Dynamics and parameterization of stably stratified turbulence: implications for estimates of mixing in geophysical flows

2014 Summer.Includes bibliographical references.This research focuses on the relationship between the observed length scales of overturns in stably-stratified shear-flow turbulence and the fundamental length scales constructed from dimensional analysis of basic physical quantities. In geophysical flows such as the ocean, overturns are relatively easy to observe while the basic quantities are not. As such, overturns provide a means of inferring basic quantities if the relationship between the observed and fundamental scales are known. In turn, inferred values of the basic quantities, namely the the turbulent kinetic energy k, and the dissipation rate of turbulent kinetic energy ϵ, can be used to estimate diapycnal diffusivity (i.e. turbulent mixing). Most commonly, the observed Thorpe length scale, LT, is assumed to scale linearly with the fundamental Ozmidov scale, LO =(ϵ/N3)1/2, so that inferred values of ϵ can be obtained and used to estimate mixing from the Osborn formulation for diapycnal diffusivity. A major goal of this research is to re-examine this and other possible scalings using dimensional analysis, direct numerical simulation (DNS), laboratory data, and field observations. The preliminary chapters constitute a fresh approach at dimensional analysis that presents the fundamental length scales, time scales, and dimensionless parameters relevant to the problem. The relationship between LT and the fundamental length scales is then examined for the simple case of homogeneously stratified turbulence (without shear) using DNS. A key finding is that the common practice of inferring ϵ from LT ~ LO, is valid at the transition between a buoyancy-dominated regime and an inertia-dominated regime where the time scale of the buoyancy oscillations, N-1, roughly matches that of the inertial motions, TL = k/ϵ. Regime definition is made possible using a non-dimensional buoyancy strength parameter NTL = Nk/ϵ. Next, the problem is generalized to consider mean shear, and thus, a shear strength parameter, STL = Sk/ϵ, and the gradient Richardson number, Ri = N2/S2, are considered along with NTL to define three regimes available to high Reynolds number stratified shear-flow turbulence: a buoyancy-dominated regime (NTL ≳ 1.7, Ri ≳ 0.25), a shear-dominated regime (STL ≳ 3.3, Ri ≲ 0.25), and an inertia-dominated regime (NTL ≲ 1.7, STL ≲ 3.3). The regimes constitute a multi-dimensional parameter space which elucidates the independent influences that shear and stratification have on the turbulence. Using a large database of DNS and laboratory results, overturns are shown to have unique scalings in the various regimes. Specifically, LT ~ k1/2N-1, LT ~ k1/2S-1, and LT ~ k3/2ϵ-1 in the buoyancy-, shear-, and inertia-dominated regimes, respectively. LT ~ LO is found only for the case of NTL = O(1) and STL ≲ 3.3, or for NTL = O(100), STL ≈ 3.3 and Ri ≈ 0.25 when shear is present. In all three regimes, LT is found to generally indicate k rather than ϵ. An alternative parameterization of turbulent diffusivity is developed based on inferred values of k with a practical eye toward field applications. When tested with DNS and laboratory data, the new model is shown to be more accurate than estimates based on inferred values of ϵ. The multi-parameter framework is broadened with consideration for the turbulent Reynolds number, ReL, thus allowing for an evaluation of existing parameterizations of diapycnal mixing efficiency, R*f. Select DNS and laboratory data sets are used in the analysis. A key finding is that descriptions of R*f based on a single-parameter are generally insufficient. It is found that Ri is an accurate parameter in the shear-dominated regime but fails in the inertia-dominated regime where turbulence is generated by external forcing (rather than mean shear). In contrast, the turbulent Froude number, FrT = (LO/LT)2/3, is an accurate parameter in the inertia-dominated regime but loses accuracy in the shear-dominated regime. Neither Ri or FrT sufficiently describe R*f in the buoyancy-dominated regime where additional consideration for ReL is needed. Another key finding is that the popular buoyancy Reynolds number, Reb = ReL(NTL)-2, is a particularly misleading parameter for describing R*f because it fails to distinguish between (i) a low-Reynolds number, weakly stratified regime of low efficiency (low ReL, low NTL, low R*f) typical of DNS flows and (ii) a high-Reynolds number, strongly stratified regime of high efficiency (high ReL, high NTL, high R*f) typical of geophysical flows. Finally, oceanic observations from Luzon Strait and the Brazil Basin are featured to examine the relationship between LT and LO in geophysical flows where turbulence is driven by overturns that are very large by open ocean standards. LT is found to increase with respect to LO as a function of the normalized overturn size LT = LTN1/2ν-1/2. When large overturns are present, dissipation rates inferred from LT ~ LO are generally larger than measured values on average. The overestimation is quantified over a spring tidal period at Luzon Strait where depth- and time-integration of inferred and measured values show that inferred energy dissipation is four times too large

Biases in Thorpe-scale estimates of turbulence dissipation. Part I : Assessments from large-scale overturns in oceanographic data

Author Posting. © American Meteorological Society, 2015. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 45 (2015): 2497–2521, doi:10.1175/JPO-D-14-0128.1.Oceanic density overturns are commonly used to parameterize the dissipation rate of turbulent kinetic energy. This method assumes a linear scaling between the Thorpe length scale LT and the Ozmidov length scale LO. Historic evidence supporting LT ~ LO has been shown for relatively weak shear-driven turbulence of the thermocline; however, little support for the method exists in regions of turbulence driven by the convective collapse of topographically influenced overturns that are large by open-ocean standards. This study presents a direct comparison of LT and LO, using vertical profiles of temperature and microstructure shear collected in the Luzon Strait—a site characterized by topographically influenced overturns up to O(100) m in scale. The comparison is also done for open-ocean sites in the Brazil basin and North Atlantic where overturns are generally smaller and due to different processes. A key result is that LT/LO increases with overturn size in a fashion similar to that observed in numerical studies of Kelvin–Helmholtz (K–H) instabilities for all sites but is most clear in data from the Luzon Strait. Resultant bias in parameterized dissipation is mitigated by ensemble averaging; however, a positive bias appears when instantaneous observations are depth and time integrated. For a series of profiles taken during a spring tidal period in the Luzon Strait, the integrated value is nearly an order of magnitude larger than that based on the microstructure observations. Physical arguments supporting LT ~ LO are revisited, and conceptual regimes explaining the relationship between LT/LO and a nondimensional overturn size are proposed. In a companion paper, Scotti obtains similar conclusions from energetics arguments and simulations.B.D.M. and S.K.V. gratefully acknowledge the support of the Office of Naval Research under Grants N00014-12-1-0279, N00014-12-1-0282, and N00014-12-1-0938 (Program Manager: Dr. Terri Paluszkiewicz). S.K.V. also acknowledges support of the National Science Foundation under Grant OCE-1151838. L.S.L. acknowledges support for BBTRE by the National Science Foundation by Contract OCE94-15589 and NATRE and IWISE by the Office of Naval Research by Contracts N00014-92-1323 and N00014-10-10315. J.N.M. was supported through Grant 1256620 from the National Science Foundation and the Office of Naval Research (IWISE Project).2016-04-0

Biases in Thorpe-Scale Estimates of Turbulence Dissipation. Part I: Assessments from Large-Scale Overturns in Oceanographic Data

Oceanic density overturns are commonly used to parameterize the dissipation rate of turbulent kinetic energy. This method assumes a linear scaling between the Thorpe length scale L[subscript]T and the Ozmidov length scale L[subscript]O. Historic evidence supporting L[subscript]T ~ L[subscript]O has been shown for relatively weak shear-driven turbulence of the thermocline; however, little support for the method exists in regions of turbulence driven by the convective collapse of topographically influenced overturns that are large by open-ocean standards. This study presents a direct comparison of L[subscript]T and L[subscript]O, using vertical profiles of temperature and microstructure shear collected in the Luzon Strait—a site characterized by topographically influenced overturns up to O(100) m in scale. The comparison is also done for open-ocean sites in the Brazil basin and North Atlantic where overturns are generally smaller and due to different processes. A key result is that L[subscript]T/L[subscript]O increases with overturn size in a fashion similar to that observed in numerical studies of Kelvin–Helmholtz (K–H) instabilities for all sites but is most clear in data from the Luzon Strait. Resultant bias in parameterized dissipation is mitigated by ensemble averaging; however, a positive bias appears when instantaneous observations are depth and time integrated. For a series of profiles taken during a spring tidal period in the Luzon Strait, the integrated value is nearly an order of magnitude larger than that based on the microstructure observations. Physical arguments supporting L[subscript]T ~ L[subscript]O are revisited, and conceptual regimes explaining the relationship between L[subscript]T/L[subscript]O and a nondimensional overturn size are proposed. In a companion paper, Scotti obtains similar conclusions from energetics arguments and simulations.Keywords: Ocean Structure, Dynamics, Observational techniques and algorithms, Turbulence, Diapycnal mixing, Small scale processes, Atm, Circulation, Models and modeling, Parameterization, Phenomena, Profilers, oceanic, Mixin

Climate Process Team on internal wave–driven ocean mixing

Author Posting. © American Meteorological Society, 2017. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 98 (2017): 2429-2454, doi:10.1175/BAMS-D-16-0030.1.Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean and, consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Away from ocean boundaries, the spatiotemporal patterns of mixing are largely driven by the geography of generation, propagation, and dissipation of internal waves, which supply much of the power for turbulent mixing. Over the last 5 years and under the auspices of U.S. Climate Variability and Predictability Program (CLIVAR), a National Science Foundation (NSF)- and National Oceanic and Atmospheric Administration (NOAA)-supported Climate Process Team has been engaged in developing, implementing, and testing dynamics-based parameterizations for internal wave–driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here, we review recent progress, describe the tools developed, and discuss future directions.We are grateful to U.S. CLIVAR for their leadership in instigating and facilitating the Climate Process Team program. We are indebted to NSF and NOAA for sponsoring the CPT series.2018-06-0

Climate Process Team On Internal Wave-Driven Ocean Mixing

The study summarizes recent advances in our understanding of internal wave–driven turbulent mixing in the ocean interior and introduces new parameterizations for global climate ocean models and their climate impacts

Climate Process Team on Internal-Wave Driven Ocean Mixing

Diapycnal mixing plays a primary role in the thermodynamic balance of the ocean, and consequently, in oceanic heat and carbon uptake and storage. Though observed mixing rates are on average consistent with values required by inverse models, recent attention has focused on the dramatic spatial variability, spanning several orders of magnitude, of mixing rates in both the upper and deep ocean. Climate models have been shown to be very sensitive not only to the overall level but to the detailed distribution of mixing; sub-grid-scale parameterizations based on accurate physical processes will allow model forecasts to evolve with a changing climate. Spatio-temporal patterns of mixing are largely driven by the geography of generation, propagation and destruction of internal waves, which are thought to supply much of the power for turbulent mixing. Over the last five years and under the auspices of US CLIVAR, a NSF and NOAA supported Climate Process Team has been engaged in developing, implementing and testing dynamics-base parameterizations for internal-wave driven turbulent mixing in global ocean models. The work has primarily focused on turbulence 1) near sites of internal tide generation, 2) in the upper ocean related to wind-generated near inertial motions, 3) due to internal lee waves generated by low-frequency mesoscale flows over topography, and 4) at ocean margins. Here we review recent progress, describe the tools developed, and discuss future directions

Outcomes from elective colorectal cancer surgery during the SARS-CoV-2 pandemic

This study aimed to describe the change in surgical practice and the impact of SARS-CoV-2 on mortality after surgical resection of colorectal cancer during the initial phases of the SARS-CoV-2 pandemic

Mortality and pulmonary complications in patients undergoing surgery with perioperative SARS-CoV-2 infection: an international cohort study

Background: The impact of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) on postoperative recovery needs to be understood to inform clinical decision making during and after the COVID-19 pandemic. This study reports 30-day mortality and pulmonary complication rates in patients with perioperative SARS-CoV-2 infection. Methods: This international, multicentre, cohort study at 235 hospitals in 24 countries included all patients undergoing surgery who had SARS-CoV-2 infection confirmed within 7 days before or 30 days after surgery. The primary outcome measure was 30-day postoperative mortality and was assessed in all enrolled patients. The main secondary outcome measure was pulmonary complications, defined as pneumonia, acute respiratory distress syndrome, or unexpected postoperative ventilation. Findings: This analysis includes 1128 patients who had surgery between Jan 1 and March 31, 2020, of whom 835 (74·0%) had emergency surgery and 280 (24·8%) had elective surgery. SARS-CoV-2 infection was confirmed preoperatively in 294 (26·1%) patients. 30-day mortality was 23·8% (268 of 1128). Pulmonary complications occurred in 577 (51·2%) of 1128 patients; 30-day mortality in these patients was 38·0% (219 of 577), accounting for 81·7% (219 of 268) of all deaths. In adjusted analyses, 30-day mortality was associated with male sex (odds ratio 1·75 [95% CI 1·28–2·40], p\textless0·0001), age 70 years or older versus younger than 70 years (2·30 [1·65–3·22], p\textless0·0001), American Society of Anesthesiologists grades 3–5 versus grades 1–2 (2·35 [1·57–3·53], p\textless0·0001), malignant versus benign or obstetric diagnosis (1·55 [1·01–2·39], p=0·046), emergency versus elective surgery (1·67 [1·06–2·63], p=0·026), and major versus minor surgery (1·52 [1·01–2·31], p=0·047). Interpretation: Postoperative pulmonary complications occur in half of patients with perioperative SARS-CoV-2 infection and are associated with high mortality. Thresholds for surgery during the COVID-19 pandemic should be higher than during normal practice, particularly in men aged 70 years and older. Consideration should be given for postponing non-urgent procedures and promoting non-operative treatment to delay or avoid the need for surgery. Funding: National Institute for Health Research (NIHR), Association of Coloproctology of Great Britain and Ireland, Bowel and Cancer Research, Bowel Disease Research Foundation, Association of Upper Gastrointestinal Surgeons, British Association of Surgical Oncology, British Gynaecological Cancer Society, European Society of Coloproctology, NIHR Academy, Sarcoma UK, Vascular Society for Great Britain and Ireland, and Yorkshire Cancer Research