scholarly journals Challenges associated with modeling low-oxygen waters in Chesapeake Bay: a multiple model comparison

2015 ◽  
Vol 12 (24) ◽  
pp. 20361-20409 ◽  
Author(s):  
I. D. Irby ◽  
M. A. M. Friedrichs ◽  
C. T. Friedrichs ◽  
A. J. Bever ◽  
R. R. Hood ◽  
...  
Keyword(s):  
Dissolved Oxygen ◽  
Chesapeake Bay ◽  
Mixed Layer ◽  
Model Comparison ◽  
Three Dimensional ◽  
Surface Mixed Layer ◽  
Multiple Model ◽  
Low Oxygen ◽  
Mixing Depth ◽  
Biogeochemical Models

Abstract. As three-dimensional (3-D) aquatic ecosystem models are becoming used more frequently for operational water quality forecasts and ecological management decisions, it is important to understand the relative strengths and limitations of existing 3-D models of varying spatial resolution and biogeochemical complexity. To this end, two-year simulations of the Chesapeake Bay from eight hydrodynamic-oxygen models have been statistically compared to each other and to historical monitoring data. Results show that although models have difficulty resolving the variables typically thought to be the main drivers of dissolved oxygen variability (stratification, nutrients, and chlorophyll), all eight models have significant skill in reproducing the mean and seasonal variability of dissolved oxygen. In addition, models with constant net respiration rates independent of nutrient supply and temperature reproduced observed dissolved oxygen concentrations about as well as much more complex, nutrient-dependent biogeochemical models. This finding has significant ramifications for short-term hypoxia forecasts in the Chesapeake Bay, which may be possible with very simple oxygen parameterizations, in contrast to the more complex full biogeochemical models required for scenario-based forecasting. However, models have difficulty simulating correct density and oxygen mixed layer depths, which are important ecologically in terms of habitat compression. Observations indicate a much stronger correlation between the depths of the top of the pycnocline and oxycline than between their maximum vertical gradients, highlighting the importance of the mixing depth in defining the region of aerobic habitat in the Chesapeake Bay when low-oxygen bottom waters are present. Improvement in hypoxia simulations will thus depend more on the ability of models to reproduce the correct mean and variability of the depth of the physically driven surface mixed layer than the precise magnitude of the vertical density gradient.

scholarly journals Challenges associated with modeling low-oxygen waters in Chesapeake Bay: a multiple model comparison

2016 ◽  
Vol 13 (7) ◽  
pp. 2011-2028 ◽  
Author(s):  
Isaac D. Irby ◽  
Marjorie A. M. Friedrichs ◽  
Carl T. Friedrichs ◽  
Aaron J. Bever ◽  
Raleigh R. Hood ◽  
...  
Keyword(s):  
Dissolved Oxygen ◽  
Chesapeake Bay ◽  
Mixed Layer ◽  
Model Comparison ◽  
Three Dimensional ◽  
Surface Mixed Layer ◽  
Multiple Model ◽  
Low Oxygen ◽  
Mixing Depth ◽  
Biogeochemical Models

Abstract. As three-dimensional (3-D) aquatic ecosystem models are used more frequently for operational water quality forecasts and ecological management decisions, it is important to understand the relative strengths and limitations of existing 3-D models of varying spatial resolution and biogeochemical complexity. To this end, 2-year simulations of the Chesapeake Bay from eight hydrodynamic-oxygen models have been statistically compared to each other and to historical monitoring data. Results show that although models have difficulty resolving the variables typically thought to be the main drivers of dissolved oxygen variability (stratification, nutrients, and chlorophyll), all eight models have significant skill in reproducing the mean and seasonal variability of dissolved oxygen. In addition, models with constant net respiration rates independent of nutrient supply and temperature reproduced observed dissolved oxygen concentrations about as well as much more complex, nutrient-dependent biogeochemical models. This finding has significant ramifications for short-term hypoxia forecasts in the Chesapeake Bay, which may be possible with very simple oxygen parameterizations, in contrast to the more complex full biogeochemical models required for scenario-based forecasting. However, models have difficulty simulating correct density and oxygen mixed layer depths, which are important ecologically in terms of habitat compression. Observations indicate a much stronger correlation between the depths of the top of the pycnocline and oxycline than between their maximum vertical gradients, highlighting the importance of the mixing depth in defining the region of aerobic habitat in the Chesapeake Bay when low-oxygen bottom waters are present. Improvement in hypoxia simulations will thus depend more on the ability of models to reproduce the correct mean and variability of the depth of the physically driven surface mixed layer than the precise magnitude of the vertical density gradient.

scholarly journals Quasigeostrophic Diagnosis of Mixed Layer Dynamics Embedded in a Mesoscale Turbulent Field

2016 ◽  
Vol 46 (1) ◽  
pp. 275-287 ◽  
Author(s):  
Cédric P. Chavanne ◽  
Patrice Klein
Keyword(s):  
Surface Layer ◽  
High Resolution ◽  
Mixed Layer ◽  
Finite Thickness ◽  
Vertical Mixing ◽  
Three Dimensional ◽  
Upper Ocean ◽  
Surface Mixed Layer ◽  
Deep Interior ◽  
Quasigeostrophic Model

AbstractA quasigeostrophic model is developed to diagnose the three-dimensional circulation, including the vertical velocity, in the upper ocean from high-resolution observations of sea surface height and buoyancy. The formulation for the adiabatic component departs from the classical surface quasigeostrophic framework considered before since it takes into account the stratification within the surface mixed layer that is usually much weaker than that in the ocean interior. To achieve this, the model approximates the ocean with two constant stratification layers: a finite-thickness surface layer (or the mixed layer) and an infinitely deep interior layer. It is shown that the leading-order adiabatic circulation is entirely determined if both the surface streamfunction and buoyancy anomalies are considered. The surface layer further includes a diabatic dynamical contribution. Parameterization of diabatic vertical velocities is based on their restoring impacts of the thermal wind balance that is perturbed by turbulent vertical mixing of momentum and buoyancy. The model skill in reproducing the three-dimensional circulation in the upper ocean from surface data is checked against the output of a high-resolution primitive equation numerical simulation.

scholarly journals Diagnosing Surface Mixed Layer Dynamics from High-Resolution Satellite Observations: Numerical Insights

2013 ◽  
Vol 43 (7) ◽  
pp. 1345-1355 ◽  
Author(s):  
Aurelien L. Ponte ◽  
Patrice Klein ◽  
Xavier Capet ◽  
Pierre-Yves Le Traon ◽  
Bertrand Chapron ◽  
...  
Keyword(s):  
High Resolution ◽  
Mixed Layer ◽  
Vertical Mixing ◽  
Mesoscale Eddy ◽  
Three Dimensional ◽  
Satellite Observations ◽  
Scaling Analysis ◽  
Surface Mixed Layer ◽  
Surface Layers ◽  
Velocity Scale

Abstract High-resolution numerical experiments of ocean mesoscale eddy turbulence show that the wind-driven mixed layer (ML) dynamics affects mesoscale motions in the surface layers at scales lower than O(60 km). At these scales, surface horizontal currents are still coherent to, but weaker than, those derived from sea surface height using geostrophy. Vertical motions, on the other hand, are stronger than those diagnosed using the adiabatic quasigeotrophic (QG) framework. An analytical model, based on a scaling analysis and on simple dynamical arguments, provides a physical understanding and leads to a parameterization of these features in terms of vertical mixing. These results are valid when the wind-driven velocity scale is much smaller than that associated with eddies and the Ekman number (related to the ratio between the Ekman and ML depth) is not small. This suggests that, in these specific situations, three-dimensional ML motions (including the vertical velocity) can be diagnosed from high-resolution satellite observations combined with a climatological knowledge of ML conditions and interior stratification.

scholarly journals Surface Mixed Layer Profile of Physical and Biogeochemical Variables in the Subpolar North-West and -East Atlantic Ocean: A Data-Model Comparison Study

2015 ◽  
Vol 05 (01) ◽  
pp. 33-44
Author(s):  
Nsikak U. Benson ◽  
Francis E. Asuquo ◽  
Oladele O. Osibanjo ◽  
Usoro M. Etesin ◽  
Adebusayo E. Adedapo
Keyword(s):  
Atlantic Ocean ◽  
Mixed Layer ◽  
Data Model ◽  
Model Comparison ◽  
Surface Mixed Layer ◽  
Comparison Study ◽  
East Atlantic ◽  
North West

Near-N Oscillations and Deep-Cycle Turbulence in an Upper-Equatorial Undercurrent Model

2012 ◽  
Vol 42 (12) ◽  
pp. 2169-2184 ◽  
Author(s):  
Hieu T. Pham ◽  
Sutanu Sarkar ◽  
Kraig B. Winters
Keyword(s):  
Mixed Layer ◽  
Turbulent Mixing ◽  
Three Dimensional ◽  
Surface Mixed Layer ◽  
Local Velocity ◽  
Equatorial Undercurrent ◽  
Frequency Matching ◽  
The Pacific ◽  
Horseshoe Vortices

Abstract Direct numerical simulation (DNS) is used to investigate the role of shear instabilities in turbulent mixing in a model of the upper Equatorial Undercurrent (EUC). The background flow consists of a westward-moving surface mixed layer above a stably stratified EUC flowing to the east. An important characteristic of the eastward current is that the gradient Richardson number Rig is larger than ¼. Nevertheless, the overall flow is unstable and DNS is used to investigate the generation of intermittent bursts of turbulent motions within the EUC region where Rig > ¼. In this model, an asymmetric Holmboe instability emerges at the base of the mixed layer, moves at the speed of the local velocity, and ejects wisps of fluid from the EUC upward. At the crests of the Holmboe waves, secondary Kelvin–Helmholtz instabilities develop, leading to three-dimensional turbulent motions. Vortices formed by the Kelvin–Helmholtz instability are occasionally ejected downward and stretched by the EUC into a horseshoe configuration creating intermittent bursts of turbulence at depth. Vertically coherent oscillations, with wavelength and frequency matching those of the Holmboe waves, propagate horizontally in the EUC where the turbulent mixing by the horseshoe vortices occurs. The oscillations are able to transport momentum and energy from the mixed layer downward into the EUC. They do not overturn the isopycnals, however, and, though correlated in space and time with the turbulent bursts, are not directly responsible for their generation. These wavelike features and intermittent turbulent bursts are qualitatively similar to the near-N oscillations and the deep-cycle turbulence observed at the upper flank of the Pacific Equatorial Undercurrent.

scholarly journals Characterization and Modulation of Langmuir Circulation in Chesapeake Bay

2015 ◽  
Vol 45 (10) ◽  
pp. 2621-2639 ◽  
Author(s):  
Malcolm E. Scully ◽  
Alexander W. Fisher ◽  
Steven E. Suttles ◽  
Lawrence P. Sanford ◽  
William C. Boicourt
Keyword(s):  
Chesapeake Bay ◽  
Mixed Layer ◽  
Vertical Velocity ◽  
Large Scale ◽  
Low Frequency ◽  
Surface Boundary ◽  
Surface Mixed Layer ◽  
Langmuir Circulation ◽  
Salinity Stratification ◽  
Velocity Skewness

AbstractMeasurements made as part of a large-scale experiment to examine wind-driven circulation and mixing in Chesapeake Bay demonstrate that circulations consistent with Langmuir circulation play an important role in surface boundary layer dynamics. Under conditions when the turbulent Langmuir number Lat is low (<0.5), the surface mixed layer is characterized by 1) elevated vertical turbulent kinetic energy; 2) decreased anisotropy; 3) negative vertical velocity skewness indicative of strong/narrow downwelling and weak/broad upwelling; and 4) strong negative correlations between low-frequency vertical velocity and the velocity in the direction of wave propagation. These characteristics appear to be primarily the result of the vortex force associated with the surface wave field, but convection driven by a destabilizing heat flux is observed and appears to contribute significantly to the observed negative vertical velocity skewness.Conditions that favor convection usually also have strong Langmuir forcing, and these two processes probably both contribute to the surface mixed layer turbulence. Conditions in which traditional stress-driven turbulence is important are limited in this dataset. Unlike other shallow coastal systems where full water column Langmuir circulation has been observed, the salinity stratification in Chesapeake Bay is nearly always strong enough to prevent full-depth circulation from developing.

SIMULATION AND EARLY WARNING OF NATURAL AND TECHNOGENIC INFLUENCES IN COASTAL AREAS IN THE SEA OF AZOV

2017 ◽  
Author(s):  
Tatiana Shulga ◽  
Tatiana Shulga ◽  
Leonid Cherkesov ◽  
Leonid Cherkesov
Keyword(s):  
Model Comparison ◽  
Three Dimensional ◽  
Storm Surges ◽  
Ecological Condition ◽  
Sea Of Azov ◽  
The Sea Of Azov ◽  
Waves And Currents ◽  
Coordinate Model ◽  
Passive Admixture ◽  
Comparison Of The Results

In this work, the waves and currents generated by prognostic wind in the Sea of Azov are investigated using a three-dimensional nonlinear sigma-coordinate model. The mathematical model was also used for studying the transformation of passive admixture in the Sea of Azov, caused by the spatiotemporal variations in the fields of wind and atmospheric pressure, obtained from the prediction SKIRON model. Comparison of the results of numerical calculations and the data of field observations, obtained during the action of the wind on a number of hydrological stations was carried out. The evolutions of storm surges, velocities of currents and the characteristics of the pollution region at different levels of intensity of prognostic wind and stationary currents were found. The results of a comprehensive study allow reliably estimate modern ecological condition of offshore zones, develop predictive models of catastrophic water events and make science-based solutions to minimize the possible damage.

Mixed Layer Models and Their Application to Water Quality Problems

1994 ◽  
Vol 29 (2-3) ◽  
pp. 221-232
Author(s):  
M.J. McCormick
Keyword(s):  
Water Quality ◽  
Mixed Layer ◽  
Thermal Structure ◽  
Mass Conservation ◽  
Eddy Diffusion ◽  
Rate Of Change ◽  
Surface Mixed Layer ◽  
Storm Events ◽  
Quality Model ◽  
Mixing Processes

Abstract Four one-dimensional models which have been used to characterize surface mixed layer (ML) processes and the thermal structure are described. Although most any model can be calibrated to mimic surface water temperatures, it does not imply that the corresponding mixing processes are well described. Eddy diffusion or "K" models can exhibit this problem. If a ML model is to be useful for water quality applications, then it must be able to resolve storm events and, therefore, be able to simulate the ML depth, h, and its time rate of change, dh/dt. A general water quality model is derived from mass conservation principles to demonstrate how ML models can be used in a physically meaningful way to address water quality issues.

scholarly journals Simulation of diurnal variability in vertical density structure using a coupled model

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
B. Yadidya ◽  
A. D. Rao ◽  
Sachiko Mohanty
Keyword(s):  
Mixed Layer ◽  
3D Model ◽  
Coupled Model ◽  
Wave Model ◽  
Andaman Sea ◽  
Surface Mixed Layer ◽  
Diurnal Variability ◽  
Rama Buoy ◽  
Vertical Displacements ◽  
Primary Advantage

AbstractThe changes in the physical properties of the ocean on a diurnal scale primarily occur in the surface mixed layer and the pycnocline. Price–Weller–Pinkel model, which modifies the surface mixed layer, and the internal wave model based on Garrett–Munk spectra that calculates the vertical displacements due to internal waves are coupled to simulate the diurnal variability in temperature and salinity, and thereby density profiles. The coupled model is used to simulate the hourly variations in density at RAMA buoy (15° N, 90° E), in the central Bay of Bengal, and at BD12 (10.5° N, 94° E), in the Andaman Sea. The simulations are validated with the in-situ observations from December 2013 to November 2014. The primary advantage of this model is that it could simulate spatial variability as well. An integrated model is also tested and validated by using the output of the 3D model to initialize the coupled model during January, April, July, and October. The 3D model can be used to initialize the coupled model at any given location within the model domain to simulate the diurnal variability of density. The simulations showed promising results which could be further used in simulating the acoustic fields and propagation losses which are crucial for Navy operations.

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