Intercomparison of the North Atlantic wave climatology from voluntary observing ships, satellite data and modelling

Phys. Chem. Earth, Vol. 23, No. 5-6, pp. 587 592, 1998

Pergamon

PI 1: S0079-1946(98)00075- 5

© 1998 Elsevier Science Ltd. All rights reserved 0 0 7 9 - 1 9 4 6 / 9 8 / $ - see f r o n t

matter

Intercomparison of the North Atlantic Wave Climatology from Voluntary Observing Ships, Satellite Data and Modelfing S. K. Gulev I, D. Cotton 2 and A. Sterl 3

ip. Shirshov Institute of Oceanology, 23 Krasikova sir., 117218, Moscow, Russia 2Southampton Oceanography Centre, Southampton, SO14 3ZH, United Kingdom 3Royal Netherlands Meteorological Institute, NL-3730 AE De Bilt, The Netherlands Received 25 April 1997; accepted 18 August 1997

Abstract. Three different sources of the wave data visual observations from the voluntary observing ships, wave hindcast from the WAM model driven by European Reanalysis Project winds, and the altimeter measurements from GEOSAT, TOPEX/POSEIDON and ERS-1 are used for the intercomparison of the North Atlantic wave fields for the period 1979-1993. Climatological spatial Patterns of significant wave height seen in all three products are consistent, although the actual quantitative values indicate both positive and negative biases of about 0.1 to 0.8 m. Sea and swell heights are intercompared separately for the voluntary observing ship and WAM model data. Best agreement between the visually observed data, the model hindcast and the altimeter measurements is obtained in the North Atlantic mid latitudes. However, long-term wave height trends in the merchant ship and the WAM model data are quite different. The nature of the differences in these estimates is discussed. © 1998 ElsevierScienceLtd. All rights reserved

1. Introduction

Climatology of ocean waves is important for weather prediction, ship and off-shore platform design, ocean modelling and climate studies. Although the visual wave data are available from the voluntary observing ship (VOS) collections of marine observations for a period covering several decades, they require the comprehensive validation against alternative sources of information to remove systematic and timedependent biases. Widely used by ~ailors and naval engineers "Global Wave Statistics" (Hogben et al., 1986) contain statistical distributions based on a limited visual wave data set for 104 areas of the World Ocean. These data are however restricted to a period in the late 1970s and have not been precisely calibrated against alternative information. Gulev and Hasse (1997) updated for the first time visual wave Correspondence to: SergeyK. Gulev 587

parameters from COADS (Comprehensive OceanAtmosphere Data Set) which is the most complete collection of the VOS observations available. During the last decade a number of satellite data sets of ocean waves derived from ERS-I, GEOSAT, and TOPEX/POSEIDON have been produced (Tournadre and Ezraty, 1990; Campbell et al., 1994; Young and Holland, 1995). These data, with near complete coverage of the oceans, provide measurements of significant wave height resulting from both sea and swell. Cotton and Carter (1994) calibrated the altimeter data from GEOSAT, ERS-1 and TOPEX/POSEIDON against NDBC (National Data Buoy Center) buoys and produced a global wave climatology spanning a period of over ten years, beginning m 1985. Global scale wave parameters are now also available from a number of numerical wave models. Recently a 15-year wave hindcast was obtained using the WAM model driven by winds from the ECMWF Reanalysis Project (ERA) (Sterl et al., 1997). The decades of 1980s and 1990s thus provide an overlap of the visually observed, measured, remotely sensed and numerically simulated wave fields and give us an exceptional chance to intercompare these independent products. Such an intercomparison can help to link different data sets, remove biases, scale back model hindcasts to the past, and create wave climate series of long continuity. This is important for the study of long-term changes in wave climatology e.g. the large increase of wave height as reported for the North Atlantic (Bacon and Caner, 1991). 2. Data sources and instnamentM measurements

intercomparison

with

Visual wave data were used for the period 1950-1993 from COADS in the form of Compressed Marine Reports (CMR-5) for 1950-1979, and Long Marine Reports (LMR) for 1980-1993 (Woodruff et al., 1997). After decoding the individual CMR and LMR, the records of visually observed height, period, and wind wave and swell directions, were

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subjected to a number of quality control checks (Gulev and Hasse, 1997). Since wave observations first appear in COADS at the end of 1963, we restricted the analysis period to 30 years (19641993). Sampled variables and computed products were averaged into 5°x5 ° boxes over the North Atlantic for each individual month from 1964 to 1993. In addition to sea and swell height estimates, we computed significant wave height (SWH) to provide relevant backgrotmd for the intercomparison. SWH Hs can be expressed in terms of spectral moments as Hs = 4~/mo, where mo is the zeroth moment of the spectrum which is equal to the sea surface variance. To estimate SWH from the visual observations Hogben (1988), Hogben et al. (1986), and many others recommend that Hs is computed as H s = (h,2~ + h2) t/2,

(1)

where hw and hs are wind sea and swell heights respectively. An alternative approach, justified by the comparison of visual observations with buoy data, recommends that the higher of sea or swell heights is used as an estimate of Hs (Wilkerson and Earle, 1990). Barratt (1991) recommends a combination of the two approaches by applying (1) when sea and swell are within the same 45* directional sector, and by taking the higher of the two components in all other cases. Since the estimate of the appropriate directional sector varies from 30* to 60 °, we have computed five different estimates of Hs; by using formula (I), by taking higher of sea or swell (Hh), or by using the combined Barratt method for the directional sectors of 30° (H3o), 45 ° (H4.0, and 60 ° (1-160). Visual observations have been compared to instrumental measurements at NDBC buoys (20 locations in the offshore regions of the North America), Ocean Weather Stations (OWS) L (57N, 20W) and C (52.5N, 35.5W), and Seven Stones Light Vessel (SSLV) (50N, 6W) (Gulev and Hasse, 1997) for varying periods between the years 1972 to 1993. Wilkerson and Earle (1990) also compared buoy and VOS data, selecting VOS observations taken simultaneously with the buoy measurements and within 25 to 100 km distance. We have computed spatial correlation functions centred on the instrumental measurements and then selected all VOS observations for each individual month which lie within the correlation ellipse of 0.8. The best estimates of SWH appear to be either Hh or H,o which give mean "ouoy minus VOS" differences of -0.03m and -0.07m respectively. SWH defined by (1) overestimates instrumental values everywhere by several tens of centimeters with a mean deviation of -0.27 m. Wilkerson and Earle (1990) also found Hh to be a better estimate than (1), although they did not consider a combined estimate. Estimate 1t3o fits better to the instrumental data in the regions with higher directional steadiness of sea and swell. The altimeter SWH data set was generated at SOC (Southampton), using data from three Ku-band altimeters: on GEOSAT, TOPEX/POSEIDON and ERS-1 (Cotton and Carter, 1994).. There are some

gaps in this altimeter data set, in 1986, and in 19901991. SWH was calculated for individual months from April 1985 to October 1994 on a 3°x3 ° grid over the entire globe from 72N to 63S. Cotton and Carter (1994) compared altimeter monthly means values with data from 24 NDBC buoys, for the period from October 1992 to September 1993 (for TOPEX and ERS-I) and from October 1985 to December 1 9 8 8 (for GEOSAT data). Linear regressions thus obtained were then applied to the data from each satellite to produce the corrected SWH. The model data were taken from a run in which the WAM wave model was driven by the ERA winds for the period from January 1979 to February 1994 (Sterl et al., 1997). Sterl et al. (1997) ran the WAM model in a low resolution (LR, 3°x3 ° grid) and a high resolution (HR, 1.5°xl.5 *) version. Only results from the HR version are used here, because they showed less scatter when compared with instrumental data and smaller biases than those from the LR version, confirming the results of Sterl et al. (1997). The HR version covers the globe from 81S to 81N and computes wave spectra in 12 directions and at 25 frequencies. Results are output every 6 hours, giving, among other quantities, heights and periods of sea, swell, and SWH. Comparison of the WAM SWH with NDBC buoys in general showed an underestimation of the large and an overestimation of the low waves in the model, although for the Atlantic Ocean primarily negative "model minus buoy" biases were obtained (Sterl et al., 1997). Verification against the altimeter data (Sterl et al., 1997) also showed some systematic biases in the WAM SWH. Thus, within the period 1979-1994, VOS data are available from 1979 to 1993, WAM data for 1979 to February 1994, and altimeter data for April 1985 to September 1994 with gaps in 1986 (OctoberDecember), 1989 (November), 1990 (whole year), and 1991 (January-September). For the triple intercomparison we used 80 months during the period from 1985 to 1993, overlapped by all three data sources. Since satellite altimeter data do not provide separate sea and swell height estimates, additional comparison for these parameters has been done using the VOS and WAM data for a longer period 1979-1993. For the comparison both WAM HR version data and the altimeter data were interpolated onto a 5°x5 * grid over the North Atlantic from the Equator to 70N to fit to the geographical area and resolution of the visual data.

3. Results of the intercomparison Figure 1 shows scatter plots of the monthly mean SWH from the altimeter, WAM, and VOS data for the 50×5° grid points over the whole North Atlantic. The WAM model gives quite a good agreement with the altimeter data within the SWH range from l to 2.5 m and underestimates the altimeter SWH by approximately 0.4-0.5 m when Hs > 2.5 m. The orthogonal regression slope of the WAM SWH

S. K. Gulev et al.: Intercomparison of the North Atlantic Wave Climatology

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Fig. 1. Scatter plots of SWH from the WAM and altimeter data (A), from the VOS observations and altimeter data (B), and from the VOS observations and WAM model (C).

589

against the altimeter data is 0.86. The most unbiased of the visual SWH estimates, when compared to the model and satellite data, is 1t3ofor the low values of SWH and Hh for the high values. This is consistent with the calibration of the VOS data against NDBC buoys and OWS data (Gulev and Hasse, 1997). When we compare SWH estimate (1) from the VOS data to the WAM hindcast and the altimeter measurements, there is a pronounced overestimation of the VOS SWH by 0.4 to 0.8 m against the model, and by 0.1 to 0.5 m against the altimeter data. Thus, we decided to use H30, as providing the best estimate of SWH derived from the VOS data. The orthogonal overall regressions of the H30 estimate against SWH from the WAM and altimeter data are 0.89 and 0.77 respectively. Overall bias of the VOS SWH is 0.32 m from the WAM SWH, and 0.14 m from the altimeter wave height. Figure 2a shows climatological SWH (H30) derived from the VOS observations for the period of overlap of the three data sets. Comparison with the respective charts derived from the altimeter data and the WAM model (not shown here) shows that the spatial patterns are similar for the three products. Mid-latitudinal absolute maxima are co-located, as well as the subtropical and the equatorial minima. At the same time, charts of the differences between SWH estimates taken from different products (Fig. 2b,c) indicate that the VOS SWH is systematically higher than the WAM SWH over the whole North Atlantic by 0.2 to 0.6 m. The largest "VOS minus WAM" differences are found in the western subtropics, in the regions close to the North American coast, and in the high latitudes. The best agreement is in the North-East Atlantic where the departure of the WAM values from VOS is less than 0.2 m. Absolute 'WOS minus altimeter" differences are smaller than the "VOS minus WAM" deviations, and they change the sign over the North Atlantic, being slightly negative (altimeter waves are higher) in mid latitudes and positive (higher VOS waves) in the subtropics and tropics. The largest deviations are observed in the western subtropics and in the Gulf region. Comparison of the WAM SWH to the altimeter data shows that SWH derived from the WAM is lower than the altimeter SWH everywhere, except for the eastern subtropics and tropics~ In the area of the maximum SWH in the mid latitudes, WAM gives 0.3 to 0.5 m lower waves than the altimeter measurements. The VOS and WAM sea and swell fields were compared for the period 1979-1993. Figure 3a,b shows mean sea and swell height from the VOS data for 1979-1993, and fig 3c,d displays the WOS minus WAM" sea and swell height differences. Spatial patterns of the sea height are comparable, but the WAM sea height is systematically lower. The largest positive "VOS minus WAM" biases are observed in the subtropics and in the equatorial area where they are larger than 0.5 m. The best agreement is in the mid latitudes where the wind waves are high. Large biases in the areas with the small sea height probably result from the fact that the smallest COADS code figure "1" corresponds to the sea height of 0.5 m, and smaller heights, even if appear, are coded as "1". Thus, we can expect that the low sea heights in the VOS data are overestimated by several tens

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al.:

lntercomparison of the North Atlantic Wave Climatology

Fig.2. Climatologicalcharts of SWH obtained from the VOS observations (A), and of the "v'OSminus WAM" (B) and "VOSminus altimeter" (C) SWH differences(meters).

height is larger than the VOS sea height by 0.1 to 0.2 m. As found in the sea height comparison, swell height patterns in the VOS climatology and WAM hindcast are also observed to be consistent. At the same time, VOS swell is systematically higher over nearly the whole North Atlantic. The largest biases are observed in the North-West Atlantic mid latitudes, where they range from 0.7 to 1.0 m with maximum absolute "VOS minus WAM" swell height difference of 1.0-1.2 m in October-November. It was interesting to intercompare the interannual variability of the wave height from the WAM runs and the VOS observations. This is important in view of the dramatic increase of SWH (2% a year) reported by two wave recorders in the North-East Atlantic (Bacon and Carter, 1991). For this intercomparison, the 15-year monthly-mean time series of SWH from the VOS and the WAM runs were broken down into the regular seasonal cycle, intra-annual short-term variations of a stochastic nature, and the long-term interannual variability. The latter was then used for the estimates of longterm linear trends. Figure 4 shows charts of the estimates of linear trends in SWH from the VOS observations and the WAM model together with the statistical significance taken from a t-test. Spatial patterns of the trend estimates are remarkably different. VOS indicates statistically significant positive trends in the mid-latitudinal North-West Atlantic and in the high latitudes where trends are higher than 20 cm per decade. Positive trends in the North-East Atlantic, where Bacon and Carter (1991) found pronounced increase of the winter wave height, are smaller and primarily insignificant. Weak negative trends in the VOS SWH height were found in the western subtropics and the open ocean tropics. The WAM SWH indicates weak and primarily insignificant trends in the North-East Atlantic and in the western tropics, and significantly negative trends in the North-West Atlantic. Sterl et al. (1997) compared trends for individual calendar months and found large variations with a change of sign between the summer and the winter seasons. We have analysed the VOS and WAM time series of SWH, sea height and swell for the Newfoundland basin, where fig. 4 demonstrates the remarkable disagreement of the trend estimates. The tendencies are comparable in the VOS and WAM during 19791986, and quite different in the later years. As a result, SWH shows opposite tendencies in the VOS and WAM during late 1980s and early 1990s. The most remarkable disagreement in the calculated interannual variability is found for swell which indicates a steady decrease in the WAM model but pronounced growth in the VOS data. 4. Conclusions

centimeters. It is hard to estimate quantitatively how this systematic error propagates into the monthly means. We can only expect the larger effect to be in the calmer areas and during summer months. Consideration of the seasonal dependence of the "VOS minus WAM" difference in sea height shows, that during summer the largest overestimation of the VOS sea height (of 0.6-0.7 m) occurs in the tropics and subtropics. In winter in mid latitudes WAM sea

A pilot intercomparison of the visually Qbserved, remotely sensed and modelled wave data" for the North Atlantic has shown that all three products have their strengths and weaknesses. The major climatological spatial patterns and the seasonal cycle in all three products are seen to be comparable, and at first glance depict the North Atlantic wave climatology quite realistically. In fact, previous

S. K. Gulev et al.: Intercomparison of the North Atlantic Wave Climatology

Fig.3. Climatological charts of the wind Sea height (A) and swell height (B) derived from the VOS observations, and charts of the "VOS minus WAM" wind sea (C) and swell (D) differences for 1979-1993 (meters).

Fig. 4. Estimates of the linear trends (era/decade) in SWH for the period 1979-1993 from the VOS data (A) and the WAM model (B). Black points indicate 95% significance (t-test).

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comparisons of different VOS based atlases have shown even higher biases with regard to each other than the biases which have been found between the three climatologies based on alternative sources of information considered here. At the same time, the differences between the VOS wave data, altimeter measurements and the model hindcast are not negligible, and the nature of biases has to be precisely studied. The smallest "VOS minus WAM" and "VOS minus altimeter" differences are found in mid latitudes, although the differences are of the opposite sign here: VOS waves overestimates the model hindcast by 0.I to 0.3 m and underestimates the altimeter SWH by approximately 0.1-0.2 m. In the tropics, where the waves and winds are smaller, VOS wave height is higher than those taken from the WAM or the altimeter by 0.3 to 0.7 m. Alternatively, the WAM SWH and the altimeter data fit better to each other in the low latitudes, and show the largest disagreement in mid latitudes where the "altimeter minus WAM" biases are from 0.3 to 0.5 m. Of the three sets of comparisons, the VOS and altimeter SWH show less scatter, whilst the largest scatter is obtained for the "VOS-WAM" comparison. It is possible to explain some of the disagreements as a consequence of "known biases" in the products. E.g. the tropical overestimation of the VOS wave heights (primarily, the sea height) which results from the problem of the code figure "1" which is applied in VOS to all waves <0.5 m. Thus, all sea heights coded as "1" should not be taken to represent a height of 0.5 m, but rather a lower value, the exact value to be deducted through a comparison with sea height frequency distributions from instrumental data. The results of wave models are very sensitive to the wind-input. Sted et al. (1997) argue that monthly mean ERA winds may be too low in areas of high wind, as the limited spatial and temporal resolution results in wind speed peaks being missed. They estimate the relative error in wind speed to be less than 5%, leading to an underestimation of SWH of about 10%. This estimate should be checked by comparison with high quality instrumental data from OWS and from the field experiments such as VSOP-NA (Kent et al. 1993) and SECTIONS (Gulev 1994). Special attention has to be paid to the late 1980s and 1990s when the tendencies are quite different in the VOS data and in the WAM hindcast. The large "VOS minus WAM" differences in the high latitudes may be a consequence of the relatively low density of VOS observations north of 60N. Figure 3 shows a large underestimation of the WAM swell in comparison with the VOS data. Obviously the definition of swell used in the WAM model is not the same as that used by the observers responsible for the VOS data set. In this context, more parameters should be involved in the intercomparison to allow a better understanding of

the nature of disagreement. These are first of all periods of sea and swell which are easily available from the VOS and WAM. The expansion of the intercomparison to the entire World Ocean will enable the depiction of regional features. This will require careful processing of the VOS data for the other oceans which do not provide as complete a coverage by VOS observations as does the North Atlantic. In some areas, which are almost free of VOS data (e.g. South Pacific), the intereomparison can help to create the VOS-based fields of wave parameters. Acknowledgements. This study was supported by Deutsche Forschungsgemeinschafl, Sonderforschungsbereich SFB 133 and Ministry of Science and Technology of the Russian Federation under the "World Ocean ~ National Programme. W A M runs were sponsored by the Dutch National Research Program on Global Air Pollution and Climate Change under contract no. 951207. Individual C O A D S data were made availableby courtesy of Steve Worley of N C A R (Boulder). W e thank both anonymous reviewers for helpful comments.

References Baeon,S. and D.J.T.Carter, Wave climate changes in the North Atlantic and North Sea. Int. J. Climatol., 11, 545-588. 1991. Bm'ratt, M.J., Waves in the North East Atlantic. UK Dept. of Report 0T190545, HMSO, London, 1991. W.J., E.J.Josberger, and N.M.Mognard, Southern wave fields during the attstral winters, 1985-1988, by GEOSAT radar altimeter. In The Polar Oceans and Their Role in Shaping the Global Environment. Geoph. Monngr. 85, AGU, 421-434, 1994. Cottl, P.D. and D.J.T.Cau'ter, Cross calibration of TOPEX, EI~-I, and GEOSAT wave heights. J. Geophys. Res., 99,

~a

C12, 25025-25033, 1994.

Gnlev, S.K., Influence of space-time averaging on the oceanatmosphere exchange estimates in the North Atlantic raid latitudes. J. Phys. Oceanogr,, 24, 1236-1255, 1994. Gnlev, S.K. and L.Hasse, North Atlantic wind waves and wind stress fields from voluntary observing ship data. J. Phys. Oceanogr., in print, 1997. Hol~,N., Experience from compilation of Global Wave Statistics. Ocean Engng., 15, 1-31, 1988. H o l ~ , N., N.M.C. Dllemdlll, ~ G.F.OItver, Global Wave Statistics. Unwin Brothers, London, 661 pp. 1986. Kent,E.C., P.K.Ta]I~r, B . $ . ~ , and J.$.HopMns, The accuracy of voluntary observing ships' meteorological observations-results of the VSOP-NA. J.Atmos.Oceanic Techn., 10, 591-608. 1993. Steal, A., G.J.Komm, ~ D.Cotla, 15 years of wave hindcasts using ERA winds: Validating the rcanalyscd winds and assessing the wave efimate. J. Geophys, Res., submitted. 1997. Torn'andS, J. and R.F.,Imlff, Local cfimatolngyof wind and sea state by means of satellite radar altimeter measurements. J. Geophys. Res., 95, 18225-18268, 1990 Wi]~mma, J.C. mid M.D.Em~, A study of differences between environmental reports by ships in the voluntary observing ~.rogram and measurements from NOAh, Buoys. C~ophys.Res., 95, 3373-3385. 1990. Womlrd, S.D., H.F.Di~, J.D.Ems, and SJ.Wm4ey, COADS Release 2 data and metadata enhancements for improvements of marine surface flux fields. Physics and Chemistry of the F.arth, 1997, in print. Y~ang, 1. IR4 G. ltoUluld, Atlas of the oceans: Wind and wave climate. Elsevier-Pergamon,Oxford, England. 1995.