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Changes in presentations EMS A. Smedman: Velodity spectra in the marine atmospheric boundary layer and EMS U. Högström: Turbulence structure of the marine boundary layer during mixed sea and growing sea conditions have been changed to

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Turbulence structure in the marine atmospheric boundary layer – influences of ocean waves Part I and part II Ann-Sofi Smedman Ulf Högström Uppsala University Uppsala Sweden

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We all know why oceans and marine boundary layers (AMBL) are so important in the climate system? Oceans occupy 70% of the Earth surface Oceans have a large heat capacity Oceans are a large sink of CO2 Ocean waves influence the turbulent transport in AMBL and thus the air-sea exchange

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In models the air-sea exchange is described through Monin-Obukhov similarity theory a theory which is well tested over land but is it valid over the ocean?

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Analysis of data from BASE (BAltic Sea Swell Experiment) Measurements Tower: turbulence and profiles ASIS bouy: turbulence and profiles, wave parameters Wave Rider Bouys: wave parameters R/V Aranda: turbulence and wave parameters Collaboration between MIUU Uppsala, Swden RSMAS, Miami, USA FMRI, Helsinki, finland

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buoys (temp, wave height, dir. and CO 2 Footprint area Tower 30 m Temp and wind profiles 5 levels Turbulence 3 levels Humidity and CO 2 at 2 levels Long term measurements in the Baltic Sea

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Turbulence, wind speed, temperature and wave parameters Short term experiments with RV Aranda and ASIS buoy In the mean, excellent agreement between tower and ASIS

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Characterizing the sea state Definition of wave age: c p /U 10 or c p /U 10 cos c p =phase speed, U =wind speed Waves c p /U cp cp Origin Growing sea Young waves <0.8SlowLocal wind Mixed sea Mature sea >0.8 &<1.2 Swell, old waves>1.2FastDistant storms

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uαuα cpcp uaua cpcp There is a small phase shift between p and w and depending on the sign the energy transport is upwards or downwards

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The equation for the form drag over a wave = =

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The data One case with growing sea defined as c p /U 8 <0.7 One case (F1) has c p /U 8 about 5 and is considered as pure swell, one case ( F2) has c p /U 8 2 (weak swell) One case has c p /U 8 0.9 and is defined as mixed sea All cases consist of about 10 hours of data

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The turbulence kinetic energy budget Note that the terms on the right hand side are found to be close to zero so the pressure transport term can be obtained as a residual of the remaining four terms, which can all be obtained from the measurements

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Results of the TKE-analysis The pure swell cases : Mechanical production, P = 0; Pressure transport, Tp positive,i.e. a gain of TKE Tp thus tends to accelerate the flow but it is balanced by molecular friction at the surface (low level wind maximum).

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Low level jet

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Turbulence spectra of the u and w components during strong swell

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Strong resemblence with free convection spectra but The main source of turbulence is not heat flux. Energy is taken from the waves and transported upwards through the pressure transport term

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Pressure transport acts directly vertically and effects the whole boundary layer Creating large eddies (boundary layer scale)

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From the turbulence energy eq the 3:e component reads, Both bouancy and pressure transport are input to the w- component

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Similarity between swell and free convection: u * 0 and eddies scale with boundary layer height Convective scaling where

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Convective boundary layer over land (Kaimal et al. 1976) but In the marine boundary layer dyring swell 2 (BASE and Utlängan) Heat flux is not the source of turbulence

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Conclusions Monin-Obukhov similarity teory is only valid for growing sea cp/U<0.7 When swell dominates (c p /U 8 large), an exponentially decreasing function for Tp fits the data well. Swell thus tends to accelerate the flow but is balanced by molecular friction at the surface (low level wind maximum).

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Velocity spectra ’Convective scaling’ can be applied for strong swell conditions and large eddies tend to move towards isotropy

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