Abstract

The interactions of atmospheric cnoidal waves with a critical level are examined using a two-dimensional numerical model. A cnoidal wave system is generated by applying a bore initial condition to a shallow surface-based inversion; the system is analyzed using various profiles of stability and shear. Under neutral conditions a critical level is shown to cause wave reflection with wave growth, as indicated by the vertical velocity, when the stability is low. Increasing the ambient stability above the cnoidal wave leads to a reduction in the reflective properties of the critical level and an increase in critical level absorption. The division between wave growth and wave decay occurs near a critical level Richardson's number of 0.25 agreeing with instability theory. When a variable profile of stability is assumed, with stable regions both below and above the critical layer and weak stability at the critical level region, the cnoidal wave system again amplifies, but not as strongly. The results conform with past analytical results pertaining to the atmospheric structure required for wave reflection and absorption. However, the occurrence of overreflection cannot be diagnosed from the simulations because of the strongly nonlinear, unsteady behavior of the cnoidal wave systems.

Abstract

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.

Abstract

Interactions between a turbulent boundary layer and nonlinear mountain waves are explored using a large-eddy simulation model. Simulations of a self-induced critical layer, which develop a stagnation layer and a strong leeside surface jet, are considered. Over time, wave breaking in the stagnation region forces strong turbulence that influences the formation and structure of downstream leeside rotors. Shear production is an important source of turbulence in the stagnation zone and along the interface between the stagnation zone and surface jet, as well as along the rotor edges. Buoyancy perturbations act as a source of turbulence in the stagnation zone but are shown to inhibit turbulence generation on the edges of the stagnation zone.

Surface heating is shown to have a strong influence on the strength of downslope winds and the formation of leeside rotors. In cases with no heating, a series of rotor circulations develops, capped by a region of increased winds. Weak heating disrupts this system and limits rotor formation at the base of the downslope jet. Strong heating has a much larger impact through a deepening of the upstream boundary layer and an overall decrease in the downslope winds. Rotors in this case are nonexistent. In contrast to the cases with surface warming, negative surface fluxes generate stronger downslope winds and intensified rotors due to turbulent interactions with an elevated stratified jet capping the rotors. Overall, the results suggest that for nonlinear wave systems over mountains higher than the boundary layer, strong downslope winds and rotors are favored in late afternoon and evening when surface cooling enhances the stability of the low-level air.

Abstract

Simulations are presented focusing on the role of temperature inversions in controlling the formation and strength of downslope wind storms. Three mechanisms are examined depending on the relative height of the inversion with respect to the mountain and the stability of vertically propagating mountain waves. For low-level inversions, flows are generated that closely resemble a reduced gravity shallow water hydraulic response with a large vertical displacement of the inversion on the lee side of the mountain. For higher-level inversion cases, simulated flows more closely followed a stratified hydraulic behavior with the inversion acting as a rigid reflective lid. In the third mechanism, downslope winds were forced by a self-induced critical layer located below the inversion height. The presence of the inversion in this case had little effect on the resulting downslope winds.

Observations made on the Falkland Islands show that downslope windstorms may preferentially occur in early morning even without synoptic-scale changes in atmospheric structure. Most windstorms on the Falkland Islands generally have a short jet length; rare, longer jet length storms typically occur in conjunction with a strong low-level inversion. Idealized numerical experiments tend to produce a similar response depending on the presence of strong low-level inversion and surface cooling. Results suggest that surface heating can have significant control on the flow response by reducing the low-level inversion strength, or by changing the stratification and wind velocity below the inversion, thereby preventing a strong downslope windstorm.

Abstract

A simple, isolated front is modeled using a turbulence resolving, large-eddy simulation (LES) to examine the generation of instabilities and inertial oscillations by surface fluxes. Both surface cooling and surface wind stress are considered. Coherent roll instabilities with 200–300-m horizontal scale form rapidly within the front after the onset of surface forcing. With weak surface cooling and no wind, the roll axis aligns with the front, yielding results that are equivalent to previous constant gradient symmetric instability cases. After ~1 day, the symmetric modes transform into baroclinic mixed modes with an off-axis orientation. Traditional baroclinic instability develops by day 2 and thereafter dominates the overall circulation. Addition of destabilizing wind forcing produces a similar behavior, but with off-axis symmetric-Ekman shear modes at the onset of instability. In all cases, imbalance of the geostrophic shear by vertical mixing leads to an inertial oscillation in the frontal currents. Analysis of the energy budget indicates an exchange between kinetic energy linked to the inertial currents and potential energy associated with restratification as the front oscillates in response to the vertically sheared inertial current. Inertial kinetic energy decreases from enhanced mixed layer turbulence dissipation and vertical propagation of inertial wave energy into the pycnocline.

Abstract

Using a two-dimensional nonhydrostatic model, experiments were performed to investigate the formation and maintenance of internal waves in the equatorial Pacific Ocean. The simulations show that internal waves are generated in the surface mixed layer by a type of Kelvin–Helmholtz instability that is dependent on both the flow Reynolds number (i.e., shear strength) and Richardson number. Because of the Richardson number dependence, the simulated internal waves exhibit a diurnal cycle, following the daily stability change in the mixed layer. The diurnal cycle is not evident when the wind stress is eastward because of a decreased mixed layer shear and corresponding Reynolds number. The amplitude, wavelength, frequency, and diurnal variability of the simulated waves are in agreement with high-resolution thermistor chain measurements. Linear theory shows that the horizontal wavelength of the internal waves depends on both the thermocline stratification and the strength of the Equatorial Undercurrent.

The simulations show that internal waves can provide an efficient mechanism for the vertical transport of horizontal momentum. In the surface mixed layer, the internal waves gain westerly momentum at the expense of the background flow. In some cases, this momentum is transferred back to the mean flow at a critical level resulting in a deceleration below the undercurrent core. Otherwise, the waves tend to decrease the current velocity above the undercurrent core.

Abstract

Interaction between mixed layer baroclinic eddies and small-scale turbulence is studied using a nonhydrostatic large-eddy simulation (LES) model. Free, unforced flow evolution is considered, for a standard initialization consisting of an 80-m-deep mixed layer with a superposed warm filament and two frontal interfaces in geostrophic balance, on a model domain roughly 5 km × 10 km × 120 m, with an isotropic 3-m computational grid. Results from these unforced experiments suggest that shear generated in narrow frontal zones can support weak three-dimensional turbulence that is directly linked to the larger-scale baroclinic waves. Two separate but closely related issues are addressed: 1) the possible development of enhanced turbulent mixing associated with the baroclinic wave activity and 2) the existence of a downscale transfer of energy from the baroclinic wave scale to the turbulent dissipation scale. The simulations show enhanced turbulence associated with the baroclinic waves and enhanced turbulent heat flux across the isotherms of the imposed frontal boundary, relative to background levels. This turbulence develops on isolated small-scale frontal features that form as the result of frontogenetic processes operating on the baroclinic wave scale and not as the result of a continuous, inertial forward cascade through the intermediate scales. Analysis of the spectrally decomposed kinetic energy budget indicates that large-scale baroclinic eddy energy is directly transferred to small-scale turbulence, with weaker forcing at intermediate scales. For fronts with significant baroclinic wave activity, cross-frontal eddy fluxes computed from correlations of fluctuations from means along the large-scale frontal axis generally agreed with simple theoretical estimates.

Abstract

Cold air outflow over the Gulf Stream is modeled using a cloud-resolving large-eddy simulation model with three classes of precipitation. Simulations are conducted in a quasi-Lagrangian framework using an idealized sounding and uniform geostrophic winds based on observations taken on 20 February 2007 as part of the World Climate Research Program Climate Variability and Predictability (CLIVAR) Mode Water Dynamics Experiment (CLIMODE) project. Two cases are considered, one with an increasing sea surface temperature (SST) representing the crossing of the Gulf Stream front, and a second case with constant SST.

Cloud systems develop in the model with strong convective plumes that spread into regions of stratus clouds at the top of the boundary layer. Simulated boundary layer growth is forced by a combination of evaporative cooling at the cloud top, upward radiative flux, and mechanical entrainment of the overlying warmer and drier air. Constant growth of the boundary layer acts to maintain a near-constant water vapor level in the boundary layer, promoting high latent and sensible heat fluxes. Frictional surface drag is distributed throughout the boundary layer by convection, causing increased shear at the cloud top, qualitatively agreeing with observed sounding profiles. Overall, the frontal case develops stronger precipitation and turbulence in comparison with the constant SST case. A near-uniform stratocumulus layer and stronger radiative cooling are produced in the constant SST case, whereas the frontal case generates open cumuliform clouds with reduced cloud coverage. Cloud evolution in the frontal case is similar to the transition from stratocumulus to shallow cumulus observed in the subtropics, as cumuliform clouds enhance cloud-top entrainment and evaporation of stratus clouds.

Abstract

A large eddy simulation (LES) model and the Advanced Regional Prediction System (ARPS) model, which does not resolve turbulent eddies, are used to study the effect of a slope angle decrease on the structure of katabatic slope flows. For a simple, uniform angle slope, simulations from both models produce turbulence kinetic energy and momentum budgets that are in good overall agreement. Simulations of a compound angle slope are compared to a uniform angle slope to demonstrate how a changing slope angle can strongly affect the strength of katabatic flows. Both ARPS and the LES model show that slopes with a steep upper slope followed by a shallower lower slope (concave shape) generate a rapid acceleration on the upper slope followed by a transition to a slower evolving structure characterized by an elevated jet over the lower slope. In contrast, the case with uniform slope (having the same total height change) yields a more uniform flow profile with stronger winds at the slope bottom. Higher average slope in the uniform slope angle case generates greater gravitational potential energy, which is converted to kinetic energy at the bottom of the slope. Analysis of the total energy budget of slope flows indicates a consistent structure where potential energy generated at the top of the slope is transported downslope and converted into kinetic energy near the slope base.

Abstract

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.