arXiv subjects

Kaushik Srinivasan

Publications and source records attributed to Kaushik Srinivasan.

5 recordsLinked to original sources

arXiv2026

Disentangling Internal Tides from Balanced Motions with Deep Learning and Surface Field Synergy

A fundamental challenge in ocean dynamics is disentangling balanced motions and internal waves. Extracting internal tidal (IT) imprints from surface data is a central part of this challenge. Traditional harmonic analysis can fail under strong incoherence and poor temporal sampling, as in global satellite observations. New wide-swath satellites provide two-dimensional spatial coverage, allowing IT extraction to be reformulated as image translation. Building on our earlier deep-learning approach for extracting IT signatures from sea surface height (SSH) in an idealized turbulent simulation, we show that a simpler, computationally cheaper algorithm performs comparably in our experiments when the learning rate is annealed during training. Using this algorithm, we test different combinations of surface inputs: SSH, surface temperature, and surface velocity. All fields contribute synergistically to disentanglement in our deterministic benchmark, with surface velocity by far the most informative. These findings underscore the value of coordinated multi-platform observations and highlight the importance of surface velocity for separating balanced motions and internal waves. Additional analysis shows that both wave-signature information and scattering-medium information aid IT extraction. To exploit large-scale, mesoscale-reaching information in the scattering medium, the algorithm must be highly non-local. Residual errors concentrate at small spatial scales near mode-2 tidal wavelengths, likely reflecting incomplete input information, uncertainty in the simulation-derived reference fields, including possible Doppler-shift contamination, and limitations of the present deterministic architecture.

physics.flu-dyn↗

arXiv2024

The High-Frequency and Rare Events Barriers to Neural Closures of Atmospheric Dynamics

Recent years have seen a surge in interest for leveraging neural networks to parameterize small-scale or fast processes in climate and turbulence models. In this short paper, we point out two fundamental issues in this endeavor. The first concerns the difficulties neural networks may experience in capturing rare events due to limitations in how data is sampled. The second arises from the inherent multiscale nature of these systems. They combine high-frequency components (like inertia-gravity waves) with slower, evolving processes (geostrophic motion). This multiscale nature creates a significant hurdle for neural network closures. To illustrate these challenges, we focus on the atmospheric 1980 Lorenz model, a simplified version of the Primitive Equations that drive climate models. This model serves as a compelling example because it captures the essence of these difficulties.

math.DS↗

arXiv2023

Gaussian processes at the Helm(holtz): A more fluid model for ocean currents

Given sparse observations of buoy velocities, oceanographers are interested in reconstructing ocean currents away from the buoys and identifying divergences in a current vector field. As a first and modular step, we focus on the time-stationary case - for instance, by restricting to short time periods. Since we expect current velocity to be a continuous but highly non-linear function of spatial location, Gaussian processes (GPs) offer an attractive model. But we show that applying a GP with a standard stationary kernel directly to buoy data can struggle at both current reconstruction and divergence identification, due to some physically unrealistic prior assumptions. To better reflect known physical properties of currents, we propose to instead put a standard stationary kernel on the divergence and curl-free components of a vector field obtained through a Helmholtz decomposition. We show that, because this decomposition relates to the original vector field just via mixed partial derivatives, we can still perform inference given the original data with only a small constant multiple of additional computational expense. We illustrate the benefits of our method with theory and experiments on synthetic and real ocean data.

stat.ME↗

arXiv2023

Turbulence closure with small, local neural networks: Forced two-dimensional and $β$-plane flows

We parameterize sub-grid scale (SGS) fluxes in sinusoidally forced two-dimensional turbulence on the $β$-plane at high Reynolds numbers (Re$\sim$25000) using simple 2-layer Convolutional Neural Networks (CNN) having only O(1000)parameters, two orders of magnitude smaller than recent studies employing deeper CNNs with 8-10 layers; we obtain stable, accurate, and long-term online or a posteriori solutions at 16X downscaling factors. Our methodology significantly improves training efficiency and speed of online Large Eddy Simulations (LES) runs, while offering insights into the physics of closure in such turbulent flows. Our approach benefits from extensive hyperparameter searching in learning rate and weight decay coefficient space, as well as the use of cyclical learning rate annealing, which leads to more robust and accurate online solutions compared to fixed learning rates. Our CNNs use either the coarse velocity or the vorticity and strain fields as inputs, and output the two components of the deviatoric stress tensor. We minimize a loss between the SGS vorticity flux divergence (computed from the high-resolution solver) and that obtained from the CNN-modeled deviatoric stress tensor, without requiring energy or enstrophy preserving constraints. The success of shallow CNNs in accurately parameterizing this class of turbulent flows implies that the SGS stresses have a weak non-local dependence on coarse fields; it also aligns with our physical conception that small-scales are locally controlled by larger scales such as vortices and their strained filaments. Furthermore, 2-layer CNN-parameterizations are more likely to be interpretable and generalizable because of their intrinsic low dimensionality.

physics.flu-dyn↗

arXiv2006

On a separation criterion for symmetric elliptic bluff body flows

A new analytical criterion that captures the onset of separation of flow past elliptic cylinders is derived by considering the variation of the wall normal velocity in Reynolds number parameter space. It is shown that this criterion can be used to calculate the separation Reynolds number Re_s for the classical problem of flow past a circular cylinder, a contentious and unresolved issue till date. The two dimensional Navier-Stokes equations are solved computationally and an exact value of Re_s is obtained by applying the aforementioned criterion.

physics.flu-dyn↗