Generalized Split-Window Algorithm for Estimate of Land Surface Temperature from Chinese Geostationary FengYun Meteorological Satellite (FY-2C) Data

Transcription

1 Senor 8, 8, enor ISSN by MDPI Full Reearch Paper Generalzed Splt-Wndow Algorthm for Etmate of Land Surface Temperature from Chnee Geotatonary FengYun Meteorologcal Satellte (FY-C) Data Bohu Tang 1,, Yuyun B 3, 4, Zhao-Lang L 1, 3,* and Jun Xa 1 1 Key Laboratory of Water Cycle and Related Land Surface Procee, Inttute of Geographc Scence and Natural Reource Reearch, Chnee Academy of Scence, Beng, 111, Chna E-mal: tangbh@gnrr.ac.cn Graduate Unverty of Chnee Academy of Scence 3 TRIO/LSIIT(UMR75 CNRS)/ENSPS, Bld Sebaten Brant, BP1413, 6741 Illkrch, France E-mal: lzl@gnrr.ac.cn 4 Inttute of Agrcultural Reource and Regonal Plannng, Chnee Academy of Agrcultural Scence, Beng, 186, Chna * Author to whom correpondence hould be addreed; E-mal: lzl@gnrr.ac.cn Receved: 3 January 8 / Accepted: 31 January 8 / Publhed: 14 February 8 Abtract: On the ba of the radatve tranfer theory, th paper addreed the etmate of Land Surface Temperature (LST) from the Chnee frt operatonal geotatonary meteorologcal atellte-fengyun-c (FY-C) data n two thermal nfrared channel (IR1, μ m and IR, μ m ), ung the Generalzed Splt-Wndow (GSW) algorthm propoed by Wan and Dozer (1996). The coeffcent n the GSW algorthm correpondng to a ere of overlappng rangng of the mean emvty, the atmopherc Water Vapor Content (WVC), and the LST were derved ung a tattcal regreon method from the numercal value mulated wth an accurate atmopherc radatve tranfer model MODTRAN 4 over a wde range of atmopherc and urface condton. The mulaton analy howed that the LST could be etmated by the GSW algorthm wth the Root Mean Square Error (RMSE) le than 1 K for the ub-range wth the Vewng Zenth Angle (VZA) le than 3 or for the ub-rang wth VZA le than 6 and the atmopherc WVC le than 3.5 g/cm provded that the Land Surface Emvte (LSE) are known. In order to determne the range for the optmum coeffcent of the GSW algorthm, the LSE could be derved from the data n MODIS channel 31 and 3

2 Senor 8, provded by MODIS/Terra LST product MOD11B1, or be etmated ether accordng to the land urface clafcaton or ung the method propoed by Jang et al. (6); and the WVC could be obtaned from MODIS total precptable water product MOD5, or be retreved ung L et al. method (3). The entvty and error analye n term of the uncertanty of the LSE and WVC a well a the ntrumental noe were performed. In addton, n order to compare the dfferent formulaton of the plt-wndow algorthm, everal recently propoed plt-wndow algorthm were ued to etmate the LST wth the ame mulated FY-C data. The reult of the ntercomparon howed that mot of the algorthm gve comparable reult. Keyword: Land urface temperature, FY-C data, Splt-wndow algorthm. 1. Introducton Land Surface Temperature (LST) not only a good ndcator of both the energy equlbrum of the Earth urface and greenhoue effect, but alo one of the key varable controllng fundamental bopherc and geopherc nteracton between the Earth urface and t atmophere. It can play ether a drect role uch a when etmatng longwave fluxe, or ndrectly a when etmatng latent and enble heat fluxe [1, ]. Moreover, many other applcaton, uch a evaportranpraton modelng [3, 4], etmatng ol moture [5], and clmatc, hydrologcal, ecologcal and bogeochemcal tudyng [6, 7] and o on, rely on the knowledge of LST. Conequently, t crucal to have acce to relable etmate of urface temperature over large patal and temporal cale. It practcally mpoble to obtan uch nformaton from ground baed meaurement, wherea the atellte obervaton n the Thermal Infra-Red (TIR) appear to be very attractve nce t can gve acce to global and temporal etmate of LST. However, the retreval of the LST from atellte data a very dffcult tak becaue, bede the radometrc calbraton and the cloud creenng procedure, three type of correcton have to be made. They are emvty correcton, atmopherc correcton and topography correcton [8]. Up to now, many algorthm for etmatng the LST from atellte obervaton have been propoed. They may be roughly grouped nto three categore: the ngle channel algorthm [9, 1], the plt wndow algorthm [11, 1] and the trple wndow algorthm [13]. The ngle channel method a mple nveron of the radatve tranfer equaton provdng that the Land Surface Emvte (LSE) and the atmopherc profle are known n advance. The trple wndow method combne two thermal wndow channel and one mddle nfrared channel to etmate the LST for nghttme atellte obervaton. The plt wndow method ued to retreve the LST baed on the dfferental water vapor aborpton n two adacent nfrared channel. Th method wa frtly propoed by McMlln (1975) [11] to etmate ea urface temperature from atellte meaurement. Snce then, a varety of plt wndow algorthm have been developed and modfed to retreve LST, and, currently, mot of them have been uccefully appled to the LST retreval from the

3 Senor 8, data oberved by the AVHRR, MODIS, and Spnnng Enhanced Vble and Infrared Imager (SEVIRI) ntrument [8, 14-]. The FengYun-C (FY-C), a geotatonary meteorologcal atellte developed by Shangha Academy of Space Flght Technology (SAST, alo known a 8 th Space Academy) and Chna Academy of Space Technology (CAST, alo know a 5 th Space Academy) and operated by Chna Meteorologcal Admntraton (CMA), wa launched on 19 October 4 and becomng fully operatonal n 6. The FY-C the Chnee frt operatonal meteorologcal atellte, whch alo the fourth atellte of the FY ere and located above the Equator at longtude 15 E, and ome 35,8 km away. The obectve of the mon to montor the temperature and the cloud above Chna and neghborng area and alo to provde meteorologcal nformaton for the Aa-Pacfc regon. The upgraded Stretched-Vble and Infrared Spn-Scan Radometer (S-VISSR) one of the maor payload onboard the FY-C. Th optcal magng radometer cont of one vble channel and four nfrared channel. The charactertc of the ntrument are hown n Table 1. It can acqure one full dc mage coverng the Earth urface from 6 N to 6 S n lattude and from 45 E to 165 E n longtude per hour and 3 mn per acquton for flood eaon. The work preented n th paper am to retreve LST from the FY-C atellte data n two thermal nfrared channel (IR1, μ m and IR, μ m ), ung the Generalzed Splt-Wndow (GSW) algorthm propoed by Wan and Dozer (1996) [1]. Secton decrbe the theory aocated wth the LST retreval ung the GSW algorthm and preent the algorthm development for FY-C data. Secton 3 gve the reult and the numercal value of the coeffcent n the GSW algorthm. The entvty and error analye n term of the uncertanty of the LSE and Water Vapor Content (WVC) n the atmophere a well a the ntrumental noe are alo preented n th ecton. In addton, n order to compare the dfferent formulaton of the plt-wndow algorthm, th ecton gve the ntercomparon of the LST etmated by everal plt-wndow algorthm. Secton 4 gve an example of retrevng LST from FY-C atellte data. The Concluon drawn n Secton 5. Table 1. Specfcaton of S-VISSR channel: pectral range and patal reoluton. Channel no. Channel name Spectral range (μm) Spatal reoluton (km) 1 IR IR IR IR VIS Theory.1. Radatve tranfer for plt-wndow algorthm On the ba of the radatve tranfer theory, for a cloud-free atmophere under thermodynamc equlbrum, the channel radance B ( T ) meaured at the Top Of the Atmophere (TOA) n a Thermal Infra-Red (TIR) channel of the enor onboard the atellte, gven wth a good approxmaton a [3]

4 Senor 8, B( T) = B( T ) τ + R + (1 ) R τ (1) atm_ atm_ where T the channel brghtne temperature oberved n channel at the TOA, B the Planck functon, B( T ) the radance meaured f the urface a black body wth urface temperature T, the channel emvty n channel, τ the total atmopherc tranmttance along the target to enor path n channel, the thermal path atmopherc upwellng radance n channel, and R atm _ R atm _ the channel downwellng atmopherc radance from the whole hemphere n channel. The frt term on the rght hand de of Eq. (1) repreent the urface emon that attenuated by the atmophere. The econd term repreent the upwellng atmophere emon toward the enor and the thrd term repreent the downwellng atmophere emon that reflected by the urface and reache the enor. Invertng Eq. (1), one can get T 1 B( T) Ratm_ (1 ) Ratm_ τ = B [ τ ] () n whch 1 B the nvere of the Planck functon. Once the channel emvty known, there are two way to etmate the LST from atellte data. One to ue Eq. () wth atmopherc radatve tranfer model uch a MODTRAN 4 [4] or 4A/OP [5], f the atmopherc profle avalable from ether conventonal radooundng or atellte oundng. The other to employ the plt-wndow algorthm developed on the ba of the dfferental water vapor aborpton n two adacent nfrared channel (McMln, 1975) [11] f the atmopherc profle not avalable. A S-VISSR enor onboard FY-C ha two adacent thermal nfrared channel (IR1 and IR), the GSW algorthm propoed by Wan and Dozer [1] adopted to etmate the LST from FY-C atellte data. Accordng to GSW algorthm, the LST can be expreed a 1 Δ T + T 1 Δ T T T a ( a1 a a3 ) ( a 4 a5 a ) = (3) wth = ( + )/ and Δ =. where T and T are the TOA brghtne temperature meaured n channel (11. μ m ) and (1. μ m ), repectvely; and are, repectvely, the land urface emvte n channel and ; the averaged emvty; Δ the emvty dfference between the two adacent channel; and a a 6 are unknown coeffcent whch wll be derved n the followng from mulated FY-C data.... Algorthm development for FY-C So far, a there no avalable databae of n tu LST meaurement n concdence wth the FY-C overpae, the only poble way to obtan the coeffcent n Eq. (3) to ue numercal mulaton for etablhng the databae ued n the tattcal regreon. To th end, the atmopherc radatve

5 Senor 8, tranfer model MODTRAN 4 wa ued to mulate the TOA radance wth the approprate thermal nfrared channel repone functon of the S-VISSR onboard FY-C. Keepng n mnd that a practcal LST algorthm hould accommodate atmopherc varaton wde enough to cover all poble real tuaton, two radoonde obervaton databae were taken nto account n our mulaton. One the latet veron of the Thermodynamc Intal Gue Retreval (TIGR) databae TIGR, whch wa contructed by the Laboratore de Meteorologe Dynamque (LMD) and repreent a worldwde et of atmopherc tuaton (311 radoundng) from polar to tropcal atmophere wth varyng water vapor amount rangng from.1 to 8 g/cm ( The other the x tandard atmopherc profle (tropcal, md-lattude ummer, md-lattude wnter, ub-arctc ummer, ub-arctc wnter, and US76) tored n the MODTRAN 4. For LST retreval, we only conder atmopherc varaton n clear-ky condton. Conequently, the profle wth relatve humdty at one of level greater than 9% n TIGR were dcarded a th eldom happen under clear-ky condton. Therefore, 1413 repreentatve atmopherc tuaton were extracted from TIGR. Fgure 1 how a plot of the atmopherc Water Vapor Content (WVC) a functon of the atmopherc temperature T n the frt boundary layer of thee elected atmophere. A hown n th fgure, the vare from 31 K to 315 K and the atmopherc WVC change from.6 g/cm to 6.44 g/cm. T 7 6 Water vapor content (g/cm ) T (K) Fgure 1. Plot of the atmopherc water vapor content a functon of atmopherc temperature n the frt boundary layer of the elected 1413 atmopherc profle n TIGR. T Takng nto account the angular dependence of the TOA radance, x dfferent Vewng Zenth Angle (VZA) (, 33.56, 44.4, 51.3, 56.5, 6 ) varyng from to 6 were ued n MODTRAN mulaton. Wth the VZA and the radooundng mentoned above a MODTRAN nput, we can obtan the channel atmopherc parameter (τ, R, R ) wth pectral ntegraton of the channel repone functon for each VZA and each atmopherc profle. In addton, n order to make the mulaton more repreentatve, the reaonable varaton of LST are vared n a wde range accordng to the atmopherc temperature T n the frt boundary layer of atm _ atm _

6 Senor 8, the atmopherc profle ued. That, LST vare from T 5 K to T + 15 K n tep of 5 K for T 9 K, and from T 5 K to T + 5 K n tep of 5 K for T < 9 K. Moreover, conderng the mot land cover, the averaged emvty varyng from.9 to 1. wth a tep of. and the emvty dfference Δ from -.5 to.15 wth a tep of.5, were ued n our mulaton [1]. Then for a gven LST, n combnaton wth the atmopherc parameter ( τ, R atm _, R atm _ ), LST ( T ) and LSE ( ), the channel brghtne temperature T at the TOA can be determned accordng to Eq. (1) wth the nvere of Planck law. At th tage, the T drectly related to the TOA meaured brghtne temperature T and T. The coeffcent a a n Eq. (3) can be obtaned through 6 tattcal regreon method. In total, for the TIGR databae and the x tandard MODTRAN 4 atmophere, dfferent tuaton were obtaned for each VZA. 3. Reult and Analy 3.1. GSW algorthm coeffcent In order to determne the coeffcent a a6n Eq. (3), Wan and Dozer (1996) [1] dvded the averaged emvty, atmopherc WVC and atmopherc urface temperature ( ) nto everal tractable ub-range for mprovng the fttng accuracy. Takng nto account the fact that the S-VISSR enor onboard FY-C ha no atmopherc oundng channel, the atmopherc urface temperature not multaneouly avalable, and thu t wll be ubttuted n th work for the determnaton of the coeffcent n Eq. (3) by the approxmate Land Surface Temperature (LST). For dfferent value of the numercal experment, n order to mprove the accuracy of the retreval LST, for each VZA a done n [1], the averaged emvty wa dvded nto two group: one vare from.9 to.96 and the other range from.94 to 1.. The WVC wa dvded nto x ub-range wth an overlap of.5 g/cm : [, 1.5], [1.,.5], [., 3.5], [3., 4.5], [4., 5.5], and [5., 6.5] g/cm. The LST, T, wa dvded nto fve ub-range wth an overlap of 5 K: T 8 K, 75 T 95 K, 9 T 31 K, 35 T 35 K, T 3 K. Then, the coeffcent a a 6 n Eq. (3) can be obtaned through tattcal regreon method for each VZA and each ub-range. A an example, Fgure dplay the coeffcent of the GSW algorthm a functon of the ecant VZA for the ub-range wth WVC from 1. g/cm to.5 g/cm, and LST varyng from 9 K to 31 K for the two emvty group. A hown n th fgure, the coeffcent a a 6 for other VZA can be lnearly nterpolated n functon of the ecant VZA. Smlar reult are obtaned for the other ubrange. T

7 Senor 8, Value of coeffcent [.9,.96] W [1.,.5] T [9K, 31K] FY-C /co(VZA) a 6 a 4 a 5 a a 1 a 3 a Value of coeffcent [.94, 1.] W [1.,.5] T [9K, 31K] FY-C /co(VZA) a 6 a 4 a 5 a 1 a a 3 a (a) (b) Fgure. Coeffcent of the generalzed plt-wndow algorthm for the ub-range wth LST varyng from 9 K to 31 K, and WVC from 1. g/cm to.5 g/cm. (a) for [.9,.96] and (b) for [.94,1.] 3.. Etmaton of LST Fgure 3 how, repectvely, the htogram of the dfference between the actual T and the T etmated ung GSW algorthm wth the coeffcent correpondng to the ub-range WV C [1.,.5], and T [9 K,31 K] for two dfferent emvty group and VZA=. The Root Mean Square Error (RMSE) between the actual and etmated T.37 K for the emvty group [.94,1.], and.48 K for the other emvty group [.9,.96]. Smlar reult were obtaned for the other VZA. [.9,.96] WVC [1.,.5] T [9K, 31K] Ba= RMSE=.48 K 3 [.94, 1.] WVC [1.,.5] T [9K, 31K] Ba= RMSE=.37 K 15 VZA= o 5 VZA= o Frequence 1 FYC Frequence 15 FYC Actual T - Etmated T (K) (a) Actual T - Etmated T (K) (b) Fgure 3. Htogram of the dfference between the actual and etmated T for the ubrange wth LST varyng from 9 K to 31 K, and WVC from 1. g/cm to.5 g/cm. (a) for [.9,.96] and (b) for [.94,1.].

8 Senor 8, LST 8K LST 95K RMSE (K).5.4 RMSE (K) /co(VZA) /co(VZA). 9 LST 31K.8 35 LST 35K RMSE (K) 1..8 RMSE (K) /co(VZA) /co(VZA) LST 3K 3. The whole range of LST.5 RMSE (K) RMSE (K) /co(VZA) /co(VZA) Fgure 4. RMSE between the actual and etmated T a functon of the ecant VZA for dfferent ub-range n two dfferent emvty group.

9 Senor 8, In addton, Fgure 4 gve the RMSE between the actual and etmated T a functon of the ecant VZA for the two emvty group wth dfferent ub-range. Takng nto account that, n realty, the lower LST uually accompaned wth much le WVC, a hown n fgure 1, Therefore, for the LST le than 8 K, the maxmum WVC.5 g/cm, whle for the LST between 75 K and 95 K, the maxmum WVC 5.5 g/cm. From fgure 4, one can ee that the RMSE ncreae wth the ncreae of the VZA. The RMSE are le than 1 K for all ub-range wth the VZA le than 3, or for all ub-range wth the VZA le than 6 and the WVC le than 3.5 g/cm. The RMSE ncreae dramatcally wth the ncreae of the VZA when the WVC larger than 3. g/cm, wth the maxmum RMSE of.7 K for the ub-range [.94,1.], WV C [5.,6.5], and T [35 K,35 K], for VZA=6. It hould be ponted out here that, n practce, the LST etmated n two tep for actual atellte data. Frtly, approxmate LST are etmated ung Eq. (3) wth the coeffcent derved for the whole range of LST provdng that the ub-range of emvty and WVC are known, and then more accurate LST are etmated once agan ung Eq. (3), but wth the coeffcent a a6 correpondng to the ub-range of LST whch determned accordng to the approxmate LST obtaned n the frt tep. Fgure 4 alo how the RMSE between the actual T and the T etmated wth the coeffcent obtaned for the whole range of LST. Keepng n mnd that the GSW algorthm alo requre LSE and WVC a model nput, the followng ecton wll preent the determnaton of thee two parameter Determnaton of the LSE The LSE n channel IR1 and IR of S-VISSR can be etmated from the LSE n channel 31 (11 μm) and 3 (1 μm) of MODIS provded by the MODIS LST product MOD11B1 at 5 km reoluton. To determne the emvty relatonhp between S-VISSR channel and MODIS 31 and 3 channel, two pectral databae, one from the Unverty of Calforna Santa Barbara (UCSB) ( and the other from the John Hopkn Unverty (JHU) ( are ued. The emvte n the two plt-wndow channel of MODIS ( 31 and 3 ) and S-VISSR ( IR1 and IR ) were calculated by the ntegral of the pectral emvty wth the channel repone functon over the pectral range of the channel. The channel repone functon of the two plt-wndow channel for MODIS and FY-C are dplayed repectvely n fgure 5. A tattcal relatonhp between MODIS channel and S-VISSR channel wa etablhed by a lnear regreon analy. A a reult, the emvte n S-VISSR channel IR1 and IR are, repectvely, related to the emvte n MODIS channel 31 and 3 by Eq. (4) and (5). 1 = (4) IR 31 = (5) IR 3

10 Senor 8, MODIS 31 MODIS 3 Spectral repone functon FY-C IR1 FY-C IR Wavelength (μm) Fgure 5. S-VISSR and MODIS plt-wndow pectral repone functon. Fgure 6 how the emvte and lnear regreon reult. Only the emvte of ol, vegetaton, water, and now/ce n JHU and UCSB databae were ncluded n th work. Some few devated pont n th fgure are due to the fact that the pectral range of S-VISSR channel IR1 and IR are broader than thoe of MODIS channel 31 and 3 a hown n fgure 5. However, a hown n fgure 6, the reult of the lnear regreon are good wth the RMSE wthn., whch ndcate that the emvte n S-VISSR channel IR1 and IR can be drectly derved from thoe n MODIS channel 31 and 3, repectvely. 1. IR1 = * IR = * 3 Emvty n FYC IR1 ( IR1 ) R-quare=.98 RMSE=. JHU + UCSB databae Emvty n FYC IR ( IR ) R-quare=.97 RMSE=. JHU + UCSB databae Emvty n MODIS channel 31 ( 31 ) Emvty n MODIS channel 3 ( 3 ) Fgure 6. Lnear fttng relatonhp of the emvte between the S-VISSR channel IR1 and IR and the MODIS channel 31 and 3, repectvely. Alternatvely, the emvte of the S-VISSR IR1 and IR channel can be etmated ether wth the land urface clafcaton a dd by Sun and Pnker (3) [13] or ung the method developed by Jang et al. (6) [6] whch combned md-nfrared and thermal nfrared data of SEVIRI to retreve LSE.

11 Senor 8, Determnaton of the atmopherc WVC The MODIS total precptable water product MOD5 provde the atmopherc column water vapor amount, whch can be ued a the model nput when the cannng tme of the enor MODIS and S-VISSR are cloe to each other. However, MODIS provde the ntantaneou WVC only four tme per day, whch can not meet the need for the temporal reoluton (an hour) of S-VISSR onboard FY- C. Snce the atmopherc WVC change wth tme, the method developed by L et al. (3) [7] can be ued to determne the WVC from S-VISSR IR1 and IR data. Accordng to L et al. (3) [7], the atmopherc WVC can be derved by the ue of the tranmttance rato of plt-wndow channel, wth and R = τ τ N k= 1 WVC c c τ = 1+ (6) τ = R (7) ( T T)( T T ) k, k, N k = 1 ( T T) k, (8) where and c are unknown coeffcent, c1 τ and τ are the atmopherc tranmttance n the pltwndow channel and, the ubcrpt k denote pxel k, and the T and T are the TOA mean (or the medan) channel brghtne temperature of the N neghborng pxel condered for channel and, repectvely. On the ba of the numercal reult obtaned n Secton., coeffcent and c can be repectvely derved a functon of ecant VZA a c1 c = VZA + VZA / co( ) 3.11/ co ( ) c = / co( VZA) 3.6 / co ( VZA) (1) (9) Fgure 7 how the curve ft of the coeffcent c 1, c a functon of ecant VZA. A noted, the fttng reult are qut well wth both R-quare equal to.999. In addton, wth the actual WVC and the tranmttance rato of plt-wndow channel IR1 and IR obtaned n Secton., the RMSE between the actual WVC and the WVC etmated ung Eq. (6), (9) and (1).17 g/cm, whch ndcate that the fttng reult are good Sentvty analy A Wan and Dozer (1996) [1] ndcated that the error of LST etmated by the GSW algorthm come manly from the uncertante of LSE, atmopherc properte and the ntrument noe. Thee three uncertante of error are taken nto account n th nvetgaton.

12 Senor 8, c 1 = /co(VZA)+3.11/co (VZA) R-quare= c = /co(VZA)-3.6/co (VZA) R-quare=.999 Value of the coeffcent c Value of the coeffcent c /co(VZA) /co(VZA) Fgure 7. Curve ft of the coeffcent c c n Eq. (6) a functon of the VZA Sentvty analy to ntrumental noe (NE T) In order to ee how gnfcant the effect of the ntrumental NE T on the retreval of LST, a Gauan random dtrbuton error of.1 K,. K and.5 K are, repectvely added to the TOA brghtne temperature T and T n Eq. (3). Then we etmate the LST ung GSW algorthm wth the noed TOA brghtne temperature. A an example, compared the actual LST wth the etmated LST for the ub-range: [.94, 1.], WV C [1.,.5], and T [9 K,31 K], the RMSE.38 K for NE T=.1 K,.43 K for NE T=. K, and.67 K for NE T=.5 K. Compared the RMSE of.37 K for no ntrumental noe, the accuracy of retreval LST can be affected by 3% for NE T=.1 K, by 16% for NE T=. K, and by 81% for NE T=.5 K Sentvty analy to LSE Accordng to the Eq. (3), the entvty of the uncertante n LSE manly dependent on the term (1 ) / and Δ /( ), whch can be wrtten a T + T T T α = a + a5 (11) T + T T T β = a3 + a6 ( 1) Two cae are condered n th nvetgaton. One the extremely dry atmopherc condton (WVC [.,1.5] ) and the other the extremely wet atmopherc condton ( WVC [5.,6.5] ). Wth the regreon coeffcent and the T and T mulated n Secton., ung equaton (11) and (1) we can obtan the varaton of α and β. Table lt the varaton of α and β for the ub-range: [.94,1.], T [9 K,31 K], WVC [.,1.5] and the ub-range [.94,1.], [9 K,31K], WVC [5.,6.5], fo r VZA=, repectvely. T

13 Senor 8, Table. Stattc of the error due to the uncertante n LSE for the ubrange [.94,1.], T [9 K,31 K ], WVC [.,1.5] and the ub-range [.94,1.], T [9 K,31 K], WV C [5.,6.5], for VZA=. Condton [.94,1.], T [9 K,31 K], VZA= Water vapor content (g/cm ) Varable WV C [.,1.5] WVC [5.,6.5] α β α β Range of Value (K) [44.8,61.3] [ ,-11.5] [11.57, 34.4] [-7.13,-19.48] Mean (K) Standard devaton (K) From table one can ee that the value of α and β n extremely dry atmopherc condton (WV C [.,1.5] ) are nearly two tme a large a thoe of α and β n extremely wet atmopherc condton, (WV C [5.,6.5] ), repectvely. Th mean that the entvty of ( 1 ) / and Δ /( ) to LST for wet atmopherc condton decreaed two tme a that for dry atmopherc condton. From equaton (3), the LST error δ LST due to the uncertanty n (1 ) / and Δ /( ) can be etmated by, 1 Δ δlst = α δ( ) + β δ( ) (13) Aumng that the uncertante of (1 ) / and Δ /( ) are around 1%, the LST error [1.3K, 1.5K] wth the mean of 1.4 K for the dry atmophere and [.K,.8K] wth the mean of.5 K for the wet atmophere Sentvty analy to the atmopherc WVC It well known that the WVC n the atmophere not ealy determned from atellte data. In order to ee how gnfcant the effect of the uncertanty of the WVC on the retreval of LST n GSW algorthm, the wrong ub-range electon of the WVC nvetgated n our work. A mentoned above n Secton 3.1, the WVC wa dvded nto x ub-range wth an overlap of.5 g/cm. The overlap WVC could be fallen nto two adacent ub-range. That, t ncluded by two ub-range and correponded to two par of coeffcent a a6. We am to analyze the effect of the overlap WVC on the retreval of LST. Fgure 8 gve an example of the uncertanty of the WVC. From fgure 8 one can ee that the overlap water vapor content WVC [1.,1.5] fallng nto two ub-range WVC [.,1.5] and WVC [1.,.5]. When we etmate the LST wth the water vapor content WV C [1.,1.5] ung the coeffcent correpondng to the ub-range [.94,1.], WV C [.,1.5], and T [9 K,31K], the RMSE between the actual and the etmated T.18 K, whle ung the coeffcent correpondng to the ub-range [.94,1.], WV C [1.,.5], and T [ 9 K,31 K], the RMSE.43 K.

14 Senor 8, ung coeffcent of ub-range: [.94, 1.] WVC [., 1.5] T [9K, 31K] 5 ung coeffcent of ub-range: [.94, 1.] WVC [1.,.5] T [9K, 31K] Frequence WVC [1.,1.5] Ba=-.1 RMSE=.18 K Frequence 4 3 WVC [1.,1.5] Ba=.6 RMSE=.43 K Actual T - Etamted T (K) Actual T - Etamted T (K) Fgure 8. Htogram of the dfference between the actual and etmated T for the overlap water vapor content WV C [1.,1.5] ung the coeffcent of dfferent ub-range Intercomparon of dfferent formulaton of the plt-wndow algorthm It well known that the LST retreval from atellte obervaton ha been ongong for everal decade. Many dfferent formulaton of the plt-wndow algorthm have been propoed. They are omewhat mlar n formulaton and everal of them are drectly npred from Becker and L (199) [1] formulaton. In order to perform the ntercomparon wth the recently propoed plt-wndow algorthm, dfferent formulaton were ued to etmate the LST wth the ame mulated FY-C data n th work. Thoe formulaton are lted n table 3: Table 3. Dfferent formulaton of plt-wndow algorthm n lterature, the averaged emvty n channel and, and Δ the dfference between the two channel emvte. Author Formulaton Prce, 1984 [8] T = a + at 1 + a( T T) + a3( T T)(1 ) + a4tδ Prata and Platt, 1991 [14] T T 1 T = a + a1 + a + a3 Vdal, 1991 [15] 1 Δ T = a + at 1 + a( T T) + a3 + a4 Ulver et al., 199 [16] T = a + at 1 + a( T T) + a3(1 ) + a4δ Sobrno et al., 1993 [17] T = a + at 1 + a( T T) + a3( T T) + a4(1 ) + a5δ Sobrno et al., 1994 [18] Δ T = a + at 1 + a( T T) + a3 + a4 Coll et al., 1997 [19] T = T + a + a1( T T) + a( T T) + a3(1 ) + a4δ

15 Senor 8, In addton, Becker and L (1995) [] further modfed ther plt-wndow algorthm (Becker and L, 199) [1] by addng atmopherc water vapor correcton a T + T T T T = A + P + M (14) wth A = a + a1w P = a + ( a3+ a4wco θ )(1 ) ( a5 + a6w) Δ M = a7 + a8w+ ( a9 + a1w) (1 ) ( a11+ a1w) Δ where = ( + ) / and Δ =, w the total precptable water amount, and θ the Vewng Zenth Angle (VZA). In order to make the ntercomparon more reaonable, the coeffcent from the above equaton have been recalculated ung the ame mulated FY-C data wthn the ame ub-range n Secton 3.1. A an example, table 4 depct the RMSE between the actual and the etmated T veru the ecant VZA for the ub-range: [.94,1.], WVC [1.,.5], and T [9 K,31 K]. From th table, one can ee that the RMSE ncreae wth the ncreae of the VZA for all algorthm. In addton, except for the algorthm propoed by Prce (1984) [8], and Prata and Platt (1991) [14], the T etmated ung the other algorthm are comparable, whch ndcate that the plt-wndow algorthm can be uccefully appled to the LST retreval from FY-C data. It hould be noted that the RMSE value n Table 4 wll be larger, epecally for BL95, n conderng the entvty to WVC error. Table 4. RMSE between the actual T and the T etmated ung dfferent formulaton of the plt-wndow algorthm for the ub-range [.94,1.], WVC [1.,.5], and T [9 K,31 K]. Author VZA( o ) GSW Prce84 Prata91 Vdal91 Ulver9 Sobrno93 Sobrno94 Coll97 BL RMSE (K) Applcaton to actual FY-C atellte data The obectve of the preent work to etmate the LST from Chnee frt operatonal geotatonary meteorologcal atellte FengYun-C (FY-C) data for cloud-free ke. Fgure 9 gve an example of the retreval LST around Beng n Chna durng FY-C atellte cannng on May 15, 6 at 11: local tme. The model nput are the TOA brghtne temperature LST, VZA, LSE,

16 Senor 8, and WVC. The TOA brghtne temperature LST and VZA are drectly extracted from the FY-C atellte data. The LSE are derved from the emvte n MODIS channel 31 and 3 provded by MODIS/Terra LST product MOD11B1, and the WVC are obtaned from MODIS total precptable water product MOD5. Symbol A, B, and C located n red, green and baby blue colored area n fgure 9 repreent bare ol, cultvated urface and ea urface, repectvely. A Beng C B Fgure 9. Map of the LST etmated from FY-C atellte data at 11: local tme on May 15, 6. In addton, table 5 lt the value of the VZA, WVC, LSE, TOA brghtne temperature, and reultant T for one repreentatve pxel n each red, green, and baby blue colored area n fgure 9. Table 5. Decrpton of ymbol A, B and C n fgure 9. A (red) B (green) C (baby blue) Longtude ( o ) 1.6 E E 1.75 E Lattude ( o ) 43.7 N N N VZA ( o ) WVC (g/cm ) IR IR T IR1 (K) T IR (K) T (K)

17 Senor 8, It hould be ponted out here that the LST etmated from the FY-C atellte data ha not been valdated wth n tu meaurement nce there are no n tu meaurement avalable. In addton, due to the extreme dffculty or mpoblty to get the LST at ground level repreentatve at 5km*5km, we wll try to cro valdate LST derved from FY-C data n the future wth the well valdated LST product provded by MODIS data. 5. Concluon In th paper, we have addreed the retreval of the Land Surface Temperature (LST) from the Chnee frt operatonal geotatonary meteorologcal atellte FengYun-C (FY-C) data n two thermal nfrared channel IR1 ( μ m ) and IR ( μ m ), ung the Generalzed Splt-Wndow (GSW) algorthm propoed by Wan and Dozer (1996) [1]. Takng nto account the fact that the S-VISSR enor onboard FY-C ha no atmopherc oundng channel, the coeffcent n the GSW algorthm were derved by dvdng the range of the mean emvty, the atmopherc Water Vapor Content (WVC), and the LST nto tractable ub-range, and were recalculated ung a tattcal regreon method from the numercal value mulated wth an accurate atmopherc radatve tranfer model MODTRAN 4 over a wde range of the atmopherc and urface condton. The mulaton analy howed that the LST could be etmated by the GSW algorthm wth the Root Mean Square Error (RMSE) le than 1 K for the ub-range wth the Vewng Zenth Angle (VZA) le than 3 or for the ub-range wth VZA le than 6 and the atmopherc WVC le than 3.5 g/cm provded that the Land Surface Emvte (LSE) are known. A the GSW algorthm requre WVC and LSE a model nput, the MODIS total precptable water product MOD5 provdng the atmopherc column water vapor amount, wa ued to obtan the WVC when the cannng tme of the enor MODIS and S-VISSR are cloe to each other. A for the other cannng tme of S-VISSR, the atmopherc WVC can be determned ung the method developed by L et al. (3) [7]. A for LSE, the MODIS/Terra LST product MOD11B1 provdng the LSE wth 5 km reoluton for the thermal nfrared channel 31 and 3, wa ued to derve the LSE n S-VISSR channel IR1 and IR, repectvely. In addton, the entvty and error analye n term of the uncertanty of the LSE and WVC a well a the ntrumental noe were alo performed n th work. The reult how that the accuracy of retreval LST can be affected by 3% for NE T=.1 K, by 16% for NE T=. K, and by 81% for NE T=.5 K for the ub-range [.94,1.], WVC [1.,.5], and T [9 K,31 K] ; gven the uncertante of (1 ) / and Δ /( ) around 1%, the LST error [1.3K, 1.5K] wth the mean of 1.4 K for the dry atmophere and [.K,.8K] wth the mean of.5 K for the wet atmophere; and the effect of the uncertanty of the WVC on the retreval LST could be around.3 K. Moreover, n order to compare the dfferent formulaton of the plt-wndow algorthm, everal plt-wndow algorthm were ued to etmate the LST wth the ame mulated FY-C data. The reult of the ntercomparon howed that mot of the algorthm gve comparable reult, whch ndcate that the plt-wndow algorthm can be uccefully appled to the LST retreval from FY-C data.

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