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Inside the Marine Atmospheric Boundary Layer (MABL) turbulence drives a large amount of physical processes through a dynamical coupling. Understanding this coupling is a key issue in weather and climate modeling, but an adapted statistical representation is still lacking. A strong limitation comes from the non-Gaussianities existing inside the MABL. We seek an approach to describe statistically the couplings across scales, which is poorly measured by the power spectrum. Recent developments in data science provide new tools as the Wavelet Scattering Transform (WST), which gives a low-variance statistical description of non-Gaussian processes and offers to go beyond the power spectrum representation.We proposes to apply WST technique on turbulent moments recorded during the EUREC4A campaign in trade-wind cloud regimes. Using WST analysis on both fluctuating time series and realistic Large-Eddy Simulations (LES), we study how the different scales in turbulent flows interact between each other in the MABL. Are the turbulent flows organised or purely random in the MABL ? Do the couplings change for different cloud organisations ? Do the models show evidence of such scale couplings ?In this presentation, we first look at the expected WST coefficients law for turbulent flows in the inertial subrange. Second, we present what are the WST coefficients for the measured turbulent moments and how they evolve for different cloud organizations.

The performance in term of tropical cyclone track and intensity prediction of the new coupled ocean-atmosphere system based on the operational atmospheric model AROME-Indian Ocean and the ocean model NEMO is assessed against that of the current operational configuration in the case of seven recent tropical cyclones. Five different configurations of the forecast system are evaluated: two with the coupled system, two with an ocean mixed layer parameterization and one with a constant sea surface temperature. For each ocean-atmosphere coupling option, one is initialized directly with the MERCATOR-Ocean PSY4 product as in the current operational configuration and the other with the ocean state that is cycled in the AROME-NEMO coupled suite since a few days before the cyclone intensification. The results show that the coupling with NEMO generally improves the intensity of cyclones in AROME-IO, reducing the mean intensity bias of the 72 h forecast of about 10 hPa. However, the impact is especially significant when the TCs encounter a slow propagation phase. For short-term forecasts (less than 36 hours), the presence of a cooling in the initial state that has been triggered by the AROME high-resolution cyclonic winds in a previous coupled forecast already improves the tropical cyclone intensity bias of 2-3 hPa for both coupled or uncoupled configurations. Key Points AROME/NEMO improves the forecast of tropical cyclone compared to the operational configuration with a 1D ocean mixed layer parameterization The improvement mostly comes from quasi-stationary or very slow moving intense cyclones The tropical cyclone forecasts are sensitive to the ocean initial conditions Plain Language Summary The ocean provides a large part of the energy for the intensification of tropical cyclones through warm sea surface temperature and sea-air heat and moisture exchanges. However, the ocean-atmosphere interactions also trigger processes which cools the sea surface temperature beneath the tropical cyclone and thus generates a negative feedback on the TC intensification. The numerical forecasts of the regional numerical weather prediction model AROME-IO are valuable guidance for the Regional Specialized Meteorological Centre for Tropical Cyclones, La Réunion. The objective of our study is to evaluate the possibility of replacing the current ocean mixed layer parameterization by a more realistic ocean able to represent more complex processes such as the explicit transport by the currents. Overall, we found that the new coupling improves the cyclone intensity in AROME-IO both in terms of bias and standard deviation. These improvements come almost entirely from tropical cyclones that encounter a slow propagation phase. For short-term forecasts, the presence of a cooling that is triggered by AROME high-resolution cyclonic winds in the initial state of the ocean already improves the TC intensity forecast, even when the ocean mixed layer parameterization is used.

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

With the flourishing of offshore wind projects there is a new socio-economic interest to better our knowledge and forecasting ability of winds within the coastal marine atmospheric boundary layer (MABL). Air-sea fluxes of enthalpy and momentum greatly influence the turbulent and mean winds in the MABL. Already at moderate but certainly at high winds, wave breaking is a key driver of air-sea fluxes and the sea spray generated by whitecaps is thought to be a crucial component when modelling air-sea interactions. Most studies so far have focused on the role of sea spray in enhancing tropical cyclone intensity. Here we investigate its impacts on the MABL under strong orographic wind forcing. A coupled model framework was developed within the scope of the CASSIOWPE project aiming at characterizing the physical environment in the Gulf of Lion (NW Mediterranean Sea) in the prospective of future floating wind farms development. It consists of the non-hydrostatic mesoscale atmospheric model of the French research community Meso-NH, the 3rd generation wave model WAVEWATCH III®, and the oceanic model CROCO. Sea-spray physics were incorporated into the Meso-NH’s surface model SURFEX. Added parametrizations will be detailed and a series of test cases will be presented to illustrate how sea spray alters the MABL under Mistral and Tramontane winds. Several sea-state dependent sea spray generation functions (SSGF) are considered in the present study. The variability in simulated fields linked to the choice of wave forcing or coupling will be showcased to evaluate their suitability in varying fetch conditions. Sea spray production remains to be adequately quantified. Existing measurement derived SSGFs span several orders of magnitude resulting in uncertainties in simulated fields which will be discussed.

The representation of the Ushant front in Meteo-France numerical models is not accurate. The aim of this study is to evaluate the impact of a better representation of this front derived from SST satellite observation data on the weather forecast in Brittany. The study consisted in finding and selecting cases from 2016 to 2018 where the Ushant front was present in satellite SST analysis (high spatial and temporal resolution ) with differences in weather pattern between North and South Brittany. Then compare this to the operational Arome model (Meteo France non hydrostatic model).Situations of disagreement between the model and the observations were selected. Some weather forecast simulations with Mesonh model (very close to Arome) were performed on these cases with a better definition of the Ushant front. We present some results.

A set of realistic coastal coupled ocean-wave numerical simulations is used to study the impact of surface gravity waves on a tidal temperature front and surface currents. The processes at play are elucidated through analyses of the budgets of the horizontal momentum, the temperature, and the turbulence closure equations. The numerical system consists of a 3D coastal hydrodynamic circulation model (Model for Applications at Regional Scale, MARS3D) and the third generation wave model WAVEWATCH III (WW3) coupled with OASIS-MCT at horizontal resolutions of 500 and 1500 m, respectively. The models were run for a period of low to moderate southwesterly winds as observed during the Front de Maree Variable (FroMVar) field campaign in the Iroise Sea where a seasonal small-scale tidal sea surface temperature front is present. Over the 2 day period considered, long fetch waves grow gradually propagating north east and east. Contrasting a stand-alone ocean run with a coupled ocean-wave run shows that waves move the Ushant front offshore by up to 4 kilometres and cool the offshore stratified side of the front by up to 1.5°C. The analysis of the temperature budget shows that the change in advection is the dominant factor contributing to the frontal shift while the contribution of wave enhanced vertical temperature diffusion is secondary. Temperature, considered to be a tracer, is advected in the coupled run by the Lagrangian current resulting from the quasi-Eulerian and Stokes drift. Although the Stokes drift is directed shorewards, changes in the quasi-Eulerian current lead to a more offshore advection in the coupled than the stand-alone run. The quasi-Eulerian current is reduced (enhanced) during the ebb (flood) flow which correspond to periods of wave-following (-opposing) currents. This is due to wave breaking enhanced vertical mixing acting on the positive vertical gradient present in the quasi-Eulerian current during both ebb and flood tides. Partially coupled runs reveal that it is the surface flux of TKE associated to wave breaking that is key rather than the changes in the surface stress. They further elucidate the role of other modelled wave related processes. Although the contribution of the Stokes–Coriolis force and the wave breaking induced enhancement in vertical mixing to the quasi-Eulerian current are of similar magnitude and sign, it does not contribute significantly to the frontal shift. This is because it partially counters the Stokes drift advection which pushes the front shorewards. All Stokes drift related processes combined thus only lead to a very slight displacement of the front.