NP3.1

Geophysical fluid dynamics systems generally involve a wide range of spatio-temporal scales. Numerical representation can only simulate some of the scales. The others, at the unresolved scales of motion, must be parameterized for each type of phenomenon (wave, eddy, current), in terms of expected effects on the resolved scales. Most developments then assume that the fluid transport velocity has a time-uncorrelated noisy component with zero mean and stationary statistics. These approximations generally simplify theoretical descriptions, numerical simulations, data comparisons or more recently model error quantifications for data assimilation.

In the present work, we will discuss the applicability of such approximations through two examples: a surface oceanic current dynamics and swell refractions by surface currents.

When the time-decorrelation assumption is valid, we propose simple and tuning-free parametric models to represent the spatial correlations of the white-in-time small-scale velocity to help simulate the geophysical system of interest. These parametric models relies on turbulence space self-similarity and their statistical properties (e.g. spectral slope) can be easily estimated from observations of larger scale fluid velocities.

When the white-in-time approximation is not valid, we extend the previous parametric models to follow self-similarity properties in both time and space.

Numerical simulations will illustrate these theoretical developments along the presentation.

How to cite: resseguier, V., Hascoet, E., Chapron, B., and Fox-Kemper, B.: Validity domains and parametrizations for white-noise and multiscale models in turbulence and wave-turbulence interactions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-692, https://doi.org/10.5194/egusphere-egu21-692, 2021.

Vertical motions are fundamental to atmospheric dynamics and our understanding of phenomena such as moist convection. A long-standing problem in atmospheric sciences is to understand the mesoscale energy spectra. Several numerical studies show that the vertical velocity spectrum has a homogeneous energy distribution across the mesoscales with a flat spectrum. Compared to the energy spectra of horizontal motion, the mechanisms that govern the spectrum of vertical velocity are less well known. In the troposphere, most of the horizontal mesoscale energy comes from divergent motions. At large scales O(100 km), vertical velocity relates, to a good approximation, to the vertically averaged divergence of horizontal motions by continuity in the incompressible limit. Recent measurements from NARVAL-2 (Next Generation Remote Sensing for Validation Studies) campaign conducted in the tropical Atlantic, unveiled that mesoscale horizontal mass divergence profiles possess a rich vertical structure and high spatio-temporal variability. Although the premise of a radiatively-balanced circulation holds on the long-term average, instantaneous deviations from this equilibrium occur in the form of wave-like oscillations. Numerical studies show that our state-of-the-art models can reproduce the observed variability in mesoscale divergence. We ask the following question in support of the previous arguments: What controls the spectrum of coherent mesoscale vertical motion? We aim to elucidate the mechanisms determining the homogeneous energy distribution across horizontal scales of vertical velocity spectra. This study designs numerical experiments, which include mechanisms-denial simulations employing the Icosahedral Nonhydrostatic (ICON) model. We conducted numerical simulations on a limited-area domain located in the western tropical Atlantic (4°S – 18°N, 64°W – 42°W). This domain has a horizontal resolution of 1.25 km and a lid at 30 km—the analysis period spans 48 hours. The experiments include the following: (i) a control run using DWD NWP physics configuration (ii) a dry atmosphere with all moist processes excluded along with the latent heat surface fluxes (iii) clouds invisible to radiation and, (iv) effects of saturation adjustment on temperature neglected while maintaining surface heat fluxes. Preliminary results show that the divergence profiles horizontally averaged over 200 km present a clear dominance of vertical wavelengths of 3 – 6 km. We found autocorrelation time-scales of around 4 – 6 hours increasing with altitude outside convective areas and consistent among all simulations. All experiments show a systematic decrease of about 50% in the temporal autocorrelation inside convective areas; therefore, moist convective processes modulate divergence's temporal variability. Moreover, we found that local moist processes contribute the most considerable fraction to the energy spectrum at scales < 200 km. The spectral response to moist processes is broad and extends into the free troposphere. The spectral response of surface fluxes instead is confined to the subcloud layer.

How to cite: Morfa-Avalos, Y.: Why is the tropical sky white? Numerical investigations to elucidate the shape of mesoscale vertical velocity spectra., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13215, https://doi.org/10.5194/egusphere-egu21-13215, 2021.

The ocean surface wind plays a crucial role in the air-sea exchanges of momentum, heat, and mass, consequently is vital to the controlling of weather and climate. Due to the extremely large range of scales of the motion of the wind field, e.g., flow structures from millimeters to thousands of kilometers, the multiscale dynamics are known to be relevant. In this work, with the help of a Wiener-Khinchine theorem-based Fourier power spectrum estimator, the scaling features of the wind field provided by several satellites, i.e., QuikSCAT, Metop-A, -B, and -C, Haiyang-2B, and China France Oceanography SATellite (CFOSAT), is examined. Power-law scaling behavior is evident in the ranges of 100 to 3000 km with a scaling exponent β varying from 5/3 to 3. The global distributions and seasonal variations of the scaling exponent β have also been considered. The results show that due to the energetic convective activities in the low-latitude zones, the scaling exponents β in these regions are closer to the value of 5/3. As for the mid-latitudes, the values of β are close to 2 and independent of the variation of longitude. Concerning the seasonal variations, for most regions, the scaling exponents measured in winter are larger than those in summer. Furthermore, the seasonal variations of β in low-latitudes are stronger than those in the mid-latitudes. Our preliminary results indicate that all satellites provide a consistent scaling feature of the ocean surface wind field.

How to cite: Gao, Y., Schmitt, F. G., Hu, J., and Huang, Y.: Scaling Feature of the Ocean Surface Wind Field, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1717, https://doi.org/10.5194/egusphere-egu21-1717, 2021.

Planktonic copepods are tiny crustaceans, with a typical size of the order of mm, living in suspension in marine or freshwaters during their entire life cycle. They have swimming and jumping abilities and are known to be well adapted to their turbulent environment. Turbulence is known to increase their contact rate and feeding flux. However too intense turbulence is believed to have a negative effect so that a qualitative bell-shape is classically invoked to represent the contact rate of copepods versus turbulence intensity. In this framework, the objective of this work is to quantify the influence of ambient turbulence on copepod’s behavior, using trajectory analysis.

In this work, the motions of copepods were filmed using an infrared high-speed camera (1000 fps) in a turbulent environment, in the dark to avoid phototropism. The custom-made experimental set-up has been built-up in order to obtain in a central zone an isotropic and homogeneous turbulence representative of the natural environment. The flow was characterized with different tracer sizes at different turbulence intensities.

Copepods are filmed and the trajectories are extracted using signal processing routines. The instantaneous velocity, tangential and centripetal accelerations, and the local curvature are extracted for each trajectory. Their pdfs are computed, as well as different statistical moments: these indicators are studied at varying the turbulence intensity level (Reynolds number). Particles of different sizes (100 and 600 microns of mean diameters) and dead copepods are compared to living copepods statistics. This strategy allows to precisely characterize the copepods behavioral activity in relation with ambient turbulence. Ecological interpretations are drawn from the experimental results.

How to cite: Le Quiniou, C., Schmitt, F., Huang, Y., Calzavarini, E., and Souissi, S.: Copepods in turbulence: laboratory velocity and acceleration studies using high speed cameras, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4860, https://doi.org/10.5194/egusphere-egu21-4860, 2021.

The surface circulation in the Gulf of Tonkin (GoT) was analyzed using 2.5 year-long dataset from the High-Frequency radar (from April 2014 to October 2016). High temporal resolution of the measurements and large coverage from HFR dataset enable us to characterize the variability of surface circulation in the GoT in a wide range of scales: from tidal to annual scale. A number of techniques of data including rotary spectral analysis (RSA), principal component analysis (PCA), harmonic tidal analysis, coherent analysis, etc. were used to identify the dominant modes of variability. The tidal motions, accounting for approximately 62% of the total variability, revealed the dominance of diurnal components (K1 and O1) with 4 times larger magnitude than that of semi-diurnal constituent (M2). At seasonal scale, the monsoon wind plays an important role in driving the surface circulation in the GoT. This was supported by a tight correlation (0.7) between the wind stress and current velocities and by a large contribution (more than 50%) of the Ekman-driven component to the total variability of currents in the offshore area. Along the shore, large seasonal variability of circulation was highlighted. During the year, the seaward extension of the coastal current is primarily controlled by the cross-shore wind stress while the flow intensity is modulated by the Red River discharge.

How to cite: Tran, M. C., Sentchev, A., and Nguyen, K. C.: Multi-scale variability of surface currents in the Gulf of Tonkin derived from HF radar observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16284, https://doi.org/10.5194/egusphere-egu21-16284, 2021.

Lake Geneva is one of the largest bodies of water in western Europe and the largest one in the Alps region. Besides its obvious touristic importance it supplies drinking water for a large portion of Switzerland and work for hundreds of commercial fishermen. It has been under constant monitoring since the 1970's, for the impact of human activities on its water quality and biodiversity. The lake is known to be a warm monomictic lake, thermally stratified through most of the year with the exception of winter, when small thermal vertical gradients permit mixing from top to bottom. In lake Geneva, thermal stratification is one of the main environmental drivers of phytoplankton communities which are widely used as bioindicators for freshwater ecosystems. Studies on thermal stratification are thus essential to better predict phytoplankton seasonality and the development of harmful species blooms. In this work we examine more than 20 years of surveillance data from the INRAE (National Research Institute for Agriculture, Food and Environment) regarding temperature vertical profiles and meteorological data. We review both the climatology and the temperature stratification history of the lake and refine the temperature depth profiles obtaining the yearly progressions of the mixed layer depths. We then discuss the fitting of the depth profiles through the use of power-law and exponential functions, finding that in 66% of the cases the power-law better describes the experimental data, and we report the probability density function of the related statistics throughout the seasons.  Finally, we discuss the implications of our results for the modelling of the lake turbulent regime and phytoplankton seasonality.

How to cite: Beltram Tergolina, V., Jiang, Y., Schmitt, F., Berti, S., Calzavarini, E., and Anneville, O.: Power-law thermal stratification in lake Geneva and its seasonal evolution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8821, https://doi.org/10.5194/egusphere-egu21-8821, 2021.