EGU2020-15534
https://doi.org/10.5194/egusphere-egu2020-15534
EGU General Assembly 2020
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

On the momentum-, heat-, and moisture-exchange on the ocean surface under strong wind conditions.

Elena Savenkova1, Vladimir Kudryavtsev1, and Bertran Chapron2
Elena Savenkova et al.
  • 1Satellite Oceanography Laboratory, Russian State Hydrometeorological University (RSHU), St.Petersburg, Russian Federation (savenkova.en@mail.ru)
  • 2Laboratoire d'Oceanographie Spatiale, Ifremer, Plouzane, France (bertrand.chapron@ifremer.fr)

We present results of the model treatment of momentum-, heat-, and moisture-exchange on the ocean surface under strong wind conditions. Despite the large amount of experimental and theoretical efforts, the mechanism and physics of the air-sea interaction at high wind conditions is still poor known and many open questions are still remained. (see e.g. Donelan et.al. 2004, Powell 2003, Kudryavtsev 2006, Jarosz 2007, Troitskaya et.al. 2011).

The model is based on extension of wind-over-wave couple model suggested by Kudryavtsev, Chapron and Makin (2014, hereinafter KCM2014). This model confirmed crucial role of wave breaking on surface drag and heat-, moister-transfer coefficients. Description of wave breaking crest roughness in KCM2014 is treated as Kolmogorov-type spectra resulting from the energy flux from the largest energetic breaking disturbances toward shorter scales. To extend KCM2014 model on high wind conditions, we introduced  Kelvin- Helmholtz instability which is able to disrupt both the crests of short regular (non-breaking) waves, and the small-scale breaking crests roughness. It is suggested that at wind speed exceeding a critical value, spectral components of both regular wind waves and breaking crests roughness are subjected to Kelvin-Helmholtz instability and aerodynamically disrupted, and thus do not contribute to the total form drag. This effect results in decrease of the surface drag, that in turn, following KCM2014, leads to  enhancement of exchange at the sea surface heat and moister transfer. As a consequence, ratio of the enthalpy to the drag coefficient increases and at wind speed above 25 m/s exceeds critical level introduced by (Emanuel, 1995). Comparison of model predictions with available data at high winds is encouraging, and suggests that accounting for the Kelvin- Helmholtz instability in the wind-over-wave coupled model provides realistic description of air-sea interaction under strong wind condition.

The work was supported by Russian Science Foundation grant No 17-77-30019.

How to cite: Savenkova, E., Kudryavtsev, V., and Chapron, B.: On the momentum-, heat-, and moisture-exchange on the ocean surface under strong wind conditions., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15534, https://doi.org/10.5194/egusphere-egu2020-15534, 2020

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Presentation version 1 – uploaded on 04 May 2020
  • CC1: Comment on EGU2020-15534, Ad Stoffelen, 08 May 2020

    Dear Elena,

    I was just too late to ask in the chat, but in your model roughness goes down above 40 m/s (slide 20), while we see the highest speeds above 40 m/s in the most intense hurricanes with scatterometers. This is, we see saturation, but no reduction. Are you looking into verification of your model with active microwave satellite data? Particularly SAR will be useful in comparison with hurricane models I guess, as the structure of hurricanes is well known. As Romain and me briefly explained, VH and HH polarizations are not a direct measure of ocean centimetric roughness, but VV polarization is a better candidate. We are embarking on some of this work for other purposes. So, if you are interested to collaborate, just drop a line.

    Cheers,

    Ad

    • AC1: Reply to CC1, Vladimir Kudryavtsev, 08 May 2020

      Dear Ad

      I will reply for Elena. I understood your point. That is what we are keeping in mind when we will test the model (surface roughness component) against the data, with priority to scatterometer and passive microwave measurements at high winds.

      At this stage we just demonstrated effect of K-H instability on wave spectrum and wave breaking (in terms of white caps coverage). You are right, spectrum of very short waves goes down. However, around C-band (k~10^2 rad/m) it almost saturated, and at Ku-and Ka-band is suppressed. If would follow Bragg-scattering paradigm, then NRCS would demonstrate the similar behaviour. However, following our previous development, we believe that there is important contribution of wave breaking, which provide non-polarized returns to VV and HH. In this context, some suppression of Bragg spectrum could be compensated by radar returns from wave breaking, with probably levelling off of the NRCS at high winds. With VH is a lttle bit more complicated. What we recently learn (our paper in IEEE TGRS in 2019), that Bragg scattering from regular waves does not matter VH, especially at high winds, but wave breaking does. In our model wave breaking do not go down (at high winds), but slightly grow up with wind speed. 

      We are working on it, and it would be great to collaborate with you on this matter.

      Regards

      Vladimir   

       
      • CC2: Reply to AC1, Ad Stoffelen, 08 May 2020

        Thanks Vladimir,

        That is clear. We are interested in spatial scaling for hurricanes to provide guidance on the many scatterometer acquisitions. Alexis Mouche did a nice empirical test in CHEFS, while we would like to do the reverse too, i.e., estimate hurricane fine-scale structure, based on coarse scatterometer winds. This help in guidance for hurricane advisories and is a practical excercise. However, since we use VV and HH signatures, I believe it could provide some information on saturation characteristics. We are now fitting Rankine vortices to SAR and have yet to start with scatterometers. So, it may be a bit early to start our collaboration? Anyway, we should certainly keep this in mind I believe.

        Have a nice weekend,

        Ad

        • AC2: Reply to CC2, Vladimir Kudryavtsev, 08 May 2020

          Ad, 

          we should finish our development to be ready for comparison. I anicipate, it will not take too much time. Anyway, I will keep this option in mind.

          Regards

          Vladimir