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Ocean Wave Forecasting at ECMWF

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Ocean Wave Forecasting at ECMWF Jean-Raymond Bidlot Marine Aspects Section Predictability Division of the Research Department European Centre for Medium-range Weather Forecasts (E.C.M.W.F.) Reading, UK Slide 1 Ocean waves: We are dealing with wind generated waves at the surface of the oceans, from gentle to rough … Ocean wave Forecasting at ECMWF Slide 2 Ocean Waves Forcing: earthquake wind moon/sun Restoring: gravity surface tension Coriolis force 10.0 1.0 0.03 3x10-3 2x10-5 1x10-5 Frequency (Hz) Ocean wave Forecasting at ECMWF Slide 3 What we are dealing with? Water surface elevation, η Wave Period, T Wave Length, λ Wave Height, H Ocean wave Forecasting at ECMWF Slide 4 Wave Spectrum l The irregular water surface can be decomposed into (infinite) number of simple sinusoidal components with different frequencies (f) and propagation directions (θ ). l The distribution of wave energy among those components is called: “wave spectrum”, F(f,θ). Ocean wave Forecasting at ECMWF Slide 5 l Modern ocean wave prediction systems are based on statistical description of oceans waves (i.e. ensemble average of individual waves). l The sea state is described by the two-dimensional wave spectrum F(f,θ). Ocean Wave Modelling Ocean Wave Modelling Ocean wave Forecasting at ECMWF Slide 6 Ocean Wave Modelling l For example, the mean variance of the sea surface elevation η due to waves is given by: 〈η2〉 = F( f ,θ)dfdθ ∫∫ l The mean energy associated with those waves is: 2 energy= ρηw g l The statistical measure for wave height, called the significant wave height (Hs): 2 H s = 4 η The term significant wave height is historical as this value appeared to be well correlated with visual estimates of wave height from experienced observers. It can be shown to correspond to the average rd 1/3 highest waves (H1/3). Ocean wave Forecasting at ECMWF Slide 7 Ocean Wave Modelling l The ocean wave modelling at ECMWF is based on the wave mode WAM cycle 4 (Komen et al. 1994), albeit with frequent improvements (Janssen 2007: ECMWF Tech. Memo 529.). l Products from different configurations of WAM are currently available at ECMWF. l Wave model wave page: http://www.ecmwf.int/products/forecasts/wavecharts/index.html#forecasts l General documentation: http://www.ecmwf.int/research/ifsdocs/CY36r1/index.html Ocean wave Forecasting at ECMWF Slide 8 ECMWF Wave Model Configurations Global models Tuesday20°E 14 March40°E 200660°E 00UTC80°E ECMWF100°E 120°E Forecast140°E t+36160°E VT: Wednesday180° 160°W 15 March140°W 2006120°W 12UTC100°W Surface:80°W significant60°W 40°W wave20°W height l Global from 81°S to 90°N, 10.29 70°N 70°N 9.75 60°N 60°N including all inland seas. 9 50°N 50°N 40°N 40°N 8.25 30°N 30°N 7.5 20°N 20°N 6.75 l Coupled to the atmospheric model 10°N 10°N 6 0° 0° 5.25 (IFS) with feedback of the sea 10°S 10°S 4.5 20°S 20°S 3.75 30°S 30°S 3 surface roughness change due to 40°S 40°S 2.25 50°S 50°S 1.5 60°S 60°S waves. 0.75 70°S 70°S 0 20°E 40°E 60°E 80°E 100°E 120°E 140°E 160°E 180° 160°W 140°W 120°W 100°W 80°W 60°W 40°W 20°W l The interface between WAM and Forecast wave height on 15/03/2006 12UTC. the IFS has been generalised to include air density and gustiness neutral wind effects on wave growth and more wind gustiness recently neutral winds. l Data assimilation Jason-2 air density altimeter wave heights. model Wave model Atmospheric roughness Ocean wave Forecasting at ECMWF Slide 9 ECMWF Wave Model Configurations Deterministic model Probabilistic forecasts (EPS) l 28 km grid spacing. l 55 km grid spacing. l 36 frequencies. l 30 25 frequencies *. l 36 directions. l 24 12 directions *. l Coupled to the TL1279 model. l Coupled to TL639 TL319 model *. l (50+1) (10+5) day forecasts from l Analysis every 6 hrs and 10 day 0 and 12Z (monthly once a forecasts from 0 and 12Z. week). * Change in resolutions after 10 days NB: also in seasonal forecast at lower resolutions Ocean wave Forecasting at ECMWF Slide 10 ECMWF Wave Model Configurations Interim reanalysis (1979 to present) (as a follow-up to ERA40 (45 year reanalysis) Comparison with buoys: l 1.0°x1.0°. l 30 frequencies. l 24 directions. l Coupled to TL255 model l Production is ongoing. l Very satisfactory performance: Operations ¤ http://www.ecmwf.int/research/era/do/get/era-interim ERA-40 ¤ ECMWF Newsletters No. 110 (Winter 2006/07) & ERA Interim 111 (Spring 2007) http://www.ecmwf/publications/newsletters Ocean wave Forecasting at ECMWF Slide 11 Wave Model Products Wave model The complete description of the sea state is given by the 2-D spectrum, however, it is a fairly large amount of data (e.g. 24x30 values at each grid point in the global model). It is therefore reduced to integrated quantities: 2-D Ø 1-D spectrum obtained spectrum by integrating the 2-D spectrum over all directions and/or over a 1-D frequency range. spectrum Ocean wave Forecasting at ECMWF Slide 12 Wave Model Products When simple numbers are required, the following parameters are available: ) peak f Ø The significant wave height ( E (Hs). Ø The peak period (period of the peak of the 1-D spectrum). area under spectrum = <η2> Ø Mean period(s) obtained 2 H = 4 η from weighted integration of s the 2-D spectrum. Ø Integrated mean direction. f fp < f > Ø Few others. (peak frequency) (mean frequency) T = 1 / f Complete list at: http://www.ecmwf.int/services/archive/d/parameters/order=/table=140/ Ocean wave Forecasting at ECMWF Slide 13 Wave Model Products Use simple parameters: total wave height and mean propagation direction 10m winds and mean sea level pressure: Wave height and mean direction: Analysis : 14 February 2009, 00 UTC Analysis : 14 February 2009, 00 UTC Ocean wave Forecasting at ECMWF Slide 14 Wave Model Products Wave periods: at the peak of the spectrum or in the mean PEAK PERIOD: MEAN WAVE PERIOD: Analysis : 14 February 2009, 00 UTC Analysis : 14 February 2009, 00 UTC Ocean wave Forecasting at ECMWF Slide 15 Wave Model Products Wave height and mean direction: Analysis : 14 February 2009, 00 UTC PEAK PERIOD: Analysis : 14 February 2009, 00 UTC Ocean wave Forecasting at ECMWF Slide 16 Wave Model Products Situation might be more complicated ! 10m winds and mean sea level pressure: Wave height and mean direction: Analysis : 15 February 2009, 00 UTC Analysis : 15 February 2009, 00 UTC Ocean wave Forecasting at ECMWF Slide 17 Wave Model Products Situation might be more complicated: Wave height and mean direction: Analysis : 15 February 2009, 00 UTC Ocean wave Forecasting at ECMWF Slide 18 Wave Model Products A scheme is used to split the global wave fields into waves which are under the direct influence of the forcing wind, the so-called windsea or wind waves, and those waves that are no longer bound to the forcing wind, generally referred to as swell. Period and mean direction are also determined for these split fields. Wave height and windsea mean direction: Wave height and swell mean direction: Analysis : 15 February 2009, 00 UTC Analysis : 15 February 2009, 00 UTC Ocean wave Forecasting at ECMWF Slide 19 Wave Model Products Windsea and swell: opposing sea Ocean wave Forecasting at ECMWF Slide 20 Wave Model Products Windsea and swell: cross sea swell Ocean wave Forecasting at ECMWF Slide 21 Wave Model Products on the web: Currently on our web: significant wave height and mean direction Ocean wave Forecasting at ECMWF Slide 22 Wave Model Products on the web Currently on our web: mean wave period and direction Ocean wave Forecasting at ECMWF Slide 23 Wave Model Products on the web: For the Severe Weather Forecast Demonstration Projects (SWFDP) for South Africa, East Africa and the Pacific windsea and swell plots are also available: e.g.: Swell Mean period and direction Swell wave height and direction Significant wave height and direction http://www.ecmwf.int/products/forecasts/d/charts/medium/special Ocean wave Forecasting at ECMWF Slide 24 Wave Model Products: EPS From the EPS wave forecasts it is possible to derive probabilities for certain wave conditions. Tuesday 6 November 2001 12UTC ECMWF EPS Probability Forecast t+120 VT: Sunday 11 November 2001 12UTC Surface: significant wave height probability >8 25°W 20°W 15°W 10°W 5°W 0° 5°E 10°E 15°E 20°E 25°E 54.47 25°E 50 50% MIKE HEIDRUN 45 DRAUGEN 40 65°N 35 20°E 25°W 30 25 25% 20 60°N 15 Significant wave height (m) at Heidrun 15°E 55°N 14 10 12 5 0 20°W 15°W 10°W 5°W 0° 5°E 10°E 10 06 Nov. 2001 12 UTC ECMWF EPS probability forecast t+120 8 8 m Significant wave height above 8 m 6 4 2 0 1 2 3 4 5 6 7 8 9 10 ECMWF Newsletter 95 – Autumn 2002 Forecast day Ocean wave Forecasting at ECMWF Slide 25 Wave Model Products on the web Currently on our web: probability for set thresholds (2m) Ocean wave Forecasting at ECMWF Slide 26 Wave Model Products on the web Currently on our web: probability for set thresholds (4m) Ocean wave Forecasting at ECMWF Slide 27 Wave Model Products on the web Currently on our web: probability for set thresholds (6m) Ocean wave Forecasting at ECMWF Slide 28 A bit more compact: Wave EPSgram: Like normal EPSgram but for wind direction, wind speed, significant wave height, mean wave direction and mean period.
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