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Author(s): T. D. Phan [sup.1] , J. P. Eastwood [sup.2] , M. A. Shay [sup.3] , J. F. Drake [sup.4] , B. U. Ö. Sonnerup [sup.5] , M. Fujimoto [sup.6] [...], Magnetic reconnection in current sheets is a magnetic-to-particle energy conversion process that is fundamental to many space and laboratory plasma systems. In the standard model of reconnection, this process occurs in a minuscule electron-scale diffusion region.sup.1,2. On larger scales, ions couple to the newly reconnected magnetic-field lines and are ejected away from the diffusion region in the form of bi-directional ion jets at the ion Alfvén speed.sup.3-5. Much of the energy conversion occurs in spatially extended ion exhausts downstream of the diffusion region.sup.6. In turbulent plasmas, which contain a large number of small-scale current sheets, reconnection has long been suggested to have a major role in the dissipation of turbulent energy at kinetic scales.sup.7-11. However, evidence for reconnection plasma jetting in small-scale turbulent plasmas has so far been lacking. Here we report observations made in Earth's turbulent magnetosheath region (downstream of the bow shock) of an electron-scale current sheet in which diverging bi-directional super-ion-Alfvénic electron jets, parallel electric fields and enhanced magnetic-to-particle energy conversion were detected. Contrary to the standard model of reconnection, the thin reconnecting current sheet was not embedded in a wider ion-scale current layer and no ion jets were detected. Observations of this and other similar, but unidirectional, electron jet events without signatures of ion reconnection reveal a form of reconnection that can drive turbulent energy transfer and dissipation in electron-scale current sheets without ion coupling.Observations of electron-scale current sheets in Earth's turbulent magnetosheath reveal electron reconnection without ion coupling, contrary to expectations from the standard model of magnetic reconnection.

The destructive power of hurricanes is unlike anything else on Earth, with the proven capability to significantly damage the U.S. economy, devastate regions, and kill thousands of people even in [...]

Atmospheric turbulence is the leading cause of serious personal injuries in nonfatal accidents of commercial aircraft. One main type of motion that causes flight injuries in atmospheric turbulence is the sudden plunging motion with an abrupt change in altitude. To assess the possibility of designing a control system to mitigate such plunging motion, the dynamic stability characteristics must be known. The main objective of the present paper is to evaluate the dynamic stability characteristics and, more generally, the flying qualities of a twin-jet transport aircraft encountering severe atmospheric turbulence through digital six-degree-of-freedom flight simulations in transonic flight. The fuzzy-logic thrust model and unsteady aerodynamic models are used to estimate the nonlinear unsteady aerodynamics while performing numerical integration of flight dynamic equations. The flying qualities are based on the instantaneous eigenvaines of all flight modes obtained during time integration. It is shown that the real part of eigenvalues for the plunging mode (i.e., damping characteristics) is largely positive and can be used as a key parameter in describing the flying qualities in plunging.

The problem of predicting loads due to turbulence for aircraft with nonlinear flight-control systems is reviewed, and related recent research in this area is described. Two alternative approaches to the problem, stochastic simulation and worst-case analysis, are compared and problem areas associated with each are identified. It is concluded that the latter method, which involves a search over a specified family of gust patterns to find the pattern that causes maximum load, is appropriate for investigating a limited number of critical cases, but that a preliminary simplified method of estimation is called for by computational considerations as a means of identifying such critical eases. A new genetic algorithm is described and proposed as the most reliable method of performing the search. It is illustrated how this search algorithm can be simplified computationally by introducing a weighting factor that penalizes the more complex gust patterns, a step that is supported by measurements of the amplitudes of gust patterns in severe turbulence.

This paper investigates the equilibration of baroclinic turbulence in an idealized, primitive equation, two-level model, focusing on the relation with the phenomenology of quasigeostrophic turbulence theory. Simulations with a comparable two-layer quasigeostrophic model are presented for comparison, with the deformation radius in the quasigeostrophic model being set using the stratification from the primitive equation model. Over a fairly broad parameter range, the primitive equation and quasigeostrophic results are in qualitative and, to some degree, quantitative agreement and are consistent with the phenomenology of geostrophic turbulence. The scale, amplitude, and baroclinicity of the eddies and the degree of baroclinic instability of the mean flow all vary fairly smoothly with the imposed parameters; both models are able, in some parameter ranges, to produce supercritical flows. The criticality in the primitive equation model, which does not have any convective parameterization scheme, is fairly sensitive to the external parameters, most notably the planet size (i.e., the fib ratio), the forcing time scale, and the factors influencing the stratification. In some parameter settings of the models, although not those that are most realistic for the earth's atmosphere, it is possible to produce eddies that are considerably larger than the deformation scales and an inverse cascade in the barotropic flow with a -5/3 spectrum. The vertical flux of heat is found to be related to the isentropic slope.

For an incidence composed of partially coherent multiple Gaussian beams, Huygens--Fresnel principle-based on-axis scintillation index is formulated in a weakly turbulent homogeneous horizontal atmospheric path. Our general formulation is applied to two examples of partially coherent annular and partially coherent flat-topped Gaussian beams. Compared to partially coherent single Gaussian beam scintillations, annular beam scintillations seem to possess higher values for all partial coherence levels, whereas flat-topped Gaussian beam intensity fluctuations are slightly larger, especially at lower coherence levels and at larger source sizes. At the same source partial coherence, annular beams exhibit smaller scintillations for larger ring sizes. For flat-topped Gaussian beams, except for very small and very large source sizes, as the number of Gaussian beams forming the flatness increases, intensity fluctuations also increase, a trend applicable for different degrees of coherence. A trend valid for both single and multiple Gaussian incidence, except for certain annular beams of large primary beam sizes, is that the scintillations decrease as the source becomes less coherent. Being applicable for all degrees of source coherences, for both beams examined, scintillations increase steadily as the Rytov plane wave scintillation index increases. OCIS codes: 010.1330, 010.1300, 290.5930.

The dual-polarization radar variables are especially sensitive to the microphysical processes of melting and size sorting of precipitation particles. In deep convective storms, polarimetric measurements of such processes can provide information about the airflow in and around the storm that may be used to elucidate storm behavior and evolution. Size sorting mechanisms include differential sedimentation, vertical transport, strong rotation, and wind shear. In particular, winds that veer with increasing height typical of supercell environments cause size sorting that is manifested as ah enhancement of differential reflectivity (ZDR) along the right or inflow edge of the forward-flank downdraft precipitation echo, which has been called the ZDR arc signature. In some cases, this shear profile can be augmented by the storm inflow. It is argued that the magnitude of this enhancement is related to the low-level storm-relative environmental helicity (SRH) in the storm inflow. To test this hypothesis, a simple numerical model is constructed that calculates trajectories for raindrops based on their individual sizes, which allows size sorting to occur. The modeling results indicate a strong positive correlation between the maximum [Z.sub.DR] in the arc signature and the low-level SRH, regardless of the initial drop size distribution aloft. Additional observational evidence in support of the conceptual model is presented. Potential changes in the [Z.sub.DR] arc signature as the supercell evolves and the low-level mesocyclone occludes are described.

The horizontal wavenumber spectra of wind and temperature near the tropopause have a steep -3 slope at synoptic scales and a shallower -5/3 slope at mesoscales, with a transition between the two regimes at a wavelength of about 450 km. Here it is demonstrated that a quasigeostrophic model driven by baroclinic instability exhibits such a transition near its upper boundary (analogous to the tropopause) when surface temperature advection at that boundary is properly resolved and forced. To accurately represent surface advection at the upper and lower boundaries, the vertical structure of the model streamfunction is decomposed into four parts, representing the interior flow with the first two neutral modes, and each surface with its Green's function solution, resulting in a system with four prognostic equations. Mean temperature gradients are applied at each surface, and a mean potential vorticity gradient consisting both of [beta] and vertical shear is applied in the interior. The system exhibits three fundamental types of baroclinic instability: interactions between the upper and lower surfaces (Eady type), interactions between one surface and the interior (Charney type), and interactions between the barotropic and baroclinic interior modes (Phillips type). The turbulent steady states that result from each of these instabilities are distinct, and those of the former two types yield shallow kinetic energy spectra at small scales along those boundaries where mean temperature gradients are present. When both mean interior and surface gradients are present, the surface spectrum reflects a superposition of the interior-dominated -3 slope cascade at large scales, and the surface-dominated -5/3 slope cascade at small scales. The transition wavenumber depends linearly on the ratio of the interior potential vorticity gradient to the surface temperature gradient, and scales with the inverse of the deformation scale when [beta] = 0.

The authors report results of a study of finescale turbulence structure in the portion of the nocturnal boundary layer known as the residual layer (RL). The study covers two nights during the Cooperative Atmosphere-Surface Exchange Study 1999 (CASES-99) field experiment that exhibit significant differences in turbulence, as indicated by the observed turbulence dissipation rates in the RL. The RL turbulence sometimes reaches intensities comparable to those in the underlying stable boundary layer. The commonly accepted concept of turbulence generation below critical values of the gradient Richardson number ([Ri.sub.g]) is scale dependent: Ri values typically decrease with decreasing vertical scale size, so that critical [Ri.sub.g], values (bbb0.25) occur at vertical scales of only a few tens of meters. The very small scale for the occurrence of subcritical Ri poses problems for incorporating experimentally determined [Ri.sub.g]-based methods in model closures in models with poor resolution. There appear to be two distinct turbulence 'regimes' in the RL: a very weak but ever-present background turbulence level with minimal temporal and spatial structure and a more intense intermittent regime during which turbulent intensity can approach near-surface nighttime turbulent intensities. It is hypothesized that the locally produced RL turbulence can be related to upward-propagating atmospheric gravity waves generated by flow over the low-relief terrain. The presence of critical layers in the RL, caused by wind turning with height, results in the generation of intermittent turbulence.

Turbulence properties of communication links (optical and microwave) in terms of log-amplitude variance are studied on the basis of a three-layer model of refractive index fluctuation spectrum in the free atmosphere. We suggest a model of turbulence spectra (Kolmogorov and non-Kolmogorov) changing with altitude on the basis of obtained experimental and theoretical data for turbulence profile in the troposphere and lower stratosphere. OCIS codes: 010.1330, 010.1300.

The mean and turbulent structures in a breaking mountain wave are considered through an ensemble of high-resolution (essentially large-eddy simulation) wave-breaking calculations. Of particular interest are the turbulent heat and momentum fluxes in the breaking wave and their roles in shaping the wave-scale and larger-scale flows. The evolution of the breaking wave in the ensemble mean is found to be broadly consistent with prior low-resolution calculations. A turbulent kinetic energy budget for the wave shows that the turbulence production is almost entirely due to the mean shear. Most of the production is at the top of the leeside shooting flow, where the mean-flow Richardson number is persistently less than 0.25. The turbulent dissipation of mean-flow wave energy is shown to result mainly from the turbulent momentum fluxes--specifically, from the tendency of these fluxes to act counter to the mean-flow disturbance wind. Of particular importance is the eddy deceleration of the leeside shooting flow. The resulting momentum dissipation leads to a mean-flow Bernoulli loss, a cross-stream mean-flow PV flux, and a permanent upward mean-flow vorticity transfer. The dependence of the turbulent fluxes on grid spacing is considered by computing a series of ensembles with grid spacings ranging from L/56 to L/3.7 (where L is the mountain half-width). At the highest resolution, the eddy fluxes are mostly resolved, but with increasing grid spacing, the resolved-scale fluxes decline and the parameterized fluxes become larger. It is shown that for the chosen parameter values, the parameterized fluxes overestimate the mean-flow PV flux: at L/3.7 the PV flux is nearly twice that computed at L/56.

The slope detection and ranging (SLODAR) method recovers atmospheric turbulence profiles from time averaged spatial cross correlations of wavefront slopes measured by Shack-Hartmann wavefront sensors. The Palomar multiple guide star unit (MGSU) was set up to test tomographic multiple guide star adaptive optics and provided an ideal test bed for SLODAR turbulence altitude profiling. We present the data reduction methods and SLODAR results from MGSU observations made in 2006. Wind profiling is also performed using delayed wavefront cross correlations along with SLODAR analysis. The wind profiling analysis is shown to improve the height resolution of the SLODAR method and in addition gives the wind velocities of the turbulent layers. OCIS codes: 010.1330, 010.7350, 110.1085.

There is a growing interest in understanding the role that moisture plays in atmospheric dynamics, particularly in its effect on predictability. Current research indicates that when moisture effects are added to an atmospheric model, the error growth produced by the new moist dynamics reduces the predictability times, especially at the scales of moist convection. The issue of moist convection's effect on predictability is addressed herein. By performing high-resolution large-ensemble runs, it is shown that although nonprecipitating moist convection is less predictable than dry convection resulting from the same forcing, this effect can be explained by the energy injected into the system through the latent heating and cooling arising from the convective motion. This extra energy is spread evenly over most scales of the convective dynamics. When the predictability times are scaled to account for the extra kinetic energy, and the resulting earlier growth of error energy, wet and dry convection have very similar error growth characteristics. Sensitivity tests are performed to ensure that the results from the large ensembles have converged and that they are consistent with either changing resolution, diffusion levels, initial error energy length scales, or forcing amplitude.

Turbulent flows are often observed to be organized into large-spatial-scale jets such as the familiar zonal jets in the upper levels of the Jovian atmosphere. These relatively steady large-scale jets are not forced coherently but are maintained by the much smaller spatial- and temporal-scale turbulence with which they coexist. The turbulence maintaining the jets may arise from exogenous sources such as small-scale convection or from endogenous sources such as eddy generation associated with baroclinic development processes within the jet itself. Recently a comprehensive theory for the interaction of jets with turbulence has been developed called stochastic structural stability theory (SSST). In this work SSST is used to study the formation of multiple jets in barotropic turbulence in order to understand the physical mechanism producing and maintaining these jets and, specifically, to predict the jet amplitude, structure, and spacing. These jets are shown to be maintained by the continuous spectrum of shear waves and to be organized into stable attracting states in the mutually adjusted mean flow and turbulence fields. The jet structure, amplitude, and spacing and the turbulence level required for emergence of jets can be inferred from these equilibria. For weak but supercritical turbulence levels the jet scale is determined by the most unstable mode of the SSST system and the amplitude of the jets at equilibrium is determined by the balance between eddy forcing and mean flow dissipation. At stronger turbulence levels the jet amplitude saturates with jet spacing and amplitude satisfying the Rayleigh-Kuo stability condition that implies the Rhines scale. Equilibrium jets obtained with the SSST system are in remarkable agreement with equilibrium jets obtained in simulations of fully developed [beta]-plane turbulence.

This paper discusses the representation of subgrid-scale turbulent mixing in bulk models of warm (ice free) clouds, which assume instantaneous adjustment to grid-scale saturation. This is a reasonable assumption for condensation of water vapor because supersaturations inside clouds are typically small (~0.1% or smaller), except near cloud bases where about an order of magnitude larger supersaturations are anticipated. For the cloud evaporation, however, instantaneous adjustment to grid-scale saturation is questionable, especially when evaporation occurs as a result of turbulent mixing between a cloud and its unsaturated environment. This is because turbulent mixing between initially separated volumes of cloudy and cloud-free environmental air proceeds through a gradual filamentation of these volumes, with progressively increasing evaporation of cloud water during the approach to final homogenization. A relatively simple model of this chain of events is included in a bulk model of moist nonprecipitating thermodynamics. The model delays adjustment to saturation for cloud evaporation following the turbulent mixing until the volume can be assumed homogeneous. An additional prognostic variable, the width of a cloudy filament, is added to represent the progress of turbulent mixing and the approach to homogenization. Theoretical developments are illustrated by idealized 2D simulations of moist thermals rising from rest and realistic large-eddy simulations of a cloud field.

Studies on the spontaneous emission of gravity waves from jets, both observational and numerical, have emphasized that excitation of gravity waves occurred preferentially near regions of imbalance. Yet a quantitative relation between the several large-scale diagnostics of imbalance and the excited waves is still lacking. The purpose of the present note is to investigate one possible way to relate quantitatively the gravity waves to diagnostics of the large-scale flow that is exciting them. Scaling arguments are used to determine how the large-scale flow may provide a forcing on the right-hand side of a wave equation describing the linear dynamics of the excited waves. The residual of the nonlinear balance equation plays an important role in this forcing.

An earlier paper proposed simple rate-equation models for subfilter-scale (SFS) scalar flux and deviatoric stress in the terra incognita--that is, in numerical modeling applications where the filter (grid-mesh) scale is of the order of the scale of the turbulence. Here the physics in these models is extended and further tested against data from the Horizontal Array Turbulence Study (HATS) experiment. It is found that extensions of the SFS scalar-flux model do not appreciably improve its performance, although an advection term (which could easily be used in modeling applications) substantially and realistically increases the fluctuation level of SFS scalar flux. The addition of buoyancy and rapid-mean-shear terms to the SFS stress model does improve its performance, bringing it to the level of the scalar-flux model.

A steady-state low-level jet observed in the Stable Atmospheric Boundary Layer Experiment in Spain1998 (SABLES-98) campaign is simulated using a large-eddy simulation model. The radiation is taken into account since its contribution to the budget of the tendency of temperature is as important as the contribution of the turbulence in the surface layer. The resulting regime is a two-layer flow separated by a change of the temperature gradient at the level of maximum speed. The upper part has turbulence originated by shear production extending up to 3 times the height of wind maximum and of the same order of magnitude as in the layer below. The jet nose is seen by the model as a decoupling layer, and the use of passive scalars allows for the inspection of the amount of transport across it, which is very small. The lower layer is very sensitive to the surface boundary conditions, but the principal patterns in the upper layer do not change significantly. The runs are compared to the observations through mean profiles, time series, and probability density functions.

A novel approach is introduced to identify potential vorticity (PV) streamers and cutoffs as indicators of Rossby wave breaking near the extratropical tropopause and to compile climatologies of these features on different isentropic surfaces. The method is based on a contour searching algorithm that identifies the dynamical tropopause [2 potential vorticity units (PVU; PVU [equivalent to] 1 x [10.sup.-6] K[kg.sup.-1][m.sup.2][s.sup.-1]) isoline] on isentropic surfaces. The contour is then analyzed to search for cutoffs and filament-like streamers. Whereas the identification of cutoffs is unambiguous, the one for streamers requires the specification of two parameters that determine the width and length of the contour feature to be classified as a streamer. This technique has been applied to the PV distribution in the Northern Hemisphere on isentropes from 295 to 360 K during the time period from 1979 to 1993 using the 15-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-15). The climatology reveals a pronounced zonal asymmetry in the occurrence of PV streamers and cutoffs. On all isentropes considered there are clear frequency maxima whose location changes with altitude. For instance, in winter and on the 300-K isentrope, stratospheric streamers and cutoffs occur most frequently near 50[degrees]-60[degrees]N over the western side of Canada and Siberia. On higher isentropes, the maxima are located farther south and at the downstream end of the storm-track regions. Considering continental areas, the Mediterranean appears as a region with particularly abundant PV features. As noted in previous studies, there is a significant seasonal cycle if considering the frequency of PV features on individual isentropes. It is shown that this is mainly due to the seasonal cycle in the location of the isentropes themselves. Comparing the streamer and cutoff frequencies during different seasons on isentropes that are comparably located in the zonal mean yields a fairly robust pattern with almost no seasonal cycle. This indicates on the one hand that care should be taken when considering the seasonal cycle of dynamical processes on isentropes and on the other hand that Rossby wave breaking occurs year-round with almost constant frequency. A quantitative statistical analysis of individual PV features reveals that stratospheric and tropospheric streamers often occur in pairs.

Stably stratified shear-driven turbulence is analyzed using the gradient Richardson number, Ri, as the stability parameter. The method overcomes the statistical problems associated with the widely used Monin-Obukhov stability parameter. The results of the Ri-based scaling confirm the presence of three regimes: the weakly and the very stable regimes and the transition in between them. In the weakly stable regime, fluxes scale in proportion with variance, while in the very stable regime, stress and scalar fluxes behave differently. At large Ri, the velocity field becomes highly anisotropic and the turbulent potential energy becomes approximately equal to half of the turbulent kinetic energy. It appears that even in the strongly stable regime, beyond what is known as the critical gradient Richardson number, turbulent motions are present.

In subgrid-scale condensation schemes of cloud models, the majority of previous authors have relied on results presented in a paper by Bougeault. In the present paper, second-order liquid water correlations are restated that differ from the former paper but are coherent with the corrigendum of Mellor. These differences are explained here through two different underlying definitions of cloud water content fluctuations; they can be summarized by whether or not unsaturated air within a grid box contributes to the eddy flux of the cloud water content. Taking into account the issue mentioned above, the 'buoyancy flux' is also derived. Although the full impact of these changes has not been evaluated here, it may become important for future global cloud-resolving climate models.

This study combines the experimental measurements with large-eddy simulation (LES) data of a neutral planetary boundary layer (PBL) documented by a 60-m lower instrumented with eight sonic anemometers, and a high-resolution Doppler lidar during the 1999 Cooperative Atmospheric and Surface Exchange Study (CASES-99) experiment. The target of the paper is (i) to investigate the multiscale nature of the turbulent eddies in the surface layer (SL), (ii) to explain the existence of a -1 power law in the velocity fluctuation spectra, and (iii) to investigate the different nature of turbulence in the two sublayers within the SL, which are the eddy surface layer (ESL; lower sublayer of the SL lying between the surface and about 20-m height) and the shear surface layer (SSL; lying between the ESL top and the SL top). The sonic anemometers and Doppler lidar prove to be proper instruments for LES validation, and in particular, the Doppler lidar because of its ability to map near-surface eddies. This study shows the different nature of turbulence in the ESL and the SSL in terms of organized eddies, velocity fluctuation spectra, and second-order moment statistics. If quantitative agreement is found in the SSL between the LES and the measurements, only qualitative similarity is found in the ESL due to the subgrid-scale model, indicating that the LES captures part of the physics of the ESL. The LES helps explain the origin of the -1 power-law spectral subrange evidence in the velocity fluctuation spectra computed in the SL using the CASES-99 dataset: strong interaction between the mean flow and the fluctuating vorticities is found in the SL, and turbulent production is found to be larger than turbulent energy transfer for [k.sub.1]z> 1 ([k.sub.1] being the longitudinal wavenumber and z the height), which is the condition of the -1 power-law existence.

The breaking of an inertia--gravity wave (IGW), initiated by its leading normal modes (NMs) or singular vectors (SVs), and the resulting small-scale eddies are investigated by means of direct numerical simulations of a Boussinesq fluid characterizing the upper mesosphere. The focus is on the primary nonlinear dynamics, neglecting the effect of secondary instabilities. It is found that the structures with the strongest impact on the IGW and also the largest turbulence amplitudes are the NM (for a statically unstable IGW) or short-term SV (statically and dynamically stable IGW) propagating horizontally transversely with respect to the IGW, possibly in agreement with observations of airglow ripples in conjunction with statically unstable IGWs. In both cases these leading structures reduce the IGW amplitude well below the static and dynamic instability thresholds. The resulting turbulent dissipation rates are within the range of available estimates from rocket soundings, even for IGWs at amplitudes low enough precluding NM instabilities. Thus SVs can help explain turbulence occurring under conditions not amenable for the classic interpretation via static and dynamic instability. Because of the important role of the statically enhanced roll mechanism in the energy exchange between IGW and eddies, the turbulent velocity fields are often conspicuously anisotropic. The spatial turbulence distribution is determined to a large degree by the elliptically polarized horizontal velocity field of the IGW.

Three two-equation turbulence models developed specifically to improve prediction of jet flowfields are investigated. These models are the Tam--Ganesan k--[epsilon] formulation, a standard k--[epsilon] model with modification for heated jets referred to as the PAB temperature correction, and a standard k--[epsilon] model employing variable diffusion for the k and [epsilon] equations. Two standard two-equation models are also investigated for comparison with the modified formulations. The standard models are the Chien k--[epsilon] and Menter shear stress transport formulations. All of the models were investigated for a reference nozzle producing heated and unheated jets at a low acoustic Mach number of 0.5 to avoid complications of large compressibility effects. The primary deficiency of the standard models was the delayed initial jet mixing rate. All of the modified turbulence model formulations provided improved mean flow predictions relative to the standard models. The improved mixing rate enabled by the Tam--Ganesan model and the variable diffusion correction resulted from increased turbulent diffusion enabled by both models. The Tam-Ganesan model and PAB temperature correction improved predictions of mean axial velocities for the heated jet, but did not improve prediction of the calculated turbulent kinetic energy fields. DOI: 10.2514/1.22650

Particle-image velocimetry experiments are performed to study the response of smooth-wall turbulent channel flow to a short fetch of roughness (10 outer length scales long). The roughness studied herein is replicated from a surface scan of a damaged turbine blade and contains both large- and small-scale surface defects attributable to pitting, deposition, and spallation. Quadrant analysis is used to investigate the characteristics of Reynolds-stressproducing events within the internal layer that develops over the roughness. The mean Reynolds stress is dramatically increased in the presence of the roughness as compared to the smooth-wall baseline owing to an increased number of extremely intense ejections and sweeps. In contrast, inward and outward interactions, as well as relatively weak ejection and sweep events, are found to be insensitive to the surface conditions. While stress and space fractions are found to be insensitive to the surface topology when all Reynolds-stress-producing events are included, stress and space fractions for only the most intense ejection and sweep events vary significantly with the local surface topology. DOI: 10.2514/1.22357

Simulations of a low-speed square cylinder wake and a supersonic axisymmetric base wake are performed using the detached eddy simulation model. A reduced-dissipation form of a shock-capturing flux scheme is employed to mitigate the effects of dissipative error in regions of smooth flow. The reduced-dissipation scheme is demonstrated on a two-dimensional square cylinder wake problem, showing a marked improvement in accuracy for a given grid resolution. The results for simulations on three grids of increasing resolution for the three-dimensional square cylinder wake are compared with experimental data and to other computational studies. The comparisons of mean flow and global flow quantities to experimental data are favorable, whereas the results for second order statistics in the wake are mixed and do not always improve with increasing spatial resolution. Comparisons to large eddy simulation are also generally favorable, suggesting detached eddy simulation provides an adequate subgrid scale model. Predictions of base drag and centerline wake velocity for the supersonic wake are also good, given sufficient grid refinement. These cases add to the validation library for detached eddy simulation and support its use as an engineering analysis tool for accurate prediction of global flow quantities and mean flow properties. DOI: 10.2514/1.22359

In this paper, direct numerical simulation results of turbulent supersonic channel flow are analyzed to address several questions that are relevant to turbulence modeling: the turbulence frequency production mechanism, wall damping effects on turbulence model parameters, and the relevance of compressibility effects. Limited support is found for usually applied models for the turbulence frequency production and wall damping effects. In contrast to that it is shown that turbulence frequency production mechanisms and wall damping effects may be explained very well on the basis of a frequency scaling that characterizes mean flow changes. The influence of compressibility is found to be relevant. DOI: 10.2514/1.22259

Large-eddy simulations of pairs of artificially generated longitudinal vortices embedded in a two-dimensional turbulent boundary layer are performed to study the dynamics of the vortical structures and to provide the unsteady inflow data for use in a future turbomachinery simulation. An immersed boundary method is employed to represent the wall-mounted half-delta wings that generate the counter-rotating vortex pairs. Two vortex pair configurations, with 'common flow' between the vortices toward the end wall (common flow down) and away from the end wall (common flow up), respectively, are investigated. Mean velocities and Reynolds stresses compare well with experimental data, demonstrating the accuracy of the numerical approach and, in particular, the efficacy of immersed boundary treatment of the vortex generators. The large coherent vortices are characterized by streamwise velocity defect and negative mean pressure. The boundary layer in the common flow region is thinned by the vortices in the common-flow-down case and thickened by them in the common-flow-up case. DOI: 10.2514/1.22043

The 'flux based' local scaling in the stably stratified boundary layer is valid only in cases with strong, continuous turbulence, when the gradient Richardson number Ri is constant and subcritical. To extend the local similarity approach to cases with weak turbulence (very stable regime), the 'gradient based' local scaling is introduced and discussed in the paper. Both types of local scaling, the flux based and the gradient based, are tested based on the data, collected from a 60-m tower during the Cooperative Atmosphere-Surface Exchange Study-1999 (CASES-99). The obtained results show that the gradient-based scaling provides a useful framework for the treatment of cases with both strong and weak turbulence and overcritical Richardson numbers.

The presented turbulence scheme was developed for the Active Tracer High-Resolution Atmospheric Model (ATHAM) to parameterize the effect of subgrid-scale turbulence. In contrast to the commonly used assumption of local isotropy in high-resolution atmospheric modeling, this scheme differentiates between horizontal and vertical turbulent exchange to represent the strong influence of buoyancy forces and vertical transports. Its computational efficiency is similar to classical turbulent kinetic energy approaches while preserving one of the main feature of higher-order schemes. The present extensions to include anisotropic effects in a turbulent kinetic energy approach do not need any ad hoc assumptions and are equivalent to the classical formulation in the isotropic limit. The presence of high tracer concentrations in a particle-laden plume is taken into account, as well as supersonic effects at low Mach numbers. The turbulent exchange coefficients used in the equations of motion are derived from a set of three coupled differential equations for the horizontal and vertical turbulent energy and the turbulent length scale. No turbulent equilibrium is assumed. All turbulent quantities are treated prognostically. Numerical simulations of convective plumes of a typical Plinian volcanic eruption with the nonhydrostatic plume model ATHAM reveal that a complex treatment of turbulent quantities is necessary in order to capture the bulk characteristics of the plume, such as the plume height, the horizontal extent, and plume development in time. Anisotropic effects of turbulence have a significant impact on the stability and internal structure of the plume. For the first time, results from a fully three-dimensional simulation of a volcanic plume are presented. Because of its general formulation the presented turbulence scheme is suitable for a wide range of atmospheric applications.

We present a detailed investigation of different methods of the characterization of atmospheric turbulence with the adaptive optics systems of the W. M. Keck Observatory. The main problems of such a characterization are the separation of instrumental and atmospheric effects and the accurate calibration of the devices involved. Therefore we mostly describe the practical issues of the analysis. We show that two methods, the analysis of differential image motion structure functions and the Zernike decomposition of the wave-front phase, produce values of the atmospheric coherence length [r.sub.0] that are in excellent agreement with results from long-exposure images. The main error source is the calibration of the wave-front sensor. Values determined for the outer scale [L.sub.0] are consistent between the methods and with typical [L.sub.0] values found at other sites, that is, of the order of tens of meters. OCIS codes: 010.1080, 010.1330, 010.7350.

Idealized numerical simulations are conducted in which an axisymmetric, moist, rotating updraft free of rain is initiated, after which a downdraft is imposed by precipitation loading. The experiments are designed to emulate a supercell updraft that has rotation aloft initially, followed by the formation of a downdraft and descent of a rain curtain on the rear flank. In the idealized simulations, the rain curtain and downdraft are annular, rather than hook-shaped, as is typically observed. The downdraft transports angular momentum, which is initially a maximum aloft and zero at the surface, toward the ground. Once reaching the ground, the circulation-rich air is converged beneath the updraft and a tornado develops. The intensity and longevity of the tornado depend on the thermodynamic characteristics of the angular momentum-transporting downdraft, which are sensitive to the ambient low-level relative humidity and precipitation character of the rain curtain. For large low-level relative humidity and a rain curtain having a relatively small precipitation concentration, the imposed downdraft is warmer than when the low-level relative humidity is small and the precipitation concentration of the rain curtain is large. The simulated tornadoes are stronger and longer-lived when the imposed downdrafts are relatively warm compared to when the downdrafts are relatively cold, owing to a larger amount of convergence of circulation-rich downdraft air. The results may explain some recent observations of the tendency for supercells to be tornadic when their rear-flank downdrafts are associated with relatively small temperature deficits.

Condensation and turbulent liquid water transport in stratocumulus clouds involve complicated interactions between turbulence dynamics and cloud microphysical processes, and play essential roles in defining the cloud structure. This work aims at understanding this dynamical-microphysical interaction and providing information necessary for parameterizations of the ensemble mean condensation rate and turbulent fluxes of liquid water variables in a coupled turbulence-microphysics model. The approach is to simulate nonprecipitating stratocumulus clouds with a coupled large eddy simulation and an explicit binmicrophysical model, and then perform a budget analysis for four liquid water variables: mean liquid water content, turbulent liquid water flux, mean cloud droplet number concentration, and the number density flux. The results show that the turbulence contribution to the mean condensation rate comes from covariance of the integral cloud droplet radius and supersaturation, which enhances condensation in turbulent updrafts and reduces evaporation in the downdrafts. Turbulent liquid water flux results from a close balance between turbulence dynamics and microphysical processes. Consequently, the flux can be parameterized in terms of the common diffusive downgradient formulation, fluxes of conservative thermodynamic variables, the turbulence mixing timescale, and the condensation timescale, which is determined by the droplet spectrum. The results also suggest that the condensation timescale regulates the turbulence fields, as does the number concentration, because it affects the condensation fluctuation, which is highly correlated with the turbulence vertical motion. A saturation adjustment cloud model, which diagnoses liquid water content at its equilibrium level, instantly condenses (evaporates) all available water vapor (liquid water) surplus. Consequently, there is likely to be a systematic difference between the turbulence field resolved with this type of model and that with a supersaturation-based cloud scheme for which a finite condensation timescale applies.

In the stable boundary layer it is often observed that turbulence is not continuous in space and time. This discontinuous, intermittent turbulence causes alterations from the mean evolution of the stratified atmospheric boundary layer, which may result in an oscillatory type of behavior of the near-surface wind speed and temperature. This paper focuses on an intermittency generating mechanism that arises from a direct interaction of the lower atmosphere (first tens of meters) with the vegetation surface, without interaction with the air aloft. This atmosphere-surface intermittency (ASI) is associated with the essential elements of the stable boundary layer (SBL): strong surface cooling, the supply of mechanical energy by the synoptic pressure gradient, and the limiting effect of stratification on mixing efficiency. In Part I it is shown that the essence of ASI can be captured by a system of three coupled nonlinear differential equations. This simplified system shows both intermittent and nonintermittent flow regimes for different circumstances. In the present paper, this system is studied analytically, following a system dynamics approach. The transition between the different flow regimes is identified as a Hopf bifurcation. This property is used to derive a dimensionless parameter, which is a function of external parameters, such as radiative forcing and pressure gradient. With this dimensionless parameter the equilibrium behavior of the system (i.e., intermittent or nonintermittent) can be predicted exactly. As such this parameter is used to classify SBL regimes. It is shown that the proposed classification parameter provides different information about the state of the SBL than other typical SBL classification parameters such as z/L and Ri.

In free-space optical communication links, atmospheric turbulence causes fluctuations in both the intensity and the phase of the received light signal, impairing link performance. In this paper, we describe several communication techniques to mitigate turbulence-induced intensity fluctuations, i.e., signal fading. These techniques are applicable in the regime in which the receiver aperture is smaller than the correlation length of fading and the observation interval is shorter than the correlation time of fading. We assume that the receiver has no knowledge of the instantaneous fading state. When the receiver knows only the marginal statistics of the fading, a symbol-by-symbol ML detector can be used to improve detection performance. If the receiver has knowledge of the joint temporal statistics of the fading, maximum-likelihood sequence detection (MLSD) can be employed, yielding a further performance improvement, but at the cost of very high complexity. Spatial diversity reception with multiple receivers can also be used to overcome turbulence-induced fading. We describe the use of ML detection in spatial diversity reception to reduce the diversity gain penalty caused by correlation between the fading at different receivers. In a companion paper, we describe two reduced-complexity implementations of the MLSD, which make use of a single-step Markov chain model for the fading correlation in conjunction with per-survivor processing. Index Terms--Atmospheric turbulence, free-space optical communication, MLSD, spatial diversity reception.

This study uses large-eddy simulation (LES) to illustrate the flow and turbulence structure and to investigate the mechanism of passive scalar transport in a street canyon. Calculations for a modeled street canyon with building-height-to-street-width ratio of unity at Reynolds number equal to 12 000 are conducted. When the approaching wind is perpendicular to the street axis, the calculation produces a primary vortex in the street canyon, similar to previous studies. An evaluation of the LES results with wind-tunnel measurements reveals good agreement for both mean and turbulence parameters of the flow and scalar fields. The computed primary vortex is confined to the street canyon and is isolated from the free stream flow such that the removal of a scalar emitted at the street level is accomplished by turbulent diffusion at the roof level. It is determined from the calculations that very little scalar is removed from the street canyon, and 97% of the scalar is retained. The scalar mixing at the roof level occurs primarily on the leeward side of the street canyon. In addition to the primary vortex, three secondary vortices are located in the corners of the street canyon at which scalar mixing is enhanced. An examination of additional simulations shows how the location of the scalar source affects the distribution of the scalar.

Turbulence in a [gamma]-mesoscale internal boundary layer (IBL) formed by a discontinuous change in surface roughness has been investigated using a large eddy simulation (LES) model to explicitly treat turbulent transport. Two cases are examined: a rough-to-smooth transition and a smooth-to-rough transition. IBL heights are identified using two absolute criteria, one in terms of horizontal stress variation and the other in terms of vertical stress variation, and the ratio of these two heights is found to be approximately constant with fetch. The IBL growth rate with fetch is essentially the same for both transitions, which is here interpreted as reflecting self-similarity of the IBL at relatively large fetches. Parameterization of IBL growth in terms of turbulent intensity is successful when the average turbulent intensity in the IBL is employed but not if the turbulent intensity at the IBL top is utilized. The effective eddy mixing length for longitudinal velocity does not experience strong variations in the vicinity of the surface discontinuity when parameterized in terms of the local turbulent kinetic energy (TKE), but the effective mixing length is somewhat larger over the smoother surface as a result of pressure gradients induced by the horizontally inhomogeneous flow. The TKE dissipation length scale is roughly equal to the mixing length scale for longitudinal velocity, so stress and TKE are strongly coupled above the surface layer. The 'quality' of turbulence, evaluated by turbulent skewness and kurtosis, indicates that turbulence above and below the identified IBL height is associated with the upstream and underlying surfaces, respectively, demonstrating that this height is a physically relevant length scale.

In the stable boundary layer (SBL) it is observed often that turbulence is not continuous in space and time. This discontinuous, intermittent turbulence causes alternations from the mean evolution of the stratified atmospheric boundary layer, which may result in an oscillatory type of behavior of the near-surface wind speed and temperature. It is well known that intermittent turbulence in the SBL can be generated by various mechanisms. This paper focuses on an intermittency generating mechanism that arises from a direct interaction of the lower atmosphere (first tens of meters) with the vegetation surface, without interaction with the air aloft. It is shown that the essence of this mechanism can be captured by a 1D bulk model of three coupled nonlinear differential equations. In the present paper, numerical runs with the model show that intermittent turbulence is most likely to occur over land surfaces with low vegetation during clear-sky conditions in the presence of a moderate to low synoptical pressure gradient. The existence of a vegetation layer has a strong influence on intermittency dynamics. Due to its small heat capacity, the vegetation temperature is able to quickly respond to rapidly changing conditions. This in turn affects the stability of the lower atmosphere, causing an important feedback mechanism. In addition, it was found that intermittent behavior of SBL models occurs for various first-order closure schemes with different stability functions. However, stability functions that allow turbulent transport beyond the critical Richardson number effectively suppress intermittent-oscillatory behavior. Currently, the latter type of formulations is often used in numerical weather prediction to prevent excessive SBL cooling in very stable conditions. The advantage of using a simplified SBL model, as proposed in the present paper, is that it allows an analytical study of the system, which, in turn, allows theoretical predictions about the occurrence of intermittent SBL behavior (see the companion paper).

The formation of stars is one of the most fundamental problems in astrophysics, as it underlies many other questions, on scales from the formation of galaxies to the formation of the solar system. The physical processes involve the turbulent behavior of a partially ionized medium containing a non-uniform magnetic field. Current debate centers around the time taken for turbulence to decay and the relative importance of the roles played by magnetic fields and turbulence. Technological advances such as millimeter-wave cameras have made possible observations of the temperature and density profiles, and statistical calculations of the lifetimes, of objects collapsing under their own self-gravity and those on the verge of collapse. Increased computing power allows more complex models to be made that include magnetic and turbulent effects. No current model can reproduce all of the observations., Stars are among the most fundamental building blocks of the universe, and yet the processes by which they are formed are not understood. Models can give different predictions for the [...]

Stabilized space--time finite element methods for the transient computational analysis of the elastodynamics of Timoshenko beams have been developed. The underlying time-discontinuous Galerkin formulation is implicit, unconditionally stable, higher-order accurate, and robust. The employed interpolations are continuous in space and time inside time slabs, but discontinuous in time between adjacent time slabs. To suppress spurious numerical oscillations near discontinuities or high gradients, Galerkin/least-squares stabilization has been applied. Further improvement of the numerical representation of the transient elastic wave propagation phenomena has been achieved by specially designed Galerkin/gradient least-squares operators. Because of the reduced phase and amplitude errors of these finite elements, much coarser spatial meshes may be used without loss of accuracy. The resulting reduction of the number of unknowns leads to a significant decrease of computer time. Furthermore, a global adaptive time-stepping strategy has been employed based on the temporal jump residual of the time finite element method as error estimator. Numerical examples of transient elastic wave propagation in Timoshenko beams involving a wide spectrum of wave numbers and frequencies demonstrate the good performance of the developed methods.

Flow characterization experiments of stagnation enthalpies up to 37 MJ/kg are carried out in the recently commissioned expansion tube facility. The features and the effective duration time of the test flowfield are determined not only from static and pitot pressures but also by observing the temporal variation of radiating emission from a shock layer around a spherical forebody. The thickness of the radiating layer matches well with the numerical results. The arrival time of the contact surface between the acceleration and test gases is better identified both as a decrease in the radiating layer thickness and from the emission spectroscopy. The useful test time is determined by integrating the mentioned measurements.

Shock-wave/turbulent boundary-layer interactions in the presence of thermal and chemical nonequilibrium phenomena are analyzed. The approach relies on a linear eddy viscosity two-equation turbulence modeling that accounts for the coupling of turbulence with chemistry and vibration, and it employs a total variation diminishing finite volume numerical methodology. The capability of the model has first been assessed for a cylinder flare configuration, and results have been compared with experiments. The model has then been applied to assess the aerodynamic performance of control surfaces of a reusable launch vehicle. In particular, the effects of wall temperature and flap deflection on the separation, aerothermal loads, and flap efficiency have been studied. The analysis shows that turbulence becomes important for flap deflection angles greater than a critical value [O (15 deg)], thus avoiding the crisis of the flap efficiency that is observed under laminar conditions and extending the operating capabilities of the control surface. The study also shows that the wall temperature affects significantly the efficiency and the operating envelope of the flap primarily under turbulent conditions.

A class of lower-upper symmetric Gauss-Seidel implicit weighted essentially nonoscillatory (ENO) schemes for solving the two- and three-dimensional compressible Navier-Stokes equations with pointwise version of Baldwin-Barth one-equation turbulence model is presented (Baldwin, B. S., and Barth, T. J., 'A One-Equation Turbulence Transport Model for High Reynolds Number Wall Bounded Flows,' AIAA Paper 91-0610, 1991). A weighted ENO (WENO) spatial operator is employed for inviscid fluxes and central differencing for viscous fluxes. A numerical flux of the WENO scheme in flux limiter form is adopted, which consists of first-order and high-order fluxes and allows for a more flexible choice of first-order dissipative methods. The computations are performed for the two-dimensional turbulent flows over NACA 0012 and Royal Aircraft Establishment 2822 airfoil and the three-dimensional turbulent flow over an ONERA M6 wing. The present solutions are compared with experimental data and other computational results and exhibit good agreement.

A direct numerical simulation is performed to examine the initial relaxation of spatially evolving turbulent channel flow subjected to uniform wall blowing and suction. Just beyond the entrance section, a uniform blowing is applied at the lower wall and a uniform suction at the upper. The relaxation processes of the mean velocity, turbulence intensities, Reynolds shear stresses, and vorticity fluctuations after the sudden wall blowing and suction are scrutinized. The effect of wall blowing and suction on the location and strength of the streamwise vortices is significant. The relaxation associated with suction is much slower than that with blowing. The initial relaxation is evaluated in terms of the turbulent kinetic energy transport. Blowing enhances the intercomponent energy transfer between the Reynolds stresses, whereas suction suppresses it.

An analysis of the condensation and evaporation processes involved in the classic Sommeria-Deardoff-Mellor turbulence cloud models is presented. A liquid water budget is derived from the diagnostic Gaussian cloud relations. It is found that the adiabatic condensation generated by turbulent eddies at the cloud base and the mean radiative cooling at the cloud top are the two major processes responsible for the condensation in the parameterizations for stratocumulus-topped boundary layers. The evaporation is directly related to the stratification of the boundary layers and the turbulence variability (variance and covariance) of the conserved thermodynamic variables. The evaporation caused by the turbulence variability plays a dominant role at the cloud top. The analysis also shows that the profile of the parameterized liquid water flux is primarily determined by the turbulence-generated condensation and evaporation in the cloud model. This model is also compared with other prognostic cloud schemes.

A generalization of the method of random wave vectors [Appl. Opt. 36, 464 (1997)] that is suitable for a simulation of turbulence-induced anisoplanatic effects is proposed. A simulation of the cross-correlated phase fluctuations produced by two initially plane waves propagating through weak turbulence is considered. The variation of [[C.sub.n].sup.2] along a propagation path and an effect of the finite outer scale of the turbulence are included in the simulation. The validity of the simulation method is verified by comparison of theoretical and simulated results. The simulation approach developed can be used in the problems related to adaptive optics, speckle inteferometry, guide stars, and imaging through turbulence.

The role of duty cycle and injection time on the penetration and mixing of fully-modulated, incompressible and turbulent transverse jets have been studied in a water tunnel. The jet flow was modulated by operating an in-line solenoid valve which cuts off jet supply during a portion of the cycle. Results indicate that complete modulation resulted in a modification of the jet flow as well as its penetration depth, dilution and mixing to various degrees. Long injection times led to the formation of axially elongated turbulent puffs that resembled a segment of the steady jet.