The European Centre for Medium-Range Weather Forecasts (ECMWF) and Institut Francais Pour la Recherche et l’Exploitation de la Mer European Remote-Sensing Satellite, named IFR2, surface wind velocity data products are compared during July 1995 over the Arabian Sea. Substantial differences were found. The central positions of the maximum isotach were separated by 450 km, and the ECMWF maximum isotach was 2 m s⁻¹ higher than that of IFR2. IFR2 wind components contained about 10 times more variance than ECMWF winds for horizontal distances from 50 to 250 km. Along the 8.5°N southern boundary of the Arabian Sea, ECMWF southward Ekman transport was higher than that of IFR2 by an amount that could be observed with current measurements. The ECMWF and IFR2 difference in upward transport of water into the Ekman layer, computed from wind stress curl, was large enough to measure

Freilich, Michael H.

Halpern, David

Weller, Robert A.

Undetermined

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JULY 1999 1619N O T E S A N D C O R R E S P O N D E N C Eq 1999 American Meteorological SocietyNOTES AND CORRESPONDENCEECMWF and ERS-1 Surface Winds over the Arabian Sea during July 1995DAVID HALPERNEarth and Space Sciences Division, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CaliforniaMICHAEL H. FREILICHCollege of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, OregonROBERT A. WELLERDepartment of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts5 November 1997 and 10 September 1998ABSTRACTThe European Centre for Medium-Range Weather Forecasts (ECMWF) and Institut Francais Pour la Rechercheet l’Exploitation de la Mer European Remote-Sensing Satellite, named IFR2, surface wind velocity data productsare compared during July 1995 over the Arabian Sea. Substantial differences were found. The central positionsof the maximum isotach were separated by 450 km, and the ECMWF maximum isotach was 2 m s21 higherthan that of IFR2. IFR2 wind components contained about 10 times more variance than ECMWF winds forhorizontal distances from 50 to 250 km. Along the 8.58N southern boundary of the Arabian Sea, ECMWFsouthward Ekman transport was higher than that of IFR2 by an amount that could be observed with currentmeasurements. The ECMWF and IFR2 difference in upward transport of water into the Ekman layer, computedfrom wind stress curl, was large enough to measure.1. IntroductionSurface wind velocity data products for the ArabianSea are available from a variety of sources: wind reportsfrom ships, operational wind field analyses produced atnumerical weather prediction (NWP) centers, and windvelocity computed from satellite scatterometer mea-surements. Ship reports do not have uniform error char-acteristics and are highly aliased in time and space. TheNWP data products suffer because of incomplete pa-rameterizations of physical and thermodynamical pro-cesses and because of sparse distribution of ship windobservations used in data assimilation. Since the July1991 launch of the first European Remote-Sensing Sat-ellite (ERS-1), surface wind velocity fields have beencreated from scatterometer data.This paper describes a comparison between ERS-1 IFR2(IFREMER 1996) and European Centre for Medium-Range Forecasts (ECMWF) 10-m height wind velocityCorresponding author address: Dr. David Halpern, Jet PropulsionLaboratory, MS 300-323, California Institute of Technology, 4800Oak Grove Drive, Pasadena, CA 91109-8099.E-mail: david.halpern@jpl.nasa.govdata products over the Arabian Sea for July 1995. We usedthe 6-h, 1.1258 3 1.1258 ECMWF wind data product. TheInstitut Francais Pour la Recherche et l’Exploitation de laMer (IFREMER) produced, in addition to IFR2, a versionof the ECMWF wind product that was collocated withIFR2 measurements (Anonymous 1996), which we namedECMWF/IFR. We chose July because it is climatologicallythe month with highest wind speeds (Hastenrath and Lamb1979) and in July 1995 several international oceanographicfield campaigns provide additional data.In this report, the level of physical oceanographicsignificance between monthly mean wind speeds is 1 ms21. For oceanographic and meteorological conditionsencountered at the Improved Meteorological (IMET)buoy (15.58N, 61.58E), where measurements of surfacemeteorological and upper-ocean variables had been re-corded (Weller 1996), a 61 m s21 change in the 1-minwind speed measured during July 1995 corresponded toa net surface heat flux of 68 W m22. This value is aboutthe precision required for studies of climate variability(Bretherton 1981).2. ResultsThe patterns and general features of the July 1995mean IFR2, ECMWF, and ECMWF/IFR wind velocities1620 VOLUME 29J O U R N A L O F P H Y S I C A L O C E A N O G R A P H YFIG. 1. Arabian Sea distributions of July 1995 scalar-averaged windspeed (contour interval 5 1 m s21) and vector-averaged wind direc-tion for (a) IFR2, (b) ECMWF/IFR, and (c) ECMWF data products.The dot at 15.58N, 61.58E represents the location of IMET windobservations.FIG. 2. Scatterplot of collocated IFR2 and ECMWF/IFR windspeeds over the Arabian Sea during July 1995. Number of collocationswas 45 363. Dash line represents perfect agreement. Solid line rep-resents least-squares orthogonal regression line. Contours (with var-iable interval) are numbers of collocations.over the Arabian Sea were qualitatively similar (Fig. 1).Boundaries of the Arabian Sea were arbitrarily definedto be 8.58N, 248N, 508E, and 778E. Distortions of theECMWF/IFR isotachs compared to ECMWF (Figs. 1band 1c) were introduced by the ERS-1 scatterometersampling scheme (Zeng and Levy 1995).In July 1995 the location and magnitude of the max-imum isotach varied with data product (Fig. 1). ForECMWF and ECMWF/IFR, the region of maximumisotach (16 m s21) was centered at 108N, 538E. TheECMWF and ECMWF/IFR maximum isotach was 2 ms21 greater than the IFR2 maximum isotach; the differ-ence was twice the acceptable uncertainty. The IFR2data had three regions enclosed with the same maximumisotach (14 m s21), with the largest area centered at138N, 568E. This site was 450 km northeast of the cen-ters of the ECMWF and ECMWF/IFR maximum iso-tach. Examination of Special Sensor Microwave/Imagerwind speeds (Wentz 1997) showed that the position andmagnitude of the maximum isotach (13 m s21) weremore similar to those computed with IFR2 data com-pared to ECMWF data.a. Regression analysisFor IFR2 speeds below (above) 8.7 m s21, ECMWF/IFR speeds were lower (higher) than IFR2 speeds (Fig.2). For IFR2 speeds between 3 and 14 m s21, the re-gression line was within 1 m s21 of the 1:1 line of perfectcorrespondence. For IFR2 speeds greater than 15 m s21,the predicted residual speed between ECMWF/IFR andIFR2 was greater than 1 m s21. The correlation coef-ficient between ECMWF/IFR and IFR2 wind speeds was0.8, which was significant at the 95% confidence level.In this report, the level of statistical significance is 95%.Large differences between collocated ECMWF/IFR andIFR2 speeds yielded a rms difference of 2.1 m s21,which was twice the desired level of uncertainty.b. Temporal variabilityThe July 1995 ECMWF/IFR and IFR2 18 3 18 windspeed variances averaged over the Arabian Sea were 4.8and 3.6 m2 s22, respectively, which were significantlydifferent according to the F test. To examine the rep-resentativeness of the ECMWF temporal structure, 6-hECMWF and IMET data were compared for July 1995.JULY 1999 1621N O T E S A N D C O R R E S P O N D E N C EFIG. 3. Frequency spectra of (a) east–west, u, and (b) north–south,y , wind velocity components for IMET and ECMWF data during July1995. The 95% represents the 95% confidence levels determined fromthe chi-square distribution.IMET data were averaged over 6 hours centered on theECMWF forecast time. The ECMWF and IMET July1995 mean wind speed difference was less than 1 ms21. However, the ECMWF u variance (3.3 m2 s22) andy variance (4.1 m2 s22) were nearly two and three times,respectively, greater than the corresponding IMET var-iances. The differences between the ECMWF and IFR2wind component variances were significant.Spectra of ECMWF and IMET u and y componentsyielded the distribution of variance with frequency (Fig.3). In contrast to the large difference in IMET u spectrallevels between low and high frequencies, the ECMWFu spectrum was almost flat. At the diurnal frequencyand at higher frequencies, the ECMWF u spectral valueswere significantly different. Significant statistical dif-ference of spectral estimates means that confidence in-tervals associated with ECMWF and IFR2 spectral val-ues at the same frequency did not overlap. In theECMWF y component, the amplitude of the diurnal-period oscillation was significant. At the diurnal period,the ECMWF rms wind speed amplitude (0.8 m s21) was2.7 times greater than that computed with IMET data.We did not expect ECMWF data to have more tem-poral variability than IMET data. In the ECMWF fore-cast–analysis system, data gaps of the surface wind fieldto be ingested into the atmosphere model were filledwith previous forecasts, which should reduce temporalvariability.c. Spatial variabilityThe IFR2 data occurred along 19 cell tracks parallelto the ERS-1 ground track. During July 1995, there were102 cell tracks, each with 72 25-km elements, in theregion bounded by 568–708E along 88N and by 608–688E along 248N. The 6-h ECMWF data between 88 and248N were interpolated to 72 24.4-km intervals alongfive meridians (618, 628, 638, 648, and 658E). Beforecomputing the periodogram, the u and y componentsalong each cell track (for IFR2) and meridian (forECMWF) were prewhitened to reduce spectral leakage.For IFR2, the spectral estimate for each wave numberki was equal to the arithmetic mean value of the 102recolored periodogram ordinates at ki. For ECMWF,spectral estimates were computed from 124 periodo-grams along each meridian, and then the five-meridianaverage spectra were computed. For ECMWF spectra,confidence limits are not easily determined because each6-h wind field was not an independent representation ofthe wind field. We arbitrarily allow the ECMWF-spec-tral confidence intervals to equal that corresponding to204 degrees of freedom, instead of 1240 degrees offreedom had each ECMWF wind field been indepen-dent.For wave lengths smaller than 400 km, IFR2 u- andy -spectral estimates were significantly larger thanECMWF with differences increasing from one-third at400 km to nearly 100 at 50 km (Fig. 4). Stoffelen (1998)reported that ERS-1 winds, which had been computedwith an algorithm different than the one we used, con-tained substantially more variance than ECMWF windson scales between 50 and 250 km. It is not surprisingthat ECMWF data have lower variability in space com-pared to IFR2 because the ECMWF assimilation scheme1622 VOLUME 29J O U R N A L O F P H Y S I C A L O C E A N O G R A P H YFIG. 4. Wavenumber spectra of (a) east–west, u, and (b) north–south, y , wind velocity components for ECMWF and IFR2 data duringJuly 1995 in the central Arabian Sea. The 95% represents the 95%confidence levels determined from the chi-square distribution.used a weighted average of all data less than 3 h apartfrom the 6-h forecast verification time and over a 500-km region. This is in marked contrast to the instantaneityand 50-km resolution of IFR2 data.The ECMWF u- and y-spectra displayed significantpeaks of approximately equal amplitude at 64 km; theIFR2 spectra contained no such spectral peak (Fig. 4).The variance at 64 km was 104 times smaller than thatat 1000 km. No hypothesis nor previous observationjustify a spectral peak at 64 km, indicating that thesmall-amplitude spectral peak is spurious.Throughout the 1800- to 50-km range, the IFR2 u-and y-spectral (log–log) slopes were 22.6 and 22.4,respectively (Fig. 4), which were nearly 25% greaterthan those computed by Freilich and Chelton (1986) forthe trade wind zone in the Pacific Ocean.d. Wind-driven ocean transportsEmploying conservation of mass, a lowest-order ap-proximation of Arabian Sea southwest monsoon wind-driven ocean circulation is meridional flow across thesouthern boundary at 8.58N, vertical exchange betweenthe thermocline and near-surface layers, and water en-tering the Arabian Sea by the narrow Somali Current.Attention is focused on Ekman transport and upwardtransport into the near-surface layer, which were com-puted by methods described in Halpern et al. (1998).Along the Arabian Sea southern boundary at 8.58N,the integrated southward Ekman transports in July 1995for ECMWF, ECMWF/IFR, and IFR2 were 217.5,216.7, and 212.5 Sv (Sv [ 106 m3 s21), respectively.The 4.2 Sv difference between ECMWF/IFR and IFR2would correspond to a southward Ekman current av-eraged over 50 m of 3 cm s21, which is a measurablequantity.The July 1995 ECMWF, ECMWF/IFR, and IFR2 ver-tical transports into the Ekman layer integrated over theArabian Sea north of 88N were 6.3, 5.2, and 3.2 Sv,respectively. The 2 Sv (or 48%) difference betweenECMWF/IFR and IFR2 corresponded to an average ver-tical velocity uncertainty of 0.6 3 1026 m s21. This valueis comparable to the error associated with vertical ve-locity estimated from the continuity equation and hor-izontal current measurements. North of 138N where ver-tical motion was upward, the average difference be-tween ECMWF/IFR and IFR2 vertical velocities was1.8 3 1026 m s21 or three times greater than the averageover the Arabian Sea.3. Concluding remarksResults included 1) ECMWF/IFR wind speeds weregreater (smaller) than IFR2 at high (low) wind speeds;2) IFR2 maximum isotach (14 m s21) was centered 450km toward the northeast and was 2 m s21 smaller com-pared to ECMWF maximum isotach (wind speed dif-ferences greater than 1 m s21 were significant); 3) at allJULY 1999 1623N O T E S A N D C O R R E S P O N D E N C Efrequencies (1.34 3 1023–8.33 3 1022 cycles per hour)ECMWF wind component spectral values were greaterthan those computed from moored-buoy wind obser-vations at 15.58N, 61.58E, and at the diurnal period theECMWF wind speed amplitude was nearly three timesgreater than that computed with wind recorder data; 4)zonal and meridional spectral values of ECMWF mo-tions were significantly smaller than IFR2 for horizontalscales from 400 to 50 km; 5) IFR2 southward Ekmantransport integrated along 8.58N was substantially great-er than that computed with ECMWF data; and 6) upwardtransport of water into the Ekman layer over the ArabianSea was less for IFR2 than ECMWF, especially over thenorthern Arabian Sea.Studies of wind-driven ocean circulation, especiallythose related to monsoon dynamics of the Arabian Sea,require accurate winds. Local wind-driven velocities,away from the coast, are essentially linearly related, ateach frequency, to the wind stress (Weller 1981), andthe higher ECMWF energy levels (Fig. 3) would resultin stronger wind-driven currents and shear at the baseof the mixed layer compared to IFR2. ECMWF south-ward Ekman transport and upward transport into theEkman layer would yield stronger meridional overturn-ing circulation in the north Indian Ocean compared toIFR2. Further studies are warranted with other NWPwind products, as well as numerical simulations utiliz-ing these products.Acknowledgments. We are extremely grateful to theIFREMER Centre ERS d’Archivage et de Traitement(CERSAT) for producing and distributing ECMWF/IFRand IFR2 data. Dr. Frank Wentz (Remote Sensing Sys-tems) kindly provided SSMI winds for July 1995. Themanuscript benefited greatly from constructive com-ments by Dr. Craig Lee (WHOI), Dr. Ad Stoffelen(KNMI), and two anonymous reviewers. Data process-ing and graphics were made by Peter Woiceshyn (JPL)in his usual proficient manner. The work described inthis paper was supported by NASA 622-47-09 (DH),NASA SeaWinds Project 229-18-41 (DH), NASANSCAT Project via JPL Contract 959351 (MHF), andONR Grant N00014-94-1-0161 (RW). The research de-scribed in this paper was performed, in part, by the JetPropulsion Laboratory, California Institute of Technol-ogy, under contract with the National Aeronautics andSpace Administration.REFERENCESBretherton, F., 1981: The ocean surface energetics—A need for cli-mate understanding. Applications of Existing Satellite Data tothe Study of the Ocean Surface Energetics, C. Gautier, Ed., Uni-versity of Wisconsin Press, 5–14.Freilich, M. H., and D. B. Chelton, 1986: Wavenumber spectra ofPacific winds measured by the Seasat scatterometer. J. Phys.Oceanogr., 16, 741–757.Halpern, D., M. H. Freilich, and R. A. Weller, 1998: Arabian Seasurface winds and ocean transports determined from ERS-1 scat-terometer. J. Geophys. Res., 103, 7799–7805.Hastenrath, S., and P. J. Lamb, 1979: Climatic Atlas of the IndianOcean. University of Wisconsin Press, 19 pp and 97 charts.IFREMER, 1996: Off-line wind scatterometer ERS products—Users’manual. CERSAT Ref. C2-MUT-W-01-IF: V2.0, Dept. Ocean.from Space, IFREMER, Plouzane, France, 85 pp. [Availablefrom IFREMER, B.P. 70, 29280 Plouzane, France.]Stoffelen, A., 1998: Towards the true near surface wind speed: Errormodeling and calibration using triple collocation. J. Geophys.Res., 103, 7755–7766.Weller, R. A., 1996: In situ observations of surface meteorology andair–sea fluxes. 1996 U.S. WOCE Implementation Rep. 8, 48 pp.[Available from the Department of Oceanography, Texas A&MUniversity, College Station, TX 77843.], 1981: Observations of the velocity response to wind forcingin the upper ocean. J. Geophys. Res., 86, 1969–1977.Wentz, F. J., 1997: A well-calibrated ocean algorithm for specialsensor microwave/imager. J. Geophys. Res., 102, 8703–8718.Zeng, L., and G. Levy, 1995: Space and time aliasing structure inmonthly mean polar-orbiting satellite data. J. Geophys. Res., 100,5133–5142.

ECMWF and ERS-1 Surface Winds over the Arabian Sea during July 1995

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ECMWF and ERS-1 Surface Winds over the Arabian Sea during July 1995

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