Observations of Atmospheric Solitary Waves in the Urban Boundary Layer

The simultaneous operation of a three-axis Doppler sodar system in the centralurban area of Rome and two similar systems in the suburban area, forming atriangle about 20 km on each side, provided evidence of solitary-type wavesin the urban boundary layer. Three events, each lasting from a few minutes toabout 30 min, and ranging in depth from the minimum range of the sodar (39 m) to over 500 m, are reported here. Two events were recognizable onall three sodar records while the third event could be observed at the urbanlocation only. Time-height acoustic echo intensity records showed no-echoregions within the wave indicating transport of trapped recirculating air.This is typical of large amplitude solitary waves. The time series plots ofsodar-derived vertical wind velocity revealed a maximum peak-to-peakvariation of about 5 m s^-1 during periods of wave-associated disturbance.The vertical velocity is found to increase with height up to the top of the closedcirculation within the wave and decreases further above. The normalisedamplitude-wavelength relationship for the two events indicates that theobserved waves are close to a strongly nonlinear regime.     

The Coherent Structure of Water Vapour Transfer in the Unstable Atmospheric Surface Layer

Observations of water vapour fluctuations over arice field show vapour ramps. Coherent structuresare first revealed by the frequently occurring ramp pattern in the vapourtrace. Wavelet and pseudo-wavelet analysis techniques were used inconditional sampling, and more than 100 hr of data have been analyzedto determine coherent structure characteristics. The most probablecoherent structure duration was in the range 2–12 sec andthe duration range of the most effective coherent structures shows somedifference between heat and water vapour transfers. Coherent structurescontribute to the major part of the total flux.     

Evidence Of Dynamical Coupling Between The Residual Layer And The Developing Convective Boundary Layer

The diurnal cycle of the atmospheric boundary layer (ABL) hasbeen documented on 8 August 1998 in the framework of the Étude et Simulation de la QUalité de l'air en Ile-de-France (ESQUIF) experiment that took place in the Paris area. The ABL structure was documented by means of a ground-based lidar, surface meteorological stations and soundings. The interaction between the residual layer and the convective boundary layer is investigated using the collected data as well as mesoscale modelling. As opposed to the generally accepted concept, we find evidence of entrainment at the top ofthe residual layer. High temporal simulations of the 8 August 1998 casemade with the mesoscale atmospheric model Meso-NH also show evidenceof mixing at the top of the residual layer (RL). This mixing is believed to be related to the presence of convective (gravity) waves in the RL.     

Nonuniform Distribution Of High-Frequency Turbulence In The Unstable Boundary Layer

Turbulent flow data of wind velocity and temperature in the unstably stratifiedatmospheric boundary layer, derived from steel tower observations in the field and wind-tunnel experiments were used to study the relationship between the plumes and the small-scale eddies in the inertial subrange. Flow visualisation experiments in the wind tunnel were also conducted to observe the structure of the flow in the plumes, and time series data were analysed by using wavelet transforms. The results show that variances of velocity and temperature due to the small-scale eddies are large in the plumes and small outside of the plumes, and that the momentum and heat fluxes due to the small-scale eddies follow the same tendency as found in the variances. The ratios of the variances caused by the small-scale eddies in the plumes to the whole of the variances caused by the small-scale eddies in and out the plumes increase with non-dimensional height -z/L in which L is the local Obukhov length. Similar ratios of the fluxes caused by the small-scale eddies also show the same tendency. These ratios can be expressed as functions of -z/L for results based on field observationand the wind tunnel experiments. This relation hardly changes even if the wavelet function is changed. The flow visualisation experiments show that the plumes have a complicated structure in which mushroom type flows are stacked on top of each other. This characteristic structure seems to increase the energy of the small-scale eddies in the plumes.     

Fluid Modelling Of Atmospheric Dispersion In The Convective Boundary Layer

A laboratory convection tank has been established following thepioneering work of Willis and Deardorff, but with many improvements and enhancements thattake advantage of modern technology. The main emphasis in the current design was toprovide the ability to conduct a virtually unlimited number of realizations under essentiallyidentical conditions in order to obtain reliable statistics on the dispersion of plumes and puffsreleased within the simulated atmospheric convective boundary layer. Described herein is the tankitself and its auxiliary systems, including a laser-induced-fluorescence and video-imaging system for makingnon-intrusive, full-field measurements of concentrations, and the interfacing of varioussubsystems with a master controller that automates essentially all operation and measurement functions.The current system provides unprecedented resolution, control, and data volumes. Exampleresults are presented from two types of releases: continuous plumes and instantaneous puffs.These data sets clearly show penetration of the highly buoyant plumes and puffs into theinversion above the convective boundary layer, gravity spreading within the inversion, andrapid diffusion within the mixed layer. They also show extreme `spottiness' in the instantaneousconcentration cross-sections.     

Local Similarity Relationships In The Urban Boundary Layer

To investigate turbulent structures in an urban boundary layer (UBL) with many tallbuildings, a number of non-dimensional variable groups based on turbulent observationsfrom a 325-m meteorological tower in the urban area of Beijing, China, are analyzedin the framework of local similarity. The extension of surface-layer similarity to localsimilarity in the stable and unstable boundary layer is also discussed. According to localsimilarity, dimensionless quantities of variables: e.g., velocity and temperature standarddeviations _i/u_*l (i=u,v,w) and_T/T_*l,correlation coefficients of uw and wT covariance, gradients of wind and temperature_m and _h, and dissipation rates of turbulent kinetic energy (TKE) andtemperature variance and _N can be represented as a functiononly of a local stability parameter z/, where is the local Obukhovlength and z is the height above ground. The average dissipation rates of TKE andtemperature variance are computed by using the u spectrum, and the uw and wTcospectra in the inertial subrange. The functions above were found to be in a goodagreement with observational behaviour of turbulence under unstable conditions, butthere were obvious differences in the stable air.     

Remote Sensing And Surface Observations Of The Response Of The Atmospheric Boundary Layer To A Solar Eclipse

On 11 August 1999, a near-total solar eclipse (80%) was observed in Campistrous, France. The influence of this particular event on the atmospheric boundary layer was observed with a UHF-RASS radar, a sodar and an instrumented mast. The changes in turbulence intensity, radar reflectivity, and temperature on the radiative budget are described in relation to collocated ground meteorological data. The impact of the eclipse induces a clear response of the atmosphere, with a time lag of 15 to 30 min, perceptible in several mean and turbulent meteorological variables up to the top of the atmospheric boundary layer.     

A Simple Model Of The Convective Internal Boundary Layer And Its Application To Surface Heat Flux Estimates Within Polynyas

A simple model of the convective (thermal) internalboundary layer has been developed for climatologicalstudies of air-sea-ice interaction, where in situobservations are scarce and first-order estimates ofsurface heat fluxes are required. It is amixed-layer slab model, based on a steady-statesolution of the conservation of potentialtemperature equation, assuming a balance betweenadvection and turbulent heat-flux convergence. Boththe potential temperature and the surface heat fluxare allowed to vary with fetch, so the subsequentboundary-layer modification alters the fluxconvergence and thus the boundary-layer growth rate.For simplicity, microphysical and radiativeprocesses are neglected.The model is validated using several case studies.For a clear-sky cold-air outbreak over a coastalpolynya the observed boundary-layer heights,mixed-layer potential temperatures and surface heatfluxes are all well reproduced. In other cases,where clouds are present, the model still capturesmost of the observed boundary-layer modification,although there are increasing discrepancies withfetch, due to the neglected microphysical andradiative processes. The application of the model toclimatological studies of air-sea interaction withincoastal polynyas is discussed.     

Growth Of The Summer Daytime Convective Boundary Layer At Anand

The heights of the daytime convective boundary layer (CBL), computed by a one-dimensional model for a bare soil surface at a semi-arid station,Anand, during the dry and hot summer month of May 1997, are presented. As input, the model requires surface heat flux, friction velocity and air temperature as functions of time. Temperature data at the one-metre level from a tower and sonic anemometer data at 9.5 m collected during the period 13–17 May 1997 in the Land Surface Processes Experiment (LASPEX-97) are used to compute hourly values of surface heat flux, friction velocity and Obukhov length following the operational method suggested by Holtslag and Van Ulden [J. Climate Appl. Meteorol. 22,517–529 (1983)]. The model has been tested with different values for the potential temperature gradient ( ) above the inversion. The model-estimated CBL heights comparefavourably with observed heights obtained from radiosonde ascents.     

Comparison Of Upwind Land Use And Roughness Length Measured In The Urban Boundary Layer

The results of wind measurements taken in two studies at an urban sitein Birmingham (UK) are presented. Displacement heights and roughnesslengths are calculated that compare well with previous measurements inthe urban canopy. Dimensionless windshear and vertical-velocity standarddeviation are also shown to obey Monin–Obukhov similarity theory. Themean roughness lengths measured at the highest measuring position (45 m)increase with the fraction of upwind urban cover calculated using asource-area model.     

An Actively Controlled Wind Tunnel And Its Application To The Reproduction Of The Atmospheric Boundary Layer

An actively controlled wind tunnel equipped with multiple fansand airfoils has been developed, mainly for the purpose of reproducing the atmospheric boundary layer (ABL) for wind engineering applications. Various fluctuating flows can be achieved in this wind tunnel by altering the input data of the fans and airfoils through computer control. In this study, the ABL is physically simulated in this wind tunnel, and particular attention ispaid to the simulation of the profile of Reynolds stress. The method of generating the fluctuating flow and the experimental results of reproducing the ABL are presented. As the results show, the spatial distribution of Reynolds stress is satisfactorily simulated, and the profiles of other statisticalturbulent parameters, such as mean velocity, turbulent intensity, integral scale and power spectrum are successfully reproduced simultaneously.     

The Influence Of Vertical Wind Direction Shear On Dispersion In The Convective Boundary Layer, And Its Incorporation In Coastal Fumigation Models

The mean concentration distributionwithin a plume released from a point source in the atmosphericboundary layer can be greatly influenced by the systematic turningof wind with height (i.e. vertical wind direction shear). Such aninfluence includes a deflection of the plume centroid, with anassociated shearing of the vertical plume cross-section, and anenhancement of dispersion, in the horizontal plane. Wind directionshear is normally not accounted for in coastal fumigation models,although dispersion observations with shear acting as acontrolling parameter are not uncommon. A three-dimensionalLagrangian stochastic model is used to investigate the influenceof uniform wind direction shear on the diffusion of a point-sourceplume within the horizontally homogeneous convective boundarylayer, with the source located at the top of the boundary layer.Parameterisations are developed for the plume deflection andenhanced dispersion due to shear within the framework of aprobability density function (PDF) approach, and compared with theLagrangian model results. These parameterisations are thenincorporated into two applied coastal fumigation models: a PDFmodel, and a commonly used model that assumes uniform andinstantaneous mixing in the vertical direction. The PDF modelrepresents the vertical mixing process more realistically. A moreefficient version of the PDF model, which assumes a well-mixedconcentration distribution in the vertical at large times, isapplied to simulate sulfur dioxide data from the Kwinana CoastalFumigation Study. A comparison between the model results and thedata show that the model performs much better when the wind-sheareffects are included.