Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session G20: Geophysical Fluid Dynamics Stratified Flow I |
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Chair: Paul Burns, University of Exeter Room: 602 |
Sunday, November 24, 2019 3:48PM - 4:01PM |
G20.00001: Dilution and Compression of Thermals in Stratified Domains Evan Anders, Daniel Lecoanet, Benjamin Brown Dense downflows are observed in the atmosphere (leading to cold pools), and may exist in stars. These downflows can be modeled as dense, negatively buoyant thermals. We present an analytical theory describing the evolution of dry thermals with depth, and verify the theory with 3D simulations of thermals in stratified atmospheres. Our results show that dense thermals fall in two categories: a stalling regime in which they slow down and expand, and a falling regime in which they accelerate and shrink as they propagate downwards. [Preview Abstract] |
Sunday, November 24, 2019 4:01PM - 4:14PM |
G20.00002: Regime transitions and energetics of sustained stratified shear flows Adrien Lefauve, Jamie Partridge, Paul Linden We describe the long-term dynamics of laboratory sustained stratified shear flows relevant to geophysical flows. The stratified inclined duct (SID) experiment sets up a sustained two-layer exchange flow in an inclined duct connecting two reservoirs containing salt solutions of different densities. Varying the two key parameters θ and Re (the tilt angle of the duct and the Reynolds number based on the density difference driving the flow) leads to four qualitatively different regimes: laminar flow; mostly laminar flow with Holmboe waves; spatio-temporally intermittent turbulence; and vigorous interfacial turbulence. In this talk we provide a quantitative basis for this regime classification and explain the power law scaling of the transitions in the (θ,Re) plane. We employ (i) newly-available, state-of-the-art simultaneous volumetric measurements of the density field and the three-component velocity field; (ii) time- and volume-averaged potential and kinetic energy budgets. We show and explain how regime transitions are caused by an increase in the non-dimensional time- and volume-averaged kinetic energy dissipation within the duct, which scales with the non-dimensional group θRe. [Preview Abstract] |
Sunday, November 24, 2019 4:14PM - 4:27PM |
G20.00003: Interaction of a tidally modulated stratified current with an isolated obstacle Pranav Suresh Puthan Naduvakkate, Masoud Jalali, Geno Pawlak, Sutanu Sarkar Three-dimensional obstacles on the ocean bottom are progenitors of turbulence and mixing. A numerical study of a flow past an idealized conical obstacle with height $h$ and bottom diameter $D$ is undertaken using the large eddy simulation (LES) technique. The flow is composed of two components: a uniform current ($U_c$) and a sinusoidal tidal modulation ($U_t \sin(\omega t)$). The velocity ratio $R=U_t/U_c$ is systematically varied (by varying $U_t$ while $U_c$ is constant) in the study keeping $\omega$ constant at M2 tidal frequency. The background stratification is strong so that the Froude number ($Fr_c = U_c/Nh$) based on the current is small. The cycle-averaged value of turbulent dissipation ($\varepsilon$), even when normalized with $U_m^3/D$ (where $U_m=U_c + U_t$ is the maximum barotropic velocity attained by the flow), increases with increasing $R$. The spatial and temporal variability of $\varepsilon$ during the tidal cycle is quantified to further understand the behavior of $\varepsilon$. Analysis of vortex dynamics reveal the formation of vortex dipoles during flow reversal, triggered by strong lateral flow and upstream acceleration of fluid in the recirculation zone. The characteristics of the internal wave field are also altered on varying $R$. [Preview Abstract] |
Sunday, November 24, 2019 4:27PM - 4:40PM |
G20.00004: ABSTRACT WITHDRAWN |
Sunday, November 24, 2019 4:40PM - 4:53PM |
G20.00005: Nonlinear resonance in the persistence of layers in stably-stratified fluids Paul Burns, Jemma Shipton, Beth Wingate In this work we look at the long-lived nature of layers in non-rotating stably-stratified flows and discuss whether our results support the idea that internal gravity waves (IGWs) are the cause of this longevity. We seek to describe the (low frequency) layer structure in space (or meanflow) using the non-linear resonance of IGWs. Our work is motivated by the phenomena of layer formation in the upper Arctic Ocean, which prevents the ice packs from melting by limiting vertical heat transfers. The research includes numerical simulations of an idealised laboratory experiment simplified to enable analytical methods. Our key analysis tool is a novel coordinate rotation, rotating our Boussinesq equations into the space of the waves and so revealing the non-linear resonance within our system. Future work will consider the effects of an external force and incorporate observations from the Beaufort Gyre Exploration program. [Preview Abstract] |
Sunday, November 24, 2019 4:53PM - 5:06PM |
G20.00006: Subsurface Suppression of Turbulence in the Bay of Bengal Rama Govindarajan, Ritabrata Thakur, Emily L Shroyer, J Thomas Farrar, Robert A Weller, James N Moum Quantifying the degree of turbulence, and mixing, in an ocean is important to understand how it responds to surface forces and distributes subsurface fluxes. The northern Bay of Bengal is highly salinity-stratified due to the discharge of numerous rivers and precipitation. From a year-long dataset of subsurface turbulent fluxes we show that it is not only surface forcing, but its interplay with an evolving complex subsurface stratification that determines the seasonality in turbulence. We observe a months-long suppression of turbulence below 40m when low-salinity water is present in the topmost layer (Thakur et al. "Seasonality and Buoyancy Suppression of Turbulence in the Bay of Bengal." Geophysical Research Letters 46.8 (2019): 4346-4355.), in spite of high winds. An implication is that the low mixing with deeper colder water can provide a heat source for tropical cyclogenesis. The stability reasons for this suppression will be discussed. [Preview Abstract] |
Sunday, November 24, 2019 5:06PM - 5:19PM |
G20.00007: Predicting Interannual Variability of Climate using Deep Learning Balasubramanya Nadiga, Changlin Jiang, Amir Farimani Predictability of climate over the interannual to decadal timescale (near term) is controlled by both natural variability related predictablity and external-forcing related predictability. Given that the field of near-term prediction of climate is in a nascent stage of development, we examine a deep learning approach to the problem. Preliminary work using a Long Short-Term Memory network architecture with added encoding and decoding is found to be capable of predicting an Earth System Model’s leading modes of global temperature variability with prediction lead times of upto a year. Related issues and further extensions are discussed. [Preview Abstract] |
Sunday, November 24, 2019 5:19PM - 5:32PM |
G20.00008: Bayesian calibration of global climate models using infrasound events Christophe MILLET, Francois LOTT, Alvaro de la Camara While stochastic parameterizations in Global Climate Models (GCMs) are promising for improving longstanding climate predictions, there is no consensus regarding the values of tunable parameters. In this work, we propose a Bayesian hierarchical approach to calibrate the input parameters of a stochastic multiwave gravity wave (GW) scheme, which is currently in use in the LMD GCM. The GW field is obtained as a combination of individual wavepackets, whose horizontal wavenumber, direction and phase speed are chosen randomly. These parameters are inferred using ground-based infrasound records as tracers of small-scale GW variability. In a sense, the acoustic signals are "back propagated" to adjust the GW sources on a daily basis, using for this a WKB approximation of the Taylor-Goldstein equation to represent the upward-propagating GWs. The method is applied using acoustic signals observed at the Norwegian station in August-September every year. These signals are known to be generated by the well-characterized daily ammunition destruction explosions that occur at the Hukkakero site, in northern Finland. The performance of the method is demonstrated by comparing the updated climatology and variability of the middle atmosphere with the reanalysis provided by the European Centre for Medium-Range Weather Forecasts [Preview Abstract] |
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