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Sensitivity of wave model predictions to wind fields in the Western Mediterranean sea
Coastal Engineering 55 (2008) 920–929
Contents lists available at ScienceDirect
Coastal Engineering j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o a s t a l e n g
Sensitivity of wave model predictions to wind fields in the Western Mediterranean sea S. Ponce de León, C. Guedes Soares ⁎ Centre for Marine Technology and Engineering, Technical University of Lisbon, Instituto Superior Técnico. Av. Rovisco Pais, 1049-001 Lisboa, Portugal
a r t i c l e
i n f o
Available online 1 July 2008 Keywords: Hindcast Wind Waves ERA-40 HIPOCAS Mediterranean Sea
a b s t r a c t The paper compares the wave hindcast in the Western Mediterranean sea using the reanalysis wind fields from HIPOCAS and ERA-40 from ECMWF for November 2001. The study has concentrated on the Mediterranean coast of Spain where there are known difficulties with the wind and wave modelling. Two winter storms have been compared. The main differences between the significant wave heights using the ERA-40 reanalysis (ECMWF) and HIPOCAS reanalysis winds were observed to increase when moving southwards in the geographical domain at the offshore locations. Systematic negative biases of Hs were obtained with the ERA-40 data at all the coastal locations analyzed, whereas positive biases are typical for the HIPOCAS reanalysis. For offshore and coastal locations when using the ERA-40 data the Hs biases increased moving to South, while this pattern was not so clear for the HIPOCAS data. The inconsistencies in the comparisons of modelled waves against measurements seem to be associated with the quality of the wind fields. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Wave predictions in deep waters have experienced significant developments during the last few decades and the skill of the state of the art models has been shown to be generally good. However, the predictions are very sensitive to the wind fields used as has been demonstrated by various authors (Teixeira et al., 1995; Holthuijsen, et al., 1996; Ponce de Leon and Ocampo-Torres, 1998). Nowadays, the wind field's quality over the oceans is generally good, but for the enclosed basins, where the surface winds are affected by land's presence the skill of wind models diminishes. In these areas the modelled surface wind speeds are almost always underestimated and the bias depends on the proximity of land. This negative effect appears in various locations of the Mediterranean Sea (Cavaleri and Bertotti, 2004, 2006). This situation becomes particularly acute in the north-western Mediterranean due to the limited fetches (the longest fetch for the coast off Barcelona is of order 600 km in the Northeast direction) and the limited time of atmospheric storms. This situation brings a challenge for wind and consequently wave forecasting since conventional models are at the limit of their performance envelope. ⁎ Corresponding author. E-mail address: [email protected] (C. Guedes Soares). 0378-3839/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.coastaleng.2008.02.023
Wave predictions near the coast are subjected to more complicated physics than in deep water, due to the greater influence of bathymetry and breaking processes near the shore, which combined with the uncertainty from wind models, renders wave modelling in coastal waters still a challenge. The EU project HIPOCAS (Hindcast of Dynamic Processes of the Ocean and Coastal Areas of Europe) produced a 44 year hindcast of atmospheric, sea level and wave data for European waters (Guedes Soares et al., 2002). It used, as input boundary conditions, the wind fields produced by NCEP for the period of 1958 to 2001, and used a regional atmospheric model to produce fine-grid wind fields in the areas around Europe. This local area model takes due account of the land topography and thus is able to produce better wind fields than the global atmospheric model that is used to force it (Weisse and Feser, 2003; Sotillo et al., 2008-this issue). Therefore, the new wind fields should be able to allow better quality wave predictions. These wind fields are used to force a local area atmospheric model, which produces fine grid and high time resolution wind data for the Atlantic coasts of Europe. To further determine the sensitivity of models to wind conditions, and in particular to assess the quality of the HIPOCAS hindcast, the present study deals with a particularly difficult area from the wind regime point of view. It compares the results of using the HIPOCAS wind fields and the ERA-40 wind fields (Caires et al., 2002) for two severe winter storms associated with different Mediterranean events
S. Ponce de León, C. Guedes Soares / Coastal Engineering 55 (2008) 920–929
Tramuntana and Mistral. One of the objectives of ERA-40 were to create high quality global analyses of atmosphere, land and oceanwave conditions for 40 years an up-to-date data assimilation system and exploiting, at any time, the maximum information from the available observational sources. The hindcast sea state is obtained with the WAM-PRO model (Monbaliu et al., 2000) and wave observations are supplied by 7 wave buoys deployed along the Spanish Mediterranean coast from the networks XIOM (Generalitat de Catalonia) and Puertos de Estado (Spain). Some recent results on the wind waves on the Western Mediterranean demonstrated that inaccuracies of different sources exist on the wind fields in this region and that the wave models are well refined but do not produce good results all the time (Ponce de León and Sánchez-Arcilla, 1999; Sánchez-Arcilla et al., 2003). 2. Wind conditions in Western Mediterranean sea The Mediterranean Sea has a structure that is very complex. (See e.g. Cavaleri et al., 1991) as the numerous peninsulas extend small basins. Deep water conditions for wind waves are omnipresent, outside of the north Adriatic Sea, the Gulf of Sirte and near river deltas (Ebro, Rhone, Nile). The Mediterranean is almost surrounded by mountain ridges, except for some river deltas. When a low-pressure area enters the Mediterranean from the Atlantic Ocean, moving east on a northern track, a south-westerly wind is given momentum at the front. This wind is called the garbino as it blows from the Arabic lands. This is often followed by a cold air stream from the northwest. Because of its strength, sailors named this wind the Mistral. This is responsible for some of the most severe storms in the Mediterranean basin. The lows that enter the Mediterranean from the Atlantic Ocean tend to dissipate moving east, with major storms taking place in the western Mediterranean. Near the Catalan coast (Garcia et al., 1993), the most frequent wind directions are E, SW and NW. The northwesterlies are the most intense. Strong northeasterlies and easterlies also called levants
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coincide with storm conditions which are associated with cyclonic activity in the Western Mediterranean. Southwesterlies - “garbins” are summer, moderate winds blowing when high-pressure systems come into Southern Europe and the Mediterranean, and in calm weather. NW winds — Mistrals are intense, developing as gradient winds or flows pushed by Atlantic high-pressure heads down the Ebro valley. The period of simulation is centred on November 2001, during which two notorious storms were observed on the Western Mediterranean characterized by the strong winds from the NE (Levant). The first one, designated here as storm 1, occurred in 9–13 of November 2001, with a noticeable north-easterly wind event (26 m/s) in the north western of the basin. The Tortosa buoy registered value of Hs of 5.62 m during this period. The second one, designated here as storm 2, occurred in 13–17 of November 2001, with Tramontana winds of 24 m/s in the Gulf of Lyons in a combination with a Mistral wind in the Delt Ebre with 15 m/s. The buoy of Tortosa registered a Hs of 5.95 m during this period. In this paper the wind field comes from two sources: the European Centre (ECMWF) ERA-40 (Caires et al., 2002) and from the HIPOCAS project (Guedes Soares et al., 2002; Sotillo et al., 2008-this issue). The wind field supplied by the ERA-40 data set from the ECMWF atmospheric model was from the operational model analysis with global resolution of 2.5° in latitude and longitude with, both wind components linearly interpolated to specify the wind at each grid point whereas the HIPOCAS wind had the same resolution as the wave model (0.25°). The wind input time step was 6 h for all simulations performed. 3. Model setup The bathymetry grid used was DbDbV with 2 min of resolution in Latitude/ Longitude, and interpolated in space to be equal to the wave grid model. The bathymetry data has been smoothed to avoid the gradients that could cause some numerical instabilities of the wave model, as done in other studies.
Fig. 1. DbDbV bathymetry grid for whole the Mediterranean Sea. Spatial resolution of .25°.
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Table 1 Geographical limits and parameters of the numerical simulation Lat. North
Lat. South
Long. W
Long. E.
# points of grids
CPU time
integration time step
46.0
30.0
−6.0
37.0
11245
40 min
600 s
The spatial grid considered for the wave model is of 15 min (0.25°). For 24 direction and 25 frequency bands the energy balance equation was integrated with an integration time step of 10 min. The lowest resolved frequency is 0.0418 Hz. The simulation domain is:− 6° W, 20° E, 34° N, 45° N (Fig. 1). The initial condition was the usual JONSWAP spectrum, with Phillip's parameter equal to 0.00001, peak frequency equal to 0.2 Hz, peak enhancement factor of 3, left peak width of 0.07, right peak width of 0.09, and averaged wave direction of 90°. The wind input time step adopted in the final set up was 6 h. The wave model interpolated internally this wind field to specify the components at every grid point in the 0.25° model mesh. The time step of output of the results was 6 h. The time step of the output boundary conditions was 2400 s. More details can be found in Table 1. 4. Comparison of wind fields Two moderate wave storms were observed in November 2001, between the dates of 9th and 16th, and at the Western Mediterranean sea wind speed of about 130 km/h was registered at the Prat Airport, 16 of November. The low pressure centre of 994 Hpa was located east of the Balearic Islands. The 9.5 m wave height was measured by the Tortosa XIOM Buoy of Local Government Generalitat de Catalonia. Several damages in the Catalan coastal sector were reported at this date by the newspapers and media.
The strong winds from NE (Levant) took place in November 2001 during a period which is included in this study and for which the fetch is largest with a notorious effect of the waves on the Catalan coastal sector. It is important to point out that previous studies indicate that in general the wind speeds by ECMWF are 10% lower than those by other models (Ardhuin, et al. 2007). The ECWMF wind field for the 10th of November 2001 at 18:00 UTC is depicted in Fig. 2 showing the strong winds from the NE in the Western Mediterranean with wind speed of the order of 18 m/s. The HIPOCAS wind field for the same time (Fig. 3) apparently presents the same spatial distribution and higher values of the magnitude in the wind speed with values of 25 m/s; some displacement can be noted comparing the centre of the absolute maxima in Figs. 2 and 3. The HIPOCAS wind velocity friction time series for the location of Mahón exhibits at the main peaks an overestimated value in relation to those values of the European Centre (ERA-40) friction velocities (Fig. 4), which indicates that the wind fields at some time instants are more inconsistent than in others. Cavaleri and Bertotti (2004) found the largest values close to the Northern coasts of the basin (50% in the more enclosed seas) and pointed out that the bias for the hindcast wave heights in the Mediterranean decreases substantially when moving southwards. In the present case the differences between wind data sets will be compared in two selected locations in a transect offshore at deep waters from the NE to SW as shown in Fig. 7. The northern location in front of the Gulf of Lyons is designated B and A is the southern one (see the Table 2 and Fig. 7). From the wind speed time series (Figs. 5 and 6) it can be seen that the wind speed from HIPOCAS and ERA-40 data decreased moving to the north, i.e. at location B at the main storm. From Fig. 5 it can be seen that the HIPOCAS wind speed values are high producing a difference of the order of 7 m/s between the values of location A and B during the
Fig. 2. ERA-40 ECMWF wind field for the date 10th of November 2001 at 18:00 UTC.
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Fig. 3. HIPOCAS wind field for the date 10th of November 2001 at 18:00 UTC.
main storm whereas the differences between the wind speed values from the same data set are lower of the order of 3 m/s (Fig. 6). One of the main differences between the HIPOCAS and ERA-40 wind fields is the spatial resolution, which is 2.5° for the ERA-40 data, whereas the HIPOCAS data had 0.25°. Lower resolution tends to smooth the wind field and this has implication in the modelled wave field. The distribution of the wave is related with the generating winds
and higher wind speed will imply higher Hs. Any error in the wind fields will be reflected in the wave field. 4.1. The statistics A statistical validation is the standard method in wave modelling. The validation statistics of models results against field data allow the
Fig. 4. Velocity Friction time series for November 2001 for the location Mahón.
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Table 2 The geographical coordinates of the locations for the time series comparisons Offshore locations
A
B
Latitude Longitude
39.5° 1°
41.8° 3.8°
quality of the model to be assessed. A set of statistical parameters in this paper was used following Heimbach et al. (1998): The absolute mean error (bias) bias=Y−X is given as difference of mean X¯ of the measurements (Xi) to mean of modelled results (Yi) X ¯¼ ðXi =NÞ X i
where N is the number of data. The Root Mean Square Error (RMSE), " RMSE ¼
1X ðYi Xi Þ2 N i
#1=2
The scatter Index (SI), defined as the standard deviation of the difference normalized by the mean of the observations: RMSE SI ¼ pffiffiffiffiffiffiffiffi XY 5. Comparison of predicted wave fields For the validation of the sea state hindcast a time period corresponding to a month of November 2001 have been selected in which the buoy measurements were available. The hindcast wave time series correspond to wave-rider buoys positions (stars in Fig. 7). The period of interest corresponds to November 2001 where three intense storms caused damages on the Catalan coasts. The buoys are located in the vicinity of Cabo Gata, Cabo de Palos, Alicante, Valencia, Tortosa, Llobregat and Mahón.
In the Hs time series (Figs. 8 and 9) at the offshore points A and B the differences increased substantially moving to the South. If one compares at the same location A or B the differences between the Hs from the simulations using the same wind field, these appear consistent each other. The last criterion is not coincident with the Cavaleri and Bertotti (2004), who found that errors decreased moving southwards in the Mediterranean basin starting from the Gulf of Lyons. The simultaneous WAM-PRO wave fields displayed in Figs. 10 and 11 obtained using the ERA-40 and HIPOCAS wind fields (Figs. 2 and 3), respectively are consistent each other. However, the maximum value of Hs just to the North of Mallorca Island obtained when using the HIPOCAS wind field is around 9 m whereas the maximum of Hs when using the ERA-40 data was about 4.3 m. The spatial distribution of the Hs maxima is located in the same region between Catalonia sector and the Balearic Islands according to the distribution of the absolute maxima of the wind speeds. The adjustment for the Cabo de Gata location for a month (February) shows a large overestimation of the Hs, despite this buoy being in deep waters at 536 m of depth (Fig. 12). HIPOCAS winds have produced an overestimation of the Hs value in around 1 m in the peak of a storm. The Mahón buoy is placed near the small Island Menorca in deep waters at 300 m of depth (Fig. 7), which is not solved by the wave propagation scheme present in the wave model WAM-PRO with the 0.250 spatial resolution. This point is taken as a sea point by the wave model and this could be the reason why using the HIPOCAS winds at this location, a good adjustment has been produced when compared with the buoy data (Fig. 13), whereas when the ERA-40 winds are used the wave model shows a noticeable underestimation of the Hs values. Previous work in the Mediterranean Sea such as the MEWAM Group (Ardhuin et al., 2007) point out Scatter Indexes of the order of 0.28 (open) and 0.36 (coast) for a different period, October 2002, for open and coastal locations using the WAM model and ECMWF wind
Fig. 5. Wind speed time series extracted from the HIPOCAS wind fields during whole the month of November 2001 at two different locations.
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Fig. 6. Wind speed time series extracted from the ERA-40 ECMWF wind fields during whole the month of November 2001 at two different locations.
fields. Strong underestimation associated with the low spatial resolution was obtained by the WAM-PRO model in most of the locations validated in the present work when using the ERA-40 data. The WAM-PRO model driven by the HIPOCAS winds led to better adjustment with the buoy data in some locations, but in other locations overestimation has occurred, as for example, in Cabo de Palos, Valencia and Llobregat (Table 3).
From Table 3 it can be seen that the best adjustment corresponds to the Tortosa location with a bias of 0.12 and the worst adjustment using the HIPOCAS winds was for Valencia (bias=0.73). The location of the Mahón buoy is near and south to the Menorca Island and from Table 4 the worst adjustment corresponds to this location in the case of the ERA-40 wind data with a negative bias −1.30 that indicates a substantial underestimation.
Fig. 7. Location of the buoys represented by numbers and the triangles represent the locations for the wind (U10) time series extracted from the HIPOCAS and ERA-40 ECMWF wind fields.
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Fig. 8. WAM-PRO Hs time series using the HIPOCAS wind fields during whole the month of November 2001 at two different locations.
At the resolution used by the ECMWF model (T511, about 40 km) the underestimation of wind speed varied from 10 to 40% according to the study of MEWAM Group (Ardhuin et al., 2007). In the present case it is easy to say that the ERA40 ECMWF wind fields are underestimated as well, when compared with the HIPOCAS wind data. The statistical
parameters calculated, are in the range of others studies applied in the region (Bolaños et al., 2006). It is usual that the atmospheric models give some inaccuries in this coastal region of the Western Mediterranean. The study makes emphasis on the old question of wind waves modelling, namely the quality of the wind field data showing that in
Fig. 9. WAM-PRO Hs time series using the ERA-40 ECMWF wind fields during whole the month of November 2001 at two different locations.
S. Ponce de León, C. Guedes Soares / Coastal Engineering 55 (2008) 920–929
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Fig. 10. WAM-PRO Hs wave field for the date 10th of November 2001 at 18:00 UTC produced with the ERA-40 ECMWF winds.
the semi-enclosed sea such the Mediterranean the sea state depends on the wind data. 6. Concluding remarks Two winter storms have been compared and the main differences between the significant wave heights using the ERA-40 reanalysis
(ECMWF) and HIPOCAS reanalysis winds were observed to increase when moving southwards in the geographical domain at the offshore locations. On the case of the coastal locations the same pattern was observed for both wind data sets used. This allows one to conclude that in general the main differences between the Hs determined using the ERA-40 and HIPOCAS wind data increased moving southwards in the geographical domain.
Fig. 11. WAM-PRO Hs height for the date 10th of November 2001 at 18:00 UTC produced with the HIPOCAS winds.
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Fig. 12. Significant Wave Height time series for the location Cabo de Gata.
It was observed that on the case of the ERA-40 data set the biases for the Hs were constantly negative values indicating the underestimation of the Hs values, whereas the biases obtained when using the HIPOCAS data are positive values except one indicating a persistent overestimation of the Hs values. In fact, the bias using ERA-40 wind fields varied between −0.38 and −1.3, while with HIPOCAS winds the range was from −0.29 to 0.73.
The range of the scatter indexes for all locations considered in this study has a range from between 0.41 and 0.95 for HIPOCAS winds. With the ERA-40 wind fields the scatter indexes varied between 0.46 and 0.89. The inconsistencies between the modelled waves and the measured data are related with the quality of the winds and their different spatial resolution. This study shows once more the importance of the accuracy in
Fig. 13. Significant wave height time series for the location Mahón.
S. Ponce de León, C. Guedes Soares / Coastal Engineering 55 (2008) 920–929 Table 3 Statistical parameters for Hs using HIPOCAS wind field for whole the November 2001 (from the South to the North in order of appearance, the depths are indicated in parentheses) parameter Cabo Gata (536 m) bias rmsdif SI slope
0.23 0.62 0.69 1.46
Cabo Palos (67 m) 0.36 1.11 0.81 1.5
Alicante Valencia (50 m) (260 m) 0.21 0.73 0.58 1.36
0.73 1.36 0.87 1.94
Tortosa (57 m) 0.12 1.03 0.61 1.30
Llobregat Mahón (50 m) (300 m) 0.52 1.26 0.95 1.91
−0.29 0.94 0.41 0.88
Table 4 Statistical parameters for Hs using ERA-40 ECMWF wind field for whole the November 2001 (from the South to the North in order of appearance) parameter Cabo Gata Cabo Palos Alicante Valencia Tortosa Llobregat Mahón bias rmsdif SI slope
− 0.38 0.5 0.67 0.52
−0.52 0.64 0.46 0.6
−0.58 0.76 0.67 0.49
−0.48 0.74 0.78 0.52
−1.13 1.38 0.89 0.32
−0.55 0.72 0.71 0.48
−1.30 1.71 0.71 0.43
the wind fields especially when the sea is semi enclosed such the Mediterranean and when some orography is present. Acknowledgments This study has been partially conducted in the scope of the project “Hindcast of Dynamic Processes of the Ocean and Coastal Areas of Europe (HIPOCAS)”, which has been partially funded by the European Union under the Program “Energy, Environment and Sustainable Development” (Contract No EVK2-CT-1999-00038). It was also partially performed within the scope of the Project SPEC-HIPOCAS sponsored by the ECMWF (European Centre for Medium-Range Weather Forecast). References Ardhuin, F., Bertotti, L., Bidlot, J.R., Cavaleri, L., Filipetto, V., Lefevre, J.M., Wittmann, P., 2007. Comparison of wind and wave measurements and models in the Western Mediterranean Sea. Ocean Engineering 34, 526–541. Bolaños-Sanchez, R., Sánchez-Arcilla, A., Cateura, J., 2007. Evaluation of two atmospheric models for wind-wave modelling in the NW Mediterranean. Journal of Marine System 65, 336–353. doi:10.1016/j.jmarsys.2005.09.014. Caires, S., Sterl, A., Bidlot, J.R., Graham, N., Swail, V., 2002. Climatological assessment of reanalysis of wave data. Proc. 7th Int. Workshop on Wave Hindcasting and Forecasting, Banff, Canada. 21–26 October.
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Cavaleri, L., Bertotti, L., 2004. Accuracy of the modelled wind and wave fields in enclosed seas. Tellus 56 A, 167–175. Cavaleri, L., Bertotti, L., 2006. The improvement of modelled wind and wave fields with increasing resolution. Ocean Engineering 33, 553–565. Cavaleri, L., Bertotti, L., Lionello, P., 1991. Wind wave-cast in the Mediterranean Sea. J. Geophy. Res. C96, 10739–10764. García, M.A., Sánchez-Arcilla, A., Sierra, J.P., Sospedra, J., Gómez, J., 1993. Wind waves off the Ebro Delta, NW Mediterranean. Journal of Marine Systems 4, 235–262. Guedes Soares, C., Weisse, R., Alvarez, E., Carretero, J.C., 2002. A 40 Years Hindcast in European Waters. Proceedings of the 21st International Conference on Offshore Mechanics and Arctic Engineering (OMAE 2002). ASME. Paper OMAE2002-2860. Heimbach, P., Hasselmann, S., Hasselmann, K.,1998. Statistical analysis and intercomparison of WAM model data with global ERS-1 SAR wave mode spectral retrievals over 3 years. Journal of Geophysical Research 103 (C4), 7931–7977 April 15, 1998. Holthuijsen, L.H., Booji, N., Bertotti, L., 1996. The Propagation of Wind Errors Through Ocean Wave Hindcasts. Journal of Offshore Mechanics and Arctic Engineering 118, 184–189. Monbaliu, J., Padilla-Hernandez, J.C., Hargreaves, J.C., Carretero-Albiach, W., Luo, M., Sclavo,, Gunther, H., 2000. The spectral wave model WAM adapted for applications with high spatial resolution. Coastal Engineering 41, 41–62. Sánchez-Arcilla, A., Espino, M., Ponce de León, A.S., Jordá, G., Gómez, J., 2003. The Catalan wave prediction system. 2003. In: Dahlin, H., Flemming, N.C., Nittis, K., Petersson, S.E. (Eds.), Building the European Capacity in Operational Oceanography. Elsevier, pp. 190–194. Ponce de León, S., Ocampo-Torres, F.J., 1998. Sensitivity of a wave model to wind variability. Journal of Geophysical Research 103 (C2), 3179–3201. Ponce de León, S., Sánchez-Arcilla, A., 1999. Sensitivity of a wave model to wind input in the Catalan Coast. In: Abdalla, Ozhan (Eds.), Proceedings of the International Conference on the Wind and Wave Climate of the Mediterranean and Black Sea, WIND & WAVE CLIMATE'99, March 30–April 2, 1999, Antalya, Turkey. ISBN: 975-429-140-3. Sotillo, M.G., Aznar, R., Valero, F., 2008. The 44-year Mediterranean HIPOCAS wind database: A useful tool to analyse offshore extreme wind events from long term regional perspective. Coastal Engineering 55, 930–943 (this issue). Teixeira, J.C., Abreu, M.P., Guedes Soares, C.,1995. Uncertainty of Ocean Wave Hindcasts due to Wind Modelling. Journal of Offshore Mechanics and Arctic Engineering 117, 294–297. Weisse, R., Feser, F., 2003. Evaluation of a method to reduce uncertainty in wind hindcasts performed with regional atmosphere models. Coastal Engineering 48, 211–225.
Contents lists available at ScienceDirect
Coastal Engineering j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c o a s t a l e n g
Sensitivity of wave model predictions to wind fields in the Western Mediterranean sea S. Ponce de León, C. Guedes Soares ⁎ Centre for Marine Technology and Engineering, Technical University of Lisbon, Instituto Superior Técnico. Av. Rovisco Pais, 1049-001 Lisboa, Portugal
a r t i c l e
i n f o
Available online 1 July 2008 Keywords: Hindcast Wind Waves ERA-40 HIPOCAS Mediterranean Sea
a b s t r a c t The paper compares the wave hindcast in the Western Mediterranean sea using the reanalysis wind fields from HIPOCAS and ERA-40 from ECMWF for November 2001. The study has concentrated on the Mediterranean coast of Spain where there are known difficulties with the wind and wave modelling. Two winter storms have been compared. The main differences between the significant wave heights using the ERA-40 reanalysis (ECMWF) and HIPOCAS reanalysis winds were observed to increase when moving southwards in the geographical domain at the offshore locations. Systematic negative biases of Hs were obtained with the ERA-40 data at all the coastal locations analyzed, whereas positive biases are typical for the HIPOCAS reanalysis. For offshore and coastal locations when using the ERA-40 data the Hs biases increased moving to South, while this pattern was not so clear for the HIPOCAS data. The inconsistencies in the comparisons of modelled waves against measurements seem to be associated with the quality of the wind fields. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Wave predictions in deep waters have experienced significant developments during the last few decades and the skill of the state of the art models has been shown to be generally good. However, the predictions are very sensitive to the wind fields used as has been demonstrated by various authors (Teixeira et al., 1995; Holthuijsen, et al., 1996; Ponce de Leon and Ocampo-Torres, 1998). Nowadays, the wind field's quality over the oceans is generally good, but for the enclosed basins, where the surface winds are affected by land's presence the skill of wind models diminishes. In these areas the modelled surface wind speeds are almost always underestimated and the bias depends on the proximity of land. This negative effect appears in various locations of the Mediterranean Sea (Cavaleri and Bertotti, 2004, 2006). This situation becomes particularly acute in the north-western Mediterranean due to the limited fetches (the longest fetch for the coast off Barcelona is of order 600 km in the Northeast direction) and the limited time of atmospheric storms. This situation brings a challenge for wind and consequently wave forecasting since conventional models are at the limit of their performance envelope. ⁎ Corresponding author. E-mail address: [email protected] (C. Guedes Soares). 0378-3839/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.coastaleng.2008.02.023
Wave predictions near the coast are subjected to more complicated physics than in deep water, due to the greater influence of bathymetry and breaking processes near the shore, which combined with the uncertainty from wind models, renders wave modelling in coastal waters still a challenge. The EU project HIPOCAS (Hindcast of Dynamic Processes of the Ocean and Coastal Areas of Europe) produced a 44 year hindcast of atmospheric, sea level and wave data for European waters (Guedes Soares et al., 2002). It used, as input boundary conditions, the wind fields produced by NCEP for the period of 1958 to 2001, and used a regional atmospheric model to produce fine-grid wind fields in the areas around Europe. This local area model takes due account of the land topography and thus is able to produce better wind fields than the global atmospheric model that is used to force it (Weisse and Feser, 2003; Sotillo et al., 2008-this issue). Therefore, the new wind fields should be able to allow better quality wave predictions. These wind fields are used to force a local area atmospheric model, which produces fine grid and high time resolution wind data for the Atlantic coasts of Europe. To further determine the sensitivity of models to wind conditions, and in particular to assess the quality of the HIPOCAS hindcast, the present study deals with a particularly difficult area from the wind regime point of view. It compares the results of using the HIPOCAS wind fields and the ERA-40 wind fields (Caires et al., 2002) for two severe winter storms associated with different Mediterranean events
S. Ponce de León, C. Guedes Soares / Coastal Engineering 55 (2008) 920–929
Tramuntana and Mistral. One of the objectives of ERA-40 were to create high quality global analyses of atmosphere, land and oceanwave conditions for 40 years an up-to-date data assimilation system and exploiting, at any time, the maximum information from the available observational sources. The hindcast sea state is obtained with the WAM-PRO model (Monbaliu et al., 2000) and wave observations are supplied by 7 wave buoys deployed along the Spanish Mediterranean coast from the networks XIOM (Generalitat de Catalonia) and Puertos de Estado (Spain). Some recent results on the wind waves on the Western Mediterranean demonstrated that inaccuracies of different sources exist on the wind fields in this region and that the wave models are well refined but do not produce good results all the time (Ponce de León and Sánchez-Arcilla, 1999; Sánchez-Arcilla et al., 2003). 2. Wind conditions in Western Mediterranean sea The Mediterranean Sea has a structure that is very complex. (See e.g. Cavaleri et al., 1991) as the numerous peninsulas extend small basins. Deep water conditions for wind waves are omnipresent, outside of the north Adriatic Sea, the Gulf of Sirte and near river deltas (Ebro, Rhone, Nile). The Mediterranean is almost surrounded by mountain ridges, except for some river deltas. When a low-pressure area enters the Mediterranean from the Atlantic Ocean, moving east on a northern track, a south-westerly wind is given momentum at the front. This wind is called the garbino as it blows from the Arabic lands. This is often followed by a cold air stream from the northwest. Because of its strength, sailors named this wind the Mistral. This is responsible for some of the most severe storms in the Mediterranean basin. The lows that enter the Mediterranean from the Atlantic Ocean tend to dissipate moving east, with major storms taking place in the western Mediterranean. Near the Catalan coast (Garcia et al., 1993), the most frequent wind directions are E, SW and NW. The northwesterlies are the most intense. Strong northeasterlies and easterlies also called levants
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coincide with storm conditions which are associated with cyclonic activity in the Western Mediterranean. Southwesterlies - “garbins” are summer, moderate winds blowing when high-pressure systems come into Southern Europe and the Mediterranean, and in calm weather. NW winds — Mistrals are intense, developing as gradient winds or flows pushed by Atlantic high-pressure heads down the Ebro valley. The period of simulation is centred on November 2001, during which two notorious storms were observed on the Western Mediterranean characterized by the strong winds from the NE (Levant). The first one, designated here as storm 1, occurred in 9–13 of November 2001, with a noticeable north-easterly wind event (26 m/s) in the north western of the basin. The Tortosa buoy registered value of Hs of 5.62 m during this period. The second one, designated here as storm 2, occurred in 13–17 of November 2001, with Tramontana winds of 24 m/s in the Gulf of Lyons in a combination with a Mistral wind in the Delt Ebre with 15 m/s. The buoy of Tortosa registered a Hs of 5.95 m during this period. In this paper the wind field comes from two sources: the European Centre (ECMWF) ERA-40 (Caires et al., 2002) and from the HIPOCAS project (Guedes Soares et al., 2002; Sotillo et al., 2008-this issue). The wind field supplied by the ERA-40 data set from the ECMWF atmospheric model was from the operational model analysis with global resolution of 2.5° in latitude and longitude with, both wind components linearly interpolated to specify the wind at each grid point whereas the HIPOCAS wind had the same resolution as the wave model (0.25°). The wind input time step was 6 h for all simulations performed. 3. Model setup The bathymetry grid used was DbDbV with 2 min of resolution in Latitude/ Longitude, and interpolated in space to be equal to the wave grid model. The bathymetry data has been smoothed to avoid the gradients that could cause some numerical instabilities of the wave model, as done in other studies.
Fig. 1. DbDbV bathymetry grid for whole the Mediterranean Sea. Spatial resolution of .25°.
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Table 1 Geographical limits and parameters of the numerical simulation Lat. North
Lat. South
Long. W
Long. E.
# points of grids
CPU time
integration time step
46.0
30.0
−6.0
37.0
11245
40 min
600 s
The spatial grid considered for the wave model is of 15 min (0.25°). For 24 direction and 25 frequency bands the energy balance equation was integrated with an integration time step of 10 min. The lowest resolved frequency is 0.0418 Hz. The simulation domain is:− 6° W, 20° E, 34° N, 45° N (Fig. 1). The initial condition was the usual JONSWAP spectrum, with Phillip's parameter equal to 0.00001, peak frequency equal to 0.2 Hz, peak enhancement factor of 3, left peak width of 0.07, right peak width of 0.09, and averaged wave direction of 90°. The wind input time step adopted in the final set up was 6 h. The wave model interpolated internally this wind field to specify the components at every grid point in the 0.25° model mesh. The time step of output of the results was 6 h. The time step of the output boundary conditions was 2400 s. More details can be found in Table 1. 4. Comparison of wind fields Two moderate wave storms were observed in November 2001, between the dates of 9th and 16th, and at the Western Mediterranean sea wind speed of about 130 km/h was registered at the Prat Airport, 16 of November. The low pressure centre of 994 Hpa was located east of the Balearic Islands. The 9.5 m wave height was measured by the Tortosa XIOM Buoy of Local Government Generalitat de Catalonia. Several damages in the Catalan coastal sector were reported at this date by the newspapers and media.
The strong winds from NE (Levant) took place in November 2001 during a period which is included in this study and for which the fetch is largest with a notorious effect of the waves on the Catalan coastal sector. It is important to point out that previous studies indicate that in general the wind speeds by ECMWF are 10% lower than those by other models (Ardhuin, et al. 2007). The ECWMF wind field for the 10th of November 2001 at 18:00 UTC is depicted in Fig. 2 showing the strong winds from the NE in the Western Mediterranean with wind speed of the order of 18 m/s. The HIPOCAS wind field for the same time (Fig. 3) apparently presents the same spatial distribution and higher values of the magnitude in the wind speed with values of 25 m/s; some displacement can be noted comparing the centre of the absolute maxima in Figs. 2 and 3. The HIPOCAS wind velocity friction time series for the location of Mahón exhibits at the main peaks an overestimated value in relation to those values of the European Centre (ERA-40) friction velocities (Fig. 4), which indicates that the wind fields at some time instants are more inconsistent than in others. Cavaleri and Bertotti (2004) found the largest values close to the Northern coasts of the basin (50% in the more enclosed seas) and pointed out that the bias for the hindcast wave heights in the Mediterranean decreases substantially when moving southwards. In the present case the differences between wind data sets will be compared in two selected locations in a transect offshore at deep waters from the NE to SW as shown in Fig. 7. The northern location in front of the Gulf of Lyons is designated B and A is the southern one (see the Table 2 and Fig. 7). From the wind speed time series (Figs. 5 and 6) it can be seen that the wind speed from HIPOCAS and ERA-40 data decreased moving to the north, i.e. at location B at the main storm. From Fig. 5 it can be seen that the HIPOCAS wind speed values are high producing a difference of the order of 7 m/s between the values of location A and B during the
Fig. 2. ERA-40 ECMWF wind field for the date 10th of November 2001 at 18:00 UTC.
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Fig. 3. HIPOCAS wind field for the date 10th of November 2001 at 18:00 UTC.
main storm whereas the differences between the wind speed values from the same data set are lower of the order of 3 m/s (Fig. 6). One of the main differences between the HIPOCAS and ERA-40 wind fields is the spatial resolution, which is 2.5° for the ERA-40 data, whereas the HIPOCAS data had 0.25°. Lower resolution tends to smooth the wind field and this has implication in the modelled wave field. The distribution of the wave is related with the generating winds
and higher wind speed will imply higher Hs. Any error in the wind fields will be reflected in the wave field. 4.1. The statistics A statistical validation is the standard method in wave modelling. The validation statistics of models results against field data allow the
Fig. 4. Velocity Friction time series for November 2001 for the location Mahón.
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Table 2 The geographical coordinates of the locations for the time series comparisons Offshore locations
A
B
Latitude Longitude
39.5° 1°
41.8° 3.8°
quality of the model to be assessed. A set of statistical parameters in this paper was used following Heimbach et al. (1998): The absolute mean error (bias) bias=Y−X is given as difference of mean X¯ of the measurements (Xi) to mean of modelled results (Yi) X ¯¼ ðXi =NÞ X i
where N is the number of data. The Root Mean Square Error (RMSE), " RMSE ¼
1X ðYi Xi Þ2 N i
#1=2
The scatter Index (SI), defined as the standard deviation of the difference normalized by the mean of the observations: RMSE SI ¼ pffiffiffiffiffiffiffiffi XY 5. Comparison of predicted wave fields For the validation of the sea state hindcast a time period corresponding to a month of November 2001 have been selected in which the buoy measurements were available. The hindcast wave time series correspond to wave-rider buoys positions (stars in Fig. 7). The period of interest corresponds to November 2001 where three intense storms caused damages on the Catalan coasts. The buoys are located in the vicinity of Cabo Gata, Cabo de Palos, Alicante, Valencia, Tortosa, Llobregat and Mahón.
In the Hs time series (Figs. 8 and 9) at the offshore points A and B the differences increased substantially moving to the South. If one compares at the same location A or B the differences between the Hs from the simulations using the same wind field, these appear consistent each other. The last criterion is not coincident with the Cavaleri and Bertotti (2004), who found that errors decreased moving southwards in the Mediterranean basin starting from the Gulf of Lyons. The simultaneous WAM-PRO wave fields displayed in Figs. 10 and 11 obtained using the ERA-40 and HIPOCAS wind fields (Figs. 2 and 3), respectively are consistent each other. However, the maximum value of Hs just to the North of Mallorca Island obtained when using the HIPOCAS wind field is around 9 m whereas the maximum of Hs when using the ERA-40 data was about 4.3 m. The spatial distribution of the Hs maxima is located in the same region between Catalonia sector and the Balearic Islands according to the distribution of the absolute maxima of the wind speeds. The adjustment for the Cabo de Gata location for a month (February) shows a large overestimation of the Hs, despite this buoy being in deep waters at 536 m of depth (Fig. 12). HIPOCAS winds have produced an overestimation of the Hs value in around 1 m in the peak of a storm. The Mahón buoy is placed near the small Island Menorca in deep waters at 300 m of depth (Fig. 7), which is not solved by the wave propagation scheme present in the wave model WAM-PRO with the 0.250 spatial resolution. This point is taken as a sea point by the wave model and this could be the reason why using the HIPOCAS winds at this location, a good adjustment has been produced when compared with the buoy data (Fig. 13), whereas when the ERA-40 winds are used the wave model shows a noticeable underestimation of the Hs values. Previous work in the Mediterranean Sea such as the MEWAM Group (Ardhuin et al., 2007) point out Scatter Indexes of the order of 0.28 (open) and 0.36 (coast) for a different period, October 2002, for open and coastal locations using the WAM model and ECMWF wind
Fig. 5. Wind speed time series extracted from the HIPOCAS wind fields during whole the month of November 2001 at two different locations.
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Fig. 6. Wind speed time series extracted from the ERA-40 ECMWF wind fields during whole the month of November 2001 at two different locations.
fields. Strong underestimation associated with the low spatial resolution was obtained by the WAM-PRO model in most of the locations validated in the present work when using the ERA-40 data. The WAM-PRO model driven by the HIPOCAS winds led to better adjustment with the buoy data in some locations, but in other locations overestimation has occurred, as for example, in Cabo de Palos, Valencia and Llobregat (Table 3).
From Table 3 it can be seen that the best adjustment corresponds to the Tortosa location with a bias of 0.12 and the worst adjustment using the HIPOCAS winds was for Valencia (bias=0.73). The location of the Mahón buoy is near and south to the Menorca Island and from Table 4 the worst adjustment corresponds to this location in the case of the ERA-40 wind data with a negative bias −1.30 that indicates a substantial underestimation.
Fig. 7. Location of the buoys represented by numbers and the triangles represent the locations for the wind (U10) time series extracted from the HIPOCAS and ERA-40 ECMWF wind fields.
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Fig. 8. WAM-PRO Hs time series using the HIPOCAS wind fields during whole the month of November 2001 at two different locations.
At the resolution used by the ECMWF model (T511, about 40 km) the underestimation of wind speed varied from 10 to 40% according to the study of MEWAM Group (Ardhuin et al., 2007). In the present case it is easy to say that the ERA40 ECMWF wind fields are underestimated as well, when compared with the HIPOCAS wind data. The statistical
parameters calculated, are in the range of others studies applied in the region (Bolaños et al., 2006). It is usual that the atmospheric models give some inaccuries in this coastal region of the Western Mediterranean. The study makes emphasis on the old question of wind waves modelling, namely the quality of the wind field data showing that in
Fig. 9. WAM-PRO Hs time series using the ERA-40 ECMWF wind fields during whole the month of November 2001 at two different locations.
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Fig. 10. WAM-PRO Hs wave field for the date 10th of November 2001 at 18:00 UTC produced with the ERA-40 ECMWF winds.
the semi-enclosed sea such the Mediterranean the sea state depends on the wind data. 6. Concluding remarks Two winter storms have been compared and the main differences between the significant wave heights using the ERA-40 reanalysis
(ECMWF) and HIPOCAS reanalysis winds were observed to increase when moving southwards in the geographical domain at the offshore locations. On the case of the coastal locations the same pattern was observed for both wind data sets used. This allows one to conclude that in general the main differences between the Hs determined using the ERA-40 and HIPOCAS wind data increased moving southwards in the geographical domain.
Fig. 11. WAM-PRO Hs height for the date 10th of November 2001 at 18:00 UTC produced with the HIPOCAS winds.
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Fig. 12. Significant Wave Height time series for the location Cabo de Gata.
It was observed that on the case of the ERA-40 data set the biases for the Hs were constantly negative values indicating the underestimation of the Hs values, whereas the biases obtained when using the HIPOCAS data are positive values except one indicating a persistent overestimation of the Hs values. In fact, the bias using ERA-40 wind fields varied between −0.38 and −1.3, while with HIPOCAS winds the range was from −0.29 to 0.73.
The range of the scatter indexes for all locations considered in this study has a range from between 0.41 and 0.95 for HIPOCAS winds. With the ERA-40 wind fields the scatter indexes varied between 0.46 and 0.89. The inconsistencies between the modelled waves and the measured data are related with the quality of the winds and their different spatial resolution. This study shows once more the importance of the accuracy in
Fig. 13. Significant wave height time series for the location Mahón.
S. Ponce de León, C. Guedes Soares / Coastal Engineering 55 (2008) 920–929 Table 3 Statistical parameters for Hs using HIPOCAS wind field for whole the November 2001 (from the South to the North in order of appearance, the depths are indicated in parentheses) parameter Cabo Gata (536 m) bias rmsdif SI slope
0.23 0.62 0.69 1.46
Cabo Palos (67 m) 0.36 1.11 0.81 1.5
Alicante Valencia (50 m) (260 m) 0.21 0.73 0.58 1.36
0.73 1.36 0.87 1.94
Tortosa (57 m) 0.12 1.03 0.61 1.30
Llobregat Mahón (50 m) (300 m) 0.52 1.26 0.95 1.91
−0.29 0.94 0.41 0.88
Table 4 Statistical parameters for Hs using ERA-40 ECMWF wind field for whole the November 2001 (from the South to the North in order of appearance) parameter Cabo Gata Cabo Palos Alicante Valencia Tortosa Llobregat Mahón bias rmsdif SI slope
− 0.38 0.5 0.67 0.52
−0.52 0.64 0.46 0.6
−0.58 0.76 0.67 0.49
−0.48 0.74 0.78 0.52
−1.13 1.38 0.89 0.32
−0.55 0.72 0.71 0.48
−1.30 1.71 0.71 0.43
the wind fields especially when the sea is semi enclosed such the Mediterranean and when some orography is present. Acknowledgments This study has been partially conducted in the scope of the project “Hindcast of Dynamic Processes of the Ocean and Coastal Areas of Europe (HIPOCAS)”, which has been partially funded by the European Union under the Program “Energy, Environment and Sustainable Development” (Contract No EVK2-CT-1999-00038). It was also partially performed within the scope of the Project SPEC-HIPOCAS sponsored by the ECMWF (European Centre for Medium-Range Weather Forecast). References Ardhuin, F., Bertotti, L., Bidlot, J.R., Cavaleri, L., Filipetto, V., Lefevre, J.M., Wittmann, P., 2007. Comparison of wind and wave measurements and models in the Western Mediterranean Sea. Ocean Engineering 34, 526–541. Bolaños-Sanchez, R., Sánchez-Arcilla, A., Cateura, J., 2007. Evaluation of two atmospheric models for wind-wave modelling in the NW Mediterranean. Journal of Marine System 65, 336–353. doi:10.1016/j.jmarsys.2005.09.014. Caires, S., Sterl, A., Bidlot, J.R., Graham, N., Swail, V., 2002. Climatological assessment of reanalysis of wave data. Proc. 7th Int. Workshop on Wave Hindcasting and Forecasting, Banff, Canada. 21–26 October.
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