On the dynamics of a solitary vortex

The dynamic equilibrium of a solitary vortex of permanent form but arbitrary shape is investigated on a β plane. The vortex is assumed to propagate in the east-west direction. Forces and moments acting on the vortex are computed and the balances between them obtained. The dynamic balances are required for the equilibrium of the vortex and represent the conditions necessary for the existence of a solitary vortex of permanent form. These conditions have two useful expressions, one as integral constraints on the vortex structure and the other as formulas for the propagation speed of the vortex. For the latter, the speed is shown to be proportional to the averaged velocity moment of the vortex and inversely proportional to its averaged relative vorticity. The propagation speed also depends on the size of the vortex; it increases with size for westward propagation and decreases with size for eastward propagation. The effect of current shear on the vortex is also investigated. The results of this study are applicable to the nonlinear barotropic vortex in a nondivergent fluid as well as to the baroclinic vortex, provided that the latter satisfies the quasigeostrophic approximation.     

Boundary-layer dynamics of the extremely arid northern part of Chile

A boundary-layer field experiment in the extremely arid northern part of Chile was carried out in July, 1970. Captive radiosonde data, pibal soundings and surface observations in a three-point zonal cross-section at 23 deg lat. S are analyzed. Evidence of a thermal response in the boundary-layer circulation and daytime subsidence enhancement is presented. Surface windspeeds seem to increase in the general upslope direction while wind components along the contour lines are apparent in the central strip, in accordance with earlier results for Pampa de la Joya, Peru.     

The dynamics of barotropic vortex merging

The merging of multiple vortices is a fundamental process of the dynamics of Earth’s atmosphere and oceans. In this study, the interaction of like-signed vortices is analytically and numerically examined in a framework of two-dimensional inviscid barotropic flows. It is shown that barotropic vortex interaction turns out to be more intricate than simple merging scenarios often assumed in previous studies. Some particular configurations exist in which the vortex merging process is never complete despite strong interaction of like-signed vortices, regardless of the strengths or distances between the vortices. While the conditions for a complete vortex merging process introduced in this study appear to be too strict for most practical applications, this study suggests that careful criteria for vortex mergers should be properly defined when simulating the interaction of vortices, because the merging may not always result in a final enhanced circulation at the end of the interaction, as usually assumed in the literature.     

Boundary-layer wind structure in a landfalling tropical cyclone

In this study, a slab boundary layer model with a constant depth is used to analyze the boundary-layer wind structure in a landfalling tropical cyclone. Asymmetry is found in both the tangential and radial components of horizontal wind in the tropical cyclone boundary layer at landfall. For a steady tropical cyclone on a straight coastline at landfall, the magnitude of the radial component is greater in the offshoreflow side and the tangential component is greater over the sea, slightly offshore, therefore the greater total wind speed occurs in the offshore-flow side over the sea. The budget analysis suggests that： （1） a greater surface friction over land produces a greater inflow and the nonlinear effect advects the maximum inflow downstream, and （2） a smaller surface friction over the sea makes the decrease of the tangential wind component less than that over land. Moreover, the boundary layer wind structures in a tropical cyclone are related to the locations of the tropical cyclone relative to the coastline due to the different surface frictions. During tropical cyclone landfall, the impact of rough terrain on the cyclone increases, so the magnitude of the radial component of wind speed increases in the offshore-flow side and the tangential component outside the radius of maximum wind speed decreases gradually.     

Boundary-layer effects on cold fronts at a coastline

The present note discusses one physical mechanism which may contribute to cold air channelling, manifest as a frontal bulge on a surface-analysis chart, in the coastal region of Victoria in southeast Australia. This involves the modification of boundary-layer air in both offshore (prefrontal) and onshore (postfrontal) flow, and the effect on cross-frontal thermal contrast. The problem is discussed in terms of a north-south-oriented cold front behaving as an atmospheric gravity current, propagating along an east-west-oriented coastline, in the presence of a prefrontal offshore stream.     

Boundary-layer development on circular cylinders

Mean-velocity measurements were made in the boundary layer on a smooth circular cylinder fitted with trip wires to model supercritical flow. The data are compared with the predictions of a boundary-layer calculation method to study the relative influence of surface curvature and low-Reynolds-number effects, and to assess the accuracy with which the location of separation can be determined. It is shown that the effects of surface curvature are most important close to separation while those of low Reynolds numbers are significant everywhere except near separation.     

Numerical Modelling of Neutral Boundary-layer Flow Across a Forested Ridge

Boundary-Layer Meteorology - Forest canopies have been shown to alter the dynamics of flows over complex terrain. Deficiencies have been found when tall canopies are represented in numerical...     

Boundary-layer trade-wind profile and stress on a tropical windward coast

Under persistent onshore trade-wind conditions, the wind profile was measured in the surface layer on a mud flat of the tidal stream of Estero de Data, Gulf of Guayaquil, Ecuador. It was found that the logarithmic wind relation fitted the observations very well and that the inferred shear velocity was a linear function of wind speed in the range of wind velocity studied.     

Boundary-layer depth and entrainment zone characterization with a boundary-layer profiler

A technique for determining the height of the convective atmospheric boundary layer (CBL) with a 915 MHz boundary-layer profiler is discussed. The results are compared with CBL heights determined from radiosonde measurements. The profiler provides continuous CBL height measurements with very good time resolution (30 minutes or less), allowing for detailed understanding of the growth and fluctuations of the CBL. In addition, the profiler provides information about the degree of definition of the CBL top and the thickness of the entrainment zone. The measurements discussed were taken during the Rural Oxidants in the Southern Environment II (ROSE II) experiment.     

Boundary-layer turbulence spectra in stable conditions

Simultaneous measurements of horizontal and vertical wind speeds and temperature fluctuations at heights up to 91 m in the stable atmospheric boundary layer are described. The power and cospectral shapes show a low-frequency peak (near the Brunt-Väisälä frequency) separated by a spectral gap from a peak at high frequency due to turbulence. Spectral shapes in the turbulence subrange at 8 m are in good agreement with the universal curves previously presented by Kaimal (1973). Further information is given on the variation of the scaling parameter, f ₀, with stability; and the applicability of the normalising procedure to the spectra from the higher levels is discussed.     

Boundary-layer pollutant concentrations over complex terrain

Given the incident profiles of wind velocity and pollutant concentration, we seek to determine the 3-dimensional concentration field of a pollutant upon a region with complex terrain. The analytic solution of the wind velocity in a 3-dimensional boundary-layer model by Walmsley et al. (1980) is utilized as a forcing function in the simplified concentration perturbation equation for a pollutant. The resulting solution applies to an isolated cosine-squared hill in a neutrally stratified boundary-layer flow with a surface type which absorbs the pollutant totally. The solution shows that the concentration perturbation field is organized in accordance with the wind field. In particular, the east-west cross-section is 180° out of phase with the velocity perturbation field. The vertical profiles of the concentration perturbations for selected grid points approach the value of the upper boundary condition very rapidly.     

On the nonlinear dynamics of the boundary layer of intense atmospheric vortex

The paper describes a stationary model of the boundary layer of a large-scale vortex. The model takes into account two types of nonlinearities: (1) advection of the momentum and centrifugal accelerations, that is related to large values of the Rossby number; (2) a nonlinear turbulent friction on the lower boundary (generally speaking, not only quadratic, but also more complicated). Nevertheless, the model may be studied analytically. This approach allows discovery and analysis of several universal regularities, which are not revealed by numerical modeling. The model provides the actual values for the speed of the surface wind and the angle of cross-isobar surface flow in the tropical cyclones.     

Two-dimensional asymmetric vortex merger: merger dynamics and critical merger distance

Two-dimensional asymmetric merger of two like-signed vorticity monopoles with different sizes and vorticities is examined by combining simplified analytical models and contour dynamics experiments. The model results can capture the key dynamics and hence allow the prediction of the critical merger distance in a number of the situations. The models ignore deformation of one of the two vortices, replacing it with a point vortex, and employ a corotating frame of reference with a rotation rate estimated by point vortices. Thus, the two vortex problem becomes two separate problems of a single vortex in a background shear flow. Vortex merger is found to happen when the vortex cannot resist the background shear flow. Vortex merger and merging processes depend on the centroid distance d, the circulation ratio, (q_i and r_i are the vorticity and radius, respectively) and initial conditions. In the lowest order, the background flow is approximated by a uniform shear field, and the behavior of an elliptical vortex can be described by the Kida (1981) equation supplemented with one describing the time evolution of the centroid distance. This model reveals that merger takes place because the natural rotation of an elliptical vortex is overcome by the background uniform shear flow; the ellipse inversely rotates and is drawn out by the background straining field. The vortex deformation in a background flow field induces an inward flow at the position of the other vortex; as a result, the centroid distance decreases and two vortices merge. The critical merger distance from this model agrees quite well with the results from contour dynamics experiments for two vortices. Inclusion of higher order non-uniform shear in the background flow extends the critical merger distance, which gives almost perfect estimates for the experiment. In the non-uniform shear flow, partial merger occurs, where the vortex sheds off a filament, but the remaining part of the vortex resumes its natural rotation.     

Study on a Boundary-layer Numerical Model with Inclusion of Heterogeneous Multi-layer Vegetational

1.IntroductionItiswellknownthattheecosystemcangreatlyinfluencebothlocalclimateandgeneralcirculation.Onthenumericalstudyoftheturbulenceinandaboveforestcanopies,alotofsignificantstudieshavebeendone.Inallthesestudies,modelsaregenerallydividedintotwotypes:oneis'K--theory'type(Waggoner,1975;Gross,1987;Gross,1988,Jietal.,1989;Schilling,I991;Dickinsonetal.,1993;Wang,1996),theotherappliesthehigher--orderclosuremethod(Wilsonetal.,1977,Yamada,1982;Yinetal.,1989)ortheLagrangianmethod(Rampach,1987;R…     

Boundary-layer heat and moisture budgets from fife

Aircraft stacks were flown upwind and downwind of the First ISLSCP Field Experiment (FIFE) site in Kansas to measure the heat and moisture budgets of the boundary layer under fairly clear skies for four daytime periods. In this paper, we evaluate the terms in the conservation equation. The vertical flux divergence and advection do not account for the difference between surface and low-level aircraft flux estimates. Budget estimates of the surface fluxes using the aircraft data agree well with surface flux measurements, but extrapolation of the aircraft fluxes gives surface fluxes that are too low. With the 5 km cutoff filter used, the aircraft underestimate for sensible heat flux is about 40%, and for the latent heat flux about 30%. Part of the underestimation is attributable to long-wavelength contributions (longer than the 5 km filter), but more investigation is needed.     

Boundary-layer flow structures associated with particle reentrainment

Measurements of near-surface longitudinal and vertical wind velocity components associated with particle reentrainment from a flat surface have been examined in a wind tunnel. Sparsely covered particle beds were used to assure that observed reentrainment events resulted primarily from the action of fluid forces. Characteristic velocity patterns were found to be associated with a majority of particle reentrainment events examined. These characteristics have been categorized and examined as ensemble averages. The flow pattern most frequently observed during particle reentrainment was termed Ejection-Sweep (E-S) and is very similar to organized fluid motions previously observed in laboratory flows and in the atmospheric boundary layer. A simple two-tiered E-S pattern recognition scheme is described which strives to identify particle reentrainment events objectively based on flow characteristics alone. The first step is to identify potential E-S patterns using criteria which identify a characteristic longitudinal acceleration, and the second step is to use threshold values of pattern characteristics to accept or reject these first-tier patterns. Pattern recognition results are presented in terms of the ability to identify reentrainment events versus false identifications, and show an exponential growth in false identifications with an increasing number of reentrainment events identified.     

Combining Non-local Scalings with a TKE Closure for Mixing in Boundary-layer Clouds

A new approach to the parametrization of the cumulus-capped boundary layer is described. It combines a traditional higher-order turbulence closure, appropriate for boundary layers where the skewness of thermodynamic variable probability distributions is low (typically stratocumulus-capped), with non-local scaled similarity functions. These are introduced in order to represent explicitly that part of the distribution arising from skewed cumulus elements and the scalings are found to work very well against equilibrium shallow cumulus large-eddy simulations. Results from a wide range of single column model simulations, from stratocumulus to shallow cumulus to cumulus rising into stratocumulus, are presented that demonstrate the validity of the approach as a means of parametrizing the cloudy boundary layer. Sensitivity tests show that enhancement of the turbulence length scales and the buoyancy production of TKE are especially important.     

Boundary-layer flow and turbulence near porous obstacles

We derive a set of governing equations for flow through porous obstacles by employing a two-step averaging processes. The Navier-Stokes equations under the Boussinesq approximation that describe the air space of the porous obstacle are subjected to high-wavenumber a veraging, which leads to a set of high-frequency (wake) turbulence equations. We then use conventional Reynolds-averaging methods to obtain statistically steady mean and turbulence equations that include interactions between wake and shear turbulence. Our method provides a theoretical basis for the cascade of turbulent kinetic energy. We use this approach to analyze the constants and parameters of simpleK-theory and higher-order closure models. We also discuss qualitatively the theory of the turbulence energy generation process and the significance of interactions between different turbulent mechanisms.     

Boundary-layer effects on mountain waves: a new look at some historical studies

Early studies of mountain waves reported various results that have rarely been investigated since. These include: large-amplitude mountain waves above an unstable boundary layer much higher than the mountains; a repeated downwind drift and upwind jump of mountain waves; and larger vertical wind magnitude near sunrise and/or sunset. These are investigated using over 3,000 radiosondes and meso-strato-troposphere (MST) radar. Superadiabatic temperature gradients are found beneath mountain waves, explainable by convection which appears to raise the mountain-wave launching height. Movement of mountain-wave patterns is studied by a new method using height–time vertical wind data. A swaying motion of mountain waves, with period of a few minutes, appears to be equally upwind and downwind, rather than asymmetric at the heights measurable. Also, vertical wind shows no change in mean, variance or extreme values near sunrise and sunset, despite the expected diurnal changes of boundary-layer structure. An explanation for differences between MST radar and other measurements and models of mountain waves is suggested in terms of more than one variety of mountain wave. Type 1 has stable air near the ground; type 2 is above a convective/turbulent boundary layer of significant height as compared to the mountains.