Large-Eddy Simulation of Flow over Two-Dimensional Obstacles: High Drag States and Mixing

A three-dimensional large-eddy simulation (LES) model was used to examine how stratified flow interacts with bottom obstacles in the coastal ocean. Bottom terrain representing a 2D ridge was modeled using a finite-volume approach with ridge height (4.5 m) and width (~30 m) and water depth (~45 m) appropriate for coastal regions. Temperature and salinity profiles representative of coastal conditions giving constant buoyancy frequency were applied with flow velocities between 0.16 and 0.4 m s⁻¹. Simulations using a free-slip lower boundary yielded flow responses ranging from transition flows with relatively high internal wave pressure drag to supercritical flow with relatively small internal wave drag. Cases with high wave drag exhibited strong lee-wave systems with wavelength of ~100 m and regions of turbulent overturning. Application of bottom drag caused a 5–10-m-thick bottom boundary layer to form, which greatly reduced the strength of lee-wave systems in the transition cases. A final simulation with bottom drag, but with a much larger obstacle height (16 m) and width (~400 m), produced a stronger lee-wave response, indicating that large obstacle flow is not influenced as much by bottom roughness. Flow characteristics for the larger obstacle were more similar to hydraulic flow, with lee waves that are relatively short in comparison with the obstacle width. The relatively strong effect of bottom roughness on the small obstacle wave drag suggests that small-scale bottom variations may be ignored in internal wave drag parameterizations. However, the more significant wave drag from larger-scale obstacles must still be considered and may be responsible for mixing and momentum transfer at distances far from the obstacle source

Turbulence beneath sea ice and leads: a coupled sea ice/large-eddy simulation study

The importance of leads, sea ice motion, and frazil ice on the wintertime ocean Boundary layer was examined by using a large-eddy simulation turbulence model coupled to a thermodynamic slab ice model. Coupling was achieved through exchange coefficients that accounted for the differing diffusion rates of heat and salinity. Frazil ice concentrations were modeled by using an ice crystal parameterization with constant crystal size and shape. Stationary ice without leads produced cellular structures similar to atmospheric convection without winds. Ice motion caused this pattern to break down into a series of streaks aligned with the flow. Eddy fluxes were strongly affected by ice motion with relatively larger entrainment fluxes at the mixed layer base under moving ice, whereas stationary ice produced larger fluxes near the top of the boundary layer. Opening of leads caused significant changes in the turbulent structure of the boundary layer. Leads in stationary ice produced concentrated plumes of higher-salinity water beneath the lead. Ice motion caused the lead convection to follow preexisting convective rolls, enhancing the roll circulation salinity and vertical velocity under the lead. Comparison of model time series data with observations from the Arctic Leads Experiment showed general agreement for both pack ice and lead conditions. Simulated heat flux carried by frazil ice had a prominent role in the upper boundary layer, suggesting that frazil ice is important in the heat budget of ice-covered oceans

A numerical study of melt ponds

High-resolution turbulence simulations are used to examine the importance of melt pond geometry in setting pond growth rates and albedo. Modeling the circulation of water in melt ponds using large-eddy simulation shows that both convective and windforced conditions generate well-mixed ponds, suggesting that stratification is not a significant factor in pond circulation. Simulations with a variety of pond shapes and sizes indicate that the basic ratio of sidewall area to bottom area, R, can be used to characterize melting rates for ponds with simple shapes. Ponds with large values of R will generally melt more rapidly in the horizontal direction at the expense of bottom melting. Consequently, small and elongated ponds will have a relatively larger lateral growth rate in comparison with large, symmetric ponds, assuming minimal lateral flux of meltwater. Simulations also show that pond shape can affect the sidewall and bottom turbulence transfer rates. Ponds with large R tend to have reduced transfer rates because of weaker circulations. A bulk pond model is developed on the basis of a rectangular geometry and an assumption of uniform mixing as suggested by the turbulence model and pond scaling using R. Comparison of the bulk model with results from the large-eddy simulation cases shows good agreement

Cloud-Resolving Large-Eddy Simulation of Tropical Convective Development and Surface Fluxes

Cloud-resolving large-eddy simulations (LES) on a 500 km × 500 km periodic domain coupled to a thermodynamic ocean mixed layer are used to study the effect of large-scale moisture convergence M on the convective population and heat and moisture budgets of the tropical atmosphere, for several simulations with M representative of the suppressed, transitional, and active phases of the Madden–Julian oscillation (MJO). For a limited-area model without an imposed vertical velocity, M controls the overall vertical temperature structure. Moisture convergence equivalent to ~200 W m⁻² (9 mm day⁻¹) maintains the observed temperature profile above 5 km. Increased convective heating for simulations with higher M is partially offset by greater infrared cooling, suggesting a potential negative feedback that helps maintain the weak temperature gradient conditions observed in the tropics. Surface evaporation decreases as large-scale moisture convergence increases, and is only a minor component of the overall water budget for convective conditions representing the active phase of the MJO. Cold pools generated by evaporation of precipitation under convective conditions are gusty, with roughly double the wind stress of their surroundings. Consistent with observations, enhanced surface evaporation due to cold pool gusts is up to 40% of the mean, but has a small effect on the total moisture budget compared to the imposed large-scale moisture convergence.Keywords: Convective clouds, Radiative-convective equilibrium, Convection lines, Madden-Julian oscillation, Atmosphere-ocean interaction, Cold poolsKeywords: Convective clouds, Radiative-convective equilibrium, Convection lines, Madden-Julian oscillation, Atmosphere-ocean interaction, Cold pool

Coastal Atmospheric Circulation around an Idealized Cape during Wind-Driven Upwelling Studied from a Coupled Ocean–Atmosphere Model

The study analyzes atmospheric circulation around an idealized coastal cape during summertime upwelling-favorable wind conditions simulated by a mesoscale coupled ocean–atmosphere model. The domain resembles an eastern ocean boundary with a single cape protruding into the ocean in the center of a coastline. The model predicts the formation of an orographic wind intensification area on the lee side of the cape, extending a few hundred kilometers downstream and seaward. Imposed initial conditions do not contain a low-level temperature inversion, which nevertheless forms on the lee side of the cape during the simulation, and which is accompanied by high Froude numbers diagnosed in that area, suggesting the presence of the supercritical flow. Formation of such an inversion is likely caused by average easterly winds resulting on the lee side that bring warm air masses originating over land, as well as by air warming during adiabatic descent on the lee side of the topographic obstacle. Mountain leeside dynamics modulated by differential diurnal heating is thus suggested to dominate the wind regime in the studied case. The location of this wind feature and its strong diurnal variations correlate well with the development and evolution of the localized lee side trough over the coastal ocean. The vertical extent of the leeside trough is limited by the subsidence inversion aloft. Diurnal modulations of the ocean sea surface temperatures (SSTs) and surface depth-averaged ocean current on the lee side of the cape are found to strongly correlate with wind stress variations over the same area. Wind-driven coastal upwelling develops during the simulation and extends offshore about 50 km upwind of the cape. It widens twice as much on the lee side of the cape, where the coldest nearshore SSTs are found. The average wind stress–SST coupling in the 100-km coastal zone is strong for the region upwind of the cape, but is notably weaker for the downwind region, estimated from the 10-day-average fields. The study findings demonstrate that orographic and diurnal modulations of the near-surface atmospheric flow on the lee side of the cape notably affect the air–sea coupling on various temporal scales: weaker wind stress–SST coupling results for the long-term averages, while strong correlations are found on the diurnal scale.Keywords: Wind, Coupled models, Coastal flows, UpwellingKeywords: Wind, Coupled models, Coastal flows, Upwellin

The Dynamics of Northwest Summer Winds over the Santa Barbara Channel

A mesoscale model is used to examine the dynamics of northwest flow over the Santa Barbara Channel region. Three cases are considered, each characterized by typical summertime synoptic conditions, but with differences in pressure gradient strength and marine boundary layer depth (MBL). The first case examines a relatively deep MBL and strong pressure gradient. Case 2 is characterized by a more shallow MBL and weaker pressure gradient, and case 3 represents a transition from a deep MBL to shallow conditions. In all cases, simulated surface winds show reasonable agreement with observations over most of the model domain, with the exception of regions near abrupt terrain changes. Results from the model indicate that the flow with a deep MBL (~400 m) and strong pressure gradient (case 1) is supercritical, causing regions of acceleration and expansion in the lee of Point Conception. When the MBL is shallow (~150 m) (case 2), a transcritical flow scenario exists with subcritical flow upstream from Point Conception and a supercritical flow region over the Santa Barbara Channel and downstream from the Channel Islands. Flow over the channel is strongly affected by diurnal heating in shallow MBL cases, reversing direction in step with a land breeze circulation induced by nighttime cooling. The land breeze forces an internal wave disturbance that propagates westward across the channel, eliminating the supercritical flow region in the lee of Point Conception. Conditions with a deep MBL (~400 m) produce less variability in the surface winds, except for the region sheltered by the Santa Ynez Mountains. An expansion fan is still evident in this case, but it is produced by the interaction of the flow with higher terrain north and east of the channel. The low hills on Point Conception and the Channel Islands do not have a large blocking effect on the surface flow when the MBL is deep. Analysis of the momentum budget supports the conclusion that the boundary layer behaves like a transcritical hydraulic flow when the MBL is shallow. Except for the open ocean region, the Coriolis term is minor in comparison with the pressure and advection terms. Diurnal heating effects are evident in the nearshore pressure term, which varies from offshore during the late evening to onshore in the afternoon. These effects are most significant when the MBL is shallow and can augment the hydraulically forced pressure pattern, causing a stronger expansion fan in the late afternoon and a collapse of the expansion fan during the early morning

Resonant Wind-Driven Mixing in the Ocean Boundary Layer

The role of resonant wind forcing in the ocean boundary layer was examined using an ocean large-eddy simulation (LES) model. The model simulates turbulent flow in a box, measuring ~100–300 m on a side, whose top coincides with the ocean surface. Horizontal boundary conditions are periodic, and time-dependent wind forcing is applied at the surface. Two wind forcing scenarios were studied: one with resonant winds, that is, winds that rotated at exactly the inertial frequency (at 45°N), and a second with off-resonance winds from a constant direction. The evolution of momentum and temperature for both cases showed that resonant wind forcing produces much stronger surface currents and vertical mixing in comparison to the off-resonance case. Surface wave effects were also examined and found to be of secondary importance relative to the wind forcing. The main goal was to quantify the main processes via which kinetic energy input by the wind is converted to potential energy in the form of changes in the upper-ocean temperature profile. In the resonant case, the initial pathway of wind energy was through the acceleration of an inertially rotating current. About half of the energy input into the inertial current was dissipated as the result of a turbulent energy cascade. Changes in the potential energy of the water column were ~7% of the total input wind energy. The off-resonance case developed a much weaker inertial current system, and consequently less mixing because the wind acted to remove energy after ~¼ inertial cycle. Local changes in the potential energy were much larger than the integrated values, signifying the vertical redistribution of water heated during the summer season. Visualization of the LES results revealed coherent eddy structures with scales from 30–150 m. The largest-scale eddies dominated the vertical transport of heat and momentum and caused enhanced entrainment at the boundary layer base. Near the surface, the dominant eddies were driven by the Stokes vortex force and had the form of Langmuir cells. Near the base of the mixed layer, turbulent motions were driven primarily by the interaction of the inertial shear with turbulent Reynolds stresses. Bulk Richardson number and eddy diffusivity profiles from the model were consistent with one-dimensional model output using the K-profile parameterization

Oceanic Turbulent Energy Budget using Large-Eddy Simulation of a Wind Event during DYNAMO

The dominant processes governing ocean mixing during an active phase of the Madden–Julian oscillation are identified. Air–sea fluxes and upper-ocean currents and hydrography, measured aboard the R/V Revelle during boreal fall 2011 in the Indian Ocean at 0°, 80.5°E, are integrated by means of a large-eddy simulation (LES) to infer mixing mechanisms and quantify the resulting vertical property fluxes. In the simulation, wind accelerates the mixed layer, and shear mixes the momentum downward, causing the mixed layer base to descend. Turbulent kinetic energy gains due to shear production and Langmuir circulations are opposed by stirring gravity and frictional losses. The strongest stirring of buoyancy follows precipitation events and penetrates to the base of the mixed layer. The focus here is on the initial 24 h of an unusually strong wind burst that began on 24 November 2011. The model shows that Langmuir turbulence influences only the uppermost few meters of the ocean. Below the wave-energized region, shear instability responds to the integrated momentum flux into the mixed layer, lagging the initial onset of the storm. Shear below the mixed layer persists after the storm has weakened and decelerates the surface jet slowly (compared with the acceleration at the peak of the storm). Slow loss of momentum from the mixed layer extends the effect of the surface wind burst by energizing the fluid at the base of the mixed layer, thereby prolonging heat uptake due to the storm. Ocean turbulence and air–sea fluxes contribute to the cooling of the mixed layer approximately in the ratio 1:3, consistent with observations.Keywords: Indian Ocean, Mixed layer, Large eddy simulations, Physical Meteorology and Climatology, Tropics, Fluxes, Models and modeling, Mixing, Geographic location/entity, Atm/Ocean Structure/Phenomen

Mesoscale and large-eddy simulation model studies of the Martian atmosphere in support of Phoenix

In late May of 2008, the NASA/JPL Phoenix spacecraft will touch down near its targeted landing site on Mars (68.2°N, 126.6°W). Entry, descent, and landing (EDL) occurs in the late afternoon (~1630 hours local solar time (LST)) during late northern spring (Ls ~ 78°). Using a mesoscale and a large-eddy simulation (LES) model, we have investigated the range of conditions that might be encountered in the lower atmosphere during EDL. High-resolution (~18 km) results from the Oregon State University Mars MM5 (OSU MMM5) are used to understand the hazards from the transient circulations prominent during this season. Poleward of ~80°N these storms produce strong winds (~35 m s–1) near the ground; however, owing to the synoptic structure of these storms, and the deep convective mixed layer equatorward of the seasonal cap boundary during EDL, our modeling suggests the spacecraft would not be in winds stronger than ~20 m s–1 at parachute separation. The storm-driven variability is much weaker at Phoenix latitudes than it is poleward of the seasonal cap edge (result from an extensive sensitivity study). The OSU MLES model is used to explicitly simulate the hazards of convection and atmospheric turbulence at very high resolution (100 m). This modeling suggests that an upper bound for the maximum expected horizontal-mean atmospheric turbulent kinetic energy (TKE) is ~12 m2 s–2, seen ~3 km above the ground at ~1430 hours LST. TKE amplitudes are greatest when the horizontal mean wind is large (shear production) and/or the surface albedo is low (a lower albedo enhances buoyancy production, mimicking decreased atmospheric stability after a storm advects colder air into the region). LES simulations predict deep mixed layers (~6–7 km), ~1.5 km deeper than the mesoscale model (~5 km). Mesoscale modeling suggests that the actual landing site differs meteorologically from other longitudes (larger-amplitude diurnal wind cycle), a consequence of the strong thermal circulations that are excited by the very large regional topography. The OSU MLES model was modified for this work to utilize time- and height-dependent geostrophic wind forcing (constructed from OSU MMM5 results). With this forcing, the OSU MLES provides a site-specific simulation, where the time/height variability of the horizontal mean LES wind field is in good agreement with the OSU MMM5. On the basis of some statistical analysis, we have good confidence that the ‘‘fullspectrum’’ wind field is within engineering guidelines for Phoenix EDL

Simulation of melt pond evolution on level ice

A melt pond model is presented that predicts pond size and depth changes, given an initial ice thickness field and representative surface fluxes. The model is based on the assumption that as sea ice melts, fresh water builds up in the ice pore space and eventually saturates the ice. Under these conditions, a water table is defined equal to the draft of the ice or sea level, and ponds are produced in ice surface depressions, much like lakes in a watershed. Pond evolution is forced by applying fluxes of heat at the pond surface and a radiative transfer model for solar radiation that penetrates the pond. Results from the model using forcing data from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment and representative pond parameters indicate that the model accurately simulates pond depth and fractional area over the summer melt season, with fractional area increasing linearly. Overall, ice albedo is affected primarily by the increase in pond coverage. Decrease in pond albedo from pond deepening has a much lower influence on the total albedo. Cases with predominately sunny conditions are shown to produce more rapid pond expansion than overcast cases. In both sunny and cloudy cases the fractional area increases linearly