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Journl of Mrine Systems 7 8 (3) 7 8 Contents lists vilble t SciVerse ScienceDirect Journl of Mrine Systems journl homepge: www.elsevier.com/locte/jmrsys Comptibility of C- nd Ku-bnd sctterometer winds: ERS- nd QuikSCAT Abderrhim Bentmy, Semyon A. Grodsky b,, Bertrnd Chpron, Jmes A. Crton b Institut Frncis pour l Recherche et l'exploittion de l Mer, Plouzne, Frnce b Deprtment of Atmospheric nd Ocenic Science, University of Mrylnd, College Prk, MD 74, USA rticle info bstrct Article history: Received 3 October Received in revised form 3 Februry 3 Accepted 5 Februry 3 Avilble online Februry 3 Keywords: Sctterometer winds SST Inter-instrument bis Globl winds provided by stellite sctterometry re n importnt spect of the ocen observing system. Mny pplictions require well-clibrted time series of winds over time periods spnned by multiple missions. But sensors on individul stellites differ, introducing differences in wind estimtes. This study focuses on globl winds from two sctterometers, ERS- (996 ) nd QuikSCAT (999 9) tht show persistent differences during their period of overlp (July-999 to Jnury ). We exmine set of collocted observtions during this period to evlute the cuses of these differences. The use of different operting frequencies leds to differences tht depend on rin rte, wind velocity, nd SST. The enhnced sensitivity to rin rte of the higher frequency QuikSCAT is mitigted by combined use of the stndrd rin flg nd removing dt for which the multidimensionl rin probbility is >.5. Generlly, ERS- wind speeds computed using the IFREMER CMODIFR geophysicl model function (GMF) re lower thn QuikSCAT winds by.6 m/s, but wind directions re consistent. This wind speed bis is reduced to. m/s fter prtil reprocessing of ERS- wind speed using Hersbch's () new CMOD5.n GMF, without ltering wind direction. An dditionl contributor to the difference in wind speed is the bises in the GMFs used in processing the two dt sets nd is empiriclly prmeterized here s function of ERS- wind speed nd direction reltive to the mid-bem zimuth. After pplying the bove corrections, QuikSCAT wind speed then remins systemticlly lower (by.5 m/s) thn ERS- over regions of very cold SSTb5 C. This difference my result from temperture-dependence in the viscous dmping of surfce wves which hs stronger impct on shorter wves nd thus preferentilly ffects QuikSCAT. 3 Elsevier B.V. All rights reserved.. Introduction Only stellite sensors, prticulrly sctterometers, cn provide globl synoptic observtions of surfce winds. Yet, while mny pplictions require well-clibrted time series of winds over time periods spnned by multiple sctterometer stellite missions, the sensors on individul stellites differ, introducing differences in the wind estimtes (Bourss et l., 9). For the period from 996 to the present, three successive sctterometer missions hve been operted: the C-bnd Remote Sensing Stellite (ERS-) (996 Jnury ) followed by the Ku-bnd QuikSCAT (mid-999 to lte-9), nd by the C-bnd Advnced SCATterometer (ASCAT) (7-onwrd). Creting wellclibrted time series from such succession of individul sensor records requires ccounting for chnges in individul sensor bises, nd this ccounting is most necessry when the sctterometers operte in different frequency bnds nd operting modes (e.g. Bentmy et l.,, ; Ebuchi et l., ). Bentmy et l. () hve exploited the existence of time overlp between missions to connect the wind records for QuikSCAT nd ASCAT. Here we use the sme pproch to ddress the connection between QuikSCAT nd the erlier ERS-. The Corresponding uthor. Tel.: + 3 45 533; fx: + 3 34 948. E-mil ddress: seny@tmos.umd.edu (S.A. Grodsky). successful result of this clibrtion exercise would be continuous record of clibrted sctterometer winds spnning the pst 3 yers. Sctterometers re microwve rdrs tht infer ner-surfce wind velocity from the strength of the normlized rdr bcksctter coefficients (NRCS, σ ) mesured t vriety of zimuth (χ) nd incidence ngles (θ). The ocen surfce rdr signl bcksctter occurs primrily from centimeter-scle cpillry/grvity wves (ripples), whose mplitude is in equilibrium with the locl ner-surfce wind. At given wind velocity, it lso depends on other prmeters governing ripple genertion such s SST-dependent wter viscosity nd ir density (ρ )(Doneln nd Pierson, 987), s well s other environmentl conditions such s se stte degree of development nd/or surfce current (e.g. Quilfen et l.,, 4). In this study, we express surfce wind speed in terms of m equivlent neutrl wind (W), which is then relted to NRCS using n empiricl geophysicl model function (GMF). Equivlent neutrl wind is the wind speed tht would be ssocited with the ctul wind stress if the tmospheric boundry lyer ws neutrlly strtified. GMFs used in current sctterometer wind products do not include SST-dependence or se-stte degree of development informtion. Becuse of the need by mny pplictions for consistent, well-clibrted wind record, there hve been number of previous efforts to combine wind records from these sctterometer missions. 94-7963/$ see front mtter 3 Elsevier B.V. All rights reserved. http://dx.doi.org/.6/j.jmrsys.3..8
A. Bentmy et l. / Journl of Mrine Systems 7 8 (3) 7 8 73 Generlly, these efforts hve tken the pproch of relting ech mission wind time series to reference wind field spnning ll missions tht is itself ssumed to be consistent nd well-clibrted. Such efforts hve used both pssive microwve winds nd renlysis winds for this referencing (e.g. Atls et l., ; Bentmy et l., 7; Wentz et l., 7). The disdvntges of this pproch lie in the ssumption tht the reference wind field is itself well-clibrted, nd in the fct tht the corrections tht re mde to the sctterometer mission winds re unrelted to the bsic physicl vribles being mesured (e.g., σ, θ, χ). Use of renlysis winds for referencing is prticulrly troubling if the renlysis winds ssimilte the sme sctterometer winds tht they re then compred to.. Dt In this section, we provide brief description of the ERS- nd QuikSCAT dt sets. Additionl detils re provided in the corresponding user mnuls (CERSAT, 994; JPL, 6). Rdr microwves from C-bnd ERS- (5.3 GHz)/Ku-bnd QuikSCAT (3.4 GHz) sctter most efficiently from short scle wves with bout 5 cm/ cm lengths, respectively, phenomenon known s Brgg scttering... ERS- The ctive microwve instrument on bord ERS- is the sme C-bnd (5.3 GHz, 5.7 cm) sctterometer on bord ERS-. It operted from April, 995 through September 5,. However, due to the on-bord recorder filure, globl dt re vilble only through erly Jnury. The sctterometer hs three ntenne looking 45 forwrd (fore-bem), perpendiculr (mid-bem), nd 45 bckwrd (ft-bem) reltive to the stellite trck nd illuminting 5 km wide swth to the right of the stellite trck. m equivlent neutrl wind speed nd direction re inferred t 5 km sptil resolution using the Center for Stellite Exploittion nd Reserch (CERSAT) GMF (Quilfen, 995) bsed on the Institut Frnçis de Recherche pour l'exploittion de l Mer (IFREMER) version GMF (CMODIFR of Bentmy et l., 999). CMODIFR ws derived by fitting ERS- winds to collocted Ntionl Dt Buoy Center (NDBC) buoy winds. CMODIFR hs been pplied to ERS- without ny djustments. Lnd, ice, nd rin contmintions re excluded using the CERSAT qulity flgs. Although this version of the ERS- winds is known for persistent wind speed underestimtion t W>5 m/s nd rre occurrence of low wind dt (Bentmy et l., ), it is the only one spnning the entire mission in the globl domin... QuikSCAT The SeWinds Ku-bnd (3.4 GHz,. cm) sctterometer on bord the NASA/QuikSCAT (referred to subsequently s QuikSCAT or QS) ws lunched in June 999. The QuikSCAT rotting ntenn hs two emitters: the H-pol inner bem t θ=46.5 nd V-pol outer bem t θ=54 with swth widths of 4 km nd 8 km, respectively, tht together cover round 9% of the globl ocen dily. QuikSCAT swth dt is binned into wind vector cells of 5 5 km. QuikSCAT winds used here re level b dt, derived from bcksctter using the empiricl QSCAT- GMF (JPL, 6) together with mximum likelihood estimtor, which selects the most probble wind solution. To improve wind direction in the middle of the swth where the zimuth diversity is poor, the direction intervl retrievl with threshold nudging lgorithm is pplied. This retrievl technique provides pproximtely m/s nd ccurcy in wind speed nd direction, respectively (e.g. Bentmy et l., ; Bourss et l., 3; Ebuchi et l., ). Becuse of its shorter wvelength, Ku-bnd sctterometers re more sensitive to impcts of rin thn longer wvelength C-bnd sctterometers. Rin perturbtions result from ttenution by rindrops in the tmosphere s well s mplifiction due to volume scttering nd chnges of se surfce roughness by impinging drops (Tourndre nd Quilfen, 3, 5). It is observed (Weissmn et l., ) tht the mplifiction effects dominte nd the impct of undetected rinfll on the higher frequency QuikSCAT enhnces bcksctter leding to positive bises in W QS of up to m/s in the riny tropicl convergence zones nd western boundry current regions even fter rin flgging is pplied (Bentmy et l., ). Two rin indices, rin flg nd multidimensionl rin probbility (MRP), re provided with the QuikSCAT dt set to mrk hevy rinfll. QuikSCAT wind overestimtion in tropics is reduced by some 3% to 4% when dt for which MRP>.5 re lso removed. This combintion of rin selection indices is thus pplied to ll QuikSCAT dt in the rest of this study. The shorter wvelength Ku-bnd rdr is lso more sensitive to the direct impct of SST, which t given vlue of wind speed, lters the mplitude of the surfce ripples through the competing effects of ρ -dependent wind wve growth rte nd SST-dependent viscous wve dissiption (Doneln nd Pierson, 987; Grodsky et l., )..3. Collocted dt The procedure we use to identify colloctions of ERS-/QuikSCAT observtions is similr to tht described in Bentmy et l. (). The period of overlp when both ERS- nd QuikSCAT provide globl ocen coverge extends from July 999 to Jnury. During this period, we identify ll pirs of observtions where the sptil seprtion between collocted ERS- nd QuikSCAT cells is less thn 5 km. The two stellites re on qusi sun-synchronous orbits, but the QuikSCAT locl equtor crossing time for scending trcks (6:3.m.) leds the ERS- locl equtor crossing time (:3.m.) by pproximtely 4 h. This implies tht sptil colloctions of the two instruments occur W BUOY (m/s) b WDir BUOY WDir ERS (degn) 5 5 5 5 W ERS (m/s) 9 45 45 9 8 9 9 8 WDir ERS AZIM (degn) 4 3 4 3 Fig.. () m equivlent neutrl buoy wind speed from NDBC nd TAO moorings versus ERS- wind speed (left-hnd xis). Histogrm of W ERS (right-hnd xis). (b) Difference between buoy nd ERS- wind directions versus ERS- wind direction reltive to the mid-bem zimuth (WDir ERS AZIM). Dshed lines indicte ±. Histogrm of ERS- reltive wind direction is lso shown (right-hnd xis). Azimuth ngles re clculted counterclockwise from north (degn). WDir ERS AZIM= corresponds to ERS- midbem looking long the wind vector. Gry shding is ±STD in ech bin.
74 A. Bentmy et l. / Journl of Mrine Systems 7 8 (3) 7 8 Tble Sttistics of differences between NDBC buoy hourly winds nd collocted sctterometer winds with number of vlid qulity control flgs (see text): record length, bis, stndrd devition, nd correltion. Vlues determined only using dt from the period of ERS-/ QuikSCAT overlp re in prentheses. Sttistics for ERS- using CMOD5.n re lso included. Wind speed Wind direction ERS- (CMODIFR) Length 9985 (3659) Bis.66 (.8) 5 ( 4) Std. (.9) 9 (9) Cor.94 (.94).8 (.79) QuikSCAT Length 57,74 (77) Bis. (.3) 3 ( 5) Std.3 (.) 6 (6) Cor.95 (.95).87 (.86) ERS/N (CMOD5.n) Length 9985 (3659) Bis.5 (.7) 5 ( 4) Std.4 (.35) 9 (9) Cor.9 (.9).8 (.79) with minimum time difference of few hours t low ltitudes. If we ccept pirs of observtions lso with temporl seprtion τ of less thn 5 h, then the resulting sptil coverge of these points is globl, with >36 million colloctions, but with the mjority of the colloctions t higher ltitudes due to the polr convergence of the orbits (Bentmy et l., ). In ddition, to compre ERS- nd QuikSCAT, we re interested in connecting ech to ground observtions. Thus, ERS- nd QuikSCAT winds (within 5 km nd h for ERS- nd 5 km nd 3 min for QuikSCAT) re lso seprtely compred to the NDBC moored buoys, nd the Tropicl Atmosphere Ocen Project (TAO) nd Pilot Reserch Moored Arry (PIRATA) moorings. Hourly verged buoy wind velocity, SST, ir temperture, nd humidity re converted to m equivlent neutrl wind using the COARE3. lgorithm of Firll et l. (3). Detils of the buoy instrumenttion re provided in Meindl nd Hmilton (99), McPhden et l. (998), nd Bourlès et l. (8). 3. ERS- wind ccurcy Our initil comprison of ERS- wind speed bsed on the CMODIFR GMF shows ERS- winds to be bised low for windsb3 m/s in comprison with in-situ winds (Fig. ), s hs been previously shown by Bentmy et l. (). At higher winds, the stellite wind speed my be bised high, but this conclusion is uncertin due to the rrity of high wind conditions. The stellite-derived wind direction is consistent with in-situ wind direction to within without evidence of bis (Fig. b). Tble presents stellite buoy comprison sttistics bsed on collocted buoy nd stellite dt with vlid qulity control flgs. In prticulr, QuikSCAT dt is selected bsed on both the rin flg nd MRPb.5, s explined in Bentmy et l. (). One should notice tht wind direction greement is defined s vector correltion, nd thus vries between nd +(Crosby et l., 993). The results show ERS- wind speed to be bised low by.6 m/s while the QuikSCAT wind speed bis is negligible. Wind direction from both sctterometers compres well with buoy wind direction (see lso Fig. b). Sttisticl comprisons of buoy stellite winds bsed on the entire period for ech mission (Mrch 996 Jnury for ERS-, nd July 999 November 9 for QuikSCAT) re in line with those bsed on the shorter period of overlp (July 999 Jnury ). This greement illustrtes the representtiveness of the common period, which is used for collocted dt. Similrity of buoy ERS- nd QuikSCAT ERS- wind speed differences lso suggests tht CMODIFR-bsed ERS- wind speed is bised low. The ERS- wind speed underestimtion seen in the previous comprisons with the buoys (Fig. ) is lso present in the globl ERS-/ QuikSCAT comprison (Fig. ). But, like the buoy comprisons, the wind direction from the two missions is consistent (Fig. b). Time W QS (m/s) 5 5 5 c WDir QS WDir ERS (degn) 5 5 5 5 5 W ERS (m/s) 8 9 9 8 WDir ERS Azim (degn) b x 6 d 3 x 6 6 4 5 5 5 W ERS (m/s) 8 9 9 8 WDir ERS Azim (degn) Fig.. Gross comprison of collocted QuikSCAT (QS) nd ERS- winds. () QS wind speed (W QS ) versus ERS- wind speed m/s bins (W ERS ). Shding shows ± STD of W QS in ech bin. (b) Histogrm of W ERS. (c) Difference between QS nd ERS wind directions binned in ERS- wind direction reltive to the mid-bem zimuth. Dshed lines indicte ±. Gry shding shows ± STD. (d) Histogrm of the reltive ERS- wind direction. Azimuth ngles re clculted counterclockwise from north (degn). Zero reltive wind direction in (c) nd (d) corresponds to ERS- mid-bem looking long the wind vector.
A. Bentmy et l. / Journl of Mrine Systems 7 8 (3) 7 8 75 men ERS- wind speed is lower thn QuikSCAT wind speed lmost everywhere (Fig. 3) except t high ltitudes where the differences re reduced. However, the improved greement t high ltitudes results from ERS- bis nd QuikSCAT bis compenstion, which is tenttively explined by regionl negtive bis in QuikSCAT winds due to the stronger viscous dissiption of the Brgg wves in cold wter tht is unccounted for (Bentmy et l., ; Grodsky et l., ). The temporl vribility of ERS- nd QuikSCAT winds is consistent with correltions exceeding.8 t most loctions except low ltitudes (Fig. 3c). The reduced correltion nd stripes of incresed STD t low ltitudes follow mjor tropicl precipittion zones (Fig. 3b, c) nd re likely the result of the presence of short-lived convective vribility nd relted rinfll, which cuses differences in the conditions viewed by the two stellites becuse of their temporl seprtion of up to 5 h. Furthermore, some rin events my not be detected by stndrd lgorithms (Tourndre nd Quilfen, 3, 5) cusingnincreseofdifference between the sctterometer retrievls, especilly in the tropics. Awy from the tropics, the STD between collocted wind speeds (Fig. 3b) significntly increses in the mid-ltitude storm trck bnds likely reflecting the impct of synoptic events. The ERS- wind bis my hve t lest two cuses: (i) uncertinties in bcksctter coefficient clibrtion nd (ii) uncertinties in GMF prmeteriztion. To the best of our knowledge, only.65 db bis in the clibrted bcksctter coefficients hs been previously reported (Crpolicchio et l., 7). We shll further discuss (i) in the Discussion section. (ii) Some impct due to GMF uncertinty is to be expected becuse, s noted bove, the GMF CMODIFR ws developed for ERS-, but pplied to ERS- without ny djustments. Since the originl processing of ERS- globl winds by IFREMER, number of C-bnd GMFs hve been specificlly designed for ERS- bcksctter. The ltest, CMOD5.n, hs been derived by Hersbch () using collocted ERS- σ triplets nd ECMWF short-rnge forecst winds. Unfortuntely no ERS- retrievls estimted from CMOD5.n re yet vilble during the period of interest (996 ). To compenste, we use simple method to reduce the wind speed bis in the ERS- winds by pplying CMOD5.n ssuming tht the wind direction determined using CMODIFR is bis-free (Figs. b, b, nd Tble ). This wind direction ssumption significntly simplifies nd speeds up computing CMOD5.n winds. It is constructed from ERS- winds by djusting the winds to minimize cost function expressing the men squre 6N ΔW 3N.5 3S.5 6S b 6N STD(ΔW) 9E 8E 7E 36E 3 3N 3S 6S c 6N 3N 3S 6S CORR 9E 8E 7E 36E 9E 8E 7E 36E.9.8.7.6.5 Fig. 3. () Time men difference between collocted QS nd ERS- wind speed (W QS W ERS ), (b) STD of the difference, nd (c) temporl correltion of instntneous collocted wind speeds t ech bin. QuikSCAT rin flg nd MRPb.5 re both pplied.
76 A. Bentmy et l. / Journl of Mrine Systems 7 8 (3) 7 8 difference between observed (σ ) nd simulted (σ CMOD5.n )bcksctter coefficients, following Quilfen (995): JW; ð χþ ¼ X3 i¼ h i : σ i σ i CMOD5:nð W; χþ ðþ Here, W is the new wind speed, nd χ is the wind direction reltive to ntenn zimuth (known from the winds produced using CMODIFR). At ech ERS- Wind Vector Cell, ERS- wind speed bsed on CMODIFR is used s the first guess for the minimiztion of Eq. (). The resulting prtil reprocessing of ERS- wind speed produced in this study is vilble only for the collocted dt nd is referred to s the new ERS, or ERS/N winds. Reduction in the ERS/N wind speed bis in comprison with the originl CMODIFR-bsed dt is seen in the reduced difference of generlly less thn. m/s with respect to NDBC wind speeds (Tble ) nd in comprison with QuikSCAT (Figs. 3 nd 4). But, lrge discrepncies re still present long the North Atlntic nd Pcific storm trcks, which my be relted to the high vribility nd thus lrge errors resulting from smpling synoptic events. Errors re lso noticeble in costl res where diurnl breezes re lso poorly smpled in the collocted dt (Bentmy et l., ). Although the globl men wind speed difference between QuikSCAT nd ERS- is reduced to bout. m/s for ERS/N in comprison with bout.6 m/s for the originl CMODIFR-bsed winds (Fig. 5b), the negtive difference becomes stronger over cold SST (Figs. 3 nd 4). But s noted erlier, the originl wek wind speed difference t high ltitudes (Fig. 3) is due to compensting errors. At those ltitudes, the globl underestimtion of CMODIFR-bsed ERS- winds compenstes for the locl underestimtion of Ku-bnd QuikSCAT winds over cold SST, thus leding to loclly wek difference between the two retrievls. The prtilly reprocessed CMOD5.n-bsed winds (ERS/N) more closely gree with QuikSCAT (Fig. 4), except t high ltitudes where the difference between QuikSCAT nd ERS/N wind speed is of the sme order s tht for QuikSCAT nd ASCAT (Bentmy et l., ). Becuse both ERS- nd ASCAT re C-bnd rdrs, the similrity of the two wind speed 6N W QS W ERS/N 3N.5 3S.5 6S 9E 8E 7E 36E b W QS (W ERS/N +ΔW) 6N 3N.5 3S.5 6S 9E 8E 7E 36E c (W QS ΔW) (W ERS/N +ΔW) 6N 3N.5 3S.5 6S 9E 8E 7E 36E Fig. 4. Time men difference between collocted QuikSCAT nd ERS- wind speeds. () ERS- wind prtilly reprocessed with CMOD5.n (ERS/N). (b) ERS/N wind corrected for GMF dependence [W ERS/N +ΔW(W ERS/N, χ)]. (c) ERS/N wind corrected for GMF dependence ΔW nd QS winds corrected for SST dependence [W QS ΔW(W QS, SST)].
m/s b.5.5 6S 4S S N 4N 6N 4 x 6 ;.56 3.5 ;.3 4;. 3.5.5 A. Bentmy et l. / Journl of Mrine Systems 7 8 (3) 7 8 3 4 dominnt) (Fig. 4). These binned differences hve positive vlues for W ERS/N b5 m/s (not shown), which result from the one-sided distribution of wind speeds for winds pproching the low wind speed cutoff nd thus should not be reflected in ΔW (Freilich, 997). Artificilly positive vlues t low winds re corrected for by multiplying the binned differences by cut-off function, tnh[(w ERS/N /5) 4 ], the result of which we gin cll ΔW. To mitigte the impct of smpling errors, we use bins contining t lest 5 smples, then we smooth ΔW by the tringulr 3 3 sptil filter, nd retin only the first 5 ngulr hrmonics (Fig. 6). ERS/N wind speed is lower thn QuikSCAT wind speed for W ERS/N >5 m/s in the up- nd down-wind directions (Fig. 6), but the difference is opposite in the two cross-wind directions. The zimuth symmetry of ΔW is unexpected becuse CMOD5.n itself hs this symmetry. This suggests the presence of inconsistency in ntenn clibrtion of the fore- nd fter-bems (discussed lter). The time men sptil pttern of ΔW depends on the distribution of locl wind speed nd direction. Adding the ΔW correction to ERS/N wind speed, W ERS/N +ΔW results in slight strengthening of the trde winds nd wekening of the midltitude westerlies (Fig. 7). This correction reduces the globl wind speed bis from. m/s to. m/s nd improves the consistency of the corrected ERS/N nd QuikSCAT winds t high ltitudes (Figs. 4,b nd 5). 77.5 differences t high ltitudes underlines the fct tht this difference is due to the physics of rdr bckscttering nd my be SST-dependent (see lso Grodsky et l., for model considertion of the effect). 4. Adjusting ERS/N nd QuikSCAT winds The zonlly verged difference between QuikSCAT nd ERS/N wind speed of bout. m/s (Fig. 5b) includes bises due to inconsistencies in the retrievl procedures (GMF-relted bis) nd due to frequency-dependence in the physics of wind inference. 4.. GMF relted bis m/s Fig. 5. () Zonlly verged collocted wind speed difference for four cses: () CMODIFR- bsed ERS- winds (W QS W ERS ), () CMOD5.n-bsed ERS- winds (W QS W ERS/N ), (3) fter pplying GMF-relted correction to ERS- (W QS (W ERS/N +ΔW)), (4) fter pplying GMF-relted correction to ERS- nd SST-relted correction to QuikSCAT, (W QS ΔW) (W ERS/N +ΔW). (b) Histogrm of collocted wind speed difference for the sme cses. Numbers re medin wind speed differences in m/s. A difference in mesuring geometry nd retrievl procedures for the two sctterometers leds to difference in wind speed (W) due to bises in the GMFs used in processing the two dt sets. Following Bentmy et l. () GMF-relted correction (ΔW) is prmeterized s function of ERS/N wind speed nd direction reltive to the mid-bem zimuth. The CMOD5.n GMF is prmeterized by truncted Fourier series of wind direction reltive to ntenn zimuth, χ, with coefficients depending on wind speed nd incidence ngle, θ. Due to the fixed orienttion of the three bem observtion geometry of ERS-, only the wind direction reltive to mid-bem zimuth is considered for the nlysis of ΔW. As previously found in the Bentmy et l. () comprison of ASCAT nd QuikSCAT winds, there is only minor dependence of ΔW on θ (not shown). Together, these observtions suggest tht the correction ΔW is function of two vribles: W ERS/N nd χ. The construction of ΔW (Fig. 5) begins by binning collocted differences W QS W ERS/N s function of W ERS/N nd χ t ltitudes equtorwrd of 5 (where the negtive SST-relted bis is not 4.. SST-relted bis After pplying the GMF-relted correction ΔW QuikSCAT wind speed remins systemticlly lower (by.5 m/s, Fig. 4b) thn corrected ERS/N wind speed mostly over regions of very cold SSTb5 C. Modeling of this SST-relted bis suggests tht it is wek in the C-bnd nd hs greter impct on shorter wves nd thus preferentilly impcts QuikSCAT, for which the mjor impct is due to the temperture-dependence of viscous dissiption of wind ripples (Grodsky et l., ). Differences tend to be more pronounced t high southern thn northern ltitudes due to the yerly distribution of low SSTb5 C in ech re (Bentmy et l., ). Here, we pply Bentmy et l.'s () estimte of the SST-relted bis (ΔW, Fig. 6b) nd subtrct it from the QuikSCAT wind speed, W QS ΔW. Tbulr vlues of ΔW s function of wind speed nd SST bins re dopted from Bentmy et l. () (see their fig. b nd section 4.3). This correction increses W QS over regions of cold SST (Fig. 7b) nd elimintes much of the wind speed difference between QuikSCAT nd corrected ERS/N winds t high ltitudes (compre Fig. 4b nd c), thus reducing the globl-time men difference to. m/s (Fig. 5b). A slight improvement occurs in comprisons of NDBC buoy nd SST-corrected QuikSCAT winds. Using only buoys moored offshore nd north of 55 N, the time men difference of W NDBC W QS is. m/s while W NDBC W QS ΔW is bout. m/s. The SST-relted correction is smll t these loctions. In fct, it becomes noticeble only t very low SSTsb5 C (Fig. 6b), which re not common t NDBC loctions. 5. Discussion Bentmy et l. () hve shown tht the overestimtion of C-bnd sctterometer winds for crosswind directions is relted to the inccurcy of CMOD5.n in this direction. However, the difference (W QS W ERS/N, Fig. 6) is not symmetric in zimuth. ERS/N wind speed overestimtion (W QS W ERS/N b) is more pronounced, up to m/s, for the wind direction of 9 (clockwise from the mid-bem) thn tht for +9 where the difference is quite low. The low wind cut-off function we hve chosen is somewht rbitrry. It is used to ensure tht the GMF-relted correction pproches zero t wek winds. The reltive number of colloctions t W ERS b5 m/s is very low becuse of the lck of low wind speeds in ERS- dt. This prevents us from developing more justifible cut-off function.
78 A. Bentmy et l. / Journl of Mrine Systems 7 8 (3) 7 8 5 ΔW.5 W ERS/N (m/s) 5.5.5 5.5 b 8 9 9 8 WDir ERS Azim (degn) ΔW 5.6.4 W (m/s) 5.. 5.4.6 5 5 5 3 SST (degc) Fig. 6. () Wind speed difference (ΔW) between collocted QuikSCAT nd ERS/N (ERS- reprocessed with CMOD5.n) plotted s function of wind speed nd wind direction reltive to the mid-bem zimuth. (b) ΔW, the SST-relted correction for QuikSCAT wind speed. Adpted from Bentmy et l. (). Similr ngulr behvior is found for NDBC buoy minus ERS/N wind speed binned s function of wind direction (not shown). Although explntion of the symmetry is still not cler, it my be consequence of inconsistency in the ERS- bems' inter-clibrtion. In n effort to understnd the directionl dependence of the wind speed differences between QuikSCAT nd ERS-, we compre observed (σ ) nd simulted (σ CMOD5.n ) NRCSs for ech ERS- bem. Fig. 8 shows the differences (σ σ CMOD5.n ) evluted for ERS- midbem (dshed), fore-bem (solid), nd ft-bem (open circle) s function of the ssocited incidence ngles. Simulted σ CMOD5.n is bsed on CMOD5.n forced by the corrected collocted QuikSCAT wind speed (W QS ΔW) nd direction. For ft-bem nd forebem, the sme σ σ CMOD5.n is expected. Indeed, they hve the sme incidence ngles, nd differences re evluted for the sme surfce wind using the sme GMF. However, σ σ CMOD5.n for fore-bem nd tht for ft-bem differ by bout. db. Such discrepncy between observed nd simulted NRCSs for outer bems my led to the zimuth symmetry seen in Fig. 6. These results gree with those of De Chir nd Hersbch (9) nd suggest the need for complete reprocessing of ERS- sctterometer bcksctter coefficients nd winds. 6. Conclusion This study represents continution of the work of Bentmy et l. () in constructing consistent sctterometer time series spnning 996 present despite chnges in sctterometer technology. Wheres Bentmy et l. () hve compred Ku-bnd QuikSCAT nd C-bnd ASCAT dt, this study focuses on comprisons of QuikSCAT nd C-bnd ERS- sctterometer winds. Following Bentmy et l. (), we identify collocted pirs of observtions from the two missions during the 8 month period of mission overlp (July 999 erly Jnury ), ech seprted by less thn 5 h nd 5 km. Exmintion of the differences of these collocted pirs s well s comprisons of the ground truth dt from buoys revels systemtic bises in the m equivlent neutrl stellite wind speed (but not in wind direction) tht re function of rdr zimuth ngle nd wind speed rnges, s well s SST nd rinfll. In prticulr, undetected rinfll preferentilly
A. Bentmy et l. / Journl of Mrine Systems 7 8 (3) 7 8 79 6N ΔW.5 3N 3S 6S b ΔW 6N 9E 8E 7E 36E.5.5 3N 3S 6S 9E 8E 7E 36E.5 Fig. 7. Time men () GMF-relted wind speed correction for ERS/N (ΔW), (b) SST-relted wind speed correction for QuikSCAT (ΔW) pplied to ll collocted differences. Units re m/s. See lso cptions in Figs. 5 nd 6 for nottion. σ σ CMOD5.n (db).5.5.5 Mid bem Fore bem Aft bem.5 5 5 3 35 4 45 5 55 6 Incidence Angle (deg) Fig. 8. Observed rdr bcksctter (σ ) minus bcksctter simulted with the corrected QuikSCAT wind speed nd direction (σ CMOD5.n ) versus incidence ngle for ech ERS- bem. ffects the higher frequency QuikSCAT by incresing the strength of bcksctter, nd thus the pprent wind speed. This error is reduced by complementing rin selection bsed on the stndrd QuikSCAT rin flg nd excluding observtions for which the multidimensionl rin probbility (MRP) is >.5. The currently vilble ERS- surfce wind product tht spns the entire mission with globl coverge uses the IFREMER version geophysicl model function CMODIFR to convert normlized bcksctter to surfce winds. Winds bsed on this GMF (derived for the erlier ERS- mission) underestimte speed by.6 m/s in comprison with QuikSCAT, lthough the directions re consistent. In contrst, Hersbch () hs shown tht the new CMOD5.n GMF leds to much reduced bis in the wind estimtes. Thus, our first step is to introduce CMOD5.n s modifiction of the current globl ERS- surfce wind product by ssuming tht wind direction remins unchnged, resulting in modified surfce wind product we cll ERS/N, which is currently vilble only for the collocted dt nlyzed in this study. Our exmintion of ERS/N wind speed shows tht the bis in this prtilly reprocessed product is reduced to. m/s.
8 A. Bentmy et l. / Journl of Mrine Systems 7 8 (3) 7 8 We next identify difference in QuikSCAT nd ERS/N winds tht we believe is remining error in CMOD5.n GMF which we determine empiriclly s function of wind speed nd direction reltive to the ERS- mid-bem zimuth. After pplying this GMF-relted correction to ERS/N winds, the globl nd time verge wind speed difference between ERS/N nd QuikSCAT winds decreses to. m/s. Even fter this correction, QuikSCAT wind speed remins systemticlly lower (by.5 m/s) thn ERS/N in regions of very cold SSTb5 C. This wind speed difference my result from temperture-dependence in the viscous dmping of surfce wves which hs greter impct on the shorter wvelengths observed by QuikSCAT. After pplying n SST-relted correction to the QuikSCAT wind speed, the globl nd time men wind speed difference between ERS/N nd QuikSCAT becomes negligible. Finlly, we return to the broder issues rised by the presence of systemtic errors in ERS- winds. One outcome of our nlysis is recognizing tht there is significnt symmetry versus the wind direction reltive to the ERS- mid-bem zimuth. This zimuth dependence cnnot be explined by errors in the GMF used for ERS- processing since ny GMF is symmetric in zimuth. Closer exmintion of the bcksctter coefficients for the ERS- bems revels n inconsistency between the fore-bem nd ft-bem, which could be responsible for this symmetry. This finding long with n pprent wind speed bis in CMODIFR-bsed product suggests the need for complete reevlution nd reprocessing of ERS- sctterometer dt. Acknowledgments This reserch ws supported by the NASA Interntionl Ocen Vector Wind Science Tem (NNXIOAD99G), nd by the TOSCA (Terre, Océn, Surfces continentles, Atmosphère) project. We thnk D. Croizé-Fillon, F. Pul, nd J. F. Piollé nd IFREMER/CERSAT for dt processing support. 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