SIMULATING SURFACE ENERGY FLUX AND SOIL MOISTURE AT THE WENJIANG PBL SITE USING THE LAND DATA ASSIMILATION SYSTEM OF THE UNIVERSITY OF TOKYO

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1 Aual Joural of Hydraulc Egeerg, JSCE, Vol.53, 9, February SIMULATING SURFACE ENERGY FLUX AND SOIL MOISTURE AT THE WENJIANG PBL SITE USING THE LAND DATA ASSIMILATION SYSTEM OF THE UNIVERSITY OF TOKYO Hu LU, Tosho KOIKE, Ku YANG 3, Xagde Xu 4, X LI 5, Hroyuk TSUTSUI, Yueqg LI 6, Xgbg ZHAO 6, ad Katsuor TAMAGAWA 7 Member of JSCE, Ph.D., Researcher, Dept. of Cvl Eg., Uv. of Tokyo (Bukyo-ku, Tokyo , Japa) Member of JSCE, Dr. Eg., Professor, Dept. of Cvl Eg., Uv. of Tokyo (Bukyo-ku, Tokyo , Japa) 3 Member of JSCE, Ph.D., Professor, Ist. of Tbet, Cha Academy of Sceces (Bejg 85, Cha) 4 Professor, Chese Academy of Met. Scece., Cha Met. Adm. (Bejg 8, Cha) 5 Ph.D., Professor, Cold ad Ard Regos Ev. ad Eg. Research Ist., CAS (Lazhou 73, Cha) 6 Professor, Isttute of Plateau Meteorology, Cha Met. Adm. (Chegdu 67, Cha) 7 Researcher, Dept. of Cvl Eg., Uv. of Tokyo (Bukyo-ku, Tokyo , Japa) Ths paper reports a applcato of a Lad Data Assmlato System (LDAS) to the Wejag ste located ear Chegdu, Cha, for the perod from Jauary to March, 8. The LDAS was frst drve by -stu observed mcrometeorologcal data. Smulated eergy fluxes were compared to hourly drect measuremets ad smulated sol mosture cotet was compared to the -stu sol mosture observatos at a depth of 4 cm. The results show that the LDAS well smulated those varables ad thus valdated the capablty of LDAS. To check the possblty of applyg LDAS globally ad smulatg surface eergy ad water budget worldwde, two sets of model output data were used as the drvg data of the LDAS: the Japa Meteorology Agecy (JMA) Model Output Local Tme Seres (), ad the Modfed JMA. The LDAS performace was ot so good whe drve by the orgal JMA data, but mproved after we used modfed data wth some smple lear regresso equatos. Ths result demostrated the feasblty of relably smulatg lad surface fluxes wth a LDAS drve by model outputs. Key Words: Lad Data Assmlato System, Eergy Fluxes, Sol Mosture, Feld Expermets. INTRODUCTION Lad surface processes through whch exchages of water, eergy ad carbo betwee the lad surface ad the atmosphere are realzed, remarkably affect weather ad clmate. Clmate smulatos are especally sestve to the dural ad seasoal cycles of the surface eergy balace ). Lad surface eergy budgets are also very mportat hydrologcal ad ecologcal modelg. The eergy flux ca be measured at a patch scale wth some specal strumets such as traxal soc aemometers, krypto hygrometers ad fe-wre thermocouples. It also ca be estmated at a regoal scale from satellte observatos whe frared mages ad acllary data are avalable. Lad surface models (LSMs) are developed to predct temporal ad spatal patters of lad surface varables,3), but the qualty of the predctos are usually ot so good because of model talzato, parameter ad forcg errors, ad adequate model physcs ad/or resoluto 4,5). The Lad Data Assmlato System (LDAS), developed by mergg observato formato (from groud-based statos, satelltes ad so o) to dyamc models (.e. LSMs), s expected to provde hgh qualty surface eergy ad water flux estmates wth adequate coverage ad resoluto. I ths study, we appled a LDAS developed at the Uversty of Tokyo (LDASUT) 6) for the Wejag ste of a JICA project where a PBL tower had bee bult. The objectves of ths study are: () to evaluate LDASUT a vegetated lad surface usg -stu observatos, ad () to check the feasblty of estmatg areal lad surface eergy ad water fluxes relably usg LDASUT drve by spatally-

2 dstrbuted forcg data. I ths study, LDASUT was drve by Japa Meteorology Agecy (JMA) Model Output Local Tme Seres () data ad smulato results were compared wth drect measuremets. I the followg secto, we brefly descrbe the materals ad methods used ths study, cludg the expermetal ste ad troduce the LDASUT. The smulato results of LDASUT drve by -stu data are descrbed secto 3. I secto 4, the LDASUT was frst drve by data, ad the by modfed MOTLS data to mprove the qualty of the smulato. Fally, we fsh ths paper wth some coclusos.. MATERIALS AND METHODS. Expermetal ste descrpto The Wejag ste s located o a flat farm feld approxmately 9 km west of Chedu cty of Schua provce, Cha. The ste has a elevato of 53 m ad s cetered at 3 44'N lattude, 3 5'E logtude. It s ear the edge of Tbeta Plateau ad the water vapor corrdor of the Asa mosoo. A PBL tower, establshed by a JICA project, was bult ths ste Feb. 7. Observatos at the PBL tower clude wd speed ad drecto at four levels, ar temperature ad humdty, turbuleces, fluxes of eergy ad CO, sol mosture ad temperature profles, sol heat flux, solar ad atmospherc radato, ad precptato.. LDASUT I ths study, the lad surface eergy ad water budget was smulated usg the LDASUT 6). Ths system cossts of a LSM to calculate surface fluxes ad sol mosture, a radatve trasfer model (RTM) to estmate mcrowave brghtess temperature, ad a optmzato scheme to search for optmal values of sol mosture through mmzg the dfferece betwee modeled ad observed brghtess temperature. The LSM s a Smple Bosphere model (SB) ). The RTM used the LDASUT has two compoets: volume scatterg ad surface scatterg parts 7). The volume scatterg part smulates the radatve trasfer process sde the sol layer by a 4-stream based RTM whch the multply scatterg effects of a dry sol medum s calculated by the dese meda radatve trasfer model (DMRT) 8). The surface scatterg part smulates the surface scatterg effects at the ladatmosphere terface by the Advaced Itegral Equato Method (AIEM) 9). The mmzato scheme s a shuffled complex evoluto method. The tal parameters of LDASUT are obtaed from a global data set; for example, the leaf area dex (LAI) from Moderate Resoluto Imagg Spectroradometer (MODIS) data; ad the sol ad vegetato parameters from The Iteratoal Satellte Lad Surface Clmatology Project (ISLSCP). The satellte observato data s from the Advaced Mcrowave Scag Radometer for the Earth Observg System (AMSR-E) brghtess temperature data. The meteorologcal drvg data of the LDASUT ca be ether weather model outputs or -stu observato..3 Statstcal aalyss of the smulato results The smulato results (M ) of the LDASUT are compared agast the -stu feld measuremets (O ), o the bass of three statstcal aalyses: MBE = ( M O ) / () RMSE = NSEE = = = = ( M O ) / () ( M O ) / ( O ) (3) = where s the total hourly observato pots; MBE s the mea bas error; RMSE s the Root Mea Square Error; ad NSEE s the Normalzed Stadard Error of the Estmato, deotg a estmato of relatve ucertaty. 3. SIMULATION DRIVEN BY IN-SITU DATA As the frst step ths study, we performed seaso log rus from Ja. to Mar. 8 (9 days) wth PBL observato as the forcg data of the LDASUT. Agrculture/C3 grasslad the stadard SB parameters for vegetato was used for the smulato. The default sol parameters (texture, thermal ad hydraulc propertes) were derved from the ISLSCP Itatve II sol data. Ths smulato s called PBL. To avod aomalous results, data are rejected whe () latet heat flux was less tha - W/m or () the resdual eergy was less tha - W/m. After data flterg, we retaed 99 data sets from the orgal 6 data sets. 3. Surface Eergy Budget Fgure shows the mothly mea dural chages of et radato (hereafter referred to as R), latet heat flux (le), sesble heat flux (Hs), ad sol heat flux (G), from the top to the bottom row, respectvely. The ope cycle represets the drect measuremets ad the sold le represets the results of PBL. From fgure a, t s clear that PBL smulated R wth hgh accuracy for both the peak ad dural patters. Ths was because -stu observed dowward radato was used as forcg data ad

3 le(w/m/m) Hs(w/m/m) G(w/m/m) 3 - R(w/m/m) (a) Mothly Mea Dural Chage of R Measurmets PBL (b) Mothly Mea Dural Chage of le (c) Mothly Mea Dural Chage of Hs (d) Mothly Mea Dural Chage of G Fg. Comparso of mothly mea dural chage of (a) R, (b) le, (c) Hs ad (d) G of PBL agast drect measuremet. SB calculates R from the four compoets of radato budgets. As show fgure b-d, t s obvous that PBL captured the temporal varato characterstcs of le, Hs ad G. Smulated_R_PBL Smulated_lE_PBL Measured R Measured le Fg. Scatterplots of R, le, Hs ad G of PBL agast drect measuremets. Fgure shows scatterplots of smulated R, G, le, ad Hs, agast drect measuremets. The squared correlato coeffcets are.99,.8,.89 ad.85. As show table, PBL slghtly overestmated G ad uderestmated le, whle t well estmated R. The overestmato of G may be to the result of measuremet errors of sol heat flux ad the uderestmato of eergy storage the upper sol layer above the heat flux plate where the heterogeety creased as crop roots developed. The dscrepaces le may be partly to the result of strumet errors. Accordg to Mauder et al. ), the accuracy of sesble heat flux measuremet s Smulated_G_PBL Smulated_Hs_PBL Measured G Measured Hs aroud -3 W/m, ad -4 W/m for latet heat flux. Moreover, cosderg the fact that PBL smulato ad -stu observato have dfferet scales, ad the fact that the resdual eergy (Re) of drect measuremet (Re=R-lE-Hs-G, show table ) s comparable to the largest RMSE of eergy compoets, the qualty of surface eergy budget smulato of PBL s acceptable. The capablty of LDASUT to smulate lad surface fluxes relably s the valdated. Table. Three moths averaged eergy compoets (ut: W/m ) R le Hs G Re PBL Surface temperature ad upward log-wave radato Temperature s a very mportat progostc state varable o the lad surface. LDASUT s able to provde vegetato, groud surface ad deep sol temperatures. Ufortuately, the frared thermometer used at the Wejag ste was broke durg the study perod ad so we do ot have drect groud surface temperature measuremets. Accordg to the Stefa-Boltzma law, upward log-wave radato (ULR) s a good surrogate of lad surface temperature. We therefore compared the smulated ULR wth the drect measuremets. ULR (W/m/m) Table. Statstc aalyss of eergy compoets of PBL MBE RMSE NSEE R (W/m ) % le (W/m ) % Hs (W/m ) % G (W/m )..9 65% Upward Logwave Radato PBL Fg. 3 Comparso of hourly log-wave radato of PBL agast drect measuremets. Fgure 3 shows a comparso of hourly ULR. It s apparet that PBL geerated cosstet temporal varatos of ULR. The squared correlato coeffcet was.88; MBE -4. W/m ; RMSE.8 W/m ad NSEE 3%. 3.3 Sol Water Cotet Fgure 4 shows a tme seres of the volumetrc sol mosture cotet observed at 4 cm depth (th le) ad those geerated by PBL (thck le). I-stu observed precptato s also plotted. We foud that the observed sol mosture dd ot chage much

4 durg ths perod, ragg from.3 to.33. PBL predcted the mosture peak good agreemet wth drect measuremets, for both the occurrg tme ad values. The gaps betwee PBL sol mosture ad observed oes get larger the dryg processes. Ths s partly due because -stu sol mosture s measured at a depth of 4 cm, whch s geerally deeper tha the peetrato depth of AMSR-E. The scale dfferece of the AMSR-E observatos ad -stu oes also cotrbute to such dscrepaces. Geerally ad statstcally, PBL estmated sol mosture wth hgh qualty, cosderg that MBE s -.; RMSE s. ad NSEE s 9%. Mv.4.3. Ra_Obs Measurmets(4cm) PBL SIMULATION DRIVEN BY MODEL OUTPUT From a aalyss of the PBL smulato secto 3, t s clear that LDASUT ca correctly smulate the surface eergy ad water budget whe t s drve by -stu observed forcg data. I clmate studes ad umercal weather predctos, the spatal dstrbuto formato of eergy ad water fluxes s very essetal. To smulate lad surface fluxes at a regoal or global scale, spatallydstrbuted meteorologcal forcg data are eeded. Such forcg data were oly avalable from model outputs, ad, as metoed secto, JMA data was selected ths study. Same as the PBL smulato, a smulato was coducted by usg the orgal as meteorologcal forcg data ad s called M_O. 4. Surface Eergy Budget of M_O Table 3 shows the statstcal results of the eergy fluxes of M_O. It s clear that the qualty of M_O s much worse tha that of PBL. The MBE of R s larger tha 5 W/m, whch s the accuracy Table. 3 Statstc aalyss of radato compoets of M_O MBE RMSE NSEE Fg. 4 Comparg smulated sol mosture cotet wth drect measuremets R (W/m ) % le (W/m ) % Hs (W/m ) % G (W/m ) % Ra(mm) of solar radato measuremet. The NSEE of Hs ad G are larger tha %. Therefore, the qualty of M_O s ot acceptable ad we ca ot drectly apply data as forcg data for LDASUT. 4. Modfcato to data To ascerta the reaso why M_O performace s ot so good, we compared forcg data wth -stu observatos (a) Mothly mea dual chage shortwave dowward radato Measuremet (b) Mothly Ja. mea dual 4 chage logwave Feb. dowward 48 radato Mar Measuremet Fg. 5 Mothly mea dural chages dow ward radato of ad -stu PBL observato From a aalyss of mothly mea dural radato (see Fg. 5a), t s clear that the peak of the dowward short wave radato of was much bgger tha that of -stu observatos, whle the dowward log-wave radato of was aroud W/m smaller tha that of PBL observatos(fg. 5b). The pressure of was slghtly larger (mea hpa) tha that of PBL observato (mea 956.9hPa). The mea ar temperature of was 8.8K, almost the same as that of PBL observato, 8.5K (a) Comparso of Precptato (mm/hour) (b) Comparso of accumulated precptato (mm) Fg. 6 Hourly precptato ad accumulated values of ad -stu observato There are some obvous dffereces betwee MOTLS precptato data ad PBL observato (see Fg. 6). gave a larger precptato tha PBL observato. The accumulated precptato of ths perod was 3.3 mm, whle that of PBL observato was just 56. mm. Fortuately,

5 as demostrated by Yag et al. 6), LDASUT s able to partly overcome such bases put precptato data, because t drectly assmlates AMSR-E brghtess data to correct the sol mosture states. Through a comparso of forcg data ad -stu observed data, t was clear that the large overestmato dowward radato s the ma reaso that M_O faled to correctly smulate the lad surface eergy budget. To mtgate such a obvous overestmato, we modfed the JMA dowward shortwave radato data by usg lear equatos acqured from the regresso aalyss of the mothly mea dural cycle data. Aalogously, the dowward log-wave radato data was modfed usg a lear regresso equato of all three moth data. The correcto equatos are as follows: RSW _ C = max[,( RSW_ O 5.) /.6435] (4) RLW _ C = ( RLW_ O ) /.9557 (5) where RSW s the dowward short wave radato, RLW s the dowward log-wave radato, _C meas the modfed value, ad _O meas the orgal value. After applyg equatos 4 ad 5 to all dowward radato data, a ew data set, modfed, was created. Aalogously, the smulato drve by the modfed data s called M_C. 4.3 Results of M_O ad M_C As show table 4, comparg table 3, t s clear that M_C estmates surface eergy fluxes better tha M_O ; as all tems table 4 are smaller tha those table 3. Ths meas that the performace of LDASUT s mproved usg the modfed stead of the orgal. Table. 4 Statstcal aalyss of radato compoets of M_C MBE RMSE NSEE R (W/m ) % le (W/m ) % Hs (W/m ) % G (W/m ) % Fgure 7 shows the mothly mea dural chages of the surface eergy compoets. Comparg M_O (dash le) ad M_C (sold le) agast the drect measuremets (ope cycles), t s clear that M_C geerally produced better results tha M_O. Ths meas the performace of the eergy budget smulato ca be mproved through a smple lear modfcato. Wth cosderg measuremet accuracy ad scale problems, the qualty of M_C s reasoable for the bg doma smulatos. Fgure 8 shows a comparso of the mothly mea dural chages of ULR. It s clear that M_O uderestmated ULR at ght tme, wth a MBE of - 7. W/m ; whle M_C estmated ULR wth better accuracy, wth a MBE of -4.4 W/m. Fgure 9 shows a tme seres of the hourly sol mosture of M_O (dash le), M_C (thck le) ad -stu observato (th le). The results of M_O ad M_C are acceptable, because the stregth of LDASUT, whch optmzed sol parameters ad assmlatg sol mosture. But sometmes M_O ad M_C dd ot follow the R(w/m/m) le(w/m/m) Hs(w/m/m) G(w/m/m) (a) Mothly Mea Dural Chage of R Measurmets M_O M_C (b) Mothly Mea Dural Chage of le (c) Mothly Mea Dural Chage of Hs (d) Mothly Mea Dural Chage of G Fg. 7 Comparso of mothly mea dural chage of (a) R, (b) le, (c) Hs ad (d) G of M_O ad M_C agast drect measuremet. ULR (w/m/m) Ra_ Obs(4cm) 7 8 M_O M_C Mv Mothly Mea Dural Chage of ULR M_O M_C Ja Feb Mar Fg. 8 Comparso of mothly mea dural chage of ULR of M_O ad M_C agast drect measuremet. Fg. 9 Comparg smulated sol mosture cotet wth drect measuremets Ra(mm)

6 tedecy of the drect measuremets. Ths s partly to the result of the bg dfferece betwee precptato ad the observed oe, as show fgure 6. Statstcally, M_O estmates sol mosture wth a MBE of., a RMSE of. ad NSEE of 8%, whle those of M_C are -.,. ad 7%, respectvely. By comparg M_O ad M_C results wth -stu measuremets, the advatages of modfed were verfed. Thus the possblty of geeratg relable spatal dstrbuto of lad surface fluxes wth LDASUT drve by modfed data ca be cofrmed. 5. CONCLUSIONS LDAS s expected to provde accurate temporal ad spatal cotuous lad surface varables that wll promote research felds such as clmate chage, weather forecastg, ad hydrologcal modelg. I ths study, the LDASUT was frstly drve by -stu observato data to valdate ts capablty to estmate lad surface fluxes (PBL). The, to check the feasblty to estmate the spatal patter of lad surface fluxes wth usg LDASUT ad model output forcg data, LDASUT was drve by two model output data sets: the orgal (M_O) ad a modfed (M_C). Smulato results of R, le, Hs, G, ULR ad sol mosture cotet were compared agast the drect measuremets. Our results show that the smulato results of PBL geerally well agreed wth the drect measuremet, ad the dffereces betwee -stu observato ad smulato are geerally smaller tha strumetal observato errors. Therefore, we valdated that LDASUT ca relably smulate lad surface fluxes. The dscrepaces betwee the smulated fluxes of M_O ad the drect measuremets are apprecable; whle M_C, a smple modfcato from M_O usg lear regresso equatos, estmated those fluxes wth mproved accuracy. Because of the uque feature of the LDASUT to optmze sol parameters ad the assmlate sol mosture, the smulated sol mosture of M_O ad M_C were good qualty. From these ecouragg results, t s possble to relably estmate lad surface varables usg the LDAS drve by model outputs. It s especally mportat for rug the GCM ad for studes remote areas where -stu mcrometeorologcal observato s ot avalable. We also foud that the qualty of the le ad G smulatos was ot as good as that of R. Ths could be the result of strumetal errors, dfferet scales of the LDASUT ad -stu observato, the heterogeety problem the calculato of eergy storage, ad the model defceces the structure ad parameters. Further efforts are eeded both expermetal ad model research. ACKNOWLEDGMENTS: Ths study was carred out as part of a JICA project; for whch the authors express ther great grattude. We also thak our local colleagues at the Wejag ste ad JMA for provdg ecessary data set. REFERENCES ) Betts AK, Ball JH, Beljaars ACM et al.: The lad surfaceatmosphere teracto: a revew based o observatoal ad global modelg perspectves. J. Geophys. Res.,, 79 75, 996. ) Sellers PJ, Radall DA, Collatz GJ et al.: A revsed lad surface parameterzato (SB) for atmospherc GCMs. Part I: model formulato. J. of Clmate, 9, , ) Gao, Z., N. Chae, J. Km, J. Hog, T. Cho, ad H. Lee: Modelg of surface eergy parttog, surface temperature, ad sol wetess the Tbeta prare usg the Smple Bosphere Model (SB), J. Geophys. Res., 9, D6, do:.9/3jd489, 4. 4) Ptma, A.J ad PILPS team co-authors: Key results ad mplcatos from phase (c) of the project for tercomparso of lad-surface parameterzato schemes, Clm. Dyamcs, , ) Ku Yag ad co-authors: Ital CEOP-based revew of the predcto skll of operatoal geeral crculato models ad lad surface models, JMSJ, Vol. 85A, pp 9-4, 7. 6) Ku YANG, Takahro WATANABE, et al: A Autocalbrato System to Assmlate AMSR-E data to a Lad Surface Model for Estmatg Sol Mosture ad Surface Eergy Budget, JMSJ, Vol. 85A, pp 9-4, 7. 7) Lu, H., T. Koke, N. Hrose, M. Morta, H. Fuj, D.N. Kura, T. Graf, ad H. Tsutsu: A basc study o sol mosture algorthm usg groud based observatos uder dry codtos. JSCE, 5, 7-, 6. 8) We, B, L. Tsag, D. P. Webreer, ad A. Ishmura: Dese meda radatve trasfer theory: comparso wth expermet ad applcato to mcrowave remote sesg ad polarmetry, IEEE Tras. o Geosc. Remote Sesg, 8, 46-59, 99. 9) K. S. Che, T. D. Wu, L. Tsag, Q. L, J. C. Sh, ad A. K. Fug: Emsso of rough surfaces calculated by the tegral equato method wth comparso to three-dmesoal momet method Smulatos, IEEE Tras. o Geosc. Remote Sesg, vol. 4, pp. 9-, 3. ) Mauder, M., C. Lebethal, M. Gockede, J.-P. Leps, F. Beyrch, ad T. Foke: Processg ad qualty cotrol of flux data durg LITFASS-3. Boudary-Layer Meteorology :67 88, 6. (Receved September 3, 8)