slar bands. (øc). The squares represent effective size calculated frm (2) Existing parameterizatins fr cirrus emissivity (flux value) with bservedata frm Heymsfield and Platt [1984], and the have been based n Mie calculatins fr spheres f equivalent curve represents the crrespnding plynmial fit as defined vlume/surface area [Ebert and Curry, 1992] r n apprxima- by (3). tins fr cyiinders [Platt and Harshvardhan, 1988]. Mre recently, Chylek and Videen [1994] used anmalus diffractin apprximatin (ADA) t calculate the absrptin and scatter- nly 5% fr the ADA absrptin crss sectin within this band. ing crss sectins fr a hexagnal clumn hrizntally riented Mrever, cirrus emittance (vertical radiance) at 11/ m, which and perpendicular t the directin f the incident radiatin. this band encmpasses, was shwn by Platt and Stephens [1980] They shw that fr hexagnal clumns, spheres f equivalent t equal bradband emittance. Within this regin, a and /3 vlume/surface area are nt a suitable apprximatin, with assume values f 2.84E-04 m 2 g-1 and 8.7E-07 m 3 g-l, respecerrrs as much as several hundred percent, but that equal tively. Figure 3a shws hw k decreases a functin f D vlume/surface area cylinders are a gd apprximatin. (dashed line). The inverse particle size dependency f k fr The present study develps a parameterizatin fr cirrus cirrus cluds is shwn by Platt [1989] t be cnsistent with emissivity by applying the results fr absrptin crss sectin bservatin. In Figure 3a, k decreases frm t 0.01 m 2 frm Chylek and Videen [1994]. The absrptin cefficient (per g-1 fr increasing values f Dee with a reductin f mre than meter) is, half ccurring fr values f Dee between 20 and 40/ m. Aircraft measurements indicate that k ranges frm t m 2 g-1 fr trpical cirrus [Griffith et al., 1980] and frm 'x = O'cs(L )n (L ) dl (4) t m 2 g-1 fr midlatitude cirrus [Smith et al., 1990]. L rnax rnln Paltridge and Platt [1981] estimate a k value f m 2 where O-cs is the absrptin crss sectin frm Chylek and Videen. The parameter used t determine cirrus emissivity is mass absrptin cefficient (m 2 g-l): ki = li -t- De (6) This methdlgy fllws that f Ebert and Curry [1992]. Fr all calculatins in this study, nly ki fr i = 10.6/xm-11.1 band (k) is used because Chylek and Videen reprt an errr f frm measurements f cirrus bserved ver New Mexic. As shwn in Figure 3a, k frm this study falls within the range f these bradband bserved values. The dependence f k n temperature (slid line) is als depicted in Figure 3a. Fr i.ncreasing temperature, k decreases frm between 0.08 and k a - rr/iwc (5) 0.09 t 0.02 m 2 g-l, with the largest decrease ccurring be- T parameterize k a in terms f effective particle size, the eight tween -60 ø and -40øC. This behavir is explained by the size distributins frm Heymsfield and Platt [1984], discretized inverse effective size dependency f k and plynmial relatinas described abve, are used in (4) t calculate - fr 20 nm ship between Dee and temperature. As shwn, the k temperspectral intervals within five IR bradbands. In this calculatin, ature relatinship agrees well with that derived by Platt [1989] values fr the imaginary index f refractin fr ice are taken fr6m bservatins; the rms difference is m 2 frm Warren [1984]. A fit f k a versus Dee fr each IR band Cirrus emissivity is given by yields the fllwing empirical relatinship: e exp [-1.66(k)IWP] (7) where 1.66 is the diffusivity factr. Figure 3b shws cirrus emissivity, calculated frm (7) with k frm (6), as a functin f IWP fr fur different values f Dee. These fur curves increase tward unity fr increasing IWP, but at a given value f IWP, emissivity decreases fr larger values f Dee. Emissivity

4 = 9504 JOSEPH AND WANG: CIRRUS RADIATIVE PARAMETERIZATION FOR GCM Temperature (NIR) ( m) bands. The SW clud ptical prperties, similar t Sling [1989], are fr cluds assumed t be entirely i (a) i i cmpsed f water drplets. The IR parameterizatin fllws Kiehl et al. [1987], which has been mdified t incrprate the effects f trace gase such as N20, CH4, and CFCs [Wang et al., 1991]. High clud emissivity is calculated as by Ramanathan et al. [1983]. The riginal treatment f a single effective clud has been eliminated. Multiple clud layers are assumed t be randmly verlapped. Tw versins f the RM are used in this study. The first versin (R1) is the reference RM that is described abve. The secnd versin (R2) is mdified t include the interactive ciri i i i rus radiative parameterizatin (ICRP). The schematic diagram in Figure 1 illustrates hw ICRP interacts with the RM. Fr the Effective Size cmparisn t bservatins (Oklahma in April) presented in sectin 4, the R2 cirrus parameterizatin is activated when (b) clud layer temperatures are less than -20øC and clud pres- Ramanathan sures are <400 mb (layer depths vary with decreasing pressure et al. '. in these calculatins; at these levels, they range frm t < 1 km). Fr the cmparisn f cirrus parameterizatins in the cntext f a glbal radiative calculatins fr GCM-generated ge = clud fields described in sectin 5, the cirrus parameterizatins are activated when clud layers are fund in the tw layers just belw the climatlgical trppause at a given seasn and latitude. The ICRP uses clud layer temperature t calculate the SW and LW clud layer ptical prperties. The SW prp erties are passed t the delta-eddingtn apprximatin t cal- Ice Water Path culate the layer transmissivity, reflectivity, and absrptivity, Figure 3. Infrared ptical prperties. (a) The dashed line which are then used in the ray-tracing parameterizatin t represents mass absrptin cefficient (m 2 g-i) versus effec- determine the upwelling and dwnwelling slar fluxes at the tive size ( m) as defined in (6). The slid line and squares are layer bundaries. The emissivity is used t calculate the upmass absrptin cefficient versus temperature (øc): The welling and dwnwelling LW fluxes f the layer. frmer is frm this study and the latter is deduced frm bservatins by Platt [1989]. (b) Emissivity versus IWP (g m-2): The fur lwer lines are emissivity (equatin(7)) fr fur 4. Mdel-Observatin Cmparisns different values f effective size ( m), and the upper mst line 4.1. ARM Data is emissivity frm Ramanathan et al. [1983], which is included The interactive cirrus radiative parameterizatin is evalufr cmparisn. ated with bservatins made during the spring 1994 and fall (SGP) site. Cntinuus measurements were made f radiatin, frm Ramanathan et al. [1983], which uses an aircraft mea- clud, and meterlgical prperties frm multiple platfrms sured k f 0.1 m -2 g, is als presented in Figure 3b. Emissivity and at varius spatial and tempral reslutins. The data used frm (7) is generally less than that frm Ramanathan et al. in this study cme frm tw primary surces: the ARM satelexcept fr thick cirrus (>60 g m 2) with small particles (---20 lite measurements [Minnis et al., 1995] and CERES/ARM/ m). Fr all cirrus cases t be cnsidered in this study, emissivity frm (7) is less than that in the reference mdel (fr a given IWP), since the reference mdel emplys emissivity frm Ramanathan et al. Several studies [e.g., Stephens, 1980, Platt and Stephens, 1980] have demnstrated that scattering and reflectin are imprtant cnsideratins when accunting fr cirrus emissivity, particularly in a trpical atmsphere. These effects, hwever, are deferred fr a mre thrugh treatment in a subsequent study. GEWEX/EXPERIMENT (CAGEX) data package (vl.l.1 and v2.0) [Charlck and Alberta, 1996]. A brief descriptin f the data fllws. Satellite clud prperties and bradband fluxes are derived frm GOES 7 narrwband measurements at the tp f the atmsphere (TOA). These measurements are available at half hurly intervals within a 0.3 ø x 0.3 ø (lngitude x latitude) hrizntal grid reslutin centered n the ARM Central Facility. Meterlgical fields, surface radiatin, and lidar clud base heights cme frm CAGEX. The lwer-level temperature 3. Atmspheric Radiatin Mdel and humidity are based n three hurly sundings taken directly at the ARM Central Facility. TIROS Operatinal Ver- The clumn radiative mdel (RM) used in this study is taken frm a GCM [Thmpsn and Pllard, 1995]. The slar radiatin parameterizatin [Thmpsn et al., 1987] includes a twstream delta-eddingtn apprximatin t determine reflectivtical Sunder (TOVS) temperatures are used abve 100 hpa and climatlgical humidities frm McClatchey et al. [1972] in vl.l.1 and SAGE in v2.0 are used abve 300 hpa (Tim Alberta, private cmmunicatin, 1998). Charlck and Alberta [1996] ity and transmissivity f mdel layers with cluds r aersls indicate that measurement f upper-trpspheric humidity is and ray tracing t treat multiple reflectin. The slar spectrum is divided int visible (VIS) ( m) and near-infrared 1995 ARM IOPs cnducted at the Suthern Great Plains difficult and that ARM plans further study. The direct and diffuse hrizntal cmpnents f the surface dwnwelling SW

5 JOSEPH AND WANG: CIRRUS RADIATIVE PARAMETERIZATION FOR GCM 9505 measured by pyrhelimeters and pyranmeters, respectively, f the Baseline Surface Radiatin Netwrk (BSRN) are used t calculate the ttal dwnwelling SW radiatin. The measured direct nrmal cmpnent is multiplied by the csine f the slar zenith angle t determine the direct hrizntal cmpnent. The diffuse hrizntal frm the raw data is added t the direct hrizntal t determine the ttal dwnwelling SW. J. Michalsky et al. (unpublished data, 1997) shw that this is a reliable methd fr calculating the ttal dwnwelling SW. These surface-based data, bserved at ARM SGP Central Facility, are averaged t cincide with the tempral intervals f the satellite. The cases chsen in the present study are cnfined t cirrus events with n underlying cluds. A characteristicase that was bserveduring a 2 hur subperid n April 14, 1994, is illustrated in Figure 4. Overcast cnditins dminated the satellite grid except fr 1930 UTC when clud amunts fell t ---80%. Clud ptical depths varied between 1 and 6; average clud layer temperature varied between -50 ø and -40øC; and clud heights varied between 6 and 12 km. These clud prperties are representative f midlatitude cirrus [Heymsfield and Platt, 1984; Heymsfield et al., 1990]. Calculatins with the RM are cnducted at the tempral intervals f the satellite measurements and cincident with the ARM SGP Central Facility. The meterlgical fields described abve are used t set the state f the mdel atm- sphere. The incming slar flux at the TOA is determined frm the slar cnstant, adjusted fr the day f the year and slar zenith angle, and the surface LW emissin is determined by the pyrgemeter-based skin temperature. The placement f cluds in specific vertical layers are determined frm satellite-derived clud tp and lidar base heights. Satellite-derived clud tp heights are used because lidar estimates, a mre accurate surce f clud tp heights particularly fr thin cirrus, were nt available. As reprted belw, sensitivity f calculated surface fluxes t uncertainty f satellite-derived clud tp and lidar base heights (---1 km fr satellite) is within the mdel bias, and mrever, this sensitivity is mre crrelated t uncertainty in base height than tp. The methd f clud representatin in the mdel is design t cnserve the bserved ttal clumn amunt and vertical extent. Values f fractinal clud amunt are assigned t cludy layers such that when randm verlap is applied the bserved ttal clumn amunt is cnserved. The fllwing equatin is used t determine the apprpriate fractinal amunts: Ai = 1 - (1 - AT) 1/N (8) where A r is the bserved ttal clumn clud amunt and N is the number f mdel layers in the cludy clumn. Cnsider 1700 UTC n April 14, 1994, as an example (Figure 4). The bserved clud amunt (A r) fr this case is 99% and the tp and base heights are 8.2 and 6.1 km, respectively. The vertical extent f the bserved clud is replicated in the mdel with fur layers (N = 4) and the fractinal amunt (Ai) fr each layer is 68%. N explicit attempt is made t treat subgrid clud variability, but cases with clud amunts less than 60% are filtered t minimize effects frm hrizntal inhmgeneities. Lcal 03 prfiles and aersl ptical depths are tw additinal parameters needed fr RM calculatins. Ozne prfiles are interplated frm TOMS measurements [Liang and Wang, 1995]. Ttal clumn aersl ptical depths are based n measurements frm a multifilter rtating shadwband radimeter - _ (a) l l 1 1 Optical Depth ß Arn (b) m-- ß.._. m T T I._L emperature \ ß -r- / ß x Clud tp _ ß '- ' _ _ m' Clu ase Hur Figure 4. Observed cirrus clud prperties versus time 1 ß (UTC). Included prperties, bserved ver 2-_ 2 hur perid l n April 14, 1994, at the ARM SGP Central Facility, are characteristic f cases identified fr this study. (a) Satellite-derived clud amunt (percent) and ptical depth. (b) Average clud layer temperature (øc) and satellite-derived clud tp and lidar measured clud base heights (kilmei:: rs). Errr bars represent standard deviatin f measured v!.,s. (MFRSR) (Jseph Michalsky, private cmmunicatin, 1995) lcated at the Central Facility. The energy-weighted averaging methd used by Ding et al. [1996] is adpted t determine the VIS and NIR bradband ttal clumn aersl ptical depths frm the measured spectral values Mdel Sensitivity t Variability in Clud 1WC and Layer Placement The mdel sensitivity t the clud IWC and clud placement is examined in this sectin. The R2 mdel calculated and bserved surface dwnwelling SW and LW fluxes during the April 14, 1994, subperid (Figure 4) are cmpared in Figure 5 t first create a cntext fr the sensitivity analysis. The calculated SW fluxes are substantially greater than thse bserved thrughut the subperid, with a calculated mean that is 90.8 W m -2 (12.73%) greater than the bserved mean ( W m-2). The dwnwelling LW, hwever, agrees well with bservatin as the mean bias is W m -2 (- 1.89%) with respect

6 _ JOSEPH AND WANG: CIRRUS RADIATIVE PARAMETERIZATION FOR GCM ß / / Mdel Observed / / // Slar ß ß ß '"' Infrared '-r-- I I I I I I Hur (UTC) perature, and secnd, the mdel's vertical reslutin is cmparable t (fr lidar) r within (fr satellite) the rugh uncertainty f measured clud heights. Recall that these measurements are used t place the clud layers in the mdel. Sensitivity t clud height is determined by perfrming an ensemble f mdel calculatins where the clud base and tp height cmbinatin is different in each calculatin. A ttal f nine cmbinatins are pssible at each time step since they are determined frm the mean base and tp heights and crrespnding standard deviatins (e.g., at 1750 UTC n April 14, 1994, pssible cmbinatins are 6.10 _ km (base) and 9.76 _+ 1 km (tp)). The mean dwnwelling surface LW and SW fluxes frm these calculatins are cmpared t bserved fluxes in Figure 6. The standard deviatins, represented by errr bars in Figure 6, reflect the upper and lwer bunds f the mdel bias attributed t clud base-tp height variability. The subperid average deviatin is _ W m -2 (5.65%) and _+6.02 W m -2 (1.69%) fr the SW and LW cases, respectively. Ntice that the degree f sensitivity is determined mre by variability in clud base heights than that in clud tp heights. This is caused by the nnlinear prprtinality f IWC and Dge t temperature. The deviatin at each time step in Figure 6 is mre clsely crrelated t that f base heights than tp heights in Figure 4b. Fr example, the crrelatin cefficient fr the frmer in the SW case is 0.97 and that fr the latter is The dminant influence f micrphysical prperties at the clud base n LW and SW transmissin has been demnstrated in bservatinal studies [e.g., McFarquhar and Figure 5. Mdel calculated and bserved dwnwelling SW Heymsfield, 1996; Spinhirne et al., 1990]. Tw additinal enand LW radiatin (W m -2) incident at the surface n April 14, 1994, at the ARM SGP Central Facility are pltted versus time (UTC). Mdel results are frm R2 with IWC as parameterized in (1). Errr bars representhe standard deviatin f radiatin calculatins fr a range f parameters in (1) chsen t crre- Mdel Observed spnd t variability in IWC bservatins. t the bserved mean ( W m-2). As a reference, calculated and bserved clear sky dwnwelling SW and LW frm April 15, 1994, and April 17, 1994, (the nly days with extended Slar clear sky perids) are cmpared (nt shwn). The mean calculated clear sky SW and LW are W m -2 (5%) larger and W m -2 (4%) smaller than their respective mean bserved values ( fr SW and fr LW). The mdel sensitivity t IWC is examined because the empirical fit (1) represents a relatinship between mean IWC and temperature. Mrever, Heymsfield and Platt [1984] reprt // // that the range between high and lw values f bserved IWC // within a particular temperature interval is large. A wide range 8 - // / f parameters (fits) in (1) are chsen under the cnstrainthat / / the resulting IWCs d nt exceed extreme values in the bserve data [Heymsfield and Platt, 1984, Table 2]. The errr bars in Figure 5 represent the standard deviatin f dwnwelling surface LW and SW frm calculatins with these IWC values. The mean f the standard deviatin at all times during the subperid is _ W m -2 (7.87%) and _+2.46 W m -2 T -- ß """""""" Infrared _j_ (0.69%) fr the SW and LW cases, respectively. Clearly, the I I I I I I SW flux is very sensitive t variability in IWC, but, even fr the upper bund IWC, a psitive bias remains. Overall, the abve results suggest a t-transmissive clud. Hur (UTC) The sensitivity f calculated surface fluxes t clud base and Figure 6. Same as Figure 5, except mdel (R2) calculatins tp heights is examined next. Clud base and tp heights are are with mean bserved clud tp and base heights. Errr bars expected t affect radiative transfer in the mdel fr at least tw reasns. First, clud radiative prperties depend n temrepresent the standard deviatin f radiatin calculatins fr an ensemble f clud base and tp height cmbinatins. T T

7 JOSEPH AND WANG: CIRRUS RADIATIVE PARAMETERIZATION FOR GCM 9507 semble calculatins are cnducted t shw the effect f cn- straining IWP and ptical depths frm varying with different base-tp cmbinatins. These results are shwn in Table 1. Cirrus Cases During ARM Spring 1994 and Fall 1995 IOPs Cnserving IWP reduces the SW and LW sensitivity by 1.2% Observed, R1 Errr, R2 Errr, and 1%, respectively. Cnserving SW ptical depth, which Radiatin W m -2 % % accunts fr the cmbined effect f IWP and Dee reduces the sensitivity f the SW bias t 2.69%. The sensitivity f LW t Lngwave Slar Direct cnserving IWP is small, thus n calculatin was dne fr Slar Diffuse cnserving ptical depth. The dwnwelling LW at the surface is nt very sensitive t Mdel errrs are with respect t bserved values. variability in IWC and clud height. The pacity f the atmsphere dampens cirrus clud effects n surface LW flux, but this dampening is far strnger in the trpics where the misture cntent is much greater as well as the clud base heights. The SW radiatin exhibits significant sensitivity t bth the IWC and clud height. A SW bias, hwever, remains. The bias is as lw as 50 W m -2 (7%) if the lwer and upper bunds f the clud base and IWC variabilities are taken int accunt, respectively. reprt a clear sky SW bias f 4% at similar midlatitude cnditins. Finally, SW flux measurements have uncertainties, which are estimated t be 1-2% (Jhn DeLuisi, private cmmunicatin, 1994). Michalsky et al. [1997] reprt disagreement amng a cmplement f radimeters (including BSRN radimeters) measuring ttal dwnwelling SW at the ARM SGP Central Facility. The disagreement was as large as 60 W m -2 at The psitive SW bias is cnsistent with previus studies [e.g., slar nn and was attributed t calibratin errrs. Fr the Stackhuse and Stephens, 1991; Kinne et al., 1997]. The underestimatin f small particle cncentratins mentined in secdirect and diffuse cmpnents, disagreements are f the rder f 5 W m -2 and 10 W m -2, respectively. These reprted tin 2 may cntribute t the SW bias, but recent bservatinal uncertainties were fr clear sky cases and an IOP different studies suggesthat this cntributin may nt be as significant frm that used in the present study. Crrespnding uncertainas previusly thught [Kinne et al., 1997; McFarquhar and Heymsfield, 1997]. Further, it was indicated in sectin 2 that the measured distributins are extraplated t accunt fr the underestimatin. The parameterized asymmetry factr may ties fr cludy cases in the present study are unknwn Imprvements Over Existing GCM Parameterizatin In this sectin, dwnward radiatin incident at the surface als cntribute t the slar bias [Stackhuse and Stephens, calculated frm R1 and R2 is cmpared with bservatins fr 1991], but calculated asymmetry values were within the range cirrus cases ccurring thrughut the spring 1994 and fall 1995 typical f midlatitude cirrus that are suggested by Takan and IOPs. Recall that the basic difference between R1 and R2 is Liu [1989] and Macke et al. [1995]. A mre prbable factr in the bias is the inadequate treatment f subgrid clud effects prevalent in current GCMs including the present. Such effects include clud verlapping, hrizntal inhmgeneities, and clud tp rgraphy. The later tw effects are particularly strng at slar zenith angles typical f, the midlatitudes. In the high clud ptical prperties, which in the frmer are parameterized fr water cluds and in the latter are parameterized fr ice cluds. As shwn in Table 2, bth mdels prduce realistic LW radiatin incident at the surface; the mean values frm R1 and R2 are greater than bserved by 9.0 W m -2 (2.53%) and 0.54 W m -2 (0.15%), respectively. The de- terms f clud verlapping, sectin 4.1 indicates that the randm methd was used. The maximum methd, which was tested fr cmparisn, reduced biases in the mean dwnwelling SW and LW by W m -2 (4%) and -- 1 W m -2 (0.4%), respectively. Even in this extreme case, a large SW bias reprted in the previus sectin. A mre meaningful sense f -- 8%(66 W m -2) remains. In additin, the use f maximum the effect f the new parameterizatin n the slar bias is verlap is prblematic because it results in significant under- gained by cmparing the direct, diffuse, VIS and NIR cmpestimatin f the dwnwelling SW fr cases where the number nents. Table 2 shws that bth R1 and R2 underestimate the f cludy layers is large. The msaic treatment was shwn by Liang and Wang [1997] t be a mre rbust apprach t the direct and verestimate the diffuse flux. A smaller clud ptical depth, hwever, is largely respnsible fr reducing the prblem f clud verlapping and will be emplyed in future studies. The verestimatin f mdel calculated clear sky surface SW fluxes may be anther surce f errr. Ding et al. [1996] Table 1. Sensitivity f Calculated Radiatin Bias t Variability in Clud Heights Ensemble Runs SW Flux LW Flux Nrmal +_5.65% (40.38 W m -2) +_ 1.69% (6.02 W m -2) IWP Cnserved +_4.86% (34.73 W m -2) +_0.51% (1.81W m -2) SW Tau Cnserved +_2.69% (19.23 W m -2)... Values are the mean standard deviatin f R2 calculatins ver the April 14, 1994, subperid fr an ensemble f clud base and tp height cmbinatins. Table 2. Cmparisn f R1 and R2 Calculated Radiatin Incident at the Surface With That Frm Observatin fr viatin fr R2 is larger than that fr R1 by 1%, which suggests slightly mre variability in the differences between R2 and bserved fluxes. Bth mdels have biases in the ttal slar flux similar t that negative bias in the direct and the psitive bias in the diffuse by 75 W m -2 (60%) and 44 W m -2 (40%) in R2, respectively. The ptical depth is smaller fr tw reasns. First, the effective particle size in R2 is larger than that in R1 since the frmer characterizes an ice crystal distributin and the latter characterizes a water drplet distributin. Secnd, the IWP in R2 is smaller than that in R1. The R2 imprvements are further examined by cmparing the difference in SW clud frcing in the atmsphere (CSA) between R1 and R2 (Table 3) during the April 14, 1994, subperid (cnditins are described in sectin 4.1). The ttal clud frcing in the atmsphere is enhanced clse t 20 W m -2 (>200%) in R2 mainly because less radiatin is lss t space with the smaller clud ptical depth. In additin t the larger ttal atmspheri clud frcing (absrptin) in R2, Table 3 als reveals significantly different spectral features fr atm-

8 9508 JOSEPH AND WANG: CIRRUS RADIATIVE PARAMETERIZATION FOR GCM Table 3. Cmparisn f Mean SW Clud Frcing in the Atmsphere During the April 14, 1994, subperid The fllwing cirrus criteria are develped fr applying the ICRP in GR2 calculatins. Cluds in the glbal ttal clud Radiatin R1 R2 distributin that ccur in the tw mdel layers immediately beneath the climatlgical trppause and with temperatures Visible less than 253 K are assumed t be cirrus cluds. Only the Near-infrared temperature criterin is applied t cluds abve. Figures 7a SW clud frcing is in W m -2. and 7c shw the altitude-latitude mean distributin f layer clud amunt fr all cludy layers in January and July, respectively, and Figures 7b and 7d shw the distributin f layer spheric absrptin between the tw calculatins. Atmspheric absrptin in R2 is reduced by abut 15 W m -2 (>80%) in the VIS and is almst 4 times larger in the NIR. These absrptin features result frm the R2 parameterized single scattering c-albed which accunts fr the spectral tendencies f the imaginary index f refractin fr ice: minimum at VIS wavelengths and maximum at NIR wavelengths [Fu, 1996; Warren, clud amunt fr all cludy layers defined as cirrus layers by the cirrus clud criteria described abve. The percentages in Figure 7 represent layer amunts. The latitudinal and vertical distributins f the layer cluds are qualitatively cnsistent with bservatins. In Figure 7, cirrus clud base heights increase with decreasing latitude frm the ples (-4.5 km) t the equatr (-12 km). Glbal and reginal cirrus climatlgy 1984]. Last, the reduced VIS and enhanced NIR atmspheric cmpiled by Lndn [1957] andappleman [ 1961], respectively, absrptin cntribute t reductin f the psitive bias in the frm surface bservatins shw a similar variatin f cirrus diffuse flux and negative bias in the direct flux, respectively. base heights with latitude as that shwn in Figures 7b and 7d. 5. Initial Radiative Frcing in Glbal Calculatins The migratin f trpical cirrus maxima assciated with the Intertrpical Cnvergence Zne (ITCZ) and mnsns and midlatitude maxima t the summer hemisphere is als evident 5.1. Methd in Figures 7b and 7d, as well as cirrus minima in the subtrpics. The ptential effect f the ICRP n a reference GCM sim- These features are cnsistent (prprtinally) with satellite ulated climate is evaluated within the cntext f initial radia- bservatins [Wdbury and McCrmick, 1986; Rssw et al., tive frcing. Initial radiative frcing is cmmnly defined as the 1985]. The cirrus amunts in 7b and 7d, hwever, underestimate instantaneus change in net flux due t an instantaneus that frm ISCCP everywher except at the high latitudes f the change in a radiative agent, fr example, greenhuse gases, summer hemisphere. Znal-mean ttal clumn (randm veraersls, r cluds [Hughtn et al., 1992]. Initial radiative frcing at the trppause defines the frcing f the trpsphere-surface system and thus is an indicatr f the relative climatic imprtance f a frcing agent [Hughtn et al., 1990]. This understanding fllws frm ne-dimensinal radiativelap assumptin) cirrus amunts are less than ISCCP by 30-70% (2-15% abslute difference) in the trpics, summer midlatitudes, and the winter hemispheres. At the high latitudes f the summer hemisphere, the amunts are cmparable t ISCCP. This discrepancy and its effects are discussed in sectin 5.3. cnvective mdel studies [e.g., Lacis et al., 1981; Wang et al., 1981], which shw that temperature change at the surface crrespnds t frcing at the trppause. The mdel used calculateslar and IR fluxes glbally fr 18 vertical layers in the atmsphere at a hrizntal grid reslutin f R15 (-4.5 ø latitude by 7.5 ø lngitude). Except fr these 5.2. Radiative Frcing Results Glbal- and time-averaged values f the radiative frcing calculatins (GR2-GR1) described abve are shwn in Table 4. The annual- and glbal-mean trpsphere-surface frcing difference is W m -2, meaning that the net flux int the features the mdel is identical t that described in sectin 3. trpsphere-surface system is reduced by this magnitude frm Tw single time step calculatins are perfrmed with the mdel: ne with the reference cirrus parameterizatin (GR1) and the ther with the new parameterizatin (GR2). Bth calculatins are based n cmmn glbal meterlgical fields and clud distributins taken frm integratin f a GCM [Liang et al., 1995] under Atmspheric Mdel Intercmparisn Prject (AMIP) standard cnditins [Gates, 1992]. As these are radiative calculatins, nly radiative differences develp. Differences between the tw calculatins (GR2-GR1) f the net flux replacing the reference cirrus parameterizatin with the ICRP in these calculatins. The magnitude f the net July frcing difference is greater than that in January by 0.19 W m -2 (9%) because the SW cmpnent f the July frcing difference is greater (0.43 W m-2; 60%) than the January cmpnent despite the smaller (0.25 W m-2; 17%) July LW frcing difference. Mst f the energy lst ccurs in the trpsphere as the trpsphericmpnent f the annual- and glbal-mean frcing difference is 2.20 W m -2 (97%) greater than the surface and f cirrus prperties that give rise t these flux differences cmpnent. The magnitude f glbal-mean trpsphereare determined and evaluated. surface frcing difference is cmparable t that frm 2 x CO2, The quality f results frm the GCM AMIP simulatin is discussed thrughly by Liang et al. [1995]. A brief summary f sme imprtant features fllws. The glbal distributin f ttal clud amunt, determined by randm verlap f all cludy layers in a given clumn, clsely resembles that frm satellite bservatins (Internatinal Satellite Clud Climatlgy Prject (ISCCP) [Rssw et al., 1985]). Like in mst GCM climates, systemati cld biases exist in the midtrpsphere t upper trpsphere and plar stratsphere. Additinally, atmspheric misture cntent is underestimated, especially ver the trpical cean. Annual surface temperature acrss Asia is als underestimated. suggesting that the new parameterizatin wuld have a measurable impact n the reference GCM climate simulatin. The difference (GR2-GR1) in altitude-latitude LW cling and SW heating distributins are shwn in Figure 8. It is evident (Figures 8a and 8b) that the difference in LW frcing is largest in the upper trpsphere (10-12 km) f the trpics and causes relative cling f as much as 0.12 deg d - (which is significant relative t the mean IR cling rate at that altitude f abut 2øC d-q). The increased emissin frm the trpsphere due t mre transmissive cirrus in GR2 results in a small relative heating (less than deg d - ) f the stratsphere between 20 and 30 km. Net heating ccurs at the high

9 JOSEPH AND WANG: CIRRUS RADIATIVE PARAMETERIZATION FOR GCM 9509 Jan Ttal Cluds (%) is (a) 16 Jan Cirrus Cluds (%) (b) N 60N 30N gq ,,,,,,,,!,,,,, i 6 1V 30N Q 30S Jul Ttal Cluds (%) (c) Jul Cirrus Cluds (%) (d) E l 2 = N 6ON 30N 30S 60S 90S 90N 60N 30N ]q,q S Figure 7. Altitude-latitude mean distributin f layer clud amunt fr all cludy layers in Figure 7a January and Figure 7c July, and fr all cludy layers in Figure 7b January and Figure 7d July defined as cirrus layers. The cntur interval is 5% (shaded >,13%) fr ttal cluds and 0.4% (shaded >1.6%) fr cirrus cluds. Altitude is in kilmeters. latitudes f the nrthern hemisphere (NH) summer (Figure 8b), which is indicative f a psitive difference in LW frcing caused by less transmissive cirrus in GR2 at these latitudes. Fr the slar case (Figures 8c and 8d) the difference in the vertical heating distributin (GR2-GR1) als change sign frm the trpics t the midlatitude t high latitude f the summer hemisphere. In the trpics, relative cling ccurs abve cirrus cluds and a slight warming ccurs belw. The abve-clud relative cling crrespnds t the decreased slar absrptin because f the reductin in the upward flux reflected by less reflective cluds in GR2. Relative warming at lwer levels is attributed t the increase in dwnward flux transmitted by the cluds. The ppsit effect is apparent at the midlatitudes t high latitudes f the summer hemisphere: Table 4. Difference (GR2-GR1) in SW, LW, and Net Glbal Mean Radiative Frcing fr the Trpsphere-Surface, Trpsphere, and Surface January July Annual System SW LW Net SW LW Net Net TRPS-SURF TRPS SURF TRPS, trpsphere; SURF, surface; TRPS-SURF, trpsphere-surface. Values are in W m -2.

10 ß 9510 JOSEPH AND WANG: CIRRUS RADIATIVE PARAMETERIZATION FOR GCM 24 Jan LW Cling (K/day) (a) (GR2-GR1) Jul LW Cling (K/day) (b) (GR2-GR1) 2O v 14 :3 12 : 1. /! :.. 'v.' /:.-,,.,- :%':'--"'- '%.',, \ ;/,., '"" -. )1 t, ; ; h ;, i '-,'', n ' '!! ' ' gq i]8 68 [t8 26 Jan SW Heating (K/day) (c) (GR2-GR1) Jul SW Heating (K/day) (d) (GR2-GR1) '! k,,,., ; \.. """ /...,,-J/'"'" -. [', 0 \,.,.,,,,,,. 60N 30N EQ N 6ON 30N EQ 30S 6OS 90S Figure 8. Altitude-latitude mean difference f (a) January and (b) July LW cling (deg d - ) and (c) January and (d) July SW heating (deg d- ). The differences are between calculatins f GR1 and GR2 (GR2-GR1). Cntur intervals are deg d - and deg d -. Shaded values are >0.1 deg d - and negative values are dashed cnturs. Altitude is in kilmeters. relative warming (relative cling) abve (belw) cirrus cluds. The abve-clud warming here is strngest in July, indicating that the increased clud reflectivity is largest in the NH summer Dminant Cirrus Prperties in Radiative Frcing The altitude-latitude heating difference distributin in Figure 8 reflects enhancement and reductin f cirrus albed and greenhuseffect at high latitudes f the summer hemisphere (with the maximum ccurring in July) and the trpics in GR2, respectively. These differences in cirrus albed and greenhuse effect derive frm differences in cirrus ptical depth and emissivity between GR1 and GR2, which is evident in the difference distributir/s (GR2-GR1) f cirrus SW ptical depth and emissivity pltted in Figure 9. The altitude-latitude mean SW ptical depth (Figure 9a and 9b) increases GR2 at high latitudes f the summer hemisphere, with the largest increase ccurring in July. Fr example, the ptical depth in GR2 increases by 0.25 (>100%) pleward f 40ø-50øS in January but increases by mre than 0.85 (> 100%) pleward f 40ø-50øN in July. Beynd these znes, the ptical depth in GR2 reduces frm that in GR1, except fr a relatively small area f increase at 80øN. The mst substantial reductins exist at the trpics (as much as 0.55; >60%) and midlatitudes t high latitudes (30 ø- 60øN) f the NH winter (>1.8; 80%). The abve changes in SW ptical depth lead t changes in cirrus albed that determine the heating difference distributins. The nly exceptins are the ptical depth increase centered at 80øN and the decrease at midlatitudes t high latitudes (30ø-60øN) f the NH winter in January. The respnse t these changes is negligible in the altitude-latitude heating (Figure 8) because the magnitude f the SW respnse is prprtinal t the amunt f insulatin received, and in January, that amunt is relatively small. The emissivity differences (GR2-GR1; Figures 9c and 9d) exhibit a similar znal pattern as that f the SW ptical depth: decreases in the trpics and midlatitudes t high latitudes f the NH winter and increases at high latitudes f the summer hemisphere. Generally, these features als explain the change in clud greenhus effect that leads t the radiative respnse

11 JOSEPH AND WANG: CIRRUS RADIATIVE PARAMETERIZATION FOR GCM Jan SW Optical Depth Difference (a) (GR2-GR1) Jul SW Optical Depth Difference (b) (GR2-GR1) E 8 6 ;" / -. 5'",, [ /l%! ',.l: 1 6 [..5- :. 0 i i i,, m, i,! 90N '60N' '30N' q '308' N 60N 30N EQ ' Jan Emissivity Difference (c) Jul Emissivity Difference 8 (GR2-GR1) (d) (GR2-GR1) E t = 8 6 / " "-. 35 ;';:...:, //¾'... ',/,,; ; ;,,,,,,,/; -.j,/,',, {( t.-'. )5 '"-; 5:'-"' N m m I I i I I i I m!!,, i,, 0,, I,,!,, I,, m,, m,, 60N 30N EQ g0s g0n 6ON 30N EQ g0s Figure 9. Altitude-latitude mean difference f (a) January and (b) July cirrus SW ptical depth and (c) January and (d) July cirrus emissivity. The differences are between calculatins f GR1 and GR2 (GR2-GR1). Cntur intervals fr ptical depth are -0.55, -0.35, _+0.05, 0.25, and 0.55; and cntur intervals fr emissivity are -0.45, -0.15, +_0.05, and Dash cnturs are negative values. Altitude is in kilmeters. depicted in Figure 8. One exceptin is the lack f crrespn- sity that varies meridinally and with height accrding t emdence between the trpics and high latitude f the summer pirically derived parameters [Kiehl et al., 1994]. In GR1 calcuhemisphere f the prprtinality in the radiative respnse t latins, the IWP glbal distributin has maximums in the the difference in emissivity. The LW radiative respnse is als trpics and winter hemisphere. The glbal distributin f IWP influenced by the temperature cntrast between cirrus clud in GR2 is determined by the glbal temperature distributin. tps and the surface which is largest in the trpics. Fr exam- Figures 10c and 10d depict the altitude-latitude mean temperple, this temperature cntrast in January pleward f 60øS is ature distributin fr January and July, respectively, superim K, whereas at the equatr it is K. The clud green- psed with the altitude-latitude mean cirrus distributin. Thuse effect is prprtinal t the magnitude f this temper- gether, the temperature and clud distributin plts ature cntrast; hence, the LW clud frcing and changes in the qualitatively explain the altitude-latitude mean distributin f frcing are larger in the trpics than at higher latitudes. IWP frm GR2. Trpical cirrus in bth mnths have temper- The cirrus SW ptical depth and emissivity differences dis- atures that range frm 190 t 225 K, and midlatitude t highcussed abve are largely determined by the IWP difference latitude cirrus reside at temperatures between 207 and 250 K. between GR1 and GR2. Figures 10a and 10b resemble that f Mrever, midlatitude cirrus between 8 and 12 km are 5-10 K the SW ptical depth and emissivity. The IWP difference dis- warmer in the summer hemisphere than in the winter hemitributin (Figures 10a and 10b; GR2-GR1) can be explained by sphere and, in the NH summer, als exist in larger amunts cnsidering the glbal distributin f IWP in each calculatin. than in the winter hemisphere. At higher latitudes, the clud The IWP parameterizatin in GR1 is a prescribed water den- distributin in the summer hemisphere has mre lwer (warm-

12 9512 JOSEPH AND WANG: CIRRUS RADIATIVE PARAMETERIZATION FOR GCM Jan IWP Difference (gm-2) Jul IWP Difference (gm'2) (a), ;, (GR2-GR1). (b) (GR2-GR1) v 12 E 10-8!'. ;...;,,,,' -.7 [/J i?.. ; c:, ':, -... I ß, t ' ,,!,,!,,!,,!,, i, 90N 8ON 30N N 60N 30N S Jan Temp (K)/Cirrus Cluds (%) Jul Temp (K)/Cirrus Cluds (%) N 60N 30N EQ 30S 60S 90S 90N 60N 30N EQ 30S 60S 90S Figure 10. Altitude-latitude mean difference (GR2-GR1) f IWP (g m -2) fr (a) January and (b) July. Cntur intervals are _+0.7,_+3.5, and _+7.7 and shaded values are greater than 7.7. Negative values are dashed cnturs. Altitude-latitude distributin f temperature (øk) fr (c) January and (d) July superimpsed with that f cirrus clud layer amunts (percent) fr January and July, respectively. The cntur interval fr temperature is 10 Køand the shaded values fr cirrus clud amunt are >0.4% and > 1.6% fr light and dark shades, respectively. Altitude is in kilmeters. er) cluds as cmpared t the winter hemisphere. The largest amunts in the NH summer are generally larger than that f IWP values frm the temperature-based IWP parameteriza- crrespnding cirrus layers in the SH summer, causing IWP t tin, therefre, crrespnd t these cmparatively warm cirrus be larger in the NH summer f GR2 calculatins. The margin in the summer hemisphere. As discussed abve, IWP values in f difference (GR2-GR1) f the resulting radiative prperties GR1 are largest in the winter hemisphere and the trpics. This is, therefre, greater in the NH summer than in the SH summer. cntrast leads t psitive differences (Figures 10a and 10b) f The negative summer hemisphere SW and trpical LW frcver 7.7 g m -2 (> 100%) in the high latitudes f the summer ing difference that manifest in the heating difference and that hemisphere and negative differences in the trpic f as much dminate the glbal frcing difference largely result frm the as -3.5 g m -2 (-80%) and in the midlatitude f the NH cntrast in IWP between GR1 and GR2. The temperature winter f mre than -7.7 g m -2 (> 100%). Figures 10c and 10d dependence f IWP in GR2 enables it t be mre lcally als explain the dminant NH summer radiative effect (e.g., respnsive than that in GR1. The reliability f IWP in GR2 at the factr f tw larger magnitude f znal-mean SW frcing high latitudes and the trpics is legitimately questined as it difference in the NH summer as cmpared t that in the derives frm bservatins f midlatitude cirrus cluds, but, at summer hemisphere (SH) summer). Cirrus temperatures and least in the trpics, it appraches values cmparable t that

13 JOSEPH AND WANG: CIRRUS RADIATIVE PARAMETERIZATION FOR GCM 9513 frm recent aircraft measurements reprted by Heymsfield and McFarquhar [1996]. At temperatures typical f cirrus layers in the trpics, the IWP in G R2 differed frm that reprted by Heymsfield and McFarquhar [1996] nly as much as 0.33 g m-2 (16%) fr a 1 km thick clud. Nte that the IWC parameterizatin in G R2 simulatins at trpical latitudes was scaled t the upper bund f the mean bserved IWCs reprted by Heymsfield and Platt [1984]. Cirrus amunts in the present study, as mentined in sectin 5.1, are t few when cmpared t ISCCP. Mre cmparable amunts are btained when ISCCP criteria fr high-level cluds arc applied t the ttal clud distributin. The difference between this ISCCP-defined cirrus distributin and that frm the present study is that the frmer has mre lwer-level cluds. The results abve, hwever, d nt change significantly with the ISCCP-defined cirrus. The present cirrus amunts are cmparable t ISCCP at high latitudes f the summer hemi- cluds replicated in the mdel. The shrtwave flux was shwn t be very sensitive t these variabilities, but a substantial shrtwave bias remained even fr the upper limits f the variabilities. The encuraging results frm the abve mdel-t-bservatin cmparisns reflect favrably n the applicability f temperature-based cirrus parameterizatins with state-f-the-art cirrus radiative treatment in climate mdels. Althugh the shrtwave bias is trubling, the results imply that achievement f better agreement may depend mre n systemic imprvements in radiative transfer treatments than imprvements f cirrus clud ptics. Glbal initial radiative frcing experiments were cnducted t estimate the ptential impact f the new parameterizatin n a reference GCM simulated climate. The new parameterizatin reduced glbal- and annual-mean radiative frcing f the surface-trpsphere system by W m -2. The lss f sphere, thus there is n change in the dminant radiative effect there. At the summer midlatitudes and trpics, many f the additinal lwer (warmer) level cluds in the ISCCP-defined distributin exceed the ice clud temperature threshld. Amng the lw level cluds that d nt exceed the threshld, slar energy in the high latitudes f the summer hemisphere and lss f infrared energy in the trpics dminated this frcing result. Enhanced clud albed and reduced clud greenhuse effect in these regins are respnsible fr the slar and infrared energy lsses, respectively. Furthermre, these differ- IWP differences (GR2-GR1) are relatively small as cmpared ences in the new cirrus radiative prperties primarily stem t that assciated with higher cluds. The additin f these cluds, therefre, is nt significant in the radiative difference frm the temperature dependent ice water path which is different frm that in the existing parameterizatin. In fact, it --2 since the dminant IWP difference assciated with the higher results in values that are larger by mre than 7.7 g m cluds is cmmn t bth distributins. In the winter hemi- (> 100%) in the high latitudes f the summer hemisphere and sphere, IWP differences are enhanced with the ISCCP-defined distributin, but the radiative cntributin f this regin t the glbal frcing difference is small because f the relatively small smaller by as much as 3.3 g m -: (80%) in the trpics and mre than -7.7 g m -2 (>100%) in the midlatitudes f the NH winter. Althugh the temperature dependent ice water path amunt f slar radiatin received. derives frm bservatins f midlatitude cirrus cluds, it was shwn that, at least in the trpics, its values apprach that frm 6. Summary and Cnclusin recent bservatins. These results reinfrce the imprtance f n-ging bservatinal effrts t characterize the vertical dis- An interactive cirrus radiative parameterizatin suitable fr glbal climate mdels was develped. The shrtwave cmpnent was adapted frm Fu [1996] with minr mdificatins. In the lngwave, a new mass absrptin cefficient was parameterized t calculate cirrus emissivity. The slar and infrared prperties are bth given as a functin f ice water cntent and effective particle size. These relatinships are based n analytical slutins that assume that cirrus cluds are primarily cmpsed f hexagnal ice crystals. Additinally, the micrphysical prperties are parameterized in terms f temperature using reprted bservatins f midlatitude cirrus cluds that tributin f clud water at lw and high latitudes. The W m -2 glbal- and annual-mean frcing difference resulting frm implementatin f the interactive cirrus parameterizatin shuld have a measurable impact n GCM climate simulatins. The magnitude f this frcing is almst cmparable t that frm 2 x CO:. Dcumentatin abunds as t the climatic imprtance f the latter frcing. The climate respnse f the frcing in the present study, hwever, will depend n the glbal heating distributin. Liang and Wang [1997], using an imprved vertical clud distributin scheme, shw that cirrus effects are greatly amplishw a systematic relatinship between particle cncentratin fied thrugh a radiative-dynamic-hydrlgic feedback prcess and temperature. This link prvides a means fr the radiative prperties t respnd interactively with the lcal climate f the mdel. In mdel-t-bservatin cmparisns, it was shwn that the mdel with the new parameterizatin calculated realistic infrared radiatin and imprved slar radiatin incident at the surface. In particular, biases in the mdel's slar direct and at lw latitudes. Lhmann and Reckner [1995] shw the sensitivity f this prcess t cirrus emissivity. The interactive cirrus parameterizatin will be cmbined with the vertical clud distributin scheme f Liang and Wang t further access the rle f clud gemetry and cirrus prperties in this imprtant prcess. Finally, the cirrus parameterizatin will be further evaluated and imprved in mdel-t-bservatin cmparisns as diffuse fluxes were reduced by 75 W m -2 (60%) and 44 W m -2 mre bserved data becme available frm ARM and ther (40%), respectively. These biases were reduced mainly because clud and radiatin measurement prgrams, particularly at the the new parameterizatin results in a shrtwave ptical depth trpics and high latitudes. that is systematically smaller than that frm the existing parameterizatin and absrptin at slar wavelengths that is mre characteristic f ice cluds. Acknwledgments. This study is part f Everette Jseph's dctral Additinal mdel-t-bservatin experiments were cn- dissertatin at SUNYA. We are grateful t Jseph Michalsky, X.-Z. Liang, and the annymus reviewers fr their many helpful cmments ducted t examine the sensitivity f the lngwave and shrtn this paper. This research is supprted by the Atmspheric Radiawave fluxes t variability in the temperature dependent ice tin Measurement Prgram (ARM), Office f Bilgical and Envirnwater cntent and in the tp and base heights f bserved mental Sciences, Department f Energy.

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