NP6.1

Earth System Models (ESM) and Numerical Weather Prediction (NWP) systems often have large biases in their representation of surface-atmosphere fluxes when compared to observations. Over sea, these biases are more pronounced due to dynamic non-linear interactions between atmosphere and sea surface waves. This non-linearity is not accurately represented by traditional semi-emiprical models like COARE. To this end, the PASM (Physically derived Air-Sea Momentum flux) model, developed by us, is the first attempt to represent air-sea exchanges by considering the two-way interaction between the ocean-waves and the atmospheric flow. It can simulate (i) the main turbulent eddies of the air-flow, and (ii) the wind-wave interactions including wave growth, transport and breaking. This model has been previously demonstrated to better predict the air-sea fluxes under 10m high wind speeds greater than 20m/s, where traditional approaches like COARE fail. In this study, we evaluate for the first time, the evolution of cyclone IDAI off the coast of Madagascar, using PASM and COARE approaches, to demonstrate the efficiency of our physically based model in better simulating the evolution and trajectory of cyclones, and thus its usefulness in ESM and NWP models.

How to cite: Fernandes, R., Redelsperger, J.-L., and Bouin, M.-N.: Evaluating the evolution of cyclone IDAI using the physically based PASM air-sea flux model, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12026, https://doi.org/10.5194/egusphere-egu22-12026, 2022.

The work is concerned with the study of the breaking surface wave impact on the scattered radar signal in laboratory conditions using optical methods for analyzing the state of the water surface.

The experiments were carried out on the reconstructed TSWiWaT wind wave flume of the IAP RAS. The channel is 12 m long, the channel cross-section varies from 0.7 x 0.7 m at the entrance to 0.7 x 0.9 m in the working section at a distance of 9 m. The airflow speed on the axis is 3-35 m/s, which corresponds to the values of the wind speed U10 of 11-50 m/s.

At the beginning of the channel, a wavemaker was installed, operating in a pulsed mode and generating a train of three long waves every 15 seconds. In front of the area under study, an inclined plate was installed under the water, simulating shallow water and stimulating the breaking of waves in the zone of optical and radar measurements. In parallel, wind waves were generated. Due to the design features of the experimental setup, the distance from the beginning of the channel to the inclined plate in the case of optical measurements was 884 cm, and for radar measurements - 781 cm.

Radar measurements were carried out using a Doppler scatterometer operating at a wavelength of 3.2 cm, with the ability to simultaneously receive two direct and two cross-polarizations (VV, HH, VH, HV). The dimensions of the observation window on the water surface varied depending on the selected incidence angles (30, 40, 50 degrees). Optical measurements were carried out independently of radar measurements using three cameras with a shooting frequency of 50 Hz. Using a specially developed algorithm based on threshold processing of the image brightness, the time dependences of the fraction of breakers on the area of the investigated water surface during the passage of a train of three waves were calculated.  Due to the different configuration of the experiments, the data of radar and optical measurements are separated in time, their synchronization was performed using correlation analysis.  Comparison of the data made it possible to find that, on cross-polarization, the received power monotonically increases with an increase in the fraction of breakers, while on direct polarization, the change in power remains within the values observed during collapses of wind waves. Further comparison of the values of the radar cross-section of the water surface and the relative area of the wave breaking will make it possible to determine the influence of the breaking on the formation of the scattered signal.

This work was supported by the Russian Science Foundation (RSF) project No. 21-17-00214.

How to cite: Rusakov, N., Baidakov, G., Kandaurov, A., Troitskaya, Y., Poplavsky, E., and Ermakova, O.: Experimental investigation of microwave signal scattered by the breaking waves, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7649, https://doi.org/10.5194/egusphere-egu22-7649, 2022.

Marine aerosol has a large impact on the earth system, including the physics and chemistry of the atmosphere over the oceans. It is a suspension in the air, consisting mainly of droplets injected from the ocean surface as a result of wave breaking in the coastal zone or during strong winds. A marine aerosol production model is an element of great importance in climate change and forecasting models.

In this work, the calculation of the production of sea aerosol using the developed parameterization of the sea spray generation function is carried out taking into account the contribution of the «bags breakup» spume droplet generation mechanism. For example, the work [1] show the decisive contribution of this type of spray to the sea spray generation function. The calculation is carried out both on the basis of reanalysis data on the global distribution of wind speed (CFSv2 [2]) and wave parameters (WAVEWATCH III® Hindcast and Reanalysis Archives [3]), and on the basis of calculation data within the WRF atmospheric model and the WAVEWATCH III wave model. An assessment of the production of sea aerosol is carried out using the example of hurricane Irma. The wind data in the calculations in the WRF model is obtained using the Large Eddy Simulation (LES) technique of the planetary boundary layer (PBL) with the boundary conditions from the CFSv2 reanalysis. Wave parameters data is obtained from calculations within the WAVEWATCH III wave model. A comparison of the resulting sea spray generation function obtained using the WRF LES and WAVEWATCH III data and the distribution obtained using the reanalysis data is carried out. Conclusions are made about the advantages of using computational models of high spatial resolution.

The work is supported by President Grant for young scientists MK-2489.2022.1.5.

[1] Troitskaya, Y., Kandaurov, A., Ermakova, O., Kozlov, D., Sergeev, D., & Zilitinkevich, S. (2018). The “bag breakup” spume droplet generation mechanism at high winds. Part I: Spray generation function. Journal of physical oceanography, 48(9), 2167-2188.

[2] Saha, S., et al. 2011, updated monthly. NCEP Climate Forecast System Version 2 (CFSv2) Selected Hourly Time-Series Products. Research Data Archive at the National Center for Atmospheric Research, Computational and Information Systems Laboratory. https://doi.org/10.5065/D6N877VB. Accessed 14 December 2021.

[3] https://polar.ncep.noaa.gov/waves/hindcasts/

How to cite: Poplavsky, E., Kuznetsova, A., Dosaev, A., and Troitskaya, Y.: Assessment of the sea aerosol production including the "bag breakup" effect in the spray generation function, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8909, https://doi.org/10.5194/egusphere-egu22-8909, 2022.

Today models of our atmosphere to study climate change become more and more important not only from a meteorological point of view but also from a global perspective to understand the large-scale motion of planetary waves that transport a large amount of energy. This study investigates numerically such large-scale flows in a simplified 2-dimensional model that is aligned to the AtmoFlow experiment. This experiment is the legacy of the GeoFlow experiment, which investigated planet mantle convection. The AtmoFlow experiment is a spherical shell that mimics a planet at a small scale, where terrestrial gravity is artificially induced by an equivalent electric central force field. This small planet can rotate synchronized or differentially by moving the inner and outer boundaries to simulated planetary rotation. Analogous to a real planet, the poles are cooled and the equator heated. The fluid used in the numerical simulation to mimic a planetary atmosphere is a dielectric fluid with an electric permittivity sensitive to temperature to induce convection similar to a terrestrial buoyancy. While the fluid is also sensitive to the temperate-dependent density, the spherical shell experiments are performed in free space and thus the experiment is planned to be operated on the International Space Station (ISS) after 2024. Flow patterns are retrieved using a Wollaston Shear Interferometry (WSI) and sent back to Earth's ground station.

To be able to investigate the flow structures recorded by the experiment, a numerical model is built. Here we only show 2-dimensional results of the shell in the equatorial plane without rotation. The boundary conditions in these simulations are set to an ideal fixed temperature where the inner shell is heated, and the outer is cooled. To induce thermo-electro-hydrodynamics convection, an electric voltage is applied at the inner shell whereas the outer is grounded. The resulting flow patterns evolve in time and are stationary, quasi-stationary, or chaotic structures. The arising convection cells can be classified using a time-averaged spatial Fast Fourier Transformation (FFT) of the temperature along the mid-gap of the domain to quantify a mode number. The heat transfer is expressed with the Nusselt number and increases with the Rayleigh number. This is reflected by the mode number increasing to a maximum before it decreases when the flow becomes unstable while maintaining a clear structure and mode shape with detaching plumes at the tangent cylinder.

How to cite: Gaillard, Y., Haun, P., Szabo, P., Sliavin, Y., and Egbers, C.: Numerical investigation of cell formation in a 2-dimensional differentially heated shell utilizing thermo-electrohydrodynamics, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-1687, https://doi.org/10.5194/egusphere-egu22-1687, 2022.

A series of experiments was carried out on the Thermostratified Wind Wave Tank of IAP RAS, to study the processes of secondary generation of spray due to the fall of the droplets onto a rough surface. The general scheme of the experiments was similar to [1]. The range of equivalent wind speed from U₁₀ is from 21 to 34 m/s. Initially, high-speed filming with shadow visualization of the rough surface from above was performed, followed by detection, marking and calculating the number of events during image processing using special programs. It has been demonstrated that the number of phenomena of falling drops into water per unit time per unit area, leading to the formation of spray, significantly exceeds the similar values for previously studied mechanisms of spray generation: liquid ligaments fragmentation type, bubbles rupture and bag-breakup fragmentation type. Further, detailed studies of this phenomenon were carried out with a higher resolution filming. Two main scenarios of this event: with the formation of a “crown” at large angles of drop incidence, and the so-called "jet" at small angles were identified by analogy with [2]. The number droplets, size and velocity distributions were obtained for different wind speeds. These results can be used to design a spray generation function due to this phenomenon.

Investigations were supported by Russian Science Foundation project 21-19-00755 (carrying out experiments), Russian Foundation Basic Research project 21-55-52005 (data processing), work of A.A. Kandaurov was partially supported by the President's grant for young scientists МК-5503.2021.1.5.

• Troitskaya, A. Kandaurov, O. Ermakova, D. Kozlov, D. Sergeev, and S. Zilitinkevich, The ‘Bag Breakup’ Spume Droplet Generation Mechanism at High Winds. Part I: Spray Generation Function, J. Phys. Oceanogr., vol. 48, no. 9, pp. 2167–2188, 2018.
• V. Gielen, P. Sleutel, J. Benschop, M. Riepen, V. Voronina, C. W. Visser, D. Lohse, J. H. Snoeijer, M. Versluis, andH. Gelderblom, Oblique drop impact onto a deep liquid pool, Phys. Rev. Fluids, vol. 2, pp. 083602, 2017.

How to cite: Kandaurov, A., Ermakova, O., Troitskaya, Y., and Sergeev, D.: Investigation of the mechanism of production of spray due to falling drops on the water surface in the framework of laboratory modeling of wind-wave interaction, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11007, https://doi.org/10.5194/egusphere-egu22-11007, 2022.

The number of spume droplets increases rapidly with wind speed [1], [2], so that under hurricane conditions the spray-mediated heat and momentum fluxes can have a significant impact on the exchanging processes between the ocean and the atmosphere. The estimation of the additional enthalpy flux, as well as latent and sensible heat fluxes, is based on the solution of the microphysics equations for a single saline droplet, detailed in [3]. In theoretical studies [4]-[6] it was shown that the evolution of the radius and temperature of a droplet can be described accurate enough using the following formulas:

T(t)=T_wb+(T_w-T_wb)e^-t/τ_T,

r(t)=r_eq+(r₀-r_eq)e^-t/τr,

where T_wb is the wet bulb temperature, r_eq is the equilibrium radius, τ_T and τ_r is the e-folding time to reach that temperature T_wb and radius r_eq, T(0)=T_w is the temperature of the water, r(0)=r₀ is the initial radius of the drop. However, the numerical solution of the microphysical equations of the droplet’s thermodynamics showed that for the characteristic conditions of a tropical cyclone at the initial stage evaporation occurs much more intensively than after reaching the wet bulb temperature, and the characteristic time of this change is the same as for a change in temperature. In the present study, we propose an updated parameterization of the evolution of the radius and temperature of a single saline droplet, which provides more accurate describing of the droplet’s thermodynamics. On its basis we obtained estimations of enthalpy, latent and sensible heat fluxes caused by droplets generated by bag break-up instability (the main source of spume droplets at extreme wind speeds [7]).

[1]      E. L. Andreas. A review of the sea spray generation function for the open ocean // Atmos. Interact. - 2002. - V. 1. - P. 1–46.

[2]      D. H. Richter and F. Veron. Ocean spray: An outsized influence on weather and climate // Phys. Today - 2016. - V. 69. - №11. - P. 34–39.

[3]      H. R. Pruppacher and J. D. Klett. Microphysics of clouds and precipitation, D. Reidel. Norwell: Mass., 2010.

[4]      E. L. Andreas. Time constants for the evolution of sea spray droplets // Tellus, Ser. B - 1990. - V. 42 B. - №5. - P. 481–497.

[5]      E. L. Andreas. Sea spray and the turbulent air-sea heat fluxes // J. Geophys. Res. - 1992. - V. 97. - №C7. - P. 11429–11441.

[6]      E. L. Andreas. Approximation formulas for the microphysical properties of saline droplets // Atmos. Res. - 2005. - V. 75. - №4. - P. 323–345.

[7]      Y. Troitskaya, A. Kandaurov, O. Ermakova, D. Kozlov, D. Sergeev, and S. Zilitinkevich. The “bag breakup” spume droplet generation mechanism at high winds. Part I: Spray generation function // J. Phys. Oceanogr. - 2018. - V. 48. - №9. - P. 2168–2188.

How to cite: Kozlov, D. and Troitskaya, Y.: Updated approximation formulas for the radius and temperature of saline droplets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4409, https://doi.org/10.5194/egusphere-egu22-4409, 2022.

Recently, we demonstrated that the temporal fetch-dependent wind-wave growth under abruptly applied wind forcing can be accurately described by considering a stochastic ensemble of multiple unstable harmonics (submitted to PRL). In that study, the two-phase viscous shear flow instability at the air-water interface was examined using the energy growth rates β and the group velocities c_g of the unstable harmonics obtained by solving the coupled Orr-Sommerfeld (OS) equations in air and water with appropriate boundary and initial conditions. The predictions of this unidirectional model compare well with measurements of random time-and space-dependent wave field performed in our laboratory (JFM 828, 459, 2017). The eigenvalues of the model equations determine β and c_g of each harmonic defined by its wavenumber; the suggested model then allows computation of variation with time and with fetch of the statistical wave field parameters such as the characteristic wave amplitude and the instantaneous dominant frequency. The eigenvalues of the OS system however depend strongly on the adopted mean vertical velocity profile in air and in water. The water velocity is assumed to decay exponentially with depth from the maximum value corresponding to the drift velocity at the interface. In air, we assumed the lin-log suggested by Miles that consists of a linear segment in the viscous sublayer connected smoothly to a logarithmic turbulent velocity profile over smooth water surface. The assumption of smooth water surface is reasonable at the onset of wind. However, emerging wind-waves render the surface rough; the surface roughness becomes more pronounced at higher wind forcing and larger fetches. In the present study, we extend our previous study and apply the developed OS solver to investigate the dependence of the viscous shear-flow stability on the shape of air velocity profile. We take advantage of the detailed wind-velocity profiles measured in our facility at various wind velocities and a number of fetches (JGR 117, C00J19, 2012)that demonstrated the significant deviations of the actual air velocity profiles over waves from the shape corresponding to smooth-surface. The surface drift velocities under different operational conditions were also measured. The effect of the evolving wind-wave field on eigenvalues of the OS system of equation and thus on the domains of instability, the energy growth rates β and the group velocities c_g is studied. These results extend our understanding of the interrelation between the varying in time and space wind-wave field and the turbulent airflow above the water surface and shed light on momentum and energy exchange between air and water.

How to cite: Geva, M. and Shemer, L.: Viscous shear instability at air-water interface as a function of wind velocity profile, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6007, https://doi.org/10.5194/egusphere-egu22-6007, 2022.

Mixing in the uppermost part of the water column is crucial for modelling air-sea interaction, yet it remains poorly understood, especially the processes under strong wind conditions. The Ekman boundary layers are a salient feature of the air-sea interface. In the the Ekman boundary layers the current velocity vectors always rotates, making two components of the basic flow vorticity comparable and, thus, the boundary layer three dimensional. Linear instabilities of the homogeneous steady Ekman layers were examined and  found to occur for sufficiently large turbulent Reynolds numbers.  Here, we derive a model of  nonlinear instabilities of 3d Ekman layer  in deep ocean taking into account also a possible weak stratification of the boundary layer caused by air entrainment due to wave breaking or solar heating. The model exploits the observation that the corresponding linearized boundary value problem  always supports a “vorticity wave” mode which is often decaying. Employing an asymptotic procedure utilizing smallness of the boundary layer thickness to the characteristic wavelength of perturbations  scaled as  inverse Reynolds number squared we derive a novel nonlinear evolution equation with a pseudo-differential dispersion.  We take into  account viscosity and weak stratification  in the boundary layer. Within the framework of this equation a  wide class of initial conditions, which we a priori specify, leads to `collapses’ of localized perturbations, that is an initial perturbation becomes more and more localised and its amplitude becomes infinite in finite time forming a point singularity. We derived a self-similar solution describing these collapses. The mechanism of collapse is essentially nonlinear. A new insight into linear instabilities has been also  obtained.  The collapses are expected to result in intense mixing and even temporary destruction of the boundary layer.

How to cite: Shrira, V. and Oloo, J.: : Collapses in the oceanic Ekman boundary layer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2071, https://doi.org/10.5194/egusphere-egu22-2071, 2022.

The interaction of small-scale turbulence with internal and surface waves is an urgent problem of hydrology and oceanology. In particular, this issue is especially important for the properties of the upper layer of the ocean and the inland waters.

Small-scale processes that exist against the background of average profiles of various hydrophysical quantities (temperature, velocity, density, and large-scale currents caused, in particular, by wind forcing) are usually nonlinear and therefore effectively interact with each other. We consider some aspects of the interaction of internal waves and turbulence in the upper layer of the ocean and inland waters within the framework of the semi-empirical theory of turbulence in a stratified fluid. The model used in this study takes into account  mutual transformation of the kinetic and potential energies of turbulent fluctuations [Ostrovsky&Troitskaya, 1987; Zilitinkevich et al., 2013]. The effects of amplification and maintenance of turbulence by low-frequency and high-frequency internal waves, quasi-stationary distributions of turbulent energy in the presence of a shear caused by a low-frequency internal wave are investigated; the role of the transformation of energies on the indicated processes is analyzed.

A modification of the k-epsilon mixing scheme is also proposed, which removes the limitation on the existence of turbulence at large values of the gradient Richardson number. Within the framework of the modification, the parameterization of the Prandtl number is used, which makes it possible to take into account the influence of density stratification and velocity shear on mixing processes.

A numerical study of the influence of vertical mixing schemes on the transfer processes of biochemical fields in an internal reservoir was also carried out. The modified scheme was implemented into a three-dimensional model of thermo-hydrodynamics and biochemistry of an inland water body [Gladskikh et al., 2021], and a series of numerical experiments was conducted.

The work was supported by the RFBR (20-05-00776; 20-05-00322; 21-05-52005), and by Moscow Center of Fundamental and Applied Mathematics (agreement with the Ministry of Science and Higher Education 075-15-2019-1621).

Ostrovsky LA, Troitskaya YuI (1987) A model of turbulent transfer and dynamics of turbulence in a stratified shear flow. Izv Akad Nauk SSSR, Fiz Atmos Okeana. 3:101–104.
Zilitinkevich SS, Elperin T, Kleeorin N, Rogachevskii I, Esau I (2013) A hierarchy of Energyand Flux-Budget (EFB) turbulence closure models for stably-stratified geophysical flow. Boundary-Layer Meteorol. 146:341–373
Gladskikh DS, Stepanenko VM, Mortikov EV (2021) The Effect of the Horizontal Dimensions of Inland Water Bodies on the Thickness of the Upper Mixed Layer. Water Resour 48:226–234

How to cite: Soustova, I., Ostrovsky, L., Troitskaya, Y., Gladskikh, D., and Mortikov, E.: On the interaction of small-scale turbulence and internal waves in the framework of the semi-empirical turbulence model in a stratified fluid, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4807, https://doi.org/10.5194/egusphere-egu22-4807, 2022.

The high-speed, wind-aligned streaks on the wind waves are geometrically similar to the low-speed streaks observed in the turbulent wall layer. It is generally accepted that the spanwise spacing between the low-speed streaks in wall-bounded turbulent flow, when scaled by the viscous length, exhibit probability distribution conforming to lognormal behavior with a universal mean value of 100 independent on the wall friction velocity. Analyses of thermal images from wind-wave flume experiments, however, reveal that the scaling between the mean streak spacing and the surface friction velocity is different from that of wall-bounded flow. For non-breaking waves, the scaled mean streak spacing becomes notably narrower than that between low-speed streaks next to the solid wall. Comparative numerical simulations reveal that the presence of surface waves intensifies the generation of quasi-streamwise vortices that form the elongated streaks, and reduces the streak spacings. For breaking wind waves, analyses of the consecutive image sequences reveal that the breakers wipe out the existing surface streaks. After the passage of the breakers, the wind-aligned streaks reform immediately, which are then destructed again by the next breaking waves. In contrast to the streaks on the non-breaking waves, the scaled mean streak spacing in the wake of breakers is close to the canonical value of 100, which approximately follows the wall-flow scaling.

How to cite: Tsai, W. and Lu, G.: Scaling of spacing between surface streaming on non-breaking and breaking wind waves, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10841, https://doi.org/10.5194/egusphere-egu22-10841, 2022.

The dynamics of a drift flow in the near-surface water layer driven by a turbulent air wind is investigated by direct numerical simulation (DNS). Comparatively low (up to 2×10⁴) bulk Reynolds numbers of the air-flow are considered when the air boundary layer is turbulent but velocity fluctuations in the water are sufficiently small and the water surface remains aerodynamically smooth. It is shown that a drift flow develops in the near-surface water layer, and its velocity grows monotonically with time. At long times there develops an instability which leads to a saturation of the growth of the drift-velocity. A threshold Reynolds number is defined in DNS under which the drift flow becomes unstable, and a parameterization of the surface drift velocity is formulated in terms of the air-flow friction velocity.

How to cite: Druzhinin, O.: On the dynamics of a drift flow under low wind, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6245, https://doi.org/10.5194/egusphere-egu22-6245, 2022.