EVALUATION OF FRESHWATER FLUX OVER THE OCEAN

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1 Interntionl Archives of the Photogrmmetry, Remote Sensing nd Sptil Informtion Science, Volume XXXVIII, Prt 8, Kyoto Jpn 2 EVALUATION OF FRESHWATER FLUX OVER THE OCEAN M. Kubot, T. Wtbe, S. Iwski School of Mrine Sceince nd Technology, Toki University, 3-2-, Orido, Shimizu, Shizuok, Shizuok, JAPAN ( Kubot,wtbe,iwski)@mercury.oi.u-toki.c.jp Commission VIII, WG VIII /9 KEY WORDS: freshwter flux, precipittion, evportion, ocen, globl, stellite dt, renlysis dt ABSTRACT: Huge mounts of evportion nd precipittion exist over the ocen nd ply n importnt role in the globl climte system. Therefore, evlution of freshwter flux is criticl for understnding the mechnism of globl climte chnge. Recently we cn use three kinds of globl freshwter flux products such s in situ dt, stellite dt nd renlysis dt. Although vrious studies were crried out using those dt, it is difficult to determine which dt is most ccurte becuse we don t know the true vlue for globl freshwter flux. One of importnt thing is to clrify the differences mong those dt nd the cuse of the differences. For this purpose we compre vrious freshwter flux products over the ocen in this study. We use GSSTF2,, nd J-OFURO2 evportion products nd, nd precipittion products s stellite-bsed products. OAflux evportion product tht is hybrid dt is lso used here. On the other hnd, we use ERA4, NRA, nd. We ssume reference freshwter flux product to be J-OFURO2-. Temporl nd sptil s re one month nd x degree, respectively. The nlysis period is Renlysis products generlly tend to overestimte freshwter flux from the ocen to the tmosphere in the mid-ltitudes nd underestimte in the low-ltitudes. We estimted integrted freshwter flux over the ocen. If we use renlysis products, the integrted vlue is considerbly lrge compred with tht of the reference dt. Moreover, if we use renlysis product for precipittion nd nother product for evportion, we obtin negtive freshwter flux tht mens freshwter flux from the ocen to the tmosphere. We investigted the trend of integrted freshwter flux. For reference dt integrted freshwter flux shows positive trend tht mens freshwter flux from the ocen to the tmosphere increses s time goes on. On the other hnd, if we use renlysis products, integrted freshwter flux shows negtive trend tht mens freshwter flux from the tmosphere to the ocen.. INTRODUCTION The globl hydrologicl cycle, which consists of mny components, is n importnt issue from not only scientific but lso socil viewpoints, relted to the erth environmentl problem. In prticulr, evportion (E) nd precipittion (P) over the ocen re very lrge compred with other components. Therefore, the ccurte estimtion nd the continuous monitoring re criticl for understnding the globl hydrologicl cycle. For these objectives we cn use both of stellite-derived nd renlysis products t present. However, there re not so mny studies relted to this issue. Meht et l. (25) studied the verge nnul cycle of the tmospheric brnch of the fundmentl globl wter cycle. They found tht 75% to 85% of the totl globl evportion nd pproximtely 7% of the totl globl precipittion occur over the ocen in ech seson. Also Schlosser nd Houser (27) ssessed the cpbility of globl dt compiltion, lrgely stellite-bsed, to depict the globl tmospheric wter cycle s men stte nd vribility by using vrious dt. They pointed out tht the men nnul cycle of globl E-P cn be up to 5 times lrger to tht of totl precipitble wter, depending on the choice of ocen evportion dt nd the ocen evportion trends re driven by trends in SSMI-retrieved ner-surfce tmospheric humidity nd wind speed, nd the lrgest yer-to-yer chnges re coincident with trnsitions in Specil Sensor Microwve Imger (SSM/I) fleet. However, both of Meht et l. (25) nd Schlosser nd Houser (27) did not use renlysis products in their studies. In this study we crry out intercomprison of globl freshwter flux (FWF) products over the ocen, including both of stellitederived nd renlysis products. Moreover, the differences mong vrious products, globl budget by ech freshwter product nd trend of evportion, precipittion nd freshwter flux re nlyzed. 2. DATA We use vrious kinds of products. Some of products provide precipittion or evportion only. On the other hnd, some of products provide both of precipittion nd evportion. The Jpnese Ocen Flux Dt Sets with Use of Remote Sensing Observtions Version 2 (J-OFURO2) (Kubot nd Tomit, 27), The new version of the Hmburg Ocen Atmosphere Prmeters nd Fluxes from Stellite Dt set 3 () (Andersson et l., 27), Goddrd Stellite-Bsed Surfce Turbulent Fluxes Version 2 (GSSTF2) (Chou et l., 23) nd Objectively nlyzed Air-se het Fluxes () (Yu et l., 24, Yu et l., 28) re used s evportion dt. The first three products re stellite dt nd the is hybrid product nd mde by merging stellite dt, renlysis dt nd in situ dt. Also we use Globl Precipittion Climtology Project Version 2 () (Adler et l., 23), Climte Prediction Center (CPC) Merged Anlysis of Precipittion () (Xie nd Arkin, 997) nd HOAPS precipittion dt. use only stellite dt, but nd use not only stellite dt but lso in situ dt. Moreover, we use renlysis product such s the Ntionl Centers for Environmentl Prediction (NCEP)/Ntionl Center for Atmospheric Reserch renlysis (NRA) (Klny et l., 996), NCEP/Deprtment of Energy renlysis () (Knmitsu et l., 2), Europen Centre for Medium-Rnge Wether Forecst (ECMWF) Renlysis (ERA4) (Gibson et l., 999 Simmons nd Gibson, 2) nd Jpn Re-Anlysis 25 yer 975

As shown in the tble, dt period nd temporl nd sptil s re different depending on ech product. Therefore, we unify temporl nd sptil s nd nlysis period.

2 Interntionl Archives of the Photogrmmetry, Remote Sensing nd Sptil Informtion Science, Volume XXXVIII, Prt 8, Kyoto Jpn 2 () (Onogi et l., 27). All of these products include precipittion nd evportion dt. The description of ech product is given in Tble. As shown in the tble, dt period nd temporl nd sptil s re different depending on ech product. Therefore, we unify temporl nd sptil s nd nlysis period. The temporl nd sptil s re monthly nd x, respectively. The nlysis period is from 988 to 2. We don t know which globl freshwter product is more ccurte. Therefore, we ssume combintion of J-OFURO2 nd to be reference freshwter flux product in this study. Figure. Averge field for the reference freshwter product (-JOFURO2) (unit: mm/month). Tble. Description of ech product. Dt Set J-OFURO2 988/-26/2 987/7-25/2 Sptil Temporl Dily 2 hours Dt Set GSSTF2 987/7-2/2 958/-28/2 Sptil Temporl Dily Dily Figure 2 shows verge difference fields between reference product nd other products. Both of GSSTF2- or products overestimte in the high-ltitudes nd underestimte in the tropicl regions. Such differences minly depend on the differences of precipittion dt. The differences for re reltively smll. However, those for ll renlysis products re considerbly lrge. All renlysis freshwter products overestimte in the mid- nd high-ltitudes nd underestimte in the tropicl regions. In prticulr ERA4 extremely overestimtes in the mid-ltitudes nd both of NRA products underestimte in the tropicl regions. We will nlyze the reson of the differences between the reference product nd other products lter. Jpnese-Ocen Flux dt sets with Use of Remote sensing Observtions Version 2 (J-OFURO2) Hmburg Ocen-Atmosphere Prmeters nd Fluxes from Stellite Dt 3 () Goddrd Stellite-Bsed Surfce Turbulent Fluxes 2 (GSSTF2) Objective Anlyzed Air-Se Fluxes () Dt Set 979/-present 979/-28/7 Sptil Temporl Dt Set Sptil 2.5 Dily NRA 948/present Gussin grid T62 (8km) 2 hours 979/present Gussin grid T62 (8km) 987/725/2 ERA4 967/922/8 Gussin grid TL59 (km) 2 hours 979/7present Gussin grid T6 (km) Temporl The Globl Precipittion Climtology Project Version.2 () Climte Prediction Center (CPC) Merged Anlysis of Precipittion () NCEP/NCAR Re-nlysis (NRA) NCEP-DOE Re-nlysis () ECMWF Re-nlysis 4 (ERA4) Jpnese Re-nlysis 25 () Figure 2. Averge difference (mm/month) between the reference product nd () GSSTF2-2, (b) -, (c) ERA4,(d), (e) NRA, (f), nd (g). Figure 3 shows meridionl profiles of freshwter flux. Horizontl xis mens monthlyccumulted vlue. Positive vlues men freshwter flux from the ocen to the tmosphere. Generl fetures re in common. We cn see lrge trnsfer from the tmosphere 3. INTERCOMPARISON Figure shows verge freshwter flux field derived from JOFURO2 evportion nd precipittion. Positive vlues men freshwter trnsfer from the ocen to the tmosphere. We cn see generl feture bout freshwter flux over the ocen. For exmple, lrge freshwter trnsfer from the tmosphere to the ocen exists in the tropicl regions corresponding to ITCZ nd SPCZ. On the other hnd, we cn see lrge freshwter trnsfer from the ocen to the tmosphere in the mid-ltitudes, so-clled ocen desert. to the ocen round N corresponding to ITCZ, nd lrger 976 Figure 3. Meridionl Profile of verge freshwter flux.

Interntionl Archives of the Photogrmmetry, Remote Sensing nd Sptil Informtion Science, Volume XXXVIII, Prt 8, Kyoto Jpn 2 trnsfer from the ocen to the tmosphere round 25 in both hemispheres.

For exmple, the difference between the mximum vlue JAR25 nd the minimum vlue HOAPS reches to more thn 5 mm Moreover, the sctter is considerbly lrge in the high-ltitudes.

3 Interntionl Archives of the Photogrmmetry, Remote Sensing nd Sptil Informtion Science, Volume XXXVIII, Prt 8, Kyoto Jpn 2 trnsfer from the ocen to the tmosphere round 25 in both hemispheres. However, the quntittive differences re not negligible. For exmple, the difference between the mximum vlue JAR25 nd the minimum vlue HOAPS reches to more thn 5 mm Moreover, the sctter is considerbly lrge in the high-ltitudes. Most products show negtive trnsfer tht is freshwter trnsfer from the tmosphere to the ocen in the high-ltitudes. However, ERA4 nd - show positive flux there. regions s mentioned before becuse they overestimte precipittion in these regions. It is interesting tht not only precipittion but lso other prmeters contribute to the differences. Wind speed nd Q contribute to the underestimtion, nd Qs contributes to the wek overestimtion. 4. ANALYSIS OF DISCREPANCIES BETWEEN VARIOUS PRODUCTS We nlyze discrepncies of freshwter flux mong vrious products in terms of precipittion nd bulk vrible relted to evportion following to Bourrs (26). dfwf is the differences between the reference product nd other products. dfwf = dp * ( ) + du U + dq Q + dq Q s s = Lρ U (Q s Q ), dp = P(other) P() du = U (other) U dq = Q (other) Q Q = Lρ U, Q s = Lρ U dq s = Q s (other) Q s dfwf = FWF(other) FWF(J OFURO2 ) where U :wind speed, Q :specific humidity, Q s :sturted specific humidity. All terms in the right hnd side re the contribution of ech prmeter to the devition between reference product nd other products. Figure 4 shows mp of the differences between the reference product (J-OFURO2- ) s shown before nd other products nd the contribution of ech prmeter. For exmple, we cn see underestimtion in the western Pcific nd overestimtion in the high-ltitudes in the difference between the reference product nd the combintion of GSSTF2 nd. On the other hnd, we cn see the similr feture in the difference between nd reference product, tht is. Also the contribution by Wind nd Qs is smll nd by Q is lrge. Therefore, we cn conclude the differences in this cse re minly cused by precipittion nd Q nd the distribution pttern is determined by precipittion. Bsiclly the contributions by wind speed nd Qs re reltively smll, while those by precipittion nd Q re criticl. Also it is interesting tht the contribution of precipittion is lrger thn tht of precipittion. Qs contributes to underestimtion nd Q contributes overestimtion in the sme regions. As result, the differences between nd the reference product re considerbly smll s shown in this figure. Figure 5. Contributions to the devition between J-OFURO2- freshwter flux nd other freshwter renlysis products. Upper:, Middle:NRA, Lower:. 5. GLOBAL BUDGET We lso investigte globl budget of evportion, precipittion nd freshwter flux over the ocen. Figure 7shows our nlysis region. We exclude GSSTF2 product from our nlysis becuse there re mny vcnt dt in this product. Tble 2 gives globl budget for evportion. As mentioned before, renlysis products except ERA4 show lrger vlues compred with other products including previous results. The minimum nd the mximum re given by J-OFURO2 nd, respectively. Tble 3 gives globl budget for precipittion. The minimum nd the mximum re given by nd ERA4, respectively. However, the differences between ech product re smll compred with evportion. Tble 4 gives globl budget for freshwter flux. If we use renlysis product for evportion, we obtin the extremely lrge freshwter flux from the ocen to the tmosphere. Also if we use ERA4 or precipittion, we obtin negtive vlues. Since negtive vlues men river crry wter from the ocen to the lnd, the results seem to be unrel. It is surprising tht the globl budget is considerbly divergent, in prticulr renlysis product shows very lrge vlues. Tble 2. Globl verge evportion ( 7 kg/yer). J-OFURO2 ERA4 NRA Tble 3. Globl verge precipittion ( 7 kg/yer). ERA4 NRA Evpo. Precipi. Tble 4. Globl budget for freshwter flux ( 6 kg/yer). J- OFURO2 ERA4 NRA Figure 4. Contributions to the devition between J-OFURO2- freshwter flux nd other freshwter products. Figure 5 shows similr figures except for renlysis products. All products underestimte freshwter flux in the tropicl ERA NRA

4 Interntionl Archives of the Photogrmmetry, Remote Sensing nd Sptil Informtion Science, Volume XXXVIII, Prt 8, Kyoto Jpn 2 Tble 6. Regression coefficients for freshwter flux of ech product (unit: 6kg/month) 6. TREND ANALYSIS Evpo. Next, we will investigte trend of globl budget. Figure 6 shows nnul time series of globl freshwter flux over the ocen by reference product. We cn see increse of freshwter flux from the ocen to the tmosphere. This trend mens river dischrge nd precipittion over the lnd increses. Figure 7 shows time series of evportion nd precipittion in ddition to freshwter flux. Since we cn see similr trend only in evportion shown by blue line, evportion is crucil to the trend of freshwter flux. Precipi. JOFURO2.4.4 ERA4 - - NRA - ERA4 NRA Figure 8 is trend mp for the reference product. Evportion trend is reltively homogeneous nd we cn see evportion increses in most regions. On the other hnd, we cn see lrge precipittion trends in the tropicl regions nd the distribution seems to be relted to ENSO. As result, the distribution for freshwter flux shows similr pttern to tht of precipittion. Figure 6. Annul time series of globl freshwter flux nomly over the ocen by reference product. Figure 8. A trend mp for the reference product. 7. SUMMARY We crried out intercomprison of vrious globl freshwter flux products including stellite-products nd renlysis products. In this study we ssume precipittion nd JOFURO2 evportion s reference products. All products cn reproduce generl common fetures, for exmple lrge freshwter flux from the ocen to the tmosphere cn be found in the mid-ltitudes so-clled ocen desert nd tht from the tmosphere to the ocen cn be found in the tropicl regions. However, the freshwter flux vlues re considerbly different depending on ech product. Most of products overestimte in the high-ltitudes nd underestimte in the tropicl regions compred with reference product. We investigte the contribution of ech prmeter to the devition between the reference product nd other products. For the stellite products the devition is mostly relted to precipittion nd ir specific humidity. However, for the renlysis products the devition is relted to ll prmeters. Wind speed nd Q contribute to the underestimtion nd Qs contributes to the wek overestimtion. We lso compre meridionl profiles. The differences round ITCZ re very lrge. Also freshwter flux vlues in the highltitudes re scttered, including positive nd negtive vlues. We investigte globl budget of evportion, precipittion nd freshwter flux. J-OFURO2 gives the minimum evportion nd gives the mximum vlue. Also gives minimum precipittion nd ERA4 gives the mximum precipittion. Renlysis products except ERA4 gives lrge freshwter fluxes compred with stellite-products becuse of lrge evportion. The reference product gives remrkble positive trend becuse Figure 7. Time series of evportion nd precipittion nomlies in ddition to freshwter flux nomly. Tble 5 shows regression coefficients for evportion nd precipittion. J-OFURO2,,, nd NRA give positive significnt trend for evportion. On the other hnd, only ERA4 nd give positive trend for precipittion. In prticulr the trend by ERA4 seems to be too lrge. Tble 5. Regression coefficients for () evportion nd (b) precipittion. JOFURO2 ERA4 NRA b. ERA4 NRA Tble 6 shows regression coefficients for freshwter flux of ech product. Although renlysis products relted to ERA4 nd show significnt negtive trend, most products shows positive trend. The positive trend suggests increse of precipittion over lnd nd river dischrge. 978

5 Interntionl Archives of the Photogrmmetry, Remote Sensing nd Sptil Informtion Science, Volume XXXVIII, Prt 8, Kyoto Jpn 2 evportion by J-OFURO2 hs positive trend nd precipittion by hs no trend. Most products show positive trend except products relted to ERA4 nd. The positive trend suggests increse of precipittion over lnd nd river dischrge.,, nd NRA nd precipittion by ERA4 nd show significnt positive trends. Both of positive nd negtive lrge trends of freshwter flux exist in the tropicl regions, similr to precipittion. 8. REFERENCES Adler, R. F., G. J. Huffmn, A. Chng, R. Ferrro, P. Xie, J. Jnowik, B. Rudolf, U. Schneider, S. Curtis, D. Bolvin, A. Gruber, J. Susskind, P. Arkin. nd E. Nelkin, 23: The Globl Precipittion Climtology Project (GPCP) Monthly Precipittion Anlysis (979-present), J. Hydrometeor., 4, pp Andersson, A., S. Bkn, nd C. Klepp, 27: The HOAPS Climtology, FLUX NEWS, CLIVAR Exchnges, pp.-2. Bourrs, D., 26: Comprison of Five Stellite-Derived Ltent Het Flux Products to Moored Buoy Dt, J. Climte, 9, pp Chou, S. H., E. Nelkin, J. Ardizzone, R. M. Atls, nd C. L. Shie, 23: Surfce turbulent het nd momentum fluxes over globl ocens bsed on the Goddrd Stellite retriecls,version 2(GSSTF2), J.Climte., 6, pp Simmonds, A. J., nd J. K. Gibson, 2: The ERA4 project pln, ERA4 project report series, Shinfield Prk, Reding, UK, 63pp. Xie, P., nd P. A. Arkin, 997: Globl Precipittion: A 7-Yer Monthly Anlysis Bsed on Guge Observtions, Stellite Estimtes, nd Numericl Model Outputs, Bull. Amer. Met. Soc., 78, pp Yu, L., R. A. Weller, nd B, Sun, 24: Men nd vribility of the WHOI dily ltent nd sensible het fluxes t In Situ flux mesurement sites in the Atlntic ocen, J. Climte., 7, pp Yu, L., X. Jin, nd R. A. Weller, 28: Multidecde Globl Flux Dtsets from the Objectively Anlyzed Air-se Fluxes () Project: Ltent nd sensible het fluxes, ocen evportion, nd relted surfce meteorologicl vribles. Woods Hole Ocenogrphic Institution, Project Technicl Report. OA-28-, 64pp. 9. ACKNOWLEDGEMENT This reserch ws supported by the Jpn Aerospce Explortion Agency (JAXA). Gibson, J. K., P. Kållberg, S. Uppl, A. Hernndez, A. Nomur, nd E. Serrno, 999: ERA-5 description, ECMWF Re- Anlysis project report series, Reding, UK, 67pp. Klney, E. C., M. Knmitsu, R. Kistler, W. Collins, D. Deven, L. Gndin, M. Iredell, S. Sh, G. White, J. Woolen, Y. Zhu, M. Chellih, W. Ebisuzki, W. Higgins, J. Jnowik, K. Mo, C. Ropelewski, J. Wng, A. Leetm, R. Reynolds, R. Jenne, nd D. Joseph, 996: NCEP/NCAR 4-yer Re-nlysis project, Bull. Amer. Meteor. Soc., 77, pp Knmitsu, M., W. Ebisuzki, J. Woollen, S. K. Yng, J. J. Hnilo, M. Fiorino, nd G. L. Potter, 22: NCEP-DOE AMIP- (R-2), Bull. Amer. Meteor. Soc., 83, pp Kubot, M., nd H. Tomit, 27: The present stte of the J- OFURO ir-se interction dt product, FLUX NEWS, CLIVAR Exchnges, pp.3-5. Meht, V. M. nd A. J. DeCndis(25): Remote-sensing-bsed estimtes of the fundmentl globl wter cycle: Annul cycle, J. Geophys. Res.,, D223, doi:.29/24jd5672. Oki, T., nd S. Kne, 26: Globl hydrologicl cycles nd world wter resources, Science, 33., pp Onogi, K., J. Tsutsui, H. Koide, M. Skmoto, S. Kobyshi, H. Htsushit, T. Mtsumoto, N. Ymzki, H. Kmhori, K. Tkhshi, S. Kdokur, K. Wd, K. Kto, R. Oym, T. Ose, N. Mnnoji, nd R. Tir, 27: The JRA-25 renlysis, J. Meteor. Soc. Jpn., 85, pp Schlosser, C. A. nd P. R. Houser, 27: Assesing stellite-er perspective of the globl wter cycle, J. Climte, 2, pp

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