Retrieval of surface radiative fluxes on the Dlarginal zone of sea ice from. operational satellite data

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Annals f Glacilgy 7 993 Internatinal Glacilgical Sciety Retrieval f surface radiative fluxes n the Dlarginal zne f sea ice frm.

1 Annals f Glacilgy Internatinal Glacilgical Sciety Retrieval f surface radiative fluxes n the Dlarginal zne f sea ice frm. peratinal satellite data CLAUDE KERGOMARD, BERNARD BONNEL AND YVES FOUQUART Labratire d'optique AtmspMrique, Universiti des Sciences et Technlgies de Lille, F Villeneuve d' Ascq Cedex, France ABSTRACT. With the develpment f cupled atmsphere-cean mdels fr the plar seas, there will be a great need f surface radiatin budget data ver partially ice-cvered sea surfaces f the marginal ice znes. This paper presents an attempt t retrieve the surface radiatin budget cmpnents ver the Fram Strait area frm peratinal satellite data, i.e. A VHRR visible and infrared radiances and SSM/I passive micrwave brightness temperatures. The clud ptical thickness, which is the main mdulatr fr the incming slar flux, is retrieved frm A VHRR visible radiances thrugh a radiative transfer mdel, assuming surface cnditins deduced frm the SSM/I ice cncentratins. The clud base emissivity, required fr the dwnwelling infrared flux cmputatin, is linked t the ptical thickness thrugh the liquid water path. The results presented shw a gd agreement with field measurements and little sensitivity t the clud and aersl prperties extracted frm the literature rather than frm the satellite data. Infrared fluxes retrievals wuld hwever require a better knwledge f the atmsphere temperature prfile and clud base altitude. INTRODUCTION The surface radiatin budget determines the surce r sink f heat fr the surface water and sea ice in the plar ceans. It is therefre an imprtant cmpnent f the mass budget f plar sea ice (Makyut, 986). Imprvements in the treatment f plar sea ice in glbal climate mdels and the develpment f reginal cupled ceanice mdels are at present in the scpe f the plar climate research cmmunity. They wuld require a systematic recrd, ver the full extent f the sea-ice field, f the surface radiatin budget with a desired accuracy better than 5-0 W m- 2 fr mnthly averages f reginal fluxes (WRCP, 99). Satellite remte sensing is the nly way t sample adequately such data ver plar ceans, but present sensrs n bard peratinal satellites prvide nly narrw-band, directinal radiances frm the tp f the atmsphere rather than large-band slar and infrared fluxes frm and t the surface. This requires extensive radiative transfer mdelling in rder t retrieve the surface radiatin budget cmpnents. The surface radiatin budget ver the plar ceans in summer (daytime) can be written as Qnet = S ( - a) - at 4 + L where S is the dwnwelling slar (shrtwave) irradiance, a the surface albed, at 4 the emitted (lngwave) upwelling flux density and L the lngwave dwnwelling flux density. The main mdulatrs fr the surface radiatin budget ver plar ceans are (I) The clud cver acting n bth the slar and lngwave dwnwelling fluxes. As pinted ut by Herman and Gdy (976), extensive layers f stratifrm cluds (Arctic stratus) are frequently bserved ver the icecvered ceans, where the mean clud amunt in summer rises up t 90%. Their effect n the incming slar flux by reflecting the incident slar radiatin back t space (albed effect) is related t their ptical thickness; their effect n the dwn welling lngwave flux by absrbing and reradiating the lngwave radiatin emitted frm the surface (greenhuse effect) is related t their emissivity and t the clud base temperature. (2 ) The ice cver acting n the surface albed and the surface temperature, that is t say the emitted lngwave flux. In the Marginal Ice Zne (MIZ) f the plar ceans, fractinal sea ice cver (ice cncentratin) may vary in a wide range frm 0 t 00% surface albed will therefre vary between 0. and 0.6 r even mre when snw cver persists n the sea ice. In summer cnditins hwever, the effect f the ice cver n the emitted lngwave radiatin is slight because the surface temperature f sea ice and water d nt vary much arund the freezing pint. The main difficulty in retrieving the surface radiatin budget frm satellite data fllws frm the fact that bth sea-ice cncentratin and clud cver prperties have t be bserved simultaneusly. Strng radiative interactins ccur between them and have t be accunted fr when deriving the radiative fluxes incming t the surface frm satellite radiances measured at the tp f the atmsphere. In this paper, we present and discuss an attempt t retrieve surface radiative fluxes frm peratinal satellite data using rather sphisticated radiative transfer mdels. We fcus n the methd and therefre limit the presented results t ne day in late summer (3 20

Fig.. NOAA-9/AVHRR channel image ver Fram Strait fr 3 August 988, 030 h UT. The latitude and lngitude grid delineates the area f interest. August 988) and t the Fram Strait area (78-82 N, 0 W-20 E).

2 Fig.. NOAA-9/AVHRR channel image ver Fram Strait fr 3 August 988, 030 h UT. The latitude and lngitude grid delineates the area f interest. August 988) and t the Fram Strait area (78-82 N, 0 W-20 E). This date and these areas were selected accrding t the availability f in situ measurements acquired during the ARCTEMIZ 88 ceangraphic campaign and A VHRR data frm Trms0 Satellite Statin selected during the same experiment. The Fram Strait area is a typical marginal ice zne (MIZ) with ice cncentratins varying in a wide range frm 0 (ice-free cean) t mre than 80% in the plar pack-ice nrth f Fram Strait. The selected satellite scene is typical fr latesummer cnditins with an extensive arctic stratus clud cver ver the pack ice and stratcumulus ver the icefree cean alng the casts f Spitsbergen; clud-free areas extend alng the ice edge (Fig. I). CLOUD OPTICAL TIDCKNESS RETRIEVAL Variatins f slar flux incming at the cean surface depend mainly n the sun angle and n the clud ptical thickness; ther factrs, like the atmspheric absrptin and scattering by mlecules and aersls, are f secnd rder and may be cnsidered cnstant ver the area. In the same manner, incming lngwave flux is mainly cntrlled by clud-base temperature and clud emissivity. In climate mdels, ptical thickness and emissivity f liquid water cluds are bth linked t the liquid water path thrugh realistic parameterizatins, which allws us t deduce emissivity frm the ptical thickness. As a first steh we therefre cncentrated 0 ptical thickness retrieval. We used a radiative transfer mdel, called GAME, specially designed t simulate directinal 202 AVHRR channel r channel 2 radiances at the tp f the atmsphere, with realistic atmspheric cnditins including gaseus absrptin and diffusin due t aersl and clud layers. The cde is based upn the discrete rdinate methde (Stamnes and thers, 988). The requested inputs are spectral reflectance f the surface, atmspheric cntent f absrbing gases (mainly zne and water vapur), aersl ptical prperties, vertical distributin and density (i.e. ptical thickness), and clud layer altitude, gemetrical thickness, drplet size distributin and ptical thickness. The standard utputs are the simulated AVHRR radiances fr the selected sun and satellite viewing-angle cnditins. In this study, the mdel has been used in an iterative scheme t btain the ptical thickness f the stratus clud cver. Sme f the requested input data have been extracted frm the literature. This is the case fr mst f the aersl parameters (ptical prperties and vertical distributin), which have been chsen accrding t D' Almeida and Kepke (990); aersl ptical thickness has been adjusted frm the AVHRR reflectance f cludless and ice-free cean pixels. Fr Arctic stratus clud prperties, including altitude, gemetrical thickness and drplets size distributin, we selected a typical case frm the large data set f Herman and Curry (984). The zne cntent f the atmsphere has been taken frm SAGE satellite data, while water vapur cntent and vertical distributin were set accrding t radisunding frm Bear Island (Bj0rn0ya), abut 500 km sutheast f Fram Strait. All these data were cnsidered cnstant ver the area. The ther input data have been btained frm the satellite data, and averaged n a plar steregraphic grid

with a spatial reslutin f 25 km. The Fram Strait area as delimited n Figure I is made f 300 pixels (r gridpints) with this reslutin, including each abut 550 AVHRR pixels. DMSP-SSM!

3 with a spatial reslutin f 25 km. The Fram Strait area as delimited n Figure I is made f 300 pixels (r gridpints) with this reslutin, including each abut 550 AVHRR pixels. DMSP-SSM!I passive micrwave data prvided seaice cncentratins fr the 300 pixels f the Fram Strait area (Fig. 2); the SSM/I 9 GHz (vertical and hrizntal plarisatins) and 37 GHz (vertical plarisatin) brightness temperatures were cnverted int ice cncentratins by using the NASA SSM!I algrithm (Cavalieri and thers, 984). Ice cncentratins prvided the surface spectral reflectance requested by GAME accrding t icefree water and sea-ice values read frm the AVHRR data. Infrared radiances frm A VHRR channels 3 and 4 were used t get the fractinal clud cver fr every gridpint the difference in daytime radiances between AVHRR Fig. 2. Ice cncentratin ver the Fram Strait area, btained frm SSM!I brightness temperatures. Fig. 4. Retrieved Arctic stratus cluds ptical thickness melting white sea-ice......[... j t t t j ,.. J..T. I. L = L F.. e i channel 3 (3.7 Jlm ) and 4 ( Jlm ) is a very efficient ice! clud discriminatr, since such water cluds as Arctic stratus are highly reflective in the lngwave part f the slar spectrum (Fig. 3) and ice r snw surfaces may be cnsidered blackbdies at the same wavelengths (Raschke and thers, 992). AVHRR channel (0.6 Jlm ) reflectances were calibrated as accurately as pssible and averaged fr every grid pint these average reflectances were cmpared with the values simulated by GAME t btain the ptical thicknesses f the stratus clud cver (Fig. 4). The main difficulty in ptical thickness retrieval arises frm the fact that, fr high ice cncentratins, i.e. high surface reflectances, reflectance bserved at the tp f the atmsphere first decreases with increasing ptical thicknesses fr small values, and then increases with higher,...[ ;, \ ', \ T-I0. I ' \ ' ;-. r.,!- T j \,' l l\,',( ,......! , f \ / T wavelength (J.lm) Fig. 3. Spectral reflectance curves fr sea ice and stratus cluds. The ice curve is fr melting white ice (Grenfell and Pervich, 984). The stratus curve is cmputed fr a typical stratus with ptical thickness T = 0 ver the sea and slar zenith angle (} = I'l S... j... L i" - ' c 0.3!il... u Optical thickness 6 20 Fig. 5. Variatins f the simulated AVHRR channel reflectance with the clud ptical thickness and ice cncentratin. (Slar zenithal angle B = 7S'. Radimeter viewing angle (}y = 0. ) 203

$The prblem has been avided by the previus fractinal clud cver determinatin using channels 3 and 4 f the A VHRR the gridpints with very lw clud cver were cnsidered clud-free and the ptical thickness$

4 Kergmard and thers Retrieval f surface radiative jluxes values (Fig. 5). This leads t tw different slutins fr the cmputatin f the stratus ptical thickness. The prblem has been avided by the previus fractinal clud cver determinatin using channels 3 and 4 f the A VHRR the gridpints with very lw clud cver were cnsidered clud-free and the ptical thickness set t 0; fr cludy gridpints, the highest ptical thickness (cnsidering a cntinuus stratus layer) was cnsidered mre likely. SURFACE RADIATIVE FLUXES RETRIEVAL Frm the retrieved ptical thickness, the incming, absrbed and reflected slar fluxes were cmputed by using a tw-stream narrw band mdel based n the methd f Zdunkwski and thers (980). This mdel accunts fr gaseus transmissin, aersl and clud absrptin and scattering in 98 spectral intervals between 0.25 and 2.8 Jl.m; its accuracy was prved within the framewrk f the Intercmparisn f Radiatin Cdes in Climate Mdels (IRCCM). As gaseus and aersl effects are cnsidered cnstant ver the area, and variatins f the slar zenithal angle are f secnd rder, the map f retrieved incming shrtwave flux is very similar t that f ptical thickness. At the time f the satellite pass (030 hut), the incming slar flux S! varies in the range f250 W m- 2 fr the clud-free areas t less than 60 W m -2 under a stratus cver with an ptical thickness arund 20. The map f the absrbed slar flux S! ( - a) (Fig. 6) is very different due t the albed f sea ice cver the higher values are fund ver clud-free and ice-free areas (up t 90 W m- 2), while the pack-ice areas with dense stratus cver shw values f 50 W m- 2 r less. The spectrally averaged albed f the sea ice cver des nt depend nly n ice cncentratin due t the sharp cntrast f the spectral reflectance f sea ice between visible and infrared wavelengths and the filtering f infrared by stratus cluds, the ttal albed f sea ice increases with the ptical thickness f the clud cver. Fig. 6. Retrieved absrbed slar flux jr 3 August 988 at 030h UT. 204 The dwnwelling lngwave flux was cmputed using a narrw-band mdel (Mrcrette, 984), where scattering effects are neglected and clud drplets are cnsidered as purely absrbing particles. In such a mdel, the lngwave flux is related t surface temperature, water vapur amunt and vertical distributin, clud height (clud base temperature) and emissivity. The first fur f these parameters were fixed accrding t surface bservatins and radisnde data fr Bear Island. The clud emissivity has been linked t the ptical thickness T thrugh the clud liquid water path (LWP) using = - exp( -klwp) and T= 3.LWP 2.re where k is the mean ver the whle lngwave spectrum f the mass absrptin cefficient f liquid water and re is the effective radius f the clud drplets (re = 8.7 J.m fr the chsen stratus). As emissivity is the nly parameter assumed t vary ver the area, the map f the retrieved lngwave flux is similar t that f the ptical thickness, but with values ranging frm 230 W m - 2 fr the clear areas t abut 300 W m- 2 under the dense stratus cver. Lking further at the results shws that the mean daily clud frcing f an Arctic stratus cver with ptical thickness T = 2.5, in late summer cnditins (3 August ), may be estimated t - 30 W m- 2, with a shrtwave frcing abut - 85 W m- 2 and a lngwave frcing abut + 55 W m- 2. Assuming a surface temperature f + I DC ver the pen sea and -I DC ver the ice, such a clud cver shuld lead t a decrease f the dailyaveraged surface radiatin budget abut -20 W m-2 ver the pen sea and t an increase f + 5 W m -2 ver a cntinuus sea-ice cver. This pints the need t accunt accurately fr clud-ice interactins in future cupled cean-atmsphere mdels. ACCURACY AND DISCUSSION Tw different methds have been used t assess the accuracy f the retrieved fluxes. The first cnsisted f a cmparisn f the retrieved values with in-situ measuremnts, the secnd in a few tests f the sensitivity f the results t the chice f parameters extracted frm the literature. Validatin data were btained fr 3 August frm a pyranmeter and a pyrgemetcr n bard the ceangraphic ship Crys, with nly a shrt break in the mrning (Fig. 7). During this day the ship mved in the area ' N t 8 Nand E. In the mrning, clud cnditins were vercast, with a lw stratus clud cver, clearing at abut 00 h UT. In the afternn the sky remained clear with ccasinal passing clud-banks. Frm satellite data, we fund that the ship crssed the edge between a stratus-cvered area with ptical thickness T 2.5, and a clud-free area. When cmparing the simulated fluxes with the measured nes, a very gd agreement was fund, within the ± IOWm- 2 interval fr bth the shrtwave and lngwave dwnwelling fluxes. Hwever, the lngwave flux appeared very sensitive t surface and clud-base temperatures we had t crrect the values prvided by the radisnde data frm Bear Island, accrding t the lcal bservatins by

--bserved SW flux _..._... bserved LW flux -2 W. m simulsw flux clear sky -. - simul. SWT-2.5 350 r-t"""'''''-'"""t""t""'r-t'''''''ii""'i'-i - - - simul.l W flux clear sky - - - simul. LWT-2.

5 --bserved SW flux _..._... bserved LW flux -2 W. m simulsw flux clear sky -. - simul. SWT r-t"""'''''-'"""t""t""'r-t'''''''ii""'i'-i simul.l W flux clear sky simul. LWT '",'. ' TIme Fig. 7. Cmparisn f bserved and cmputed radiative jluxes fr 3 August 988. the crew f the Crys, in rder t retrieve reasnable values f the lngwave flux. Amng the parameters used in the clud ptical thickness retrieval, plar aersl ptical prperties and stratus clud drplet-size distributin were extracted frm the available literature. Althugh the particular chice we made appears the mst reasnable, it remains arbitrary; we thus tested the sensitivity f the retrieved shrtwave irradiance t these parameters. W e cmpared the retrieved fluxes btained in the mst likely case f plar aersl (single scattering albed W = 0.964) with values retrieved assuming a highly absrbing aersl (w = 0.8), like st r industrial dust (Fig. 8a ); the mdified value has been used in bth the clud ptical thickness retrieval and flux cmputatin. In the same manner, we cmpared the fluxes cmputed in ur case f lw stratus (drplets effective radius re = 8.7 Jl.m ) t thse btained assuming high cluds with cnsiderably smaller drplets (re = 5 )lm) (Fig. 8b). The results shw a slight sensitivity f the retrieved values t these changes, with differences in the retrieved flux es smaller than ± lowm- 2 Frm the study as a whle we cnclude that shrtwave fluxes ver plar ceans might be btained n a climatlgical basis frm jint use f AVHRR and SSM/I data, with the desired accuracy fr cupled cean- atmsphere mdels. It implies that the different clud types ccurring in high latitudes were previusly discriminated frm the A VHRR imagery and accurately parameterized, as we did fr Arctic stratus. Infrared radiatin fluxes cmputatin is mre prblematic and wuld require accurate additinal data n surface and clud-base temperature. In a further wrk, we will test if the present satellite sunding sensrs, like the TOVS n bard the NOAA satellites, culd prvide this additinal infrmatin with the desired accuracy. Cess and Vulis (989) prpsed t infer the net surface slar flux directly frm satellite bservatins f the radiatin budget at the tp f the atmsphere. This methd is independent f the surface albed, and is thus particularly interesting in the case f highly variable surface albed, fr example in marginal ice znes. Hwever, satellite bservatins d nt prvide a direct measurement f radiatin fluxes, i.e. spectrally integrated irradiances, but radiances in narrw spectral intervals. The prblem thus lies in the cnversin f narrw band t brad band and f radiances t irradiances. These cnversins are strngly dependant n the spectral characteristics f the scene. With the availability f brad band bservatins frm the Earth Radiatin Budget Experiment (ERBE), the apprach f Cess and Vu lis may be the mst accurate; hwever ERBE data are available nly fr a limited perid f time and the accuracy f the radiance t irradiance cnversin ver sea ice is knwn t be ne f the prest. Finally, this a - "I. W.m QC) ci I 2 W.m 300r ,..., ,..., 200 ; i- f ' j..._ j effective radius f clud drplets re m W.m [.' 200. J.... l....! T. 50 l. j i 00...,......,.., !... '...[.....'' "'... l l l l '" b aersl single scattering albed r W.m- 2 Fig. B(a). Sensitivity tests fr the parameters extracted frm the literature. Aersl single scattering albed W cmparisn between retrieved slar fluxes with a single scattering albed W = ( ur case frm the literature) and a single scattering albed W = 0.8 (highly absrbing aersl). ( b). Sensitivity tests fr the parameters extracted frm the literature. Clud drplets siz.e cmparisn between retrieved slar fluxes with a typical drplet siz.e distributin fr lw Arctic stratus ( ur case frm literature ) with effective radius re = 8.7)lm and a case f higher cluds with smaller drplets (re = 5p.m). 205

6 apprach makes cmparisns with surface bservatins extremely difficult, since ne needs t measure the spatially averaged net surface fluxes, i.e. dwnward and upward fluxes. Since the upward fluxes are extremely dependent n surface cnditins, experimental valid a tins are nearly impssible in such inhmgeneus regins as the marginal ice znes. It is hwever pssible t cmpare surface net fluxes as derived frm bth methds; this will be dne in a frthcming paper. ACKNODGEMENTS The wrk f Ph. Dubuissn, wh set up the GAME radiative transfer cde, is gratefully acknwledged. ]. C. Vanhutte, M. Fily and the crew and scientific team f the ARCTEMIZ 88 experiment were invlved in cllecting the field measurements n bard NjO Crys. A VHRR imagery was prvided by the Trms0 Satellite Statin in Nrway and SSM/I data are extracted frm the CD-ROM distributed by NSIDC in Bulder, CO. This wrk was supprted by the Centre Natinal de la Recherche Scientifique under cntract f the Prgramme Natinal TeIedetectin Spatiale. REFERENCES Cavalieri, D.J., P. Glersen and W.J. Campbell Determinatin f sea ice parameters with the NIM BUS 7 SMMR. J. Gephys. Res., 89(D4), Cess, R. D. and I. L. Vulis Inferring surface slar absrptin frm bradband satellite measurements. ]. Climate, 2, Curry, J. A. and G. F. Herman Infrared radiative prperties f summertime Arctic stratus cluds. J. Climate Appl. Meterl., 24, D'Almeida, G. A. and P. Kepke. Unpublished. Atmspheric aersls, glbal climatlgy and radiative characteristics. Reprt, University f Munich. Fuquart, Y.,]. C. Buriez, M. Herman and R. S. Kandel The influence f cluds n radiatin a climatemdeling perspective. Rev. Gephys., 28 (2), Grenfell, T. C. and D. K. Pervich Spectral albeds f sea ice and incident slar irradiance in the suthern Beaufrt Sea. ]. Gephys. Res., 89 (C3 ), Herman, G. F. and]. A. Curry Observatinal and theretical studies f slar radiatin in Arctic stratus cluds. ]. Climate Appl. Meterl., 23, Herman, G. F. and R. M. Gdy Frmatin and persistence f summertime Arctic stratus cluds. ]. Atms. Sci., 33, Maykut, G. A. 986 The surface heat and mass balance. In Untersteiner, N., ed. The gephysics f sea ice. New Yrk, Plenum Press, (NATO ASI Series. Ser. B. Physics, 46.) Mrcrette, J.J On the radiative parameterisatin in a general circulatin mdel. (Ph.D. thesis, Universite de Lille.) Raschke, E., P. Bauer and H.]. Lutz Remte sensing f cluds and surface radiatin budget ver plar regins. Int. J. Remte Sensing, 3( ), Schmetz, J. 99. Retrieval f surface radiatin fluxes frm satellite data. Dyn. Atms. Oceans, 6,6-72. Stamnes, K., S. C.Tsay, W. Wiscmbe and K.Jayaweera Numerically stable algrithm fr discrete rdinate methd f radiative transfer in a multiple scattering and emitting layered media. Appl. Opt., 2, Wrld Climate Research Prgramme. 99. Reprt f the Twelfth Sessin f the Jint Scientific Cmmittee, Bremen, Germany, 8-23 March 99. Geneva, Wrld Meterlgical Organizatin Secretariat. (WMO jtd 432.) Zdunkwski, W. G., R. M. Welch and G. Krb An investigatin f the structure f typical tw-stream methds fr the calculatin f slar fluxes and heating rates in cluds. Beitr. Phys. Atmsph., 53, The accuracy f riferences in the text and in this list is the respnsibility f the authrs, t whm queries shuld be addressed. 206

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