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Changes in presentations EMS2009-427 A. Smedman: Velodity spectra in the marine atmospheric boundary layer and EMS2009-573 U. Högström: Turbulence structure.

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Presentation on theme: "Changes in presentations EMS2009-427 A. Smedman: Velodity spectra in the marine atmospheric boundary layer and EMS2009-573 U. Högström: Turbulence structure."— Presentation transcript:

1 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

2 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

3 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

4 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?

5 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

6 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

7 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

8 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

9 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

10 The equation for the form drag over a wave = =

11 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

12 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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17 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).

18 Low level jet

19 Turbulence spectra of the u and w components during strong swell

20 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

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

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

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

24 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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29 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).

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


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