Parameterization Of Vertical Mixing Research Articles

Evaluating CMIP5 simulations of mixed layer depth during summer

The ability of CMIP5 models in simulating surface mixed layer depth (MLD) during summer is assessed using 45 climate models. Their ocean models differ greatly in terms of vertical mixing parameterizations and model configurations. In some models, effects of surface waves, Langmuir circulations, submesoscale eddies, as well as additional wind mixing are included to improve upper-ocean simulation. Similar to findings by previous studies, the summer MLDs are significantly underestimated in most of the models. Compared with the observation, only five of these models have deeper summer MLDs in the Southern Ocean, eight models have deeper summer MLDs in the central North Atlantic Ocean, and nine models have deeper summer MLDs in the central North Pacific Ocean. This underestimation of MLD is not caused by sea surface forcing, because most of the models tend to overestimate the surface wind stress, while they underestimate the net surface heat flux. Therefore, insufficient vertical mixing in the upper ocean may still be one of the potential reasons for this systematic underestimation of MLD in the climate models.

수직 혼합 모수화 기법과 탁도에 따른 황해 수온 민감도 실험

Accurate prediction of sea water temperature has been emphasized to make precise local weather forecast and to understand change of ecosystem. The Yellow Sea, which has turbid water and strong tidal current, is an unique shallow marginal sea. It is essential to include the effects of the turbidity and the strong tidal mixing for the realistic simulation of temperature distribution in the Yellow Sea. Evaluation of ocean circulation model response to vertical mixing scheme and turbidity is primary objective of this study. Three-dimensional ocean circulation model(Regional Ocean Modeling System) was used to perform numerical simulations. Mellor- Yamada level 2.5 closure (M-Y) and K-Profile Parameterization (KPP) scheme were selected for vertical mixing parameterization in this study. Effect of Jerlov water type 1, 3 and 5 was also evaluated. The simulated temperature distribution was compared with the observed data by National Fisheries Research and Development Institute to estimate model`s response to turbidity and vertical mixing schemes in the Yellow Sea. Simulations with M-Y vertical mixing scheme produced relatively stronger vertical mixing and warmer bottom temperature than the observation. KPP scheme produced weaker vertical mixing and did not well reproduce tidal mixing front along the coast. However, KPP scheme keeps bottom temperature closer to the observation. Consequently, numerical ocean circulation simulations with M-Y vertical mixing scheme tends to produce well mixed vertical temperature structure and that with KPP vertical mixing scheme tends to make stratified vertical temperature structure. When Jerlov water type is higher, sea surface temperature is high and sea bottom temperature is low because downward shortwave radiation is almost absorbed near the sea surface.

A Unified Model for Moist Convective Boundary Layers Based on a Stochastic Eddy-Diffusivity/Mass-Flux Parameterization

Abstract A single-column model (SCM) is developed for representing moist convective boundary layers. The key component of the SCM is the parameterization of subgrid-scale vertical mixing, which is based on a stochastic eddy-diffusivity/mass-flux (EDMF) approach. In the EDMF framework, turbulent fluxes are calculated as a sum of the turbulent kinetic energy–based eddy-diffusivity component and a mass-flux component. The mass flux is modeled as a fixed number of steady-state plumes. The main challenge of the mass-flux model is to properly represent cumulus clouds, which are modeled as moist plumes. The solutions have to account for a realistic representation of condensation within the plumes and of lateral entrainment into the plumes. At the level of mean condensation within the updraft, the joint pdf of moist conserved variables and vertical velocity is used to estimate the proportion of dry and moist plumes and is sampled in a Monte Carlo way creating a predefined number of plumes. The lateral entrainment rate is modeled as a stochastic process resulting in a realistic decrease of the convective cloudiness with height above cloud base. In addition to the EDMF scheme, the following processes are included in the SCM: a pdf-based parameterization of subgrid-scale condensation, a simple longwave radiation, and one-dimensional dynamics. Note that in this approach there are two distinct pdfs, one representing the variability of updraft properties and the other one the variability of thermodynamic properties of the surrounding environment. The authors show that the model is able to capture the essential features of moist boundary layers, ranging from stratocumulus to shallow-cumulus regimes. Detailed comparisons, which include pdfs, profiles, and integrated budgets with the Barbados Oceanographic and Meteorological Experiment (BOMEX), Dynamics and Chemistry of Marine Stratocumulus (DYCOMS), and steady-state large-eddy simulation (LES) cases, are discussed to confirm the quality of the present approach.

In-canopy gas-phase chemistry during CABINEX 2009: sensitivity of a 1-D canopy model to vertical mixing and isoprene chemistry

Abstract. Vegetation emits large quantities of biogenic volatile organic compounds (BVOC). At remote sites, these compounds are the dominant precursors to ozone and secondary organic aerosol (SOA) production, yet current field studies show that atmospheric models have difficulty in capturing the observed HOx cycle and concentrations of BVOC oxidation products. In this manuscript, we simulate BVOC chemistry within a forest canopy using a one-dimensional canopy-chemistry model (Canopy Atmospheric CHemistry Emission model; CACHE) for a mixed deciduous forest in northern Michigan during the CABINEX 2009 campaign. We find that the base-case model, using fully-parameterized mixing and the simplified biogenic chemistry of the Regional Atmospheric Chemistry Model (RACM), underestimates daytime in-canopy vertical mixing by 50–70% and by an order of magnitude at night, leading to discrepancies in the diurnal evolution of HOx, BVOC, and BVOC oxidation products. Implementing observed micrometeorological data from above and within the canopy substantially improves the diurnal cycle of modeled BVOC, particularly at the end of the day, and also improves the observation-model agreement for some BVOC oxidation products and OH reactivity. We compare the RACM mechanism to a version that includes the Mainz isoprene mechanism (RACM-MIM) to test the model sensitivity to enhanced isoprene degradation. RACM-MIM simulates higher concentrations of both primary BVOC (isoprene and monoterpenes) and oxidation products (HCHO, MACR+MVK) compared with RACM simulations. Additionally, the revised mechanism alters the OH concentrations and increases HO2. These changes generally improve agreement with HOx observations yet overestimate BVOC oxidation products, indicating that this isoprene mechanism does not improve the representation of local chemistry at the site. Overall, the revised mechanism yields smaller changes in BVOC and BVOC oxidation product concentrations and gradients than improving the parameterization of vertical mixing with observations, suggesting that uncertainties in vertical mixing parameterizations are an important component in understanding observed BVOC chemistry.

Three-dimensional modelling of wave-induced current from the surf zone to the inner shelf

Abstract. We develop and implement a new method to take into account the impact of waves into the 3-D circulation model SYMPHONIE (Marsaleix et al., 2008, 2009a) following the simplified equations of Bennis et al. (2011) which use glm2z-RANS theory (Ardhuin et al., 2008c). These adiabatic equations are completed by additional parameterizations of wave breaking, bottom friction and wave-enhanced vertical mixing, making the forcing valid from the surf zone through to the open ocean. The wave forcing is performed by wave generation and propagation models WAVEWATCH III® (Tolman, 2008, 2009; Ardhuin et al., 2010) and SWAN (Booij et al., 1999). The model is tested and compared with other models for a plane beach test case, previously tested by Haas and Warner (2009)and Uchiyama et al. (2010). A comparison is also made with the laboratory measurements of Haller et al. (2002) of a barred beach with channels. Results fit with previous simulations performed by other models and with available observational data. Finally, a realistic case is simulated with energetic waves travelling over a coast of the Gulf of Lion (in the northwest of the Mediterranean Sea) for which currents are available at different depths as well as an accurate bathymetric database of the 0–10 m depth range. A grid nesting approach is used to account for the different forcings acting at different spatial scales. The simulation coupling the effects of waves and currents is successful to reproduce the powerful northward littoral drift in the 0–15 m depth zone. More precisely, two distinct cases are identified: When waves have a normal angle of incidence with the coast, they are responsible for complex circulation cells and rip currents in the surf zone, and when they travel obliquely, they generate a northward littoral drift. These features are more complicated than in the test cases, due to the complex bathymetry and the consideration of wind and non-stationary processes. Wave impacts in the inner shelf are less visible since wind and regional circulation seem to be the predominant forcings. Besides, a discrepancy between model and observations is noted at that scale, possibly linked to an underestimation of the wind stress. This three-dimensional method allows a good representation of vertical current profiles and permits the calculation of the shear stress associated with waves and currents. Future work will focus on the combination with a sediment transport model.

Distribution of vertical turbulent mixing parameter caused by internal tidal waves and solitary waves in the South Yellow Sea

Many observations show that in the Yellow Sea internal tidal waves (ITWs) possess the remarkable characteristics of internal Kelvin wave, and in the South Yellow Sea (SYS) the nonlinear evolution of internal tidal waves is one of the mechanisms producing internal solitary waves (ISWs), which is different from the generation mechanism in the case where the semidiurnal tidal current flows over topographic drops. In this paper, the model of internal Kelvin wave with continuous stratification is given, and an elementary numerical study of nonlinear evolution of ITWs is made for the SYS, using the generalized KdV model (GKdV model for short) for a continuous stratified ocean, in which the different effects of background barotropic ebb and flood currents are considered. Moreover, the parameterization of vertical turbulent mixing caused by ITWs and ISWs in the SYS is studied, using a parameterization scheme which was applied to numerical experiments on the breaking of ISWs by Vlasenko and Hutter in 2002. It is found that the vertical turbulent mixing caused by internal waves is very strong within the upper layer with depth less than about 30m, and the vertical turbulent mixing caused by ISWs is stronger than that by ITWs.

Comparison between vertical shear mixing and surface wave‐induced mixing in the extratropical ocean

Most parameterizations of vertical mixing are associated with local shear instability, which do not explicitly include the effects of surface waves. Here, we compared the performance of vertical mixing induced by vertical shear of the mean current and that by nonbreaking surface waves in the upper ocean through three numerical experiments. The vertical mixing from vertical shear alone was too weak especially in the extratropical ocean, and failed to produce a reasonable mixed layer depth and seasonal thermocline, which resulted in a large cold bias and an unrealistic seasonal cycle in the subsurface. Surface waves can enhance the vertical mixing in the upper ocean, and induce vertical mixing to sustain a reasonable upper ocean temperature structure especially in the extratropical ocean. Both the temperature structure and seasonal cycle were significantly improved by including the nonbreaking surface wave–induced vertical mixing, no matter whether shear effect was included or not. These results indicate that the vertical mixing from surface waves is more important than that associated with velocity shear of the mean current for the upper ocean especially in the extratropics.

Are there inescapable issues prohibiting the use of terrain-following coordinates in climate models?

The use of σ-coordinate ocean models has historically been considered a disadvantage for large-scale climate studies. The main reason resides in the non-alignment of the vertical coordinate isosurfaces with either geopotential surfaces or isopycnals making it harder to accurately compute the horizontal pressure gradient, advection, and isoneutral tracer diffusion. Moreover, this class of model requires a vertical mixing parameterization robust to large changes in the vertical resolution between shallow and deep areas. In this paper we show that, with some adjustments of the tracer advection, the surface boundary layer parameterization and the vertical grid, a σ-coordinate model can achieve an accurate representation of the oceanic interior and mixed-layer dynamics. To do so, a new way of handling the temporal discretization of the rotated biharmonic operator is used to achieve tracer variance dissipation in an adiabatic and computationally efficient way. Furthermore, a redesign of the K-Profile surface layer Parameterization (KPP) to improve the regularity of the solution and the overall numerical efficiency of the scheme is introduced. To validate the new algorithmic developments, we perform a set of coarse-resolution realistic basin-scale Pacific simulations. Besides improving the conservation of water mass properties, the use of an isoneutral tracer hyperdiffusion is shown to have a negative feedback on the circulation error growth rate, thus significantly reducing the sensitivity of the model solution to the degree of topographic smoothing. The overall validation of our simulations, focusing on the key characteristics of the circulation in the Pacific Ocean, provides some evidence of the efficacy of a terrain-following coordinate for large scale applications.

Evaluation and Sensitivity Analysis of an Ocean Model Response to Hurricane Ivan

Abstract An ocean model response to Hurricane Ivan (2004) over the northwest Caribbean Sea and Gulf of Mexico is evaluated to guide strategies for improving performance during strong forcing events in a region with energetic ocean features with the ultimate goal of improving coupled tropical cyclone forecasts. Based on prior experience, a control experiment is performed using quasi-optimal choices of initial ocean fields, atmospheric forcing fields, air–sea flux parameterizations, vertical mixing parameterizations, and both horizontal and vertical resolutions. Alternate experiments are conducted by altering one single model attribute and comparing the results to SST analyses and moored ADCP current measurements to quantify the sensitivity to that attribute and identify where to concentrate model improvement efforts. Atmospheric forcing that does not resolve the eye and eyewall of the storm (scales >10 km) substantially degrades the ocean response. Ordering other model attributes from greatest to least sensitivity, ocean model initialization with regard to the accuracy of upper-ocean temperature–salinity profiles along with accurate location of ocean currents and eddies is the most important factor for ensuring good ocean model performance. Ocean dynamics ranks second in this energetic ocean region because a one-dimensional ocean model fails to capture important physical processes that affect SST cooling. Wind stress drag coefficient parameterizations that yield values exceeding 2.5 × 10−3 at high wind speeds or that remain <2.0 × 10−3 over all wind speeds reduce the realism of wind-driven current profiles and have a large impact on both SST cooling and the heat flux from ocean to atmosphere. Turbulent heat flux drag coefficient parameterizations substantially impact the surface heat flux while having little impact on SST cooling, which is primarily controlled by entrainment at the mixed layer base. Vertical mixing parameterizations have a moderate impact on SST cooling but a comparatively larger impact on surface heat flux. The impacts of altering the horizontal and vertical resolutions are small, with horizontal resolution of ≈10 km and vertical resolution of ≈10 m in the mixed layer being adequate. Optimal choices of all attributes for simulating the ocean response to Ivan are identified.

Frontal dynamics in a California Current System shallow front: 2. Mesoscale vertical velocity

This is the second paper investigating the three‐dimensional dynamics from two consecutive, quasi‐synoptic surveys of a shallow front in the California Current System. The mesoscale vertical velocity (w) is obtained by solving a generalized ω equation using density and horizontal velocity observations. Highly nonlinear dynamics emerge from the ageostrophic forcing terms for w in an adiabatic generalized ω equation. The two main processes driving w are (1) wind‐induced cross‐frontal ageostrophic circulation (survey 1) and (2) ageostrophic kinematic deformation during frontogenesis (surveys 1 and 2). The horizontally averaged heat fluxes are positive in the whole water column with maxima at ∼50 m, which warms (cools) the upper (lower) water column and upwells (downwells) light (dense) water. Wind‐induced currents interact with the front, cooling the upper ocean and creating a divergent potential vorticity (PV) flux at the Ekman layer base which weakens the vertical heat and PV fluxes in survey 1. Vertical velocity extrema reach ∼10 m d−1 in both surveys. A diabatic ω equation is derived, which introduces an important new idea: the relation of the frictional w with the vertical diffusivity of the differential ageostrophic vorticity. This term is not found in the quasi‐geostrophic ω equation. By including vertical mixing, ∣w∣ is enhanced by a factor of 2 in the upper ∼100 m and reduced below. This effect is pronounced when the wind blows in the direction of the frontal jet, but it is sensitive to the vertical mixing parameterization.

Modeling tides and vertical tidal mixing: A reality check

Recently, there has been a great interest in the tidal contribution to vertical mixing in the ocean. In models, vertical mixing is estimated using parameterization of the sub-grid scale processes. Estimates of the vertical mixing varied widely depending on which vertical mixing parameterization was used. This study investigated the performance of ten different vertical mixing parameterizations in a terrain-following ocean model when simulating internal tides. The vertical mixing parameterization was found to have minor effects on the velocity fields at the tidal frequencies, but large effects on the estimates of vertical diffusivity of temperature. Although there was no definitive best performer for the vertical mixing parameterization, several parameterizations were eliminated based on comparison of the vertical diffusivity estimates with observations. The best performers were the new generic coefficients for the generic length scale schemes and Mellor-Yamada's 2.5 level closure scheme.

Helium isotopic constraints on simulated ocean circulations: implications for abyssal theories

There is ongoing controversy as to the dynamical significance of geothermal heat flow in shaping the abyssal circulation. In this paper, we gauge the impact of geothermal heating and vertical mixing parameterizations in the general circulation model OPA. The experiments are evaluated by comparing simulated mantle 3He with observations collected during the GEOSECS and WOCE programs. This tracer is particularly adapted to the validation of our numerical simulations because its injection into the ocean interior is tightly linked to geothermal processes. In agreement with previous studies, the model circulation is found very sensitive to the parameterization of the vertical mixing. The meridional overturning circulation (MOC) is globally intensified when moving from a constant mixing to a version with enhanced mixing near the ocean bottom, with the most drastic variation observed for AABW (+ 50%). Adding the geothermal heat flux mainly affects AABW circulation in the model, enhancing it all the more as the meridional circulation is slow (low vertical mixing), but proportionally less so when it is more vigorous (enhanced vertical mixing). This can be understood from the requirement of the abyssal ocean to maintain heat balance. The evaluation with mantle 3He simulations reveals that the version with low vertical mixing, with its sluggish circulation, produces unrealistically high a 3He isotopic composition. However, with a vertical mixing that is enhanced at depth, the 3He distribution falls within an acceptable range of values in the deep ocean. Finally, adding the geothermal heating to this enhanced mixing case provides a substantial improvement of the simulation of AABW in all basins but the Indian Ocean. 3He isotopic composition is then in good agreement with the observations. Taken jointly with observational estimates of the MOC intensity, these independent isotopic constraints suggest that both geothermal heating and enhanced diapycnal mixing at depth are key ingredients in the realistic simulation of abyssal circulation.

Effects of vertical mixing parameterization on the mixed layer depth over the North Pacific in a global OGCM

We present MLD variability over the North Pacific Ocean in a global ocean general circulation model and impacts of three different vertical mixing schemes on it, based on statistical measures (annual mean difference, root-mean-square difference and correlation coefficient). The constant vertical mixing scheme tends to underestimate MLD over the whole basin. The Pacanowski-Philander scheme tends to overestimate MLD (> 20 m) in the mid- to high latitude during summer, implying that vertical mixing in the mid- to high latitude may not be represented properly by simple internal mixing mechanisms such as stratification or vertical shear of horizontal velocity. On the other hand, the new vertical mixing scheme (Noh et al. 2002) gives the most consistent MLD and its seasonal and spatial variability when compared with observation. These results suggest that parameterization of vertical mixing has significant effects on simulation of the seasonal and spatial variability of MLD over the North Pacific Ocean.

Investigating the impact of surface wave breaking on modeling the trajectories of drifters in the northern Adriatic Sea during a wind-storm event

An accurate numerical prediction of the oceanic upper layer velocity is a demanding requirement for many applications at sea and is a function of several near-surface processes that need to be incorporated in a numerical model. Among them, we assess the effects of vertical resolution, different vertical mixing parameterization (the so-called Generic Length Scale –GLS– set of k– ε, k– ω, gen, and the Mellor–Yamada), and surface roughness values on turbulent kinetic energy ( k) injection from breaking waves. First, we modified the GLS turbulence closure formulation in the Regional Ocean Modeling System (ROMS) to incorporate the surface flux of turbulent kinetic energy due to wave breaking. Then, we applied the model to idealized test cases, exploring the sensitivity to the above mentioned factors. Last, the model was applied to a realistic situation in the Adriatic Sea driven by numerical meteorological forcings and river discharges. In this case, numerical drifters were released during an intense episode of Bora winds that occurred in mid-February 2003, and their trajectories compared to the displacement of satellite-tracked drifters deployed during the ADRIA02-03 sea-truth campaign. Results indicted that the inclusion of the wave breaking process helps improve the accuracy of the numerical simulations, subject to an increase in the typical value of the surface roughness z 0. Specifically, the best performance was obtained using α CH = 56,000 in the Charnok formula, the wave breaking parameterization activated, k– ε as the turbulence closure model. With these options, the relative error with respect to the average distance of the drifter was about 25% (5.5 km/day). The most sensitive factors in the model were found to be the value of α CH enhanced with respect to a standard value, followed by the adoption of wave breaking parameterization and the particular turbulence closure model selected.

Employing Satellite-Derived Sea Ice Concentration to Constrain Upper-Ocean Temperature in a Global Ocean GCM

Abstract The quality of Southern Ocean sea ice simulations in a global ocean general circulation model (GCM) depends decisively on the simulated upper-ocean temperature. This is confirmed by assimilating satellite-derived sea ice concentration to constrain the upper-layer temperature of a sea ice–ocean GCM. The resolution of the model’s sea ice component is about 22 km and thus comparable to the pixel resolution of the satellite data. The ocean component is coarse resolution to afford long-term integrations for investigations of the deep-ocean equilibrium response. Besides improving the sea ice simulation considerably, the simulations with constrained upper-ocean temperature yield much more realistic global deep-ocean properties, in particular when combined with glacial freshwater input. Both outcomes are relatively insensitive to the passive-microwave algorithm used to retrieve the ice concentration being assimilated. The sensitivity of the long-term global deep-ocean properties and circulation to the possible freshwater input from ice shelves and to the parameterization of vertical mixing in the Southern Ocean is reevaluated under the new constraint.

Design and Validation of an Offline Oceanic Tracer Transport Model for a Carbon Cycle Study

Abstract An offline passive tracer transport model with self-operating diagnostic-mode vertical mixing and horizontal diffusion parameterizations is used with assimilated ocean currents to find the chlorofluorocarbon (CFC-11) cycle in oceans. This model was developed at the National Institute for Environmental Studies (NIES) under the carbon cycle research project inside the Greenhouse Gas Observing Satellite (GOSAT) modeling group. The model borrows offline fields from precalculated monthly archives of assimilated ocean currents, temperature, and salinity, and it evolves a prognostic passive tracer with prescribed surface forcing. The model’s performance is validated by simulating the CFC-11 cycle in the ocean starting from the preindustrial period (1938) with observed anthropogenic perturbations of atmospheric CFC-11 to comply with the Ocean Carbon-Cycle Model Intercomparison Project Phase-2 (OCMIP-2) flux protocol. The model results are compared with ship observations as well as the results of candidate models of OCMIP-2 and a performance is assessed. The model simulates the deep-ventilation processes in the Atlantic Ocean appreciably well and yields a good agreement in the column inventory of the CFC-11 amplitude and phase compared to the observation. The statistical skill test shows that this model outperforms other candidate models of OCMIP-2 because of its higher resolution and assimilated offline input feeding, and it shows a potential role in improving transport calculation in the ocean with cost-effective computation.

Simulation of entrainment and transport of dust particles within North America in April 2001 (“Red Dust Episode”)

A size‐resolved, multicomponent, regional‐scale particulate‐matter (PM) model named AURAMS (A Unified Regional Air‐quality Modelling System) has been used to study the entrainment and transport of dust from the southwestern United States and northwestern Mexico to eastern North America during the so‐called “Red Dust Episode” in April, 2001. Two different wind‐blown‐dust emission schemes, the Marticorena‐Bergametti‐Alfaro (MBA) scheme and the Shao scheme, were incorporated into AURAMS to simulate dust generation, and sensitivity analyses for various dust‐emission‐scheme parameters were performed. Comparison of the model results with satellite observations and surface measurements showed that the model simulation reasonably reproduced the temporal and spatial distribution of wind‐blown dust particles during the episode period in the downwind area of Oklahoma but not in the source region. Both dust‐emission schemes captured the main features of the dust transport. The dust‐emission‐scheme parameter most responsible for inaccurate prediction of wind‐blown‐dust emission in the source region in this study appeared to be soil moisture content. The soil grain size distribution and the soil plastic pressure were also shown to be important parameters that should be accurately estimated for better model performance. For further validation and reliable use of wind‐blown‐dust emission schemes, accurate field and remote sensing measurements of those parameters are imperative. The unusually fast transport of dust during the episode appeared to be due to vigorous vertical mixing and uplift of emitted dust. Appropriate parameterization of additional vertical mixing by sub‐grid‐scale convection may help to better predict the long‐range transport of dust storms.

High-resolution sea ice in long-term global ocean GCM integrations

The resolution of the sea-ice component of a coarse-resolution global ocean general circulation model (GCM) has been enhanced to about 22 km in the Southern Ocean. The ocean GCM is designed for long-term integrations suitable for investigations of the deep-ocean equilibrium response to changes in southern hemisphere high-latitude processes. The space and time scales of the high-resolution sea-ice component are commensurate with those of the resolution of satellite passive-microwave sea-ice data. This provides the opportunity for a rigorous evaluation of simulated sea-ice characteristics. It is found that the satellite-derived continuous high ice concentration of the interior winter ice pack can only be captured when vertical oceanic mixing is modified in a way that less local, intermittent convection occurs. Furthermore, the width and the variability of the coastal polynyas around the Antarctic continent and its ice shelves are best captured when some form of ice-shelf melting is accounted for. The width of the wintertime ice edge is reasonably reproduced, while its variability remains underestimated, closely following the coarse-grid pattern of the ocean model due to its high dependence on ocean temperature. Additional variability besides daily winds, e.g. in form of idealized tidal currents, improves the temporal and spatial ice-edge variability, while leads in the interior ice pack become more abundant, more in line with the fine-scale satellite-derived texture. The coast- or ice-shelf line is described on the fine grid based on satellite passive-microwave data. This method requires parts of a coarse coastal ocean grid cell to be covered by an inert layer of “fast ice” or “ice shelf”. Reasonable long-term global deep-ocean properties can only be achieved when these areas are not inert, i.e. are exposed to heat flux and ice growth, or when the vertical mixing parameterization allows for excessive open-ocean convection. The model area exposed to cold high-latitude atmospheric conditions thus being most decisive for a realistic representation of the long-term deep-ocean properties, suggests that high-latitude coastlines are definitely in need of being represented at high resolution, including ice sheets and their effects on the heat and freshwater flux for the ocean.

Entrainment and Transport in Idealized Three-Dimensional Gravity Current Simulation

Abstract A purely z-coordinate Dietrich/Center for Air Sea Technology (DieCAST) ocean model is applied to the Dynamics of Overflow Mixing and Entrainment (DOME) idealized bottom density current problem that is patterned after the Denmark Strait. The numerical results show that the background viscosity plays a more important role than the chosen coordinate system in the entrainment and mixing if the background viscosity is not small enough. Both higher horizontal viscosity and coarser resolution leads to slower along-slope propagation. Reducing vertical mixing parameterization also leads to slower along-slope propagation with thicker plume size vertically. The simulation gives consistent results for the moderate- and fine-resolution runs. At a very coarse grid the dense water descends more slowly and is mainly dominated by diffusion. Time-averaged downstream transport and entrainment are not very sensitive to viscosity after the flow reaches its quasi-steady status. However, more realistic eddies and flow structures are found in low-viscosity runs. The results show good convergence of the resolved flow as expected and clarify the effects of numerical dissipation/mixing on overflow modeling. Larger numerical dissipation is not required nor recommended in z-coordinate models.

Modeling internal tides over Fieberling Guyot: resolution, parameterization, performance

Terrain-following ocean models are being used to simulate baroclinic tides and provide estimates of the tidal fields for circulation and mixing studies. These models have successfully reproduced elevations with most of the remaining inaccuracies attributed to topographic errors; however, the replication of barotropic and baroclinic velocity fields has not been as robust. Part of the problem is the lack of an adequate observational dataset in the simulated regions to compare the models. This problem was addressed using a dataset collected during the Flow over Abrupt Topography initiative at Fieberling Guyot. To evaluate the capability of the Regional Ocean Model System (ROMS) to simulate baroclinic tidal velocities, the combined tides for four constituents, M2, S2, K1, and O1, were modeled over Fieberling Guyot. Model inputs, numerical schemes, and parameterizations were varied to improve agreement with observations. These included hydrography, horizontal resolution, and the vertical mixing parameterization. Other factors were evaluated but are not included in this paper. With the best case, semidiurnal baroclinic tides were well replicated with RMS differences between the model estimates and the observations of 1.85 and 0.60 cm s−1 for the major axes of the tidal ellipses for M2 and S2, respectively. However, diurnal K1 baroclinic tides were poorly simulated with RMS differences of 4.49 cm s−1. In the simulations, the K1 baroclinic tides remained bottom-trapped unlike the observed fields, which had free waves due to the contribution of the mean velocity to the potential vorticity. The model did not adequately simulate the mean velocity, and the K1 tides remained trapped. A resolution of 1 km most accurately reproduced the major axes and mean velocities; however, a 4-km resolution was sufficient for a qualitative estimate of where baroclinic tidal generation occurred. Nine vertical mixing parameterizations were compared. The vertical mixing parameterization was found to have minor effects on the velocity fields, with most effects occurring over the crown of guyot and in the lower water column; however, it had dramatic effects on the estimation of vertical diffusivity of temperature. Although there was no definitive best performer for the vertical mixing parameterization, several parameterizations could be eliminated based on comparison of the vertical diffusivity estimates with observations. The best performers were Mellor–Yamada and three generic length scale schemes.