Displays

Spray produced in Tropical Cyclones affects the dynamic and heat fluxes between the atmosphere and ocean, and thus can influence the Cyclone intensity in a number of ways. Measurements of the Sea Spray Generation Function (SSGF) in situ, however, are extremely challenging and correspondingly rare, and uncertainties in quantifying SSGF reach 1000 times.

In the presentation, measurements of the total volume of spray by means of a laser array in Tropical Cyclones Olwyn (2015) and Veronica (2019) in the Indian Ocean will be reported. They are used to develop a parameterisation of SSGF at wind speeds ranging from light to extreme. It is argued that the spray is produced by wind-over-the-waves, and therefore wave properties are also accounted for in the parameterisation.

How to cite: Babanin, A., Ma, H., Xu, X., and Qiao, F.: Filed observations of spray production function during Tropical Cyclones Olwyn and Veronica, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7481, https://doi.org/10.5194/egusphere-egu2020-7481, 2020.

Despite the powerful influence that sea spray has on air-sea enthalpy and momentum fluxes, most state-of-the-art tropical cyclone forecast models do not incorporate the microphysics of sea spray evaporation into their boundary layer flux schemes. Since the air-sea enthalpy and momentum fluxes control a tropical cyclone’s intensification rate, increasing the accuracy of the associated bulk parameterizations is crucially important for improving forecast skill. New microphysics-based bulk parameterizations for enthalpy and momentum flux through the tropical cyclone boundary layer are developed from a set of prognostic evaporation equations and numerical simulations of evaporating, multiphase flow subject to extreme wind speeds. The microphysics-based parameterizations are computationally inexpensive and are functions of the local environmental conditions; these features allow forecast models to efficiently vary the air-sea enthalpy and momentum fluxes in space and time. By developing microphysics-based bulk parameterizations, the influence that sea spray exerts on tropical cyclone intensification can be more accurately simulated and intensity forecasts could be improved.

How to cite: Sroka, S. and Emanuel, K.: Microphysics-Based Bulk Parameterizations of Enthalpy and Momentum Fluxes for Tropical Cyclones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4383, https://doi.org/10.5194/egusphere-egu2020-4383, 2020.

The objective of the present study is to investigate sensible and latent heat transfer mediated by evaporating saline droplets in a turbulent air flow over a waved water surface by performing direct numerical simulation. Equations of the air-flow velocity, temperature and humidity are solved simultaneously with the two-way-coupled equations of individual droplets coordinates and velocities, temperatures and masses. Two different cases of air and water surface temperatures,T_a = 27 ⁰C, T_s = 28 ⁰C,  and T_a = -10 ⁰C, T_s = 0 ⁰C, are considered and conditionally termed as "tropical cyclone" (TC) and "polar low"  (PL) conditions, respectively. Droplets-mediated sensible and latent heat fluxes, Q_S and Q_L, are integrated along individual droplets Lagrangian trajectories and evaluated as distributions over droplet diameter at injection, d, and also obtained as Eulerian, ensemble-averaged fields. The results show that under TC-conditions, the sensible heat flux from droplets to air is negative whereas the latent heat flux is positive, and thus droplets cool and moisturize the carrier air. On the other hand, under PL-conditions, Q_S and Q_L  are both positive, and Q_L – contribution is significantly reduced as compared to Q_S - contribution. Thus in this case, droplets warm up the air. In both cases, the droplet-mediated enthalpy flux, Q_S + Q_L , is positive, vanishes for sufficiently small droplets (with diameters d ≤ 150 μm) and further increases with d. The results also show that the net fluxes are reduced with increasing wave slope.

This work is supported by the Ministry of Education and Science of the Russian Federation (Task No. 0030-2019-0020). Numerical algorithms were developed under the support of RFBR (Nos. 18-05-60299, 18-55-50005, 18-05-00265, 20-05-00322). Postprocessing was performed under the support of the Russian Science Foundation (No. 19-17-00209).

How to cite: Druzhinin, O.: Investigation of droplet-mediated sensible and latent heat fluxes in a turbulent air flow over a waved water surface by direct numerical simulation , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7249, https://doi.org/10.5194/egusphere-egu2020-7249, 2020.

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.

More than 200 Sentinel-1 acquisitions over Tropical Cyclones (TC) eyes have been accumulated since 2016 thanks to the SHOC scheme (Satellite Hurricane Observation Campaign) operated in collaboration with ESA ground segment. These high-resolution observations have shown the great potential offered by S1 constellation in dual-polarization to monitor TC along their lifetime and to provide numerous observable parameters such as maximum sustained wind speed (up to 80 m/s) and TC structure (e.g. wind radii, eye geometry and position). Co-locations with the Stepped Frequency Microwave Radiometer (SFMR) confirm that, even for extreme cases, S1-derived ocean wind speeds are found in agreement and able to provide consistent measurements in the eyewall. Similarly, co-locations between SMOS and S1-wind degraded at a similar medium resolution are in good agreement. Also, Hurricane experts listed in their recommendations at the 40^th WMO Hurricane Committee for USA/Caribbean region that “Special acquisitions plans during Irma, Jose and Maria having demonstrated the high value of kilometric-scale information provided by Sentinel-1 SAR data, HC40 recommends that these data are made available to help monitor critical aspects of the TC structure”.

Based on this demonstration, a new ESA-funded project called CYMS (CYclone Monitoring service with S-1) starts in February 2020, with the objective of scaling up the SHOC initiative for its potential integration as part of a Copernicus Service. One objective is the operational delivery of tailored S1-derived TC observations to tropical cyclone forecasters of all tropical cyclone Regional Specialized Meteorological Centres (RSMCs) and Tropical Cyclone Warning Centres (TCWCs). Besides, S1 TC observations will contribute to a new database for science applications.

In order to continuously keep improving the S1-derived TC observations, current limitations in the wind field retrieval are recalled and perspectives to overcome them are proposed. First, the presence of rain signatures over SAR images requires a fine pre-processing filtering of these non-wind related features in order not to interpret them as wind speed. Second, the current inversion using the co- and cross-polarized NRCS channels via a noise-dependent mixing can show some limitations for wind speed around 30m/s. Alternative schemes are proposed to mitigate this issue. Third, S1 wind directions are mostly influenced by the co-located atmospheric model which can show some significant shifts with respect to the actual situation. Pre-processing methods based on the exploitation of wind rolls signatures, ubiquitous under intense TC, are presented to improve the wind direction retrieval. Finally, improvements of the current cross-polarized Geophysical Model Function (GMF), MS1A, are proposed taking advantage of a more complete dataset of S1 TC observations since its first estimation in 2017.

Overall, the current and future developments for S1 wind field retrieval aim at integrating all valuable observations as inputs. Additional candidate parameters of interest are the Doppler Centroïd anomaly, which is related to the radial wind, and the Co-Cross Polarization Coherence (CCPC), which is related to both the wind speed and direction.

How to cite: Husson, R., Mouche, A., Longepe, N., Berger, H., Archer, O., and Soulat, F.: TC observations from Synthetic Aperture Radar: short term perspectives, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17958, https://doi.org/10.5194/egusphere-egu2020-17958, 2020.

A particularly pressing requirement in the Ocean Surface Vector Wind (OSVW) community is to obtain reliable extreme winds in hurricanes (> 30 m/s) from wind scatterometers, since extreme weather classification, surge and wave forecasts for societal warning are a high priority in nowcasting and in numerical weather prediction (NWP). A main goal of the EUMETSAT C-band High and Extreme-Force Speeds (CHEFS) study is therefore to consolidate an in-situ wind reference for assessing scatterometer high and extreme-force wind capabilities.

Scatterometers have proven to have very good performances when retrieving low to moderate winds. However, measuring high and extreme winds is still challenging as vicarious calibration is needed and calibrated in situ reference winds are scarce.

Moored buoy data are usually used as absolute reference to calibrate the scatterometer Geophysical Model Functions (GMF), however, for very high and extreme winds above 25 m/s, moored buoys may not be reliable. Moreover, controversy exists in the OSVW satellite community on the quality of moored buoys above 15 m/s rather than 25 m/s. Hence, the quality of buoy winds between 15 m/s and 25 m/s is thoroughly evaluated. The buoy wind performance, estimated with triple collocation analyses of buoy, ASCAT and ERA5 winds, shows that the quality of buoy wind vectors up to 25 m/s is within 2 m/s, indicating that buoy winds can indeed be used for wind scatterometer GMF calibration in the mentioned wind range.

The NOAA hurricane hunters fly into hurricanes to drop sondes, and thus obtain wind profiles in the lowest few kilometers of hurricanes, and operate dedicated microwave instrumentation on aircraft to obtain detailed wind patterns in hurricanes, such as the Stepped-Frequency Microwave Radiometer (SFMR). Ideally, local dropsonde winds may be statistically used to calibrate SFMR as they have similar spatial representation (“footprint”). SFMR, in turn, after spatial aggregation to scatterometer footprints, may be used to calibrate satellite scatterometers and radiometers in overflights.

The so-called WL150 algorithm is operationally used to estimate 10-m surface winds from dropsonde wind profiles. The measured radiosonde 10-m winds are a more direct calibration resource for the 10-m surface wind than WL150 estimates. However, an improved assessment of the position processing of the sonde near the surface, where its deceleration is maximum, is needed.

The air mass density needs to be considered to calibrate scatterometer winds in hurricanes, as these mainly occur at low pressures and hence low air mass density, i.e., so-called stress-equivalent winds should be used for comparison.

Finally, ASCAT winds show sensitivity to high winds, but lack good GMF calibration due to the lack of a consolidated in-situ wind reference. The saturation of the GMF at extreme winds is somehow compensated by the high calibration stability of the ASCAT instrument. As a result, further backscatter calibration refinements will support the retrieval of good-quality ASCAT winds in extreme conditions. In addition, GMF development and wind retrieval studies will be useful to improve high and extreme winds, in particular after a consolidated in-situ wind reference has been established.

How to cite: Stoffelen, A., Mouche, A., Polverari, F., van Zadelhoff, G.-J., Sapp, J., Portabella, M., Chang, P., Lin, W., and Jelenek, Z.: An In-situ Reference for High and Extreme Winds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19174, https://doi.org/10.5194/egusphere-egu2020-19174, 2020.

Dam-break flows are not only an important practical problem in civil and hydraulic engineering, but also a fundamental problem of fluid mechanics. Due to property damage and the loss of numerous lives, it is critically important to have an exhaustive understanding of the landslide dam-break flow and sedimentation. The main objective of this study is a detailed analysis of the mechanisms of dam breaking flows through physical and theoretical modeling.     

       Our experimental work was focused on the initial stages of dam-break flow in the water channel, where a thin plate separating water at different levels is impulsively withdrawn in the vertical direction upwards, and as a result, a hydrodynamic bore is formed.

       The theoretical model of the dam-break flow is based on Benney’s shallow water equations. We separately studied the regimes of a breaking and non-breaking bore front. On the hydrodynamic bore, the laws of conservation of mass, momentum and energy were required to be fulfilled,contrary to the classical solutions of Ritter and Stoker, in which the law of energy was not considered at all.

      The non-breaking flow includes several zones: a shock wave and a shear vortex flow after it, a contact surface and a continuous discharge zone. The bore in our solution moves faster than the classical bore, which, in turn, propagates faster than the contact surface.

      The breaking bore is characterised by the generation of a “mushroom jet” structure, including a pair of vortexes, oppositely directed, and a forerunner formed by the plunging jet directed forward. We found that the forerunner of the breaking bore has a speed significantly higher than the speed of the bore.

       The experiments carried out in the wave flume of the Tainan Hydraulics Laboratory confirmed the theoretical predictions of the proposed dam breaking flow model for various initial conditions.

How to cite: Shugan, I., Chen, Y.-Y., and Hsu, C.-J.: The flood bore problem and the mushroom jet formation in the dam-break flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7515, https://doi.org/10.5194/egusphere-egu2020-7515, 2020.

Results of measurements of the drift currents induced by waves and wind at the wavy water surface are presented. The measurements were executed by means of surface floats in a large tank with the dimensions of 32.5x1x2 m³. Three cases were studied: (i) regular (narrow-band) mechanical waves; (ii) irregular (wide-band) mechanical waves; and (iii) wind waves.

The measured surface-drift currents induced by mechanical waves, U_d, are compared with the Stokes drift at the surface, U_St, estimated by the well-known formula with the integral over a wave spectrum. In this case, it was found that ratio U_d / U_St is varying in the range 0.5 – 0.93 and slightly growing with the decrease of wave steepness, having no visible dependence on the breaking intensity. These estimations are used to separate the wind-induced drift current, U_dw, from the total drift at the presence of wind.

In the case of wind waves, the wind-induced part of the surface drift, U_dw, is compared with the friction velocity, u_*. In our measurements, the ratio U_dw / u_* varies systematically in the range 0.65 – 1.2. Taking into account the percentage of wave breaking, Br, the wave age, A, and the wave steepness, Ϭ = ak_p, it was found the parameterization:  U_dw = (Br + Ϭ A) u_*, which corresponds to the observations with the mean error less than 10%. For the first time, this ratio provides the dependence of the surface wind drift on the surface wave parameters.

How to cite: Polnikov, V. and Ma, H.: Empirical Parameterization of the Wind-induced Drift Currents , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5400, https://doi.org/10.5194/egusphere-egu2020-5400, 2020.

Oceanic and Atmospheric jets with sufficiently strong anticyclonic vorticity are subject to centrifugal instabilities. This mechanism is relatively fast in comparison to barotropic and baroclinic instabilities and require non-conservative forces that mix the fluid properties. In this work, we present a novel approach to compute the linear stability characteristics of both barotropic and baroclinic jets. This enables us to compute the growth rates and spatial structures very accurately and efficiently. Subsequently, by integrating the fully nonlinear, non-hydrostatic dynamics using the spectrally accurate numerical model SPINS, we validate the predictions of the linear theory and then investigate the nonlinear equilibration that results. Depending on the Reynolds number of the flows, there are instances where a secondary instability occurs that eventually produces vortical structures, some of which are themselves subject to centrifugal instabilities. This idealized investigation quantifies the effects of centrifugal instabilities as an initial step to determine how to parameterize them.

How to cite: Poulin, F., Harris, M., and Lamb, K.: Centrifugal Instability of a Geostrophic Jet, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3610, https://doi.org/10.5194/egusphere-egu2020-3610, 2020.

Over the last decade, the concept of topological wave has spread over all fields of physics. These ideas were initially developed in condensed matter to describe peculiar electronic transport properties in exotic materials; it has now become clear that topological tools apply as well to classical systems, and thus to geophysical fluid dynamics.  Topology predicts the emergence of unidirectional modes trapped along interfaces or boundaries, depending on broken discrete symmetries, and on the twisting of bulk eigenmodes. It guarantees the robustness  of undirectional trapped modes against disorder, such as random topography or small scale turbulence. We will explain how to compute such topological features, discuss possible experimental realizations, and present three recent applications to geophysical flows :

1. The emergence of equatorially trapped topological modes in Laplace tidal equations as a consequence of  Coriolis force breaking time-reversal symmetry [1,2].
2. The  emergence of Lamb-like waves that connect acoustic wave bands to internal gravity waves bands in compressible-stratified fluids, as a consequence of gravity breaking miror symmetry, with potential applications in helioseismology [3] .
3. A new manifestation of these topological features in geophysical ray tracing : when computing first order corrections to ray tracing, we find that rays  or wave packets are deflected by an effective field corresponding to the so-called Berry curvature. To our knowledge, the effect of this Berry curvature had up to now been overlooked in geophysical context [4].

[1] P. Delplace, J.B. Marston, A. Venaille, Topological origin of equatorial waves, Science 2017 
[2] C. Tauber, P. Delplace, A. Venaille, A bulk-interface correspondence for equatorial waves, Journal of Fluid Mechanics 2019
[3] M. Perrot, P. Delplace, A. Venaille, Topological transition in stratified fluids, Nature Physics  2019
[4] N. Perez, P. Delplace, A. Venaille Manifestation of Berry curvature in geophysical ray tracing, in prep. 2020

How to cite: Venaille, A.: Topological Waves in Astrophysical and Geophysical Flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3311, https://doi.org/10.5194/egusphere-egu2020-3311, 2020.

Motions on planetary spatial scales in the atmosphere are governed by
the planetary geostrophic equations. However, not much attention has
been paid to the interaction between the baroclinic and barotropic
flow within the planetary geostrophic scaling. This is the focus of
the present study by utilizing planetary geostrophic equations for a
Boussinesq fluid supplemented by an asymptotically derived evolution
equation for the barotropic flow. The latter is effected by meridional
momentum flux due to baroclinic flow and drag by the surface wind. The
barotropic wind on the other hand affects the baroclinic flow through
buoyancy advection. By relaxing towards a prescribed buoyancy profile
the model produces realistic major features of the zonally symmetric
wind and temperature fields. We show that there is considerable
cancelation between the barotropic and the baroclinic surface zonal
mean zonal wind. The linear and nonlinear model response to steady
diabatic zonally asymmetric forcing is investigated. The arising
stationary waves are interpreted in terms of analytical solutions. We
also study the problem of baroclinic instability on the sphere within
the present model.

Reference: Dolaptchiev, S. I., Achatz, U. and Th. Reitz, 2019: Planetary
geostrophic Boussinesq dynamics: barotropic flow, baroclinic
instability and forced stationary waves, Quart. J. Roy. Met. Soc., 145: 3751-3765.

How to cite: Dolaptchiev, S., Achatz, U., and Reitz, T.: Planetary geostrophic Boussinesq dynamics: barotropic flow, baroclinic instability and forced stationary waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9366, https://doi.org/10.5194/egusphere-egu2020-9366, 2020.

The differentially heated rotating annulus is a widely studied experimental set-up designed to model mid-latitude circulation in the atmosphere and the ocean. By installing an insulating "meridional" barrier in this cylindrical tank, one can construct a minimal model of the large-scale flow phenomena in the Southern Ocean with a closed Drake Passage, imitating the situation before the Eocene-Oligocene transition (EOT) ca. 34 million years ago. We find that in this "closed" case a persistent azimuthal temperature gradient emerges whose magnitude scales linearly with the "meridional" temperature contrast. Furthermore, seemingly contradicting paleoclimatic data, the presence of the barrier appears to yield lower values of "sea surface temperature" in the tank than those in the "opened" control experiments (whereas the actual opening of the passage coincides with a major cooling event). This difference points to the importance of the role ice-albedo feedback plays in an EOT-like transition, an aspect that is not captured in the laboratory setting. This idea appears to be confirmed by numerical simulations conducted in a medium complexity GCM, where the comparison of "closed" on "opened" configurations could be made both with and without sea ice feedback. These runs indeed yielded opposite effects on sea-surface temperature and are therefore consistent with both the laboratory experiment and the paleoclimate data. This finding may well be relevant for the better understanding of the actual EOT dynamics.

How to cite: Vincze, M., Bozóki, T., Herein, M., Borcia, I. D., Rodda, C., Pálfy, J., Nyerges, A., and Harlander, U.: Climate impact of the Drake Passage opening: lessons from a minimalistic laboratory experiment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5004, https://doi.org/10.5194/egusphere-egu2020-5004, 2020.

We introduce a reduced-order model of the underlying dynamics of zonal jets in the Southern Ocean. The model is based on multi-scale decomposition in the vorticity equation and explains how large-scale forcing breaks down into mesoscale eddies and alternating zonal jets. In this reduced-order model, we average the vorticity equation both in time and in the zonal direction and utilize eddy viscosity parametrization for turbulence closure. For verification, we compare our results with two high-fidelity models: i) the quasi-geostrophic model of a shear-driven periodic channel flow and ii) primitive equation HYCOM (HYbrid Coordinate Ocean Model) simulations of the Southern Ocean.

How to cite: Naghibi, E., Karabasov, S., and Kamenkovich, I.: A reduced-order model of the zonal jets problem in the Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20799, https://doi.org/10.5194/egusphere-egu2020-20799, 2020.

Rossby radius and Rhines scale are two popular scaling arguments for eddy length scale. They have not been tested in a well-controlled experiment with increased vertical stratification and unchanged jet. This is done using the linear response function of an idealized dry atmosphere calculated by Hassanzadeh and Kuang (2016). The resulting change in zonal wind is mostly less than 0.2m/s when temperature near surface is cooled by more than 2K. In such experiment, energy-containing zonal scale decreases, which is against the prediction of Rossby radius but consistent with the prediction of Rhines scale. Eddy kinetic energy decreases for all wavenumbers and latitudes, but eddy momentum flux strengthens locally around zonal wavenumber 8 and 40°S. This local strengthening is associated with a stronger Pearson correlation between u and v.

How to cite: Chan, P.-W., Hassanzadeh, P., and Kuang, Z.: Macroturbulence Response to Vertical Stratification Change Using Linear Response Function of an Idealized Dry Atmosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21191, https://doi.org/10.5194/egusphere-egu2020-21191, 2020.

Quasi-streamwise vortices within aqueous shear layer beneath wind waves are found to contribute significantly to the scalar transfer across the air-water interface. These streamwise vortices manifest themselves by inducing distinct elongated high-speed streaks on the interface. The density of these streaks, which can be quantified by the transverse spacing of streaks, thus characterizes the interfacial scalar transfer contributed by the quasi-streamwise vortices. Thermal imageries of laboratory wind waves and flow fields obtained from numerical simulations of turbulent shear flows bounded by stress-driven flat boundary and wavy surface are utilized to study the characteristics of streak spacings and their dependence on wind speed. Consistent with previous studies, analyses of the thermal imageries of laboratory wind waves confirm that the streak spacings conform closely to a lognormal distribution, and the mean streak spacing d decreases as the wind speed increases. Revisiting the nondimensional mean spacing scaled by the viscous length, d⁺=du^*/ν, where u^* is the shear velocity of water and ν is the kinematic viscosity of water, however, reveals the different interpretations from the previous studies. For low to immediate wind-speed range,u^*< 0.5 cm/s, the nondimensional mean spacing does not follow the scaling of d⁺≈ 100 observed in the turbulent wall layer; the scaled mean spacing d⁺< 100. This is also observed in numerical simulation of turbulent shear flow bounded by a stress-driven flat surface. For immediate wind-speed range, 0.5 cm/s < u^*< 1.2 cm/s, within which surface waves become significant, the nondimensional mean streak spacings derived from the thermal imageries of wind waves remain to be less than the universal value of 100. The scaled mean streak spacing of simulated turbulent shear layer next to a stress-driven plane boundary, however, increases with the wind speed and approaches the value of 100 at this immediate wind-speed range. Imposing surface waves on the simulated turbulent shear flow significantly reduces the nondimensional streak spacing as observed on the wind-wave surfaces. Such reduction of streak spacings in finite-amplitude wind waves can be attributed to the additional wave stress arising in the oscillatory boundary layer, and the turning and stretching of turbulent vortices by the Lagrangian drift of progressive waves. At high wind speeds, u^* > 1.2 cm/s, despite the occurrence of wave breaking, the scaled mean spacing approaches the universal value of 100 observed in the turbulent wall layer. This work was supported by the Taiwan Ministry of Science and Technology (MOST 107-2611-M-002 -014 -MY3).

How to cite: Tsai, W. and Lu, G.: The spacing of streaks in wind waves from low to high wind, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4709, https://doi.org/10.5194/egusphere-egu2020-4709, 2020.

The water surface under high wind condition is characterized by elongated high-speed streaks and randomly emerged low-speed streaks, which are attributed to underneath coherent vortical motions. These vortical structures within aqueous turbulent boundary layer plays a critical role in turbulent exchange, their characteristics and statistics are therefore of interest in this study. Direct numerical simulation of an aqueous turbulent flow bounded by a stress-driven flat free surface was performed. Simulation results of cases with high wind condition (surface friction velocity = 1.22 cm/s) as well as weak wind condition (surface friction velocity = 0.71 cm/s) are analyzed. To identify the underlying vortical structures, an indicator of swirling strength derived from local velocity-gradient tensor is adopted. A formal classification scheme, based on the topological geometry of the vortex core, is then applied to classify the identified structures. Surface layers with the two wind conditions reveal similar results in statistics and spatial distribution of vortical structures. Two types of characteristic vortices which induce the surface streaks are extracted, including quasi-streamwise vortex and reversed horseshoe vortex (head pointing upstream), most inclining at about 10 to 20 degrees. Quasi-streamwise vortices are the dominant structure, and both high- and low-speed streaks are fringed with such vortices; they adjoin the surface streaks as counter-rotating arrays in either staggered or side-by-side spatial arrangement. The length of quasi-streamwise vortices, however, are significantly shorter than the corresponding surface streaks, only 10% of the extracted quasi-streamwise vortices are longer than 150 wall units. Reversed horseshoe vortices, associated with downwelling motions and surface convergence, are located beneath the high-speed streaks. In contrast to the turbulent boundary layer next to a flat wall, typical forward horseshoe vortices (head pointing downstream) associated with upwelling motions are barely found within the free-surface turbulent shear flow.

This work was supported by the Taiwan Ministry of Science and Technology (MOST 107-2611-M-002 -014 -MY3).

How to cite: Chen, P.-C. and Tsai, W.: Spatial and statistical analysis of coherent vortical structures within a stress-driven free-surface turbulent shear flow, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6571, https://doi.org/10.5194/egusphere-egu2020-6571, 2020.

The recent experimental study [1], [2] identify ‘‘bag breakup’’ fragmentation as the dominant mechanism by which spume droplets are generated at hurricane wind speeds. These droplets can significantly affect the exchanging processes in the air-ocean boundary layer. In order to estimate spray-mediated heat, momentum and mass fluxes we need not only reliable experimental data, but a theoretical model of this process. The “bag-breakup” fragmentation is a strongly non-linear process, and we focus only on its first stage which includes the small-scale elevation of the water surface.

Our model of the bag’s initiation is based on a weak nonlinear interaction of a longitudinal surface wave and two oblique waves propagating at equal and opposite angles to the flow as it was done in [3], [4]. All of these waves have the same critical layer where cross velocities of oblique waves become infinite making inviscid analysis invalid. So we took into account viscous effects. As a result, it has been established that for a piecewise continuous velocity profile explosive growth of wave amplitudes is possible at the wind speeds exceeding the critical one.

The present model let us find the dependency of “bag’s” transverse size on the wind speed and estimate its lifetime.

 Acknowledgements

This work was supported by the RSF project 19-17-00209 and the RFBR projects 19-05-00249, 19-35-90053, 18-05-00265.

References:

How to cite: Kozlov, D. and Troitskaya, Y.: Non-linear resonant instability of short surface waves as the first stage “bag-breakup” process at the air-sea interface at high winds , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7591, https://doi.org/10.5194/egusphere-egu2020-7591, 2020.

Wind-wave interaction at extreme wind speed is of special interest now in connection with the problem of explanation of the sea surface drag saturation at the wind speed exceeding 30 m/s. Now it is established that at hurricane wind speed the sea surface drag coefficient is significantly reduced in comparison with the parameterization obtained at moderate to strong wind conditions.

The subject of this work is investigation of aerodynamic resistance of the waved water surface under severe wind conditions (up to U10 ≈ 50 m/s). Laboratory experiments were carried out at the new high-speed wind-wave flume in the Large Thermally Stratified Tank (at the Institute of Applied Physics, Russia) built in 2019. The main difference between the new wind-wave flume and the old one is the absence of a pressure gradient along the main axis of the new flume. Aerodynamic resistance of the water surface was measured by the profile method with Pitot tube. A method for data processing taking into account the self-similarity of the air flow velocity profile in the aerodynamic tube was applied for retrieving wind friction velocity and surface drag coefficients. Simultaneously with the airflow velocity measurements, the wind-wave field parameters in the flume were investigated by system of wire gauges.

Analysis of the wind velocity profiles and wind-wave spectra showed tendency to decrease for surface drag coefficient for wind speed exceeding 25 m/s simultaneously with the mean square slope and significant wave height.

Acknowledgments
This work was carried out with the financial support of the RFBR according to the research project 18-55-50005, 20-05-00322, 18-35-20068, 18-05-00265. Data processing was carried out with the financial support of Russian Science Foundation grant 19-17-00209.

How to cite: Vdovin, M., Baydakov, G., Sergeev, D., and Troitskaya, Y.: Laboratory investigation of air-sea momentum transfer under severe wind conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8589, https://doi.org/10.5194/egusphere-egu2020-8589, 2020.

Many features of nonlinear water wave dynamics can be explained within the assumption that the motion of fluid is strictly potential. At the same time, numerically solving exact equations of motion for a three-dimensional potential flow with a free surface (by means of, for example, boundary integral method) is still often considered too computationally expensive, and further simplifications are made, usually implying limitations on wave steepness. A quasi-three-dimensional model, put forward by V. P. Ruban [1], represents another approach at reducing computational cost. It is, in its essence, a two-dimensional model, formulated using conformal mapping of the flow domain, augmented by three-dimensional corrections. The model assumes narrow directional distribution of the wave field and is exact for two-dimensional waves. It was successfully applied by its author to study a nonlinear stage of of Benjamin-Feir instability and rogue waves formation.

The main aim of the present work is to explore the behaviour of the quasi-three-dimensional model outside the formal limits of its applicability. From the practical point of view, it is important that the model operates robustly even in the presence of waves propagating at large angles to the main direction (although we do not attempt to accurately describe their dynamics). We investigate linear stability of Stokes waves to three-dimensional perturbations and suggest a modification to the original model to eliminate a spurious zone of instability in the vicinity of the perpendicular direction on the perturbation wavenumber plane. We show that the quasi-three-dimensional model yields a qualitatively correct description of the instability zone generated by resonant 5-wave interactions. The values of the increment are reasonably close to those obtained from the exact equations of motion [2], despite the fact that the corresponding modes of instability consist of harmonics that are relatively far from the main direction. Resonant 5-wave interactions are known to manifest themselves in the formation of the so-called “horse-shoe” or “crescent-shaped” wave patterns, and the quasi-three-dimensional model exhibits a plausible dynamics leading to formation of crescent-shaped waves.

This research was supported by RFBR (grant No. 20-05-00322).

[1] Ruban, V. P. (2010). Conformal variables in the numerical simulations of long-crested rogue waves. The European Physical Journal Special Topics, 185(1), 17-33.

[2] McLean, J. W. (1982). Instabilities of finite-amplitude water waves. Journal of Fluid Mechanics, 114, 315-330.

How to cite: Dosaev, A. and Troitskaya, Y.: Quasi-three-dimensional simulation of crescent-shaped waves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8767, https://doi.org/10.5194/egusphere-egu2020-8767, 2020.

This study shows the comparison between the sea-breeze circulation (SBC) day and normal day turbulent characteristics and the Reynolds stress anisotropy in the lower atmospheric region. The Reynolds stress tensor is responsible for the dissipation and transport of mean kinetic energy. The variability of the turbulent kinetic energy due to the Reynolds stress anisotropy modulates the air quality. A 20 Hz Ultrasonic anemometer was deployed in the coastal area of northern France to measure the temporal wind variability for the duration of one year five months.

The SBC was detected by a change in wind direction from the West to the East during the day time. We found that the axial component of the turbulent kinetic energy is higher than the other two through an axisymmetric expansion, and energy ellipsoid has a cigar shape due to SBC. During this time the dominance of small scale zonal turbulent motions was observed. Also, the probability of a higher degree of wind anisotropy due to SBC generates large mean kinetic energy within the lower troposphere. Moreover, the production of larger negative turbulent kinetic energy due to SBC was evident.

How to cite: Roy, S., Sentchev, A., Schmitt, F. G., Augustin, P., and Fourmentin, M.: Comparison between the sea-breeze circulation day and normal day Reynolds stress anisotropy in the lower atmospheric region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9341, https://doi.org/10.5194/egusphere-egu2020-9341, 2020.

Recently, much attention has been paid to the study and numerical simulation of wind waves in the Arctic regions of the oceans. Their distinctive feature is the presence of ice cover of various types, which can significantly affect the processes of wind wave interaction, including momentum exchange. A detailed study of such processes under natural conditions is very difficult, especially for the forming ice (including pancake ice), therefore, laboratory simulation is preferable. Previously studies of the influence of floating ice on the evolution of waves that were generated by wavemakers were carried out only. In this paper we present preliminary results of studies performed on the AELOTRON circular wind wave flume of the University of Heidelberg, where the interaction of air flow with a water surface was simulated for the first time in the presence of forming ice of the pancake type. Synchronous measurements of wave characteristics were carried out using a laser wavegauge, as well as airflow velocity fields were measured with PIV-methods. Shims made of rubber with a diameter of 7 cm and a thickness of 1 cm with a density of about 0.8 kg /m³ were used as elements of artificial ice. The measurements were carried out in clean water and at three concentrations of artificial ice: maximum, 2/3 of the maximum, 1/3 of the maximum. Ice covered about half the surface at maximum concentration. The measurements were carried out in the range of equivalent wind speeds U10 from 7 to 16  m/s. The threshold character of excitation of long waves was obtained (the length is much greater than the average distance between the elements of ice). The higher the density, the higher the threshold for wind speed. According to the results of processing the velocity fields, the dependence of the aerodynamic drag coefficient on the equivalent wind speed was constructed. It is shown that the presence of ice weakly affects the momentum exchange for all concentrations and over the entire wind speed range.

Investigation was supported by Russian Found Basic Research project # 18-05-60299 Arctic.

How to cite: Sergeev, D., Troitskaya, Y., and Kandaurov, A.: Laboratory simulation of the pancake ice influence on the wind wave interaction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11860, https://doi.org/10.5194/egusphere-egu2020-11860, 2020.

It is important to measure the momentum flux across the air–water interface in the droplet- and bubble-laden turbulent flow at extremely high-wind speeds. Generally, the momentum flux is measured by a profile method, eddy correlation method, or momentum budget (balance) method at normal wind speeds. We assessed the usage of three measurement method at extremely high wind speeds in three wind-wave tanks, Kyoto, Kindai, and Kyushu Universities, JAPAN. Here, the Kyoto tank is 15 m long, 0.8 m wide, 0.8 m high and the maximum wind speed is 68 m/s. The Kyushu tank is 64 m long and the max. speed is 40 m/s. Moreover, we will show the preliminary results for the effects of the fetch on the momentum flux.

How to cite: Takagaki, N., Suzuki, N., Matsuda, K., Komori, S., and Troitskaya, Y.: Momentum flux across breaking air-water interface, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12916, https://doi.org/10.5194/egusphere-egu2020-12916, 2020.

In the framework of the modernized RANS model of turbulent closure [1], the evolution in the pycnocline and shear flow in the upper mixed layer of the ocean is studied. For this purpose, one of the variants of the model situation is considered, which consists in studying the mutual transformation of the buoyancy frequency, shear flow, as well as the kinetic and potential turbulence energies determined at the initial time at different depths. It is shown that the kinetic energy of turbulence increases with time, and its maximum shifts to the maximum of the the horizontal shear flow. However, unlike the standard gradient scheme, in the beginning there is a mutual transformation of the kinetic and potential turbulence energies, after which they quickly reach a stationary equilibrium level (at large values of the Richardson numbers). A significant change in stratification, initially having a maximum at a certain depth, was also found in the process of establishing a stationary turbulence regime.

The work was financially supported by the Russian Foundation for Basic Research (projects № 18-05-00292, 18-35-00602).

References:

1. Ostrovsky, L.A., Troitskaya, Yu.I., The model of turbulent transport and the dynamics of turbulence in a stratified shear flow/ Izvestiya, Atmospheric and Oceanic Physics., 1987. v.3. pp. 1031–1040

How to cite: Soustova, I., Gladskikh, D., Troitskaya, Y., and Ostrovsky, L.: On the features of the dynamics of the upper mixed layer of the ocean in the presence of shear flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15854, https://doi.org/10.5194/egusphere-egu2020-15854, 2020.

The drag coefficient is generally expressed as functions only of the wind speed U₁₀. However, there exists considerable disagreement among the observed values of the drag coefficient. In this study, we observed the wind stress at the coastal tower of Hiratsuka Offshore Experimental Tower of the University of Tokyo in Japan. The 3-axis sonic anemometer was installed on the top of the tower, which was 20 m above mean sea level. The observation periods were from September 15, 2015 to December 31, 2019. The eddy correlation method was used to calculate the friction velocity every 10 minutes. The variation of the drag coefficient plotted against the wind speed U₁₀ has very large using the all period data. The variation of the drag coefficient was reduced by excluding large fluctuation of wind speed in time series within one hour. Furthermore, the sudden changes of the wind speed and direction was also found to affect the variation of the drag coefficient. These results show that the wind speed fluctuation influenced the variation of the drag coefficient. We also investigate the effect of waves on the drag coefficient.

How to cite: Suzuki, N., Waseda, T., and Takagaki, N.: Investigation of the effect of wind speed fluctuation on the drag coefficient, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20860, https://doi.org/10.5194/egusphere-egu2020-20860, 2020.

   Typhoon intensity changes according to the momentum and enthalpy flux supplied from the boundary layer. MPI theory uses the ratio between a drag coefficient and an enthalpy exchange coefficient, which are indexes that indicate how much momentum or enthalpy is exchanged between the air and the sea. Each is a coefficient depending on wind speed, temperature and SST.

However, Lighthill (1999) is shown that latent heat exchange varies because sea spray generated from the sea surface evaporates in the boundary layer. In addition, Barenblatt (2005), inspired by Lighthill (1999), showed that the Karman constant changes according to the Froude number and the drag coefficient changes. Since both changes can change the MPI theory, it is necessary to quantitatively evaluate the effect of the droplets generated from the sea surface in order to grasp both accurately. In addition, it is necessary to consider the effects of rainfall in actual storms, which often involve rainfall.

In this study, to evaluate the flux exchange in the boundary layer quantitatively, we show the drag coefficient and the enthalpy exchange coefficient taking into account sea spray and rain. In addition, we show the results of observation of sea spray and rain using disdrometer and X-band radar.

How to cite: Okachi, H. and Yamada, T.: Effects of momentum and enthalpy exchange on the typhoon intensity in the atmospheric boundary layer considering sea spray and rainfall, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21662, https://doi.org/10.5194/egusphere-egu2020-21662, 2020.

A lot of experimental works is devoted to studying behaviour of a droplet in the flow of the external medium. It is shown in [1] that mode of the deformation of droplet in the stationary flow is affected by the Weber number and the Reynolds number. The authors distinguish two types of the droplet deformation in the external flow: the dome-shaped deformation and the bowl-shaped one.

Using the Basilisk software package, direct numerical simulation of the process of deformation of liquid drop in the gas stream was carried out. We examined the problem of the following geometry: a drop of liquid with diameter of 5 mm was placed in the gas stream at the speed of 30 m/s. The density of liquid and gas correspond to the density of water and air, the viscosity of liquid is equal to the viscosity of water. The viscosity of gas and the surface tension at the interface between liquid and gas are determined by the set values of the Reynolds (50 - 3000) and the Weber (2 - 30) numbers. Two main modes of the drop deformation were observed: the dome-shaped deformation and the bowl-shaped one, there is a transitional deformation mode between them. The map of deformation modes is constructed for comparison with the experimental data available in the literature. It was found that the dependence of the Weber number corresponding to the transition from one deformation mode to another on the Reynolds number is well described by the power law proposed in the literature.

This work was supported by the RFBR projects 19-05-00249, 18-35-20068, 18-55-50005, 18-05-60299, 20-05-00322 (familiarization with the Basilisk software package) and the Grant of the President No. MK-3184.2019.5, work on comparison with experimental data was supported by the RSF project No. 18-77-00074, carrying out numerical experiment was supported by the RSF project No. 19-17-00209, A.N. Zotova is additionally supported by the Ministry of Education and Science of the Russian Federation (Government Task No. 0030-2019-0020). The authors are grateful to the FCEIA employee: UNR - CONICET (Rosario, Rep. Argentina) Dr. Ing. César Pairetti.

[1] Hsiang, L.-P., Faeth, G. M., Int. J. Multiphase Flow 21(4), 545-560 (1995).

How to cite: Zotova, A., Troitskaya, Y., Sergeev, D., and Kandaurov, A.: Direct numerical simulation of the droplet deformation in the external flow at various Reynolds and Weber numbers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7775, https://doi.org/10.5194/egusphere-egu2020-7775, 2020.

Arctic storms pose a great danger to developing commercial and passenger shipping, coastal infrastructure, and also for oil production from offshore platforms. This is primarily due to high waves and extreme winds. Such episodes of adverse weather conditions due to their rapid development are poorly predicted by modern models. For this purpose, the representation of the event of polar law is studied in the wave model WAVEWATCH III.

Wind waves were simulated under conditions of polar depression on ice-free water. To simulate wind waves under conditions of polar depression, the Barents Sea was selected, where, according to the data of [1, 2], a large number of polar hurricanes are observed. Among the identified polar hurricanes, for example, in [3], a hurricane that took place on 05.02.2009, observed at coordinates 69 N 40 E is chosen. The preliminary results in the wave model are obtained without the ice influence consideration. The developed model was configured using the CFSR wind reanalysis data. The resulting distribution of significant wave heights is obtained. Then, to consider the attenuation by sea ice, the reanalysis data of the Arctic System Reanalysis Version 2 (ASRv2), which is based on Polar WRF with a resolution of 15 km for the Arctic region, is used. Modeling the destruction of ice by waves during an intense arctic storm will be implemented using WW3 models with an IS2 module.

The work is supported by RFBR grant 18-05-60299.

1. Smirnova, J. E., Golubkin, P. A., Bobylev, L. P., Zabolotskikh, E. V., & Chapron, B. (2015). Polar low climatology over the Nordic and Barents seas based on satellite passive microwave data. Geophysical Research Letters, 42(13), 5603-5609.
2. Smirnova, J., & Golubkin, P. (2017). Comparing polar lows in atmospheric reanalyses: Arctic System Reanalysis versus ERA-Interim. Monthly Weather Review, 145(6), 2375-2383.
3. Noer, G., & Lien, T. (2010). Dates and Positions of Polar lows over the Nordic Seas between 2000 and 2010. Norwegian Meteorological Institute Rep.

How to cite: Kuznetsova, A., Poplavsky, E., Rusakov, N., and Troitskaya, Y.: Wind waves modeling in the polar law weather conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8111, https://doi.org/10.5194/egusphere-egu2020-8111, 2020.

Whitecap coverage were retrieved from high-speed video recordings of the water surface obtained on the unique laboratory faculty Heidelberg Small-Scale Air-Sea Interaction Facility, the Aeolotron (annular wind-wave facility, 60 cm width, 2.4 m height, circumference of 27.3 m at the inner wall; water depth during experiments 1.0 m, water volume 18.0 m³, air space volume 24 m³; wind was generated by two axial fans mounted into the ceiling).

Records were made in the vertical direction (from top to bottom) in a shadowgraph configuration with backlight located under the channel. On the annular channel, regimes with an abrupt start of wind under an unperturbed surface condition were implemented, including the case of butanol presence in water simulating salinity. At the same time, the wave parameters varying depending on the time elapsed after the wind was turned on, made it possible to study the characteristics of the generation of spray at various effective fetches.
As a result of semi-automatic processing of image sequences using specially developed software that allows marking the moment and position of the bag-breakup formation on the videos, the dependences of the frequency of occurrence of these phenomena per unit surface area versus time after turning on the wind were obtained. From the same images, using the developed software for automatic detection of areas of wave breaking, the values of the whitecap coverage area were obtained. In this case, automatic image processing was performed using morphological analysis in combination with manual processing of part of the frames for tweaking the algorithm parameters: for each mode (water characteristics and wind speed), manual processing of several frames was performed, based on the results of which automatic algorithm parameters were selected to ensure that the resulting whitecap coverage corresponded. Since the same high-speed surface images were used to study the statistics of occurrence of events leading to the spray generation and the dependences of the whitecap coverage on time after turning on the wind for each regime were obtained, we were able to estimate the average number of fragmentation events per unit area of the collapse area.

The work was supported by the RFBR grant 18-35-20068 (conducting an experiment), President grant for young scientists MK-3184.2019.5 (software development) and the RSF grant No. 18-77-00074 (data processing).

How to cite: Kandaurov, A., Troitskaya, Y., Sergeev, D., and Kozlov, D.: Whitecap coverage measurements in laboratory modeling of wind-wave interaction, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15621, https://doi.org/10.5194/egusphere-egu2020-15621, 2020.

Currently, one-dimensional and three-dimensional models are widely used to model thermohydrodynamic and biochemical processes in lakes and water rеreservoirs. One-dimensional models are highly computationally efficient and are used to parameterize land water bodies in climate models, however, when calculating large lakes and reservoirs with complex geometry, such models may incorrectly reproduce processes associated with horizontal heterogeneity. This becomes especially important for the prediction of water quality and euthrophication.

A three-dimensional model of thermohydrodynamics and biochemistry of an inland water obect is presented, which is based on the hydrostatic RANS model [1-3], and the parameterization of biochemical processes is implemented by analogy with the scheme for calculating biochemistry in the one-dimensional LAKE model [4]. Thus, the three-dimensional model is supplemented by a description of the transport of substances such as oxygen (O₂), carbon dioxide (CO₂), methane (CH₄), as well as phyto- and zooplankton. The effect of turbulent diffusion and large-scale water movements on the distribution of a methane concentration field is studied.

To verify the calculation results, idealized numerical experiments and comparison with the measurement data on Lake Kuivajärvi (Finland) were used.

The work was supported by grants of the RF President’s Grant for Young Scientists (MK-1867.2020.5, MD-1850.2020.5) and by the RFBR (18-05-00292, 18-35-00602, 20-05-00776). 

References:
[1] Mortikov E.V. Numerical simulation of the motion of an ice keel in stratified flow // Izv. Atmos. Ocean. Phys. 2016. 52. P. 108-115.
[2] Mortikov E.V., Glazunov A.V., Lykosov V.N. Numerical study of plane Couette flow: turbulence statistics and the structure of pressure-strain correlations // Russian Journal of Numerical Analysis and Mathematical Modelling. 2019. V. 34, N 2. P. 119-132.
[3] D.S. Gladskikh, V.M. Stepanenko, E.V. Mortikov, On the influence of the horizontal dimensions of inland waters on the thickness of the upper mixed layer. // Water Resourses. 2019. 18 pages. (submitted)
[4] Victor Stepanenko, Ivan Mammarella, Anne Ojala, Heli Miettinen, Vasily Lykosov, and Vesala Timo. LAKE 2.0: a model for temperature, methane, carbon dioxide and oxygen dynamics in lakes. Geoscientific Model Development, 9(5): 1977–2006, 2016.

How to cite: Gladskikh, D., Mortikov, E., and Stepanenko, V.: On the modeling of thermohydrodynamic and biogeochemical processes in the inland water objects, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10902, https://doi.org/10.5194/egusphere-egu2020-10902, 2020.

This paper models the impact of the presence of foam on the short-wave component of surface waves and momentum exchange in the atmospheric boundary layer at high winds. First, physical experiments were carried out in a wind-wave flume in which foam can be artificially produced at the water surface. Tests were conducted under high wind-speed conditions where equivalent 10-m wind speed, U10, ranged 12–38 m/s, with measurements made of the airflow parameters, the frequency-wavenumber spectra of the surface waves, the foam coverage of the water surface, and the distribution of the foam bubbles.

Microwave measurements were performed using a coherent Doppler X-band scatterometer with a wavelength of 3.2 cm and a sequential reception of linearly polarized radiation. It was shown that the presence of foam reduces the NRCS of the agitated water surface. Foam formations are concentrated mainly on the ridges and front slopes of wind waves, which make the main contribution to the scattering of radio waves. This may explain the effect of reducing the total NRCS: foam, which has less reflective properties, masks the main diffusers on the water surface. The second mechanism is associated with the effect of foam on short waves, by analogy with surfactant films.

The effect of foam on the shape of the Doppler spectrum of a microwave signal scattered by the water surface was investigated. In the case of weak wind, the presence of foam on the surface leads to a decrease in the short-wave part of the spectrum of surface waves and, as a result, to a decrease in the scattered signal. In addition, a mirror component appears in the Doppler spectrum corresponding to the fundamental frequency of the wave. In the case of a stronger wind, the grouping of additional scatterers (foam) on the crests of the waves leads to a shift of the Doppler spectra to the high-frequency region.

The work was supported by the RFBR (grants 18-35-20068, 19-05-00366, 19-05-00249) and the RF President’s Grant for Young Scientists (MK-144.2019.5).

How to cite: Baydakov, G., Olga, E., Maxim, V., Daniil, S., and Yuliya, T.: Laboratory investigation of the effect of sea foam on the scattering of microwave radiation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8764, https://doi.org/10.5194/egusphere-egu2020-8764, 2020.

Active microwave sensing using satellite instruments has great advantages, since in this range the absorption by clouds and atmospheric gases is noticeably reduced, it allows for round-the-clock and all-weather monitoring of the ocean. One of the main problems is concerned with obtaining the dependency between the RCS of radar signal scattered by the wavy water surface and the parameters of the atmospheric boundary layer in hurricane conditions. To obtain this dependence, we used field measurements of wind speed in a hurricane from falling NOAA GPS-sondes and SAR images from the Sentinel-1 satellite. However, there is the problem of correct collocation of remote sensing data with field measurements of the atmospheric boundary layer parameters, since they are separated in time and space. In this regard, the amount of data suitable for analysis is very limited, which forces us to look for new data sources for processing. A six-channel SFMR radiometer is also installed on board of NOAA research aircraft that measures the emissivity of the ocean surface beneath the aircraft. Thus, it becomes possible to relate the radiometric measurements of SFMR with the parameters of the atmospheric boundary layer in a tropical cyclone obtained from wind velocity profiles, since they are carried out as close as possible in time and space. Using this relation, the SFMR data and the hurricane radar images were analyzed together and an alternative method was found for constructing the dependence of the RCS on the parameters of the boundary layer.

This work was supported by the RFBR projects No. 19-05-00249, 19-05-00366, 18-35-20068 (remote sensing data analysis) and RSF No. 19-17-00209 (GPS-sondes data assimilation and processing).

How to cite: Rusakov, N., Poplavsky, E., Ermakova, O., Troitskaya, Y., Sergeev, D., and Balandina, G.: Atmospheric boundary layer parameters retrieval from Stepped Frequency Microwave Radiometer measurements in tropical cyclones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9379, https://doi.org/10.5194/egusphere-egu2020-9379, 2020.

The current investigation is concerned with the study of the dependence of the scattered cross-polarized microwave signal from the Sentinel-1 satellite on the parameters of the marine atmospheric boundary layer based on data obtained from falling NOAA GPS-sondes under tropical cyclone conditions.
Field measurements and remote sensing data for hurricanes in the Atlantic and Pacific oceans were analyzed for the period 2016 - 2018. Based on the analysis of data measured by GPS-sondes, averaged wind speed profiles were obtained, while the parameters of the atmospheric boundary layer (drag coefficient and wind friction velocity) were retrieved using the self-similarity property of velocity profiles from measurements in the “wake” part.
Sentinel-1 SAR images were used as remote sensing data. Images with cross polarization have a high level of thermal noise (NESZ), which leads to errors when retrieving the NRCS. In this regard, preliminary image processing was performed in the SNAP application.
Using the obtained parameters of the atmospheric boundary layer, the data of GRS-sonde measurements and Sentinel-1 SAR images on cross polarization were collocated and the dependences of the NRCS on the parameters of the atmospheric boundary layer were obtained.

How to cite: Poplavsky, E., Rusakov, N., Ermakova, O., Troitskaya, Y., Sergeev, D., and Balandina, G.: On the use of cross-polarized SAR and GPS-sonde measurements for wind speed retrieval in tropical cyclones, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9628, https://doi.org/10.5194/egusphere-egu2020-9628, 2020.

Insufficient knowledge of the atmosphere layer momentum, heat and moisture transfer between the wavy water surface and marine atmospheric boundary layer under hurricane conditions lead to the uncertainties while using weather forecasting models and models of climate. In particular, there is a significant lack of data for heat and moisture exchange coefficients. In this regard, it is necessary to analyze and process the vertical profiles of wind speed and thermodynamic quantities. The present study is concerned with the analysis and processing of measurements from the NOAA falling GPS-sondes for hurricanes of categories 4 and 5 of 2003-2017, which represent an array of data on wind speed, temperature, altitude, coordinates, etc.

The proposed approach for describing a turbulent boundary layer formed in hurricane conditions is based on the use of the self-similarity properties of the velocity and enthalpy profiles in the atmospheric boundary layer, which includes a layer of constant flows, transferring into its “wake” part with height. Based on the proposed approach, the aerodynamic drag coefficients Cd and the enthalpy exchange coefficient Ck for a selected group of hurricanes were restored. As the value of Ck/Cd represents a determining factor in the formation of a hurricane, the dependence of this ratio on the wind speed was constructed.

This work was supported by the RFBR projects No 19-05-00249, 19-05-00366, 18-35-20068 (remote sensing data analysis) and RSF No 19-17-00209 (GPS-sonde data assimilation and processing).

How to cite: Ermakova, O., Rusakov, N., Poplavsky, E., Troitskaya, Y., Sergeev, D., and Balandina, G.: Exchange coefficients derived from GPS-sonde and SFMR measurements in hurricane conditions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8799, https://doi.org/10.5194/egusphere-egu2020-8799, 2020.

It appears that oceanographers and meteorologists have different pictures in their minds when they speak about internal waves. The reason might be that in both communities different paradigmatic gravity wave models based on different simplifying assumptions are in use.  For the oceanographer, internal wave beams are rather common, a feature virtually unknown to the atmospheric scientist.  In contrast, wave packets traveling upwards in the atmosphere is the standard picture for the meteorologist.  The mathematical origin of such a different view is that for time harmonic waves, the underlying boundary value problem for internal waves in the ocean is hyperbolic but elliptic for atmospheric flows.

In the present paper we discuss the consequences that result from these two different types of boundary value problems. Wave focusing is a rather 
generic process for hyperbolic problems and we argue that the latter should also be of interest to meteorologists in view of new findings that indeed 
a significant part of the internal waves in the atmosphere travel downward. We further apply some of our findings to new laboratory data on inertial modes arguing that an additional shear flow leads to an elliptic boundary value problem and beam-like wave fields, typical for the inertial waves without a shear flow, become mode-like.       

How to cite: Harlander, U.: Comparison of paradigmatic gravity wave models for ocean and atmosphere, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7222, https://doi.org/10.5194/egusphere-egu2020-7222, 2020.

 ‘Dead water’ phenomenon, which is essentially a ship-wave in a stratified fluid is studied experimentally in a laboratory tank. Interfacial waves are excited by a moving ship model in a quasi-two-layer fluid, which leads to the development of a drag force that reaches the maximum at the largest wave amplitude in a critical ‘resonant’ towing speed, whose value depends on the structure of the vertical density profile. We utilize five ships of different lengths but of the same width and wet depth. The experimental analysis focuses on the variability of the interfacial wave amplitudes and wavelengths as a function of towing speed in different stratifications. Data evaluation is based on linear two- and three-layer theories of freely propagating interfacial waves and lee waves. We observe that although the internal waves have considerable amplitude, linear theory still gives a surprisingly adequate description of subcritical to supercritical transition and the associated amplification of internal waves.

How to cite: Medjdoub, K., Jánosi, I. M., and Vincze, M.: Experimental and numerical study of the resonant feature of internal gravity waves in the case of ‘dead water’ phenomenon , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8302, https://doi.org/10.5194/egusphere-egu2020-8302, 2020.

The surface quasigeostrophic (SQG) model is developed to describe the dynamics of flows with zero potential vorticity in the presence of one or two horizontal boundaries (Earth surface and tropopause). Within the framework of this model, the problems of linear and nonlinear stability of zonal spatially periodic flows are considered. To study the linear stability of flows with one boundary, two approaches are used. In the first approach, the solution is sought by decomposing into a trigonometric series, and the growth rate of the perturbations is found from the characteristic equation containing an infinite continued fraction. In the second approach, few-mode Galerkin approximations of the solution are constructed. It is shown that both approaches lead to the same dependence of the growth increment on the wavenumber of perturbations. The existence of instability with a preferred horizontal scale on the order of the wavelength of the main flow follows from this dependence. A similar result is obtained within the framework of the SQG model with two horizontal boundaries. The Galerkin method with three basis trigonometric functions is also used to study the nonlinear dynamics of perturbations, described by a system of three nonlinear differential equations similar to that describing the motion of a symmetric top in classical mechanics. An analysis of the solutions of this system shows that the exponential growth of disturbances at the linear stage is replaced by a stage of stable nonlinear oscillations (vacillations). The results of numerical integration of full nonlinear SQG equations confirm this analysis.

The work was supported by the Russian Foundation for Basic Research (Project 18-05-00414) and the Russian Science Foundation (Project 19-17-00248).

How to cite: Kalashnik, M., Kurgansky, M., and Kostrykin, S.: Instability of surface quasigeostrophic spatially periodic flows, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9872, https://doi.org/10.5194/egusphere-egu2020-9872, 2020.

The experimental investigation of large-scale flows on atmospheric circulation and climate such as Earth, Mars or even distant exoplanets are of great interest in geophysics. Gaining the fundamental knowledge of the origin of planetary waves or global cell formation is interesting from a meteorological point of view but up till now difficult to reproduce in laboratory scale. The limitation is based on the central force field which may be induced by the dielectrophoretic effect. However, the established radial force field is overpowered by the gravitational field unless experiments are conducted in a microgravity environment. The AtmoFlow project provides the possibility to study convective flow patterns in a spherical shell under microgravity conditions, planned after 2022, on the International Space Station (ISS) and is in fact the follow-up experiment of the GeoFlow project which served between 2008 and 2016 on the ISS.

Without losing the overall focus of complex planetary atmospheres, the AtmoFlow experiment is able to model the intake and outtake of energy (e.g. radiation) and the rotational forcing via rotating or co-rotating boundaries. The gap is filled with a Fluor-based fluid with physical properties sensitive to temperature and electric fields. With an electric potential applied between the spherical shells a central force field is established that is based on the above mentioned dielectrophoretic effect. By adjusting rotation, thermal forcing and strength of the applied electric potential the AtmoFlow experiment can simulate different planetary atmospheres to investigate local pattern formation or global planetary cells. An interferometry system similar to the one used in the GeoFlow experiment uses the Wollaston shearing technique (WSI) to record the evolving temperature fields.

To provide a benchmark solution for the experimentally recorded WSI interferograms a ground experiment is used to develop a validation method and to find the best postprocessing method for the AtmoFlow experiment. The ground experiment consists of a thermally forced baroclinic wave tank with a corresponding WSI setup and an infrared (IR) camera that are used to record the evolving temperature field. Here, we present first numerical simulations of the ground experiment that include the formation of the convective wave patterns and the numerical evaluated interferograms and IR pictures. The numerical calculated data will then be compared to the experimental recorded data to find a technique to best process the recorded WSI interferograms of the AtmoFlow project.

How to cite: Szabo, P., Zaussinger, F., Haun, P., Travnikov, V., Meier, M., and Egbers, C.: Complementary numerical and experimental study in the baroclinic annulus for the microgravity experiment AtmoFlow , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20956, https://doi.org/10.5194/egusphere-egu2020-20956, 2020.

This study aims at introducing a simple and physically consistent method for identification and analysis of turbulent layers in the free atmosphere that can supplement the traditional methods (Richardson number criterion, Thorpe scale). The method is based on differences between the observed and hydrostatically derived (with floating level of initialization) pressure. In the paper we derive the method analytically from the Navier Stokes equations and propose a methodology how to isolate information on turbulence from an internal gravity wave and atmospheric structure signal in the pressure differences. Finally we apply the methodology on high vertical-resolution radiosonde data to demonstrate the utility of the novel method by contrasting the results with traditional diagnostics. 

How to cite: Šácha, P. and Pišoft, P.: A new method for the detection of incompressible turbulence as a deviation from the hydrostatic balance assumption, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4987, https://doi.org/10.5194/egusphere-egu2020-4987, 2020.

The baroclinic rotating annulus is a classic experiment to investigate the transition from regular waves to complex flows.  A well documented transition via Amplitude Vacillation leads to low-dimensional chaos through a sequence of canonical bifurcations.  However, the transition to geostrophic turbulence is usually through a regime of 'Structural Vacillation' (SV) which retains the overall spatial structure of regular waves but includes small-scale variability.  Even though the SV vacillation occurs with a clear time scale, the dynamics of SV cannot usually be described by low-dimensional dynamics.  For example, attractor dimension estimations tend to fail: they may not show any scaling region or converge to an unrealistic values.  Explanations of the origin of SV have variously invoked higher radial modes of the fundamental baroclinic waves, local secondary instabilities in the baroclinic waves caused by high thermal gradients (gravity waves) or velocity shear (barotropic instability), or instabilities within the side-wall (Stewartson) boundary layers.

The aim of this paper is to identify where within the fluid different signals of variability are located at different stages in the transition from a steady wave to pronounced SV.   To this end, a set of experiments in a water-filled rotating annulus with a free surface (inner radius 45 mm, outer radius 120 mm, fluid depth 140 mm) was carried out covering a temperature difference between the heated outer wall and the cooler inner wall of between 6 and 9.5 K, and a range of rotation rates from 0.84 to 2.29 rad/s (Ta= 4.75 x 10⁷ - 3.53 x 10⁸ and Θ = 0.0617 - 0.629).   The flow was observed through an infrared camera capturing the temperatures of the free surface.  Images of the flow were recorded for a period of 15 minutes at a sampling rate of 1 Hz at the lower rotation rates and 2 Hz at the higher rotation rates.

The initial processing of the time series of temperature images involved normalisation of the temperatures followed by rotation of the images to a coordinate system co-rotating with the main baroclinic wave mode. The resulting images were separated into the time-mean wave field and the fluctuation field, resulting in a set of normalised temperature fluctuations at fixed points relative to the main baroclinic wave.   Each of the time series was then used to calculate the power spectrum at that location.  The low-frequency part of the spectra (up until half the tank rotation frequency) was used in a k-means cluster analysis to identify clusters of similar spectral shape and, from this, create a map of which spectral shape was found at which location in the flow field.

The results show isolated locations of a high frequency peak near the inner boundary at the onset of visible fluctuations.  Further into the regime of clear structural vacillations, areas of pronounced variability at lower frequencies become visible at the lee shoulder of the cold jets in the fluid interior, followed by activity where the end of the cold jet interacts with the hot jet emanating from the outer boundary layer.

How to cite: Fruh, W.-G., Szabo, P., Egbers, C., and Uwe, H.: Locating sources of variability in the transition to Structural Vacillation in the baroclinic annulus, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7262, https://doi.org/10.5194/egusphere-egu2020-7262, 2020.

Two common approximations to the full Shallow Water Equations (SWEs) are non-divergence and quasi-geostrophy, and the degree to which these approximations lead to biases in numerical solutions are explored using the testbed of barotropic instability. Specifically, we examine the linear stability of strong polar and equatorial jets and compare the growth rates obtained from the SWEs along with those obtained from the Non-Divergent barotropic vorticity (ND) equation and the Quasi-Geostrophic (QG) equation. The main result of this paper is that the depth over which a layer is barotropically unstable is a crucial parameter in controlling the growth rate of small-amplitude perturbations and this dependence is completely lost in the ND equation and is overly weak in the QG system. Only for depths of 30 km or more are the growth rates predicted by the ND and QG systems a good approximation to those of the SWEs, and such a convergence for deep layers can be explained using theoretical considerations. However, for smaller depths, the growth rates predicted by the SWEs become smaller than those of the ND and QG systems and for depths of between 5 and 10 km they can be smaller by more than 50%. For polar jets, and for depths below 2 km the mean height in geostrophic balance with the strong zonal jet becomes negative and hence the barotropic instability problem is ill-defined. While in the SWEs an equatorial jet becomes stable for layer depths smaller than ~3-4 km, in the QG and ND approximation it is unstable for layer depths down to 1 km. These results may have implications for the importance of barotropic instability in Earth's upper stratosphere and perhaps also other planets such as Venus.

How to cite: Shamir, O., Paldor, N., and Garfinkel, C.: Barotropic instability of a zonal jet on the sphere: From non-divergence through quasi-geostrophy to shallow water, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4137, https://doi.org/10.5194/egusphere-egu2020-4137, 2020.

The main purpose of the work is to improve the ocean general circulation model (OGCM) by including new parameterizations of heat, salt and momentum vertical turbulent exchange, which significantly affects quality of reproducing the ocean circulation and thermohaline structure using the OGCMs based on the primitive equation system. The main instrument of the research is the σ-model of the oceanic and marine circulation INMOM (Institute of Numerical Mathematics Ocean Model) developed at the Marchuk Institute of Numerical Mathematics of RAS. The basic equation set in the incompressibility, hydrostatics and Boussinesq approximations is supplemented with the equations for the k-ω and k-ε vertical turbulent exchange parameterizations, which are solved using the splitting with respect to the physical processes. The total equations are split into the stages describing transport-diffusion of the turbulent characteristics and their generation-dissipation. At the generation-dissipation stage, the equations for turbulent characteristics can be solved analytically. This approach allows one to solve the turbulence equations with the time step used in the OGCM. To estimate quality of these two vertical turbulent exchange parameterizations, the joint circulation of the North Atlantic and Arctic Ocean is numerically simulated and the upper ocean layer characteristics are studied. It is shown that the structure of large-scale fields in the North Atlantic and Arctic Ocean is sensitive to the choice between these two vertical turbulence models. In particular, application of the k-ε parameterization is accompanied by a noticeably higher rate of water involvement within the seasonal pycnocline in the developed turbulence zone than that resulting from application of the k-ω model.

The investigation is carried out in the INM RAS and MHI RAS under support of the Russian Science Foundation (grant No 17-77-30001).

References

Moshonkin, S., Zalesny, V. and Gusev, A., 2018, Journal of Marine Science and Engineering, 6(95), https://doi.org/10.3390/jmse6030095

Zalesny, V.B., Moshonkin, S.N., Perov, V.L. and Gusev, A.V., 2019, Physical Oceanography, 26(6), 455-466, https://doi.org/10.22449/1573-160X-2019-6-455-466

How to cite: Gusev, A. and Zalesny, V.: Analytical solution technique in k-omega and k-epsilon turbulence parameterizations and their implementation in the OGCM, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17693, https://doi.org/10.5194/egusphere-egu2020-17693, 2020.