Search

Simple rate equation models for subfilter-scale scalar and momentum fluxes have previously been developed for application in the so-called 'terra ineognita' of atmospheric simulations, where the model resolution is comparable to the scale of turbulence. The models performed well over land, but only the scalar flux model appeared to perform adequately over the ocean. Analysis of data from the Ocean Horizontal Array Turbulence Study (OHATS) reveals a need to account for the moving ocean-air interface in the subfilter stress model. The authors develop simple parameterizations for the effect of surface-induced pressure fluctuations on the subfilter stress, leading to good predictions of subfilter momentum flux both over land and in OHATS.

Winds and waves in marine boundary layers are often in an unsettled state when fast-running swell generated by distant storms propagates into local regions and modifies the overlying turbulent fields. A large-eddy simulation (LES) model with the capability to resolve a moving sinusoidal wave at its lower boundary is developed to investigate this low-wind/fast-wave regime. It is used to simulate idealized situations with wind following and opposing fast-propagating waves (swell), and stationary bumps. LES predicts momentum transfer from the ocean to the atmosphere for wind following swell, and this can greatly modify the turbulence production mechanism in the marine surface layer. In certain circumstances the generation of a low-level jet reduces the mean shear between the surface layer and the PBL top, resulting in a near collapse of turbulence in the PBL. When light winds oppose the propagating swell, turbulence levels increase over the depth of the boundary layer and the surface drag increases by a factor of 4 compared to a flat surface. The mean wind profile, turbulence variances, and vertical momentum flux are then dependent on the state of the wave field. The LES results are compared with measurements from the Coupled Boundary Layers Air-Sea Transfer (CBLAST) field campaign. A quadrant analysis of the momentum flux from CBLAST verifies a wave age dependence predicted by the LES solutions. The measured bulk drag coefficient Co then depends on wind speed and wave state. In situations with light wind following swell, [C.sub.D] is approximately 50% lower than values obtained from standard bulk parameterizations that have no sea state dependence. In extreme cases with light wind and persistent swell, [C.sub.D] < 0.

This manuscript describes numerical experiments investigating the influence of 2-30-km striplike heterogeneity on wet and dry convective boundary layers coupled to the land surface. The striplike heterogeneity is shown to dramatically alter the structure of the convective boundary layer by inducing significant organized circulations that modify turbulent statistics. The impact, strength, and extent of the organized motions depend critically on the scale of the heterogeneity A relative to the boundary layer height [z.sub.i]. The coupling with the land surface modifies the surface fluxes and hence the circulations resulting in some differences compared to previous studies using fixed surface forcing. Because of the coupling, surface fluxes in the middle of the patches are small compared to the patch edges. At large heterogeneity scales ([lambda][z.sub.i] ~18) horizontal surface-flux gradients within each patch are strong enough to counter the surface-flux gradients between wet and dry patches allowing the formation of small cells within the patch coexisting with the large-scale patch-induced circulations. The strongest patch-induced motions occur in cases with 4 < [lambda]/[z.sub.i] Velocity and scalar fields respond differently to variations of heterogeneity scale. The patch-induced motions have little influence on total vertical scalar flux, but the relative contribution to the flux from organized motions compared to background turbulence varies with heterogeneity scale. Patch-induced motions are shown to dramatically impact point measurements in a free-convective boundary layer. The magnitude and sign of this impact depends on the location of the measurement within the region of heterogeneity.

A Lagrangian dispersion model driven by velocity fields from large-eddy simulations (LESs) is presented for passive particle dispersion in the planetary boundary layer (PBL). In this combined LES-Lagrangian stochastic model (LSM), the total velocity is divided into resolved or filtered and unresolved or subfilter-scale (SFS) velocities. The random SFS velocity is modeled using an adaptation of Thomson's LSM ill which the ensemble-mean velocity and velocity variances are replaced by the resolved velocity and SFS variances, respectively. The random SFS velocity forcing has an amplitude determined by the SFS fraction of the total turbulent kinetic energy (TKE): the fraction is about 0.15 in the bulk of the simulated convective boundary layer (CBL) used here and reaches values as large as 0.31 and 0.37 in the surface layer and entrainment layer, respectively. For the proposed LES-LSM, the modeled crosswind-integrated concentration (CWIC) fields are in good agreement with the 1) surface-layer similarity (SLS) theory for a surface source in the CBL and 2) convection tank measurements of the CWIC for an elevated release in the CBL surface layer. The second comparison includes the modeled evolution of the vertical profile shape with downstream distance, which shows the attainment of an elevated CWIC maximum and a vertically well-mixed CWIC far downstream, in agreement with the tank data. For the proposed model, the agreement with the tank data and SLS theory is better than that obtained with an earlier model in which the SFS fraction of the TKE is assumed to be 1, and significantly better than a model that neglects the SFS velocities altogether.

A new dynamical subgrid model for turbulent flow in the surface layer of the planetary boundary layer is presented. In this formulation, the flow is decomposed into small (subgrid) and large (resolved) scales in the spirit of large-eddy simulation. The subgrid model, however, is derived directly from the governing equations and involves no adjustable parameters. The evolution of the small scales is not postulated, but computed as a function of the large scales using a semiclassical method extending rapid distortion theory to account for the intermittency of the small scales. At the same time, feedback from the small scales to the large scales can also be computed via the corresponding subgrid stresses. The model can be generalized to include other physical effects, and thus could potentially be used to estimate momentum and temperature fluxes in complex systems like the planetary boundary layer, which impacts larger-scale models. A priori tests using 3D large-eddy and direct simulations of unstable, neutral, and stable flows support the assumptions in the model.

A new dynamical subgrid model for turbulent flow is used to derive the structure of the heat and momentum fluxes, mean wind and temperature profiles, and temperature and velocity variances, as a function of the Richardson number, in the surface layer of the planetary boundary layer. Analytical solutions are obtained for stationary, homogeneous surface layers and are compared with field observations. The theory allows for analytic nonperturbative solutions in any turbulent surface layer and can be generalized in a straightforward fashion to include other effects, for example, rotation, and thus could potentially be used to estimate momentum and temperature fluxes in the planetary boundary layer for larger-scale (e.g., climate) models.

Planetary boundary layer (PBL) flows are known to exhibit fundamental differences depending on the r combination of wind shear and buoyancy forces. These differences are not unexpected in that shear in occur locally, while buoyancy force sets up vigorous thermals, which result in nonlocal transport of momentum. At the same time, these two forces can act together to modify the flow field. In this stud large-eddy simulations (LESs) spanning the shear and buoyancy flow regimes were generated; two corre to the extreme cases of shear and buoyancy-driven PBLs, while the other two represent intermediate P both forces are important. The extreme cases are used to highlight and quantify the basic difference shear and convective PBLs in 1) flow structures, 2) overall statistics, and 3) turbulent kinetic ene budget distributions. Results from the two intermediate LES cases are used to develop and verify a v scaling and a TKE budget model, which are proposed for the intermediate PBL. The velocity variances variance fluxes (i.e., third moments) normalized by this velocity scaling are shown to become quanti the order of one, and to lie mostly between those of the two extreme PBL cases. The proposed TKE bud model is shown to adequately reproduce the profiles of the TKE budget terms and the TKE.

Large-eddy simulations of a smoke cloud are examined with respect to their sensitivity to small scales as manifest in either the grid spacing or the subgrid-scale (SGS) model. Calculations based on a Smagorinsky SGS model are found to be more sensitive to the effective resolution of the simulation than are calculations based on the prognostic turbulent kinetic energy (TKE) SGS model. The difference between calculations based on the two SGS models is attributed to the advective transport, diffusive transport, and/or time-rate-of-change terms in the TKE equation. These terms are found to be leading order in the entrainment zone and allow the SGS TKE to behave in a way that tends to compensate for changes that result in larger or smaller resolved scale entrainment fluxes. This compensating behavior of the SGS TKE model is attributed to the fact that changes that reduce the resolved entrainment flux (viz., values of the eddy viscosity in the upper part of the PBL) simultaneously tend to increase the buoyant production of SGS TKE in the radiatively destabilized portion of the smoke cloud. Increased production of SGS TKE in this region then leads to increased amounts of transported, or fossil, SGS TKE in the entrainment zone itself, which in turn leads to compensating increases in the SGS entrainment fluxes. In the Smagorinsky model, the absence of a direct connection between SGS TKE in the entrainment and radiatively destabilized zones prevents this compensating mechanism from being active, and thus leads to calculations whose entrainment rate sensitivities as a whole reflect the sensitivities of the resolved-scale fluxes to values of upper PBL eddy viscosities.

The authors use large-eddy simulation (LES) to investigate entrainment and structure of the inversion layer of a clear convectively driven planetary boundary layer (PBL) over a range of bulk Richardson numbers, Ri. The LES code uses a nested grid technique to achieve fine resolution in all three directions in the inversion layer. Extensive flow visualization is used to examine the structure of the inversion layer and to illustrate the temporal and spatial interaction of a thermal plume and the overlying inversion. It is found that coherent structures in the convective PBL, that is, thermal plumes, are primary instigators of entrainment in the Ri range 13.6 [less than or equal to] Ri [less than or equal to] 43.8. At Ri = 13.6, strong horizontal and downward velocities are generated near the inversion layer because of the plume-interface interaction. This leads to folding of the interface and hence entrainment of warm inversion air at the plume's edge. At Ri = 34.5, the inversion's strong stability prevents folding of the interface but strong horizontal and downward motions near the plume's edge pull down pockets of warm air below the nominal inversion height. These pockets of warm air are then scoured off by turbulent motions and entrained into the PBL. The structure of the inversion interface from LES is in good visual agreement with lidar measurements in the PBL obtained during the Lidars in Flat Terrain field experiment. A quadrant analysis of the buoyancy flux shows that net entrainment flux (or average minimum buoyancy flux [Mathematical Expression Omitted]) is identified with quadrant [Mathematical Expression Omitted] motions, that is, warm air moving downward. Plumes generate both large negative quadrant [Mathematical Expression Omitted] and positive quadrant [Mathematical Expression Omitted] buoyancy fluxes that tend to cancel. The maximum vertical gradient in potential temperature at every (x, y) grid point is used to define a local PBL height, [z.sub.i](x, y). A statistical analysis of z[z.sub.i] shows that skewness of [z.sub.i] depends on the inversion strength. Spectra of [z.sub.i] exhibit a sensitivity to grid resolution. The normalized entrainment rate [w.sub.e]/[w.sub.*], where [w.sub.e] and [w.sub.*] are entrainment and convective velocities, varies as [ARi.sup.-1] with A [approximately equal to] 0.2 in the range 13.6 [less than or equal to] Ri [less than or equal to] 43.8 and is in good agreement with convection tank measurements. For a clear convective PBL, the authors found that the finite thickness of the inversion layer needs to be considered in an entrainment rate parameterization derived from a jump condition.

The effect of horizontal temperature heterogeneity of the underlying surface on the turbulence structure and mixing intensity in the stably stratified boundary layer (SBL) is analyzed using large-eddy simulation (LES). Idealized LESs of flows driven by fixed winds and homogeneous and heterogeneous surface temperatures are compared. The LES data are used to compute statistical moments, to estimate budgets of the turbulence kinetic energy (TKE), of the temperature variance and of the temperature flux, and to assess the relative importance of various terms in maintaining the budgets. Unlike most previous studies, the LES-based second-moment budgets are estimated with due regard for the subgrid-scale contributions. The SBL over a heterogeneous surface is more turbulent with larger variances (and TKE), is better vertically mixed, and is deeper compared to its homogeneous counterpart. The most striking difference between the cases is exhibited in the temperature variance and its budget. Because of surface heterogeneity, the turbulent transport term (divergence of the third-order moment) not only redistributes the temperature variance vertically but is a net gain. The increase in the temperature variance near the heterogeneous surface explains the reduced magnitude of the downward buoyancy flux and the ensuing increase in TKE that leads to more vigorous mixing. Analysis of the temperature flux budget shows that the transport term contributes to net production/destruction. Importantly, the role of the third-order transport cannot be elucidated if the budgets are computed based solely on resolved-scale fields. Implications for modeling (parameterizing) the SBL over thermally heterogeneous surfaces are discussed. [ABSTRACT FROM AUTHOR]

The effect of longwave radiative cooling at the planetary boundary layer (PBL) top in determining the entrainment rate was examined in this study. The entrainment rate equation that accounts for longwave radiative cooling can be formally derived from the following steps: 1) Derive the mean buoyancy budget in the thin layer between the averaged entrainment flux (i.e., the minimum buoyancy flux) level and the top of the entrainment zone where turbulent fluxes vanish. 2) Use the Deardorff entrainment closure assumption that the entrainment buoyancy flux is proportional to the vertically averaged buoyancy flux over the whole PBL, which is a generalized form of a widely accepted entrainment closure for the surface-heated convective PBL. This leads to an entrainment velocity that depends linearly on both the inverse of the interfacial Richardson number and the radiative flux divergence above the entrainment buoyancy flux level. The relative importance of these two terms was examined through large eddy simulations (LESs) of several smoke-cloud-topped PBLs with various radiative forcings, radiative properties, temperature inversion strengths, and numerical advective schemes. These PBLs are driven only by cloud-top radiative cooling. The LESs were performed with a fine-grid nesting layer in the entrainment zone where the grid size is about 16 m and 8 m in the horizontal and vertical, respectively, which should be sufficient to resolve entrainment processes, at least for cases with weak capping inversions. The LESs showed that the contribution to entrainment rate from the radiative flux divergence term was either larger than or about equal to that of the interfacial Richardson number term. The LESs also showed that this radiative flux divergence occurs within cloudy regions below the local cloud tops. The analysis was extended to the stratocumulus-topped PBL by using a stratocumulus-like smoke layer. and the result showed that the radiative flux contribution to the entrainment rate was still significant. based on the physical understanding that this portion of the radiative flux divergence occurs within the upper half of cloud (or smoke) hummocks, the authors were able to analytically derive a relationship that links the radiative flux divergence to the cloud-top fluctuations, which were then empirically related to the interfacial Richardson number.