OS4.2

Despite many investigations/studies on the surface wave-induced stress, the global feature of the wave-induced stress has not been obtained previously as that requires a simultaneous observation of wave spectra and wind on a global scale. The China France Oceanography Satellite (CFOSAT) provided an opportunity for the first time to evaluate the global wave-induced stress and its contribution to the total wind stress. In this study, the global spatial distributions of wave-induced stress and its correlated index for August to November in 2019 are presented using the simultaneous ocean surface winds and wave spectra from the CFOSAT. The main results show that the wave-induced stress is fundamentally dependent on the wind and wave fields on a global scale and shows significant temporal and spatial variations. Further analyses indicate that there is an upward momentum flux under strong swells and low wind speeds (below approximately 5 m/s), and an anti-correlation between the dimensionless wave-induced stress and the proportion of swell energy to the total. Finally, the variations of the surface wave induced wind stress are clear asymmetric between northern and southern hemispheres in late summer but symmetric in late fall, which are closely associated with the seasonal changes in large-scale atmospheric circulation.

How to cite: Chen, S.: On the First Observed Wave-induced Stress over the Global Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3869, https://doi.org/10.5194/egusphere-egu21-3869, 2021.

Despite of significant improvement in modelling of the atmosphere after years of research, the accuracy of predicting cyclone/typhoon waves still remains highly challenging. Evidence shows that the air-sea-waves interaction over the ocean surface can significantly impact on the coupled atmosphere-ocean systems, through momentum, mass, and energy exchanges. In particular, the momentum exchanges have been found to affect both the structure of the wave boundary layer and the sea state, through the wave dissipation and wave breaking. For many decades, studies suggested different parameterizations of the momentum fluxes, through drag coefficient (C_d) and the roughness length (z₀). In recent years, research has been focused on the theoretical approaches of the momentum parameterization within the Wave Boundary Layer (WBL) in order to obtain the best C_d and z₀ (Hara and Belcher 2002,2004; Moon et al. 2004; Du et al. 2017,2019). In this study, based on the works of Du et al. (2017, 2019), we introduce a new approach of the parameterization of the momentum flux using the roughness length. The potential of the scheme is analysed with extreme wind and wave events and the results are validated against buoy observations.

How to cite: Makrygianni, N., Pan, S., Bidlot, J., and Bray, M.: A new approach of implementation of Wave Boundary Layer in OpenIFS, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9617, https://doi.org/10.5194/egusphere-egu21-9617, 2021.

Abstract: Wave hindcasts of long time series ( > 30 years) have been instrumental in understanding the wave climate. However, it is still difficult to have a consistent reanalysis suitable for study of trends and interannual variability. Here we explore the consistency of wave hindcast with independent observations from moored buoys, satellite altimeters, and  microseism data. We use the ECMWF 5th generation re-analysis (ERA5) winds to drive two wave models, using either ECMWF WAM (Bidlot et al. 2019) or WAVEWATCH III (The WAVEWATCH III Develoment Group 2019, Alday et al. 2020). We also use seismic data in the dominant double-frequency band, around 5 s period, that are generated by opposing waves of equal frequencies and compare these to modeled microseims. We find that the inter-platform corrections in the ESA CCI Version 1.1 dataset (Dodet et al. 2020) introduced a trend that differs from the microseism trends. However, the results converge when using a revised correction of this dataset. We also look at the microseism spectral signature of large storms in the North Atlantic and discuss how we may compare the severity of different storms that move over different ocean bathymetry with different wave to microseism conversions. 

References: Stopa, J. E., Ardhuin, F., Stutzmann, E., & Lecocq, T. (2019).  Sea state trends and variability: Consistency between models, altimeters, buoys, and seismic data (1979–2016). Journal of Geophysical Research: Oceans, 124. https://doi.org/10.1029/2018JC014607 

Dodet, G., Piolle, J.-F., Quilfen, Y., Abdalla, S., Accensi, M., Ardhuin, F., Ash, E., Bidlot, J.-R., Gommenginger, C., Marechal, G., Passaro, M., Quartly, G., Stopa, J., Timmermans, B., Young, I., Cipollini, P., and Donlon, C.: The Sea State CCI dataset v1: towards a sea state climate data record based on satellite observations, Earth Syst. Sci. Data, 12, 1929–1951, https://doi.org/10.5194/essd-12-1929-2020 , 2020.

How to cite: Alday, M., De Carlo, M., Dodet, G., Accensi, M., Stutzmann, E., Ardhuin, F., and Bidlot, J.: Sea state trends and variability: consistency between the ESA Sea State Climate Change Inititative dataset, ERA5 winds and microseisms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15014, https://doi.org/10.5194/egusphere-egu21-15014, 2021.

We consider the evolution of directional spectra of waves generated by constant and changing wind, modelling it by direct numerical simulation (DNS), based on the Zakharov equation. Results are compared with numerical simulations performed with the Hasselmann kinetic equation and the generalised kinetic equation, and with airborne measurements of waves generated by offshore wind, collected during the GOTEX experiment off the coast of Mexico. Modelling is performed with wind measured during the experiment, and the initial conditions are taken as the observed spectrum at the moment when wind waves prevail over swell after the initial part of the evolution.

Directional spreading is characterised by the second moment of the normalised angular distribution function, taken at selected wavenumbers relative to the spectral peak. We show that for scales longer than the spectral peak the angular spread predicted by the DNS is close to that predicted by both kinetic equations, but it underestimates the corresponding measured value, apparently due to the presence of swell. For the spectral peak and shorter waves, the DNS shows good agreement with the data. A notable feature is the steady growth of angular width at the spectral peak with time/fetch, in contrast to nearly constant width in the kinetic equations modelling. Dependence of angular width on wavenumber is shown to be much weaker than predicted by the kinetic equations. A more detailed consideration of the angular structure at the spectral peak at large fetches shows that the kinetic equations predict an angular distribution with a well-defined peak at the central angle, while the DNS reproduces the observed angular structure, with a flat peak over a range of angles.

In order to study in detail the differences between the predictions of the DNS and the kinetic equations modelling under idealised conditions, we also perform numerical simulations for the case of constant wind forcing. As in the previous case of forcing by real wind, the most striking difference between the kinetic equations and the DNS is the steady growth with time of angular width at the spectral peak, which is demonstrated by the DNS, but is not present in the modelling with the kinetic equations. We show that while the kinetic theory, both in the case of the Hasselmann equation and the generalised kinetic equation, predicts a relatively simple shape of the spectral peak, the DNS shows a more complicated structure, with a flat top and dependence of the peak position on angle. We discuss the approximations employed in the derivation of the kinetic theory and the possible causes of the found differences of directional structure.

How to cite: Annenkov, S., Shrira, V., Romero, L., and Melville, K.: Long-term evolution of directional spectra of wind waves modelled by DNS and kinetic equations, and comparison with airborne measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10435, https://doi.org/10.5194/egusphere-egu21-10435, 2021.

The CFOSAT (China France Oceanography Satellite) mission launched in 2018 now routinely provides at the global scale, directional spectra of ocean waves. The principle is based on the analysis of the normalized radar cross-section measured by the instrument SWIM (Surface Waves Investigation and Monitoring), a near-nadir pointing Ku-Band real-aperture scanning radar. From the ocean wave spectra derived from SWIM, the principal parameters of ocean wave spectra as significant wave height, peak wavelength, and peak direction are now available to better characterize the sea-state. However, it is known that these principal parameters are not sufficient not fully characterize the distribution of wave energy and understand or validate the physical processes impacting its evolution during growth order decay. Here we show that the parameters characterizing the shape of the wave spectra (e.g directional and frequency spread) can be estimated at the global scale from the SWIM measurements. We also show that they can provide consistent values of the Benjamin-Feir index, an index proposed to estimate the probability of extreme waves. Similarities of differences with the shape parameters of the MFWAM numerical wave model are also discussed.

How to cite: Hauser, D., Le Merle, E., Aouf, L., and Peureux, C.: Directional and frequency spread of surface ocean waves from CFOSAT/SWIM satelllite measurements, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2544, https://doi.org/10.5194/egusphere-egu21-2544, 2021.

The ambient sound near the ocean surface is controlled by many processes, while wave breaking becomes the dominant factor once it occurs. Laboratory experiment shows that a severer breaker will result in a higher sound level and a larger mean bubble size. This relationship indicates a potential to extract information about wave breaking from acoustic records. Based on both laboratory and field experiments, a passive acoustic method has been developed to determine the wave breaking dissipation rate across the spectrum which had been extremely difficult to obtain in the open sea. The laboratory experiments were carried out in a flume at the University of Adelaide. Waves of different amplitudes and periods were generated and triggered to break by an underwater obstacle. The wave profiles before and after breaking were measured by two capacitance probes to calculate their breaking severities. The acoustic noise emitted by bubbles was recorded by a hydrophone located right under the breaking zone and the mean bubble sizes were computed on the basis of the relationship between bubble radius and acoustic frequency. A non-dimensional empirical formula between breaking severity and mean bubble size was established then applied to acoustic measurements in Lake George, New South Wales, Australia. Acoustic pulse amplitude, power spectral density of acoustic spectrum and the ratio between acoustic pulse amplitude and period were analyzed to identify the acoustic pulses truly produced by bubbles. The mean bubble sizes of each breaker were deduced from the acoustic records and further converted into their breaking severities. Combined with the wave scale information extracted from wave surface records, the spectral dissipation rates in Lake George were finally obtained. The acoustic based results are compared with various kinds of whitecapping dissipation source terms of WAVEWATCH III® and their differences are discussed.

How to cite: Zou, X. and Babanin, A.: Passive acoustic determination of wave breaking dissipation rate across the spectrum, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13507, https://doi.org/10.5194/egusphere-egu21-13507, 2021.

Ocean wave information is of major importance for a number of applications including climate studies, safety at sea, marine engineering (offshore and coastal), and coastal risk management. Depending on the scales and regions of interest, several data sources may be considered (e.g. in situ data, VOS observations, altimeter records, numerical wave model), each one with its pros and cons. In order to optimize the use of multiple source wave information (e.g. through assimilation scheme in NWP), the error characteristics of each measurement system need to be investigated and inter-compared. In this study, we use triple collocation technique to estimate the random error variances of significant wave height from in situ, altimeter and model data. The buoy dataset is a selection of ~100 in-situ measuring stations provided by the CMEMS In-Situ Thematic Assembly Center. The altimeter dataset is composed of the ESA Sea State CCI V1.1 L2P product. The model dataset is the result of WW3 Ifremer hindcast run forced with ERA5 winds using the recently updated T475 parameterization. In comparisons to previous studies using similar techniques, the large triple collocation dataset (~450 000 matchups in total) generated for this study provides some new insights on the error variability within in situ stations, satellite missions and upon sea state conditions.Moreover, the results of the triple collocation technique help developing improved calibration of the altimeter missions included in the ESA Sea State CCI V1.1 dataset.

How to cite: Dodet, G., Bidlot, J.-R., Accensi, M., Alday, M., Abdalla, S., Piolle, J.-F., and Ardhuin, F.: Error estimation of buoy, altimeter, and model significant wave height from triple collocation technique, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14753, https://doi.org/10.5194/egusphere-egu21-14753, 2021.

Wind-waves exert stress on coastal environment and wave power is a better representative of that stress rather than wave height alone. This study inspects the global changes in seasonal wave power by the end of the 21st century, compared to the 1979–2005 period as a result of projected climate change. We use multi-model wave climate simulations from WAVEWATCH-III, forced with surface winds simulated by 7 different CMIP5 Models. Our analysis of wave power reveals decreases over the Northern Hemisphere and increases over the tropics and Southern Hemisphere, with substantial seasonal and regional variations. We analyzed five different terms of differential wave power representing contribution from wave height and/or period. Although wave height changes dominantly control wave power change, contribution of wave period is pronounced over Southern hemisphere extra-tropics, remarkably over Indian Ocean sector during austral winter. Wave period increase is strikingly higher in austral winter than summer, which resembles with wave height of swells components generated in the Southern Ocean. Strong positive inter-model relationship between future change in wave power and SAM over the Southern Hemisphere is consistent with previously reported intensification of wind belt related to more frequent occurrences of positive SAM in future.  Northern Hemisphere decrease can be attributed to reduced storm activity rising from lowered meridional temperature gradient, and lacked swell activity owing to smaller fraction of sea with respect to land than the Southern Hemisphere.  

How to cite: Patra, A. and Min, S.-K.: Hemispheric asymmetry in future seasonal wave power changes, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8180, https://doi.org/10.5194/egusphere-egu21-8180, 2021.

Sea ice seasonally covers the Sea of Okhotsk, a marginal Arctic basin nested between Russia and Japan, but its extent is predicted to decrease by 40% by 2050 leaving larger ice free areas over which waves can form. In the highly dynamical seasonal ice zone, i.e. where waves and ice interact, ice formation and breakup, and wave attenuation mutually affect each other via complex feedback mechanisms. To shed light into these interactions, wave measurements were conducted in the winter seasonal ice zone in the Southern Okhotsk Sea, North of Hokkaido, from onboard the P/V Soya using a stereo camera system. Data show that wave energy penetrates even in high ice concentration (>85%), where contemporary wave models predict complete attenuation of wind waves. Consistently with physical experiments and field observations of waves in the Arctic and Antarctic marginal ice zones, the measurements also show that the ice cover is more effective in attenuating short wave components and, consequently, the dominant wave period in ice is significantly increased compared to corresponding open ocean waves. The present data can inform calibration of wave models in the rapidly evolving seasonal ice zone in the Sea of Okhotsk.

How to cite: Alberello, A., Nose, T., Kodaira, T., Nishizawa, K., Nelli, F., Toyota, T., and Waseda, T.: Observation of wave propagation in ice using stereo imaging in the Sea of Okhostk, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9589, https://doi.org/10.5194/egusphere-egu21-9589, 2021.

The role of surface ocean waves becomes substantial in the upper ocean layer mixing. Due to turbulence induced by the surface waves (both broken and unbroken waves), the upper ocean mixing is enhanced, and important upper ocean parameters are affected such as lowering of sea surface temperature (SST), deepening of mixed layer depth (MLD) and most interestingly, the changes in oceanic biogeochemistry. The main objective of this study is to analyze the effect of wave induced turbulence on oceanic biogeochemistry such as the supply and distribution of nutrients to tiny plants in the ocean called phytoplanktons, and how it affects their concentrations. Marine phytoplanktons formed the basis of marine ecosystem which accounts for about 45 percent of global net primary productivity and play an important part in global carbon cycle. The population of phytoplanktons depends mainly on nutrients (both micro and macro), availability of sunlight and grazing organisms. For this study, we use global coupled ocean-sea ice model ACCESS-OM2 with biogeochemical module called WOMBAT to estimate the effect of wave induced turbulence and study the difference between ‘with waves’ and ‘without waves’ effect on oceanic biogeochemistry. The same effect of wave induced turbulence on oceanic biogeochemistry are also studied by incorporating the change in wave climate such as increase in significant wave height and wind speed. From the investigation of merged satellite ocean color data from ESA’s GlobColour project for the period of 23 years between 1997 and 2019, it was found that chlorophyll-a (Chl-a, an index of phytoplankton biomass) concentration showed increasing trend of 0.015 mg/m3 globally and 0.062 mg/m3 in the Southern Ocean (SO) for the study period with p-value less than 0.01. It was also found that most of the increasing trends are shown spatially in the open ocean and decreasing trend in the coastal regions during the study period.

How to cite: Tensubam, C. M. and Babanin, A. V.: Wave induced turbulence effect on oceanic biogeochemistry and study of ocean color response to changing wave climate, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9364, https://doi.org/10.5194/egusphere-egu21-9364, 2021.

The Southern ocean is a complex ocean region with uncertainties related to surface wind forcing and fluxes exchanges at the air/sea interface. The improvement of wind wave generation in this ocean region is crucial for climate studies. With CFOSAT satellite mission, the SWIM instrument provides directional wave spectra for wavelengths from 70 to 500 m, which shed light on the role of correcting the wave direction and peak wave number of dominant wave trains in the wind-waves growth phase. This consequently induced a better energy transfer between waves and a significant bias reduction of wave height in the Southern Ocean (Aouf et al. 2020). The objective of this work is to extend the analysis of the impact of the assimilation of wave number components from SWIM wave partitions on the ocean/wave coupling. To this end, coupled simulations of the wave model MFWAM and the ocean model NEMO are performed during the southern winter period of 2019 (May-July). We have examined the MFWAM/NEMO coupling with and without the assimilation of the SWIM mean wave number components. Several coupling processes related to Stokes drift, momentum flux stress and wave breaking inducing turbulence in the ocean mixing layer have been analyzed. We also compared the coupled runs with a control run without wave forcing in order to evaluate the impact of the assimilation. The results of coupled simulations have been validated with satellite Sea Surface Temperature and available surface currents data over the southern ocean. We also investigated the impact of the assimilation during severe storms with unlimited fetch conditions.

Further discussions and conclusions will be commented in the final paper.

Aouf L., New directional wave satellite observations : Towards improved wave forecasting and climate description in Southern Ocean, Geophysical Research Letters, DOI: 10.1029/2020GL091187 (in production).

How to cite: Aouf, L., Hauser, D., Law-Chune, S., chapron, B., Dalphinet, A., and Tourain, C.: New directional wave observations from CFOSAT : impact on ocean/wave coupling in the Southern Ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7412, https://doi.org/10.5194/egusphere-egu21-7412, 2021.

Accurate prediction over the North Pacific, especially for the key parameter of sea
surface temperature (SST), remains a challenge for short-term climate prediction. In
this study, seasonal predicted skills of the First Institute of Oceanography Earth System
Model version 1.0 (FIO-ESM v1.0) over the North Pacific were assessed. Ensemble
adjustment Kalman filter (EAKF) and Projection Optimal Interpolation (Projection-OI) data
assimilation schemes were used to provide initial conditions for FIO-ESM v1.0 hindcasts
that were started from the first day of each month between 1993 and 2017. Evolution
and spacial distribution of SST anomalies over the North Pacific were reasonably
reproduced in EAKF and Projection-OI assimilation output. Two hindcast experiments
show that the skill of FIO-ESM v1.0 with the EAKF data assimilation scheme to predict
SST over the North Pacific is considerably higher than that with Projection-OI data
assimilation for all lead times of 1–6 months, especially in the central North Pacific where
the subsurface ocean temperature in the initial conditions is significantly improved by
EAKF data assimilation. For the Kuroshio–Oyashio extension (KOE) region, the errors
in the initial conditions have more rapid propagation resulting in large discrepancies
between simulated and observed values, which are reduced by inducing surface
waves into the climate model. Incorporation of realistic initial conditions and reasonable
physical processes into the coupled model is essential to improving seasonal prediction
skill. These results provide a solid basis for the development of operational seasonal
prediction systems for the North Pacific.

How to cite: Song, Y. and Yin, X.: Evaluation of FIO-ESM v1.0 Seasonal Prediction Skills Over the North Pacific, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3826, https://doi.org/10.5194/egusphere-egu21-3826, 2021.

    Surface gravity waves play an important role in sediment transport. Previous studies have focused on the role of bottom shear enhanced by the surface wave orbital velocity. In this study, we embedded the University of New South Wales Sediment model into the Princeton Ocean Model, which includes a three-dimensional wave module to study sediment dynamics near a sandy spit in Sanniang Bay in the South China Sea. The simulated results for the deposition rate show that wave-induced currents play a dominant role in the maintenance of the sandy spit. The spit tip was formed as a result of the separation of wave-induced coastal flow. The spit tip was shown to be a barrier to the dominant wave-induced current, and the spit base was simulated to form via sand accumulation in the shelter of the spit tip. The deposition is mainly in the low-energy region behind the tip of the spit, which can counter the erosion effect of dominant wave-induced currents. The dominant wave-induced current prompts the lateral infilling of the spit tip when both the spit tip and base are above the water surface. The sediment carried by the coastal current is deposited along the flow branch of separation and forms the spit tip, which indicates that the sediment is deposited where the longshore current changes into an offshore current. As the water depth increases along the separated flow spindle, the bottom shear stress decreases, contributing to the deposition of the spit tip.

How to cite: Lu, J.: Sediment dynamics near a sandy spit with wave-induced coastal currents, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4459, https://doi.org/10.5194/egusphere-egu21-4459, 2021.

The "bag breakup" fragmentation is the dominant mechanism for generating spray in hurricane winds, which parameters substantially affect the exchange processes between the ocean and the atmosphere and, thereby, the dynamics of the development of sea storms. This fast process can only be studied in lab using sophisticated experimental techniques based on high-speed video filming. In such circumstances, the transfer of laboratory data to field conditions requires a kind of theoretical model that describes how the initiation of disturbances occurs, which then lead to fragmentation events, what is the threshold for fragmentation, what is the volume of liquid, which determines the size of spray droplets, that undergoes fragmentation, and how it depends on wind parameters, etc. The conclusions of the model can be first verified in the laboratory experiment and then applied to field conditions.

In the present work, such a model is proposed. First of all, a linear theory of small-scale disturbances on the water surface under the action of a strong wind has been built, which makes it possible to describe their structure, dispersion properties and determine the threshold value of the dynamic air flow velocity at which such disturbances become growing. These disturbances comprise small-scale ripples concentrated within the thin surface layer and growing fast due to shear instability of the wind drift flow in the water. The peculiarity of the structure of these disturbances enables one to consider the nonlinear stage of their evolution within the Riemann simple wave equation modified to describe the increasing disturbances. The analytical solution of the obtained equation suggests the scaling of the volume of liquid undergoing the "bag-breakup" fragmentation, to estimate the scale of the formed droplets and the speed of their injection into the atmosphere. The scaling correctly describes the dependencies of these quantities on the wind friction velocity obtained in laboratory experiments.

The obtained results are applied for the construction of the fetch-dependent spray generation function, which is applicable in the field. Within the Lagrangian stochastic model for the inertial droplets in the marine boundary layer, the momentum, heat, moisture and enthalpy exchange coefficients are calculated. One should notice substantial  feedback effect on the atmosphere caused by the presence of spray in hurricane conditions.

This work was supported by RFBR grant 19-05-00249 and RSF grant 19-17-00209.

How to cite: Troitskaya, Y.: Shear instability of wind drift as the initiation mechanism for the bag-breakup fragmentation of the air-water interface at high winds., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8539, https://doi.org/10.5194/egusphere-egu21-8539, 2021.

Under tropical cyclones, sea spray is produced by breaking waves and direct disruption of the air-sea interface. The influence of sea spray on tropical cyclone intensity and intensification has not been well understood. There are serious questions regarding the most appropriate methods for the incorporation of sea spray in tropical cyclone models. These include momentum and enthalpy fluxes at the air-sea interface due to spray, the airborne sea-salt particles inducing boundary layer convection and clouds (Woodcock  1958, Spund  et al. 2014), and other related factors. Here, we study the effect of spray on thermodynamics of tropical cyclones using a Volume of Fluid to Discrete Phase (VOF to DPM) transition model. Due to dynamic remeshing, VOF to DPM resolves spray particles ranging in size from tens of micrometers to a few millimeters. The generated water particles that satisfy the condition of asphericity are converted into Lagrangian particles involved in a two-way interaction with the airflow. This model has been partially verified at the UM RSMAS Surge Structure Atmosphere Interaction facility (Vanderplow et al. 2020). A recent addition of the ANSYS Fluent Evaporation-Condensation model also allows us to model spray evaporation and related heat and enthalpy fluxes. A substantial part of the smallest particles was suspended in the turbulent airflow and evaporated, and thus contributed less to the total air-sea enthalpy flux. The temperature of the largest particles was close to the temperature of the water layer, which contributed more to the enthalpy flux. This resembled the effect of negative feedback on the enthalpy flux (Peng and Richter 2019). Results of the numerical simulation showed a dramatic increase of spray generation under major tropical cyclones (Cat. 3-5). Under major tropical cyclones, most sea spray (including large particles-spume) is suspended in the turbulent airflow and is then subject to the negative feedback. Consequently, in major tropical cyclones the effect of sea spray is expected to be more significant in the momentum budget rather than enthalpy flux at the air-sea interface. This result may explain the nearly constant enthalpy exchange coefficient observed in laboratory and oceanic experiments on tropical cyclones. This is also consistent with the formation of an “aerodynamic drag well” around a wind speed of 60 m/s, which can explain the process of rapid storm intensification (Soloviev et al. 2017). 

How to cite: Soloviev, A., Vanderplow, B., Lukas, R., Haus, B., Sami, M., and Ginis, I.: Sea Spray in Air-Sea Enthalpy and Momentum Exchanges in Tropical Cyclones, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13353, https://doi.org/10.5194/egusphere-egu21-13353, 2021.

Recent studies stress the importance of considering sea surface wave characteristics in sea spray generation functions (SSGFs). To this end, the effect of interacting winds and waves on sea spray generation was studied using data collected during the Marine Aerosol Tunnel Experiments (MATE2019) conducted at the OSU-Pytheas large wind-wave tunnel facility at Luminy, Marseille (France) (Study detailed in Bruch et al., in review). A total of 20 wind and wave combinations were tested, with wind speeds between 8 and 20 m s^-1 combined with pure wind waves and waves generated by a wavemaker, allowing for a range of wave characteristics and wave ages. Similar wind speed profiles and whitecapping behavior between the laboratory and the field suggest that the laboratory is appropriate for the study of sea spray production. The sea spray generation flux was estimated from logarithmic vertical sea spray concentration profiles using a flux-profile method using Monin and Obukhov (1954) theory. Results show that the production of larger droplets at 20-35 µm radius is well correlated with the wave slope variance <S²>, whilst the wind friction velocity cubed u_*³ performs best over 7-20 µm. Two SSGFs are proposed.

The original work presented here is an assessment of the validity of the two SSGFs in the field. The two laboratory-derived SSGFs are tested in two numerical models; the stationary Marine Aerosol Concentration Model (MACMod) (used in Laussac et al., 2018), and the non-hydrostatic mesocale atmospheric model Meso-NH (jointly developed by the LA - UMR 5560 - and the CNRM - UMR 3589). The <S²> necessary required by both SSGFs is estimated using a wind-dependent formulation (Cox and Munk, 1956) and a spectral spectral model (Elfouhaily et al., 1997). Results show that the numerical simulations offer good results relative to sea spray measurements obtained in the North-West Mediterranean in fetch-limited conditions (Laussac et al., 2018), as well as other existing SSGFs in the literature. These results suggest that wind-wave tunnel facilities present an interesting alternative for determining the sea spray generation flux, especially in high wind speed conditions in which deployment in the field is difficult.

References :

Bruch, W., Piazzola, J., Branger, H., van Eijk, A. M. J., Luneau, C., Bourras, D., Tedeschi, G. (In review). Sea Spray Generation Dependence on Wind and Wave Combinations : A Laboratory Study. Submitted in : Boundary Layer Meteorology.

Cox, C., & Munk, W. (1956). Slopes of the sea surface deduced from photographs of sun glitter. University of California Press. Vol. 6,9,401-488.

Elfouhaily, T., Chapron, B., Katsaros, K., & Vandemark, D. (1997). A unified directional spectrum for long and short wind driven waves. Journal of Geophysical Research: Oceans, 102(C7),15781-15796.

Monin, A. S., & Obukhov, A. M. (1954). Basic laws of turbulent mixing in the surface layer of the atmosphere. Contrib. Geophys. Inst. Acad. Sci. USSR,151(163),e187.

Laussac, S., Piazzola, J., Tedeschi, G., Yohia, C., Canepa, E., Rizza, U., & Van Eijk, A. M. J. (2018). Development of a fetch dependent sea-spray source function using aerosol concentration measurements in the North-Western Mediterranean. Atmospheric Environment,193,177-189.

How to cite: Bruch, W., Piazzola, J., Branger, H., van Eijk, A. M. J., Luneau, C., Yohia, C., Bourras, D., and Tedeschi, G.: Field validation of wave-wind-dependent sea spray generation functions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16292, https://doi.org/10.5194/egusphere-egu21-16292, 2021.

Accurate modelling of air-sea surface exchanges is crucial for reliable extreme surface wind forecasts.  While atmosphere-only weather forecast models represent ocean and wave effects through sea-state independent parametrizations, coupled multi-model systems capture sea-state dynamics by integrating feedbacks between atmosphere, ocean and wave model components.

Here, we present the results of studying the sensitivity of extreme surface wind speeds to air-sea exchanges at kilometre scale using coupled and uncoupled configurations of the Met Office's UK Regional Coupled Environmental Prediction (UKC4) system. The case period includes the passage of extra-tropical cyclones Helen, Ali, and Bronagh, which brought maximum gusts of 36 ms^-1 over the UK.

Compared to the atmosphere-only results, coupling to ocean decreases the domain-average sea surface temperature by up to 0.5 K. Inclusion of coupling to waves decreases the 98th percentile 10-m wind speed by up to 2 ms^-1 as young, growing wind waves decrease wind speed by increasing the sea aerodynamic roughness. Impacts on gusts are more modest, with local reductions of up to 1ms ^-1, due to enhanced boundary-layer turbulence which partially offsets air-sea momentum transfer.

Using a new drag parametrization based on the COARE~4.0 scheme, with a cap on the neutral drag coefficient and decrease for wind speeds exceeding 27 ms^-1 , the atmosphere-only model achieves equivalent impacts on 10-m wind speeds and gusts as from coupling to waves. Overall, the new drag parametrization achieves the same 20% improvement in forecast 10-m wind skill as coupling to waves, with  the  advantage  of saving the computational cost of the ocean and wave models. 

How to cite: Gentile, E. S., Gray, S. L., Barlow, J. F., Lewis, H. W., and Edwards, J. M.: The impact of atmosphere-ocean-wave coupling on extreme surface wind forecasts, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1273, https://doi.org/10.5194/egusphere-egu21-1273, 2021.

How to cite: Benetazzo, A., Barbariol, F., Bergamasco, F., Bertotti, L., Cavaleri, L., Yoo, J., and Shim, J.-S.: On the space-time maximum oceanic waves and related sea-state parameters during the tropical storm Kong-rey (2018), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14606, https://doi.org/10.5194/egusphere-egu21-14606, 2021.

Prediction of severe natural hazards requires accurate forecasting systems. Recently, there is a tendency to move towards more integrated solutions, where different components of the Earth system are coupled to better reproduce the physical feedbacks between them. Atmosphere–wave coupling should, in principle, improve the momentum flux because there is more detail in the two-way feedback due to the atmosphere receiving a more realistic picture of the surface roughness. However, the coupling between the ocean surface and the wind might become less efficient at transferring momentum during large storms.

This study focuses on rapidly developing waves under extratropical storms to understand the sensitivity in atmosphere–wave present generation source terms and coupling strategies. Here, we analyse the effect of momentum transfer to fast growth waves during both long and fetch limited conditions using the Met Office regional atmosphere–ocean–wave coupled research system for the northwestern (NW) European shelf (UKC4).

Two different sets of numerical experiments are conducted focusing on the atmosphere–wave components. The first one explores the sensitivity to two different wave source parameterizations, ST4 and ST6, and uses a two-way feedback coupling strategy (A2W) where a sea-state dependent surface roughness modifies the atmospheric momentum budget. In the second set of simulations, the impact of the coupling strategy is assessed. The A2W approach using ST6 physics is compared against a simpler one-way strategy (A1W) where no wave feedback on the atmospheric model exists and the wind stress is directly passed to the wave model (WAVEWATCHIII) ensuring conservation of momentum.

Results demonstrate that ST6 physics allows for a faster wave growth than the currently used ST4 parameterization but might degrade low to mid energy wave states for the NW shelf. ST6 versus ST4 difference in wave growth is larger for higher wind speeds and short fetches. The experiment with ST4 and A2W consistently under-predicts the wave growth in those locations across the NW shelf where fetch dependence is an important factor (i.e., seas at the E of Ireland and the UK for storms coming from the NW-WNW). The implementation in the wave model of physics that depend solely in the wind input (ST6) with the A1W coupling strategy appears to improve growth of young wind-seas, reducing bias in those locations where the storms are underestimated. The analysis of the transfer of momentum across the air-sea boundary layer shows that forecasts of large wave events may require a different coupling approach. The slower wave growth seems to be related to an underestimation of the momentum transfer computed by the wave model when coupling the wind speeds (A2W). This suggests that coupling the wind speeds to the wave model and allowing this to calculate the momentum transfer from the atmosphere to waves and ocean underestimates the transfer by a few percent. For very young to young wind seas, this can be overcome when the surface stress is computed by the atmospheric model and directly passed to the ocean (A1W).

How to cite: Valiente, N. G., Saulter, A., Edwards, J., Lewis, H., Castillo, J. M., Bruciaferri, D., and Bunney, C.: The impact of wave source terms and coupling strategies on the accuracy of the UKC4 regional coupled atmosphere–ocean–wave forecasting system during extreme events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2996, https://doi.org/10.5194/egusphere-egu21-2996, 2021.

The interaction between waves, surges and tides is one of the main drivers of coastal total water levels (TWL).  Understanding this interaction is crucial for studying high TWL formation near shore, and to do this it is important to not only evaluate how high the TWL is but also when and where it occurs.

In this study we use a high resolution (1.5 km) three-way coupled (waves-atmosphere-ocean) numerical model developed by the MetOffice (UKC4) to study coastal conditions at the UK coast during the extreme events of winter 2013, which was chosen as case study because of the amount of flooding that occurred in relation to storms and surges during this period.

For each coastal grid point the ten strongest storms of that winter, ranked by the significant wave height (Hs) magnitude, were selected. During these storm periods, the number of hours in which Hs and surges exceeded the 90^th percentile of winter 2013 were evaluated considering what tidal stage they occurred on. The same was done for instances where high Hs and surges occurred simultaneously. The aim is to understand if specific areas were predominantly affected by one of the TWL components and how Hs and surges interacted with the tide. What was the spatial distribution of the waves, surges, and tides during winter 2013? Did extreme Hs and Surges occur more often over specific stages of the tidal cycle? Did they occur simultaneously? 

In this study we show that during the winter 2013, Hs and surges above the 90^th percentile value did occur simultaneously at all stages of the tidal cycle. They more often occurred together over the rising tide with in average 8.7% and 8.6% of instances found two and three hours before high tide. In 7.7% of cases high wave and surges also concurred at high tide.

How to cite: Rulent, J.: Extreme waves and surges interaction with tides during storms in winter 2013/2014., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3160, https://doi.org/10.5194/egusphere-egu21-3160, 2021.