Spectral Dissipation Term for Wave Forecast Models, Experimental Study

Transcription

1 pectral Dissipation Term or Wave Forecast Models, Experimental tudy Alexander Baban, Ian Young, Richard Manasseh and Eric chultz wburne University o Technology, Melbourne, Australia CIRO, Melbourne, Australia Australian Bureau o Meteorology, Melbourne, Australia ponsors Australian Research Council Oice o Naval Research, UA

2 Whitecappg Dissipation, θ, is traditionally regarded as a tung knob recent experimental advances brought much more certaty to physics o whitecappg dissipation Threshold behaviour terms o the wave spectrum: ~ ( F F ) Two-phase behaviour: dissipation at smaller scales depen on breakg/modulation at larger scales ( ) a ( F ( ) F )) A ( ) + b p ( F ( g ) At high wd spee, dissipation depen on the wd ( F ( g )) A ( g ) dg At high requencies (cumulative term domates), turbulent viscosity is more signiicant than breakg dissipation At low spectral densities (below the eshold), dissipation may persist without breakg, but has to be described by separate terms Little i any new experimental knowledge implemented the models b n

3 White Cap Dissipation, θ, passive acoustic metho have a potential advantage + strumentation is cheap, robust and easy to mata + hydrophones are deployed below the surace and escape distructive power o breakg waves + can be operated on long-term or regular basis two passive acoustic metho to study spectral dissipation - segmentg a record to breakg and non-breakg segments - usg acoustic signatures o dividual bubble-ormation events

4 the photo is curtesy o Fabrice Ardhue, France are we prepared to describe the surace like this? the description is necessary i we want to orecast the waves

5 Radiative Transer Equation, θ, tot b Represents the temporal and spatial evolution o the wave energy spectrum E(k,,θ,θ) tot b all physical processes which aect the energy transer energy put rom the wd dissipation due to wave breakg noear teraction between spectral components dissipation due to teraction with the bottom

6 Lake George - Canberra 20 km x 10km uniorm ite water depth (0.3m - 2.2m) steep waves p > 0.3 Hz strongly orced waves 1 < U/c p < 8, θ, b

7 Instrumentation, θ, b wc - White-cap dissipation 3 Acoustic Doppler Current Meters Doppler spatial current proiler Hydrophone Video images Manual taggg

8 recordg the breakg de(k,,θ, t ) b passive acoustic metho have a potential advantage + strumentation is cheap, robust and easy to mata + hydrophones are deployed below the surace and escape destructive power o breakg waves + can be operated on long-term or regular basis

9 pectrogram method, θ, b egmentg the record - 50% breakg rate - tras o dozens o breakg waves ollowed by dozens o non-breakg waves - stationary, ully-developed, constant depth case - U m/s, p 0.4 Hz - succession o breakg waves considered a tra o cipient breakers - succession o non-breakg waves considered a tra o broken waves - segments are rom hal a mute to a ew mutes long

10 pectrogram method, θ, b Cumulative eect () ΔF()/Δt d D i -D p (Δ F()/Δd Young & Baban, JPO, 2006

11 pectrogram method, θ, b Directional dissipation p 2 p p

12 Bubble-detection method, θ, b - dividual bubbles oscillate volumetrically: ω 0 ~1/R - bubbles passively emit sound at the natural requency when ormed or collapse - dividual bubbles rg at requencies khz - rgg lasts cycles - what humans perceive as a contuous noise is many discrete events - suiciently short time wdow triggered on a signal peak contas ormation about the bubble - appropriately esholded acoustic data generates statistic time on number o bubbles and bubble size

13 Bubble-detection method, θ, b Frequency distributions o breakg probability Breakg severity Manasseh et al., JTec, 2006

14 Bubble-detection method, θ, b Cumulative eect Dependence on the wd two-phase behaviour o spectral dissipation: - lear dependence o on the spectrum at the peak - cumulative eect at smaller scales b T depen on the wd or U 10 > 14 m/s

15 Bubble-detection method, θ, Frequency distributions o breakg probability b

16 Whitecappg dissipation, θ, b aturation eshold σ ( ) Phillips (2π ) 4 2g 5 2 F( ) σ ( ) σ ( ) A( Phillips ) F 2 2g (2π ) 4 σ A( ) 5 σ ( ) const ( ) a ρ g (( F( ) F ( )) A( )) a2 1 w ρw g p (( F( q) F ( q)) A( q)) dq

17 Dissipation, θ, b The duced dissipation can be caused by orced breakg o shorter waves due to the domant breakg/modulation, or by enhanced turbulent viscosity due to the domant breakg, or both. comparg with the Melville & Matusov dissipation based on distributions o the breakg crests importance o the turbulent viscosity contribution to the cumulative dissipation is evident ( ) a (( F( ) F ( )) A( )) n + b p ( F( g) F ( g)) A( g) dg

18 Whitecappg Dissipation, θ, b spectral dissipation was approached by two dependent means based on passive acoustic metho i the wave energy dissipation at each requency were due to whitecappg oy, it should be a unction o the excess o the spectral density above a dimensioess eshold spectral level, below which no breakg occurs at this requency. This was ound to be the case around the wave spectral peak (domant breakg) dissipation at a particular requency above the peak demonstrates a cumulative eect, dependg on the rates o spectral dissipation at lower requencies ( ) a (( F( ) F ( )) A( )) n + b p ( F( g) F ( g)) A( g) dg dimensioess saturation eshold value o σ ( ) should be used to obta the dimensional spectral eshold F () at each requency comparisons dicate that the turbulent viscosity becomes signiicant when the cumulative term domates