Vertjcal Profjles of WaveCoherent Momentum Flux and Velocity - PowerPoint PPT Presentation

Vertjcal Profjles of Wave–Coherent Momentum Flux and Velocity Variances in the Marine Atmospheric Boundary Layer Lichuan Wu 1 , Tihomir Hristov 2 , and Anna Rutegersson 1 Lichuan.wu@geo.uu.se 1 Deapartment of Earth Sciences, Uppsala University, Sweden 2 Department of Mechanical Engineering, Johns Hopkins University, Baltjmore

Introductjon The atmosphere turbulence facilitates momentum and energy exchange between the atmosphere and ocean. Many studies suggest that the Monin-Obukhov Similarity Theory needed to be revised in order to considering wave infmuences. The momentum fmux over waves can be expresses as: 0 turbulence momentum fmux wave-induced momentum fmux parameterized as a functjon of parameterized as a functjon of Hogstrom et al., 2015. JAS the wind speed. wave parameters.

Introductjon The total momentum fmux over waves is roughly constant near the surface. Sullivan et al., 2014. JAS; Högström et al., 2015. JAS Measure the momentum fmux at the ocean surface is not possible! Interpolation?  The wave-coherent momentum fmux decays exponentjally with height ? surface wave-induced peak wave number momentum flux 2 Semedo et al. (2009) and Song et al. (2015) 10 Hanley and Belcher (2008) coefficient 1.4 Högström et al (2015) 5 for short waves Makin and Mastenbroek (1996)

Aims  Do the wave-coherent momentum fmux and variances decay exponentjally with height?  If so, A uw , A u and A w are constant or a functjon of wave parameters?

The height that wind speed is equal to the wave phase speed Critjcal layer model  For neutral convectjve and wave aligned with wind conditjons over waves, the wave correlated fmuctuatjons can be described using Rayleigh equatjon : a transfer function for the air flow’s response Ψ  Based on logarithmic wind profjle, the wave-coherent fmuctuatjons: Hristov et al., 2003. Nature

Results Using wave slope spectrum: wave slope wave age at the spectal peak Theoretical model Exponentical decay fit The wave-coherent momentum fmux decays approximately exponentjally with height.

Results Ω = 0.01 Ω = 0.02 Ω = 0.003

Measurements-RED Rough Evaporatjon Duct experiment

Methods Methods to fjlter out the wave-induced momentum fmux  Projectjng the measured velocity onto the Hiblert space of the wave-coherent signals: (Hristov et al. 1998, 2003)  Wave coherent signals are calculated from spectral analysis: (Veron et al. 2008; Grare et al. 2013)

Results

Results

Results Exponential fit line 95% confident line Only the case with |z/L|<0.1 and regression coeffjcient R 2 >0.7 are shown.

Wave impacts on wind profjle Shear induced stress: Wind profile over waves: The ratio between the wave-induced wind and logarithmic wind:

Wave impacts on wind profjle

Conclusions  The wave-coherent momentum fmux and variances decay approximately exponentjally with height.  The exponentjal decay coeffjcient decays with the peak wave number.  The wave-coherent momentum fmux with the small decay coeffjcient has a larger impact on the logarithmic wind profjle.  The surface roughness length impact on the wave impact wind only on the near surface layer.

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