An Analytical Study on the Urban Boundary Layer

A two-layer model based on the linearized time-independent atmospheric dynamical equations is proposed in this paper. The analytical solutions of the vertical, the horizontal motions and the potential temperature field induced by the anthropogenic source of urban surface heating are obtained, therefore the heat island circulation existing in unstable boundary layer is verified theoretically. From the analytical solutions, some conclusions can be drawn. (1) The vertical motion induced by urban heat island consists of two parts, namely, the cross-hill wave and the lee wave; (2) The cross-hill wave only exists in the unstable boundary layer, and varies with height according to exponential function law; (3) The vertical motion induced by heat island reaches the maximum at the top of the unstable boundary layer; (4) The wave generated by heat island not only propogates to the downwind district but also travels to the upwind area; (5) γ≠0 is not the necessary condition of the lee wave generation.

ANALYSIS OF THE SECONDARY STABILITY OF COMPRESSIBLE BOUNDARY LAYER FLOWS OVER A TWO-DIMENSIONAL PLATE

This paper used the Floquet's three _dimensional linear stability theory in the analysis of two_dimensional compress ible boundary layer, a set of stability equations is constructed, the effect of three dimensional linear small perturbation on the two_dimensional compressible boundary layer transition is studied, and the effect of coming flow Ma number on growth and development of the subharmonics is calculated. It can be seen from t he calculations, the effect caused by the interaction of two_dimensional and thr ee_dimensional perturbation waves on the development of two_dimensional compress ible laminar boundary layer.

ON THE MECHANISM OF TURBULENT COHERENT STRUCTURE（II）──A PHYSICAL MODEL OF COHERENT STRUCTURE FOR THE ROUGH BOUNDARY LAYER

In this paper, the differences of turbulent coherent structure beween the smooth and rough boundary lavers are analysed.Based on the discussing the transient properties from the smooth wall tothe rough wall,the physicalmodel of coherent structure for the rough boundary layer are established.The width of slwly-moving turbulent spot and the bursting time are obtained,which are in agreement with experimental results.

Analytical solutions of turbulent boundary layer beneath forward-leaning waves

As a typical nonlinear wave,forward-leaning waves can be frequently encountered in the near-shore areas,which can impact coastal sediment transport significantly.Hence,it is of significance to describe the characteristics of the boundary layer beneath forward-leaning waves accurately,especially for the turbulent boundary layer.In this work,the linearized turbulent boundary layer model with a linear turbulent viscosity coefficient is applied,and the novel expression of the near-bed orbital velocity that has been worked out by the authors for forward-leaning waves of arbitrary forward-leaning degrees is further used to specify the free stream boundary condition of the bottom boundary layer.Then,a variable transformation is found so as to make the equation of the turbulent boundary layer model be solved analytically through a modified Bessel function.Consequently,an explicit analytical solution of the turbulent boundary layer beneath forward-leaning waves is derived by means of variable separation and variable transformation.The analytical solutions of the velocity profile and bottom shear stress of the turbulent boundary layer beneath forward-leaning waves are verified by comparing the present analytical results with typical experimental data available in the previous literature.

Responses of the field-aligned currents in the plasma sheet boundary layer to a geomagnetic storm

Geomagnetic storms can result in large magnetic field disturbances and intense currents in the magnetosphere and even on the ground.As an important medium of momentum and energy transport among the solar wind,magnetosphere,and ionosphere,field-aligned currents(FACs)can also be strengthened in storm times.This study shows the responses of FACs in the plasma sheet boundary layer(PSBL)observed by the Magnetospheric Multiscale(MMS)spacecraft in different phases of a large storm that lasted from May 27,2017,to May 29,2017.Most of the FACs were carried by electrons,and several FACs in the storm time also contained sufficient ion FACs.The FAC magnitudes were larger in the storm than in the quiet period,and those in the main phase were the strongest.In this case,the direction of the FACs in the main phase showed no preference for tailward or earthward,whereas the direction of the FACs in the recovery phase was mostly tailward.The results suggest that the FACs in the PSBL are closely related to the storm and could be driven by activities in the tail region,where the energy transported from the solar wind to the magnetosphere is stored and released as the storm is evolving.Thus,the FACs are an important medium of energy transport between the tail and the ionosphere,and the PSBL is a significant magnetosphere–ionosphere coupling region in the nightside.

Different Turbulent Regimes and Vertical Turbulence Structures of the Urban Nocturnal Stable Boundary Layer

Turbulence in the nocturnal boundary layer(NBL)is still not well characterized,especially over complex underlying surfaces.Herein,gradient tower data and eddy covariance data collected by the Beijing 325-m tower were used to better understand the differentiating characteristics of turbulence regimes and vertical turbulence structure of urban the NBL.As for heights above the urban canopy layer(UCL),the relationship between turbulence velocity scale(VTKE)and wind speed(V)was consistent with the“HOckey-Stick”(HOST)theory proposed for a relatively flat area.Four regimes have been identified according to urban nocturnal stable boundary layer.Regime 1 occurs where local shear plays a leading role for weak turbulence under the constraint that the wind speed V<VT(threshold wind speed).Regime 2 is determined by the existence of strong turbulence that occurs when V>VT and is mainly driven by bulk shear.Regime 3 is identified by the existence of moderate turbulence when upside-down turbulence sporadic bursts occur in the presence of otherwise weak turbulence.Regime 4 is identified as buoyancy turbulence,when V>VT,and the turbulence regime is affected by a combination of local wind shear,bulk shear and buoyancy turbulence.The turbulence activities demonstrated a weak thermal stratification dependency in regime 1,for which within the UCL,the turbulence intensity was strongly affected by local wind shear when V<VT.This study further showed typical examples of different stable boundary layers and the variations between turbulence regimes by analyzing the evolution of wind vectors.Partly because of the influence of large-scale motions,the power spectral density of vertical velocity for upsidedown structure showed an increase at low frequencies.The upside-down structures were also characterized by the highest frequency of the stable stratifications in the higher layer.

Tide-Induced Mixing in the Bottom Boundary Layer in the Western East China Sea

The East China Sea(ECS)boasts a vast continental shelf,where strong tidal motions play an important role in the substance transport and energy budget.In this study,the tide-induced mixing in the bottom boundary layer in the western ECS is analyzed based on records measured by moored acoustic Doppler current profilers from June to October 2014.Results show that the M_(2) tide is strong and shows a barotropic feature,whereas the O_(1) tide is much weaker.Based on the M_(2) tidal currents,the eddy viscosity in the bottom Ekman boundary layer is estimated with three schemes.The estimated eddy viscosity values vary within 10^(-4)–10^(-2)m^(2) s^(−1),reaching a maximum at approximately 5 m height from the bottom and decreasing exponentially with the height at all three stations.Moreover,the shear production of turbulent kinetic energy is calculated to quantify the mixing induced by different tidal constituents.The results show that the shear production of the M_(2) tide is much stronger than that of the O_(1) tide and shows a bottom intensified feature.

Three-Dimensional Direct Numerical Simulation of Flow past A Near-Wall Circular Cylinder:Combined Effects of Gap Ratio and Boundary Layer Thickness on Flow Profiles and Pressure Distribution

Three-dimensional direct numerical simulations of the wake flow downstream of a near-wall circular cylinder at different gap ratios and boundary layer thicknesses are carried out by using the iterative immersed boundary method.The non-dimensional gap between the cylinder and the wall,G/D=0.2,0.6 and 1.0,the non-dimensional boundary layer thickness,δ/D=0.0,0.7 and 1.6,the Reynolds number,Re=350,and the aspect ratio of the cylinder,L/D=25are adopted.High-resolution visualizations of the complex vortex structures at differentδ/D and G/D are presented.The transition of the streamwise vortex mode,the combined effects ofδ/D and G/D on the flow statistics,the pressure and shear stress distribution and the hydrodynamic forces are analyzed.Results show that with decreasing G/D and increasingδ/D,the gap flow and its vortex-shedding are significantly weakened,together with an elongated wake and an enlarged low-velocity area near the wall,leading to the wake mode transition from the two-sided to one-sided vortex-shedding.Different relative positions of the cylinder regarding the boundary layer alter the flow features of the shear layers.With an increase inδ/D,the front stagnation point shifts to the upper surface,and the distance between the flow divergence point and the maximum pressure position increases.The mean drag coefficient and r.m.s.values of drag and lift coefficients decrease with a decrease in G/D and an increase inδ/D,while the mean lift coefficient increases with decreasing G/D but decreases with increasingδ/D.

Characteristics of atmospheric boundary layer structure and its influencing factors under different sea and land positions in Europe

This study identifies quantitatively the dominant contributions of meteorological factors on the development of the boundary layer heights(BLH)in the European region,based on 32 years(1990-2021)of radiosonde observations.The spatial variability of the BLH is further discussed by location,by classifying recording stations as inland,coastal,or bay.We find that the BLH in Europe varies considerably from day to night and with the seasons.Nighttime BLH is higher in winter and lower in summer,with the highest BLH recorded at coastal stations.Daytime BLH at coastal stations shows a bimodal structure with peaks in spring and autumn;at inland and bay stations,daytime BLH is lower in winter and higher in summer.The daily amplitudes of BLH at the inland and bay stations are stronger than those at coastal stations.Based on our multiple linear regression analysis and our decoupling analysis of temperature and specific humidity,we report that the development of the nighttime BLH at all types of stations is strongly dominated by the variations of surface wind speed(and,at coastal stations,wind directions).The main contributors to daytime BLH are the near-surface temperature variability at most coastal and inland stations,and,at most bay stations,the variation of the near-surface specific humidity.

Effect of local wall temperature on hypersonic boundary layer stability and transition

Wall temperature significantly affects stability and receptivity of the boundary layer. Changing the wall temperature locally may therefore be an effective laminar flow control technique. However, the situation is complicated when the wall temperature distribution is nonuniform, and researchers have experimentally found that local wall cooling may delay the onset of transition. We attempt to clarify the physical mechanisms whereby the local wall temperature affects the transition and the stability of a hypersonic boundary layer. A numerical investigation of the disturbance evolution in a Mach-6 sharp cone boundary layer with local wall heating or cooling is conducted. Direct numerical simulation(DNS) is performed for the single-frequency and broadband disturbance evolution caused by random forcing. We vary the local wall temperature and the location of heating/cooling, and then use the eNmethod to estimate the transition onset. Our results show that local wall cooling amplifies high-frequency unstable waves while stabilizing low-frequency unstable waves, with local heating amplifying all unstable waves locally. The disturbance amplitude and second-mode peak frequency obtained by DNS agree well with the previous experimental results. Local cooling/heating has a dual effect on the stability of the hypersonic boundary layer. For local cooling, while it effectively inhibits the growth of the low-frequency unstable waves that dominate the transition downstream, it also further destabilizes the downstream flow. In addition, while upstream cooling can delay the transition, excessive cooling may promote it;local heating always slightly promotes the transition.Finally, recommendations are given for practical engineering applications based on the present results.

Turbulent boundary layer control with DBD plasma actuators

The flat-plate turbulent boundary layer at Reτ=1140 is manipulated using a spanwise array of bidirectional dielectric barrier discharge(DBD)plasma actuators.Based on the features of no moving mechanical parts in the DBD plasma control technology and hot-wire anemometer velocity measurements,a novel convenient method of local drag reduction(DR)measurement is proposed by measuring the single-point velocity within the linear region of the viscous sublayer.We analyze the premise of using the method,and the maximum effective measurement range of-73.1%<DR<42.2%is obtained according to the experimental environment in this work.The local drag decreases downstream of the center of two adjacent upper electrodes and increases downstream of the upper electrodes.The magnitude of the local DR increases with increasing voltage and decreases as it moves away from the actuators.For the spanwise position in between,the streamwise distribution of the local DR is very dependent on the voltage.The variable-interval time-average detection results reveal that all bursting intensities are reduced compared to the baseline,and the amount of reduction is comparable to the absolute values of the local DR.Compared with previous results,we infer that the control mechanism is that many meandering streaks are combined together into single stabilized streaks.

Wavelet Multi-Resolution Interpolation Galerkin Method for Linear Singularly Perturbed Boundary Value Problems

In this study,a wavelet multi-resolution interpolation Galerkin method(WMIGM)is proposed to solve linear singularly perturbed boundary value problems.Unlike conventional wavelet schemes,the proposed algorithm can be readily extended to special node generation techniques,such as the Shishkin node.Such a wavelet method allows a high degree of local refinement of the nodal distribution to efficiently capture localized steep gradients.All the shape functions possess the Kronecker delta property,making the imposition of boundary conditions as easy as that in the finite element method.Four numerical examples are studied to demonstrate the validity and accuracy of the proposedwavelet method.The results showthat the use ofmodified Shishkin nodes can significantly reduce numerical oscillation near the boundary layer.Compared with many other methods,the proposed method possesses satisfactory accuracy and efficiency.The theoretical and numerical results demonstrate that the order of theε-uniform convergence of this wavelet method can reach 5.

Observations of boundary layer parameters and suspended sediment transport over the intertidal flats of northern Jiangsu, China

A current-turbidity monitoring system (CTMS) was deployed on the intertidal flat at Wanggang, northern Jiangsu during October 16-17, 2000, to measure the tidal current speeds and seawater turbidities at 5 levels above the seabed. Based upon the logarithmic-profile equation, the boundary layer parameters, i.e., u, z0 and C60, were obtained for 247 tidal flow velocity profiles. Around 90% of the profiles were logarithmic according to the critical correlation coefficient. Internal consistency analysis shows that these parameters derived by different methods are consistent with each other. In addition, the height of the bedforms observed is close to the seabed roughness lengths calculated from the velocity profiles, indicating that the boundary layer parameters obtained can reveal the conditions at the sediment-water interface on the intertidal flats. Suspended sediment concentrations were obtained from the 5 CTMS turbidity meters using laboratory and in-situ calibrations. The results show that the in-situ calibrated SSCs have a much higher accuracy than the laboratory calibrated ones. Calculation of suspended sediment fluxes on the intertidal flats, with a magnitude of 104 kg/m per spring tidal cycle, indicates that suspended sediment moves towards the northwest, which is reversal to the transport pattern controlled by the southward Northern Jiangsu Coastal Current in the sub-tidal zone and adjacent shallow waters.

Characteristics of the Boundary Layer Structure of Sea Fog on the Coast of Southern China

Using boundary layer data with regard to sea fog observed at the Science Experiment Base for Marine Meteorology at Bohe,Guangdong Province,the structure of the atmospheric boundary layer and the characteristics of the tops of the fog and the clouds were analyzed.In addition,the effects of advection,radiation,and turbulence during sea fog were also investigated.According to the stability definition of saturated,wet air,the gradient of the potential pseudo-equivalent temperature equal to zero was defined as the thermal turbulence interface.There is evidence to suggest that two layers of turbulence exist in sea fog.Thermal turbulence produced by long-wave radiation is prevalent above the thermal turbulence interface,whereas mechanical turbulence aroused by wind shear is predominant below the interface.The height of the thermal turbulence interface was observed between 180 m and 380 m.Three important factors are closely related to the development of the top of the sea fog：（1） the horizontal advection of the water vapor,（2） the long-wave radiation of the fog top,and（3） the movement of the vertical turbulence.Formation,development,and dissipation are the three possible phases of the evolution of the boundary-layer structure during the sea fog season.In addition,the thermal turbulence interface is the most significant turbulence interface during the formation and development periods;it is maintained after sea fog rises into the stratus layer.

The Influence of Convergence Movement on Turbulent Transportation in the Atmospheric Boundary Layer

Classical turbulent K closure theory of the atmospheric boundary layer assumes that the vertical turbulent transport flux of any macroscopic quantity is equivalent to that quantity's vertical gradient transport flux. But a cross coupling between the thermodynamic processes and the dynamic processes in the atmospheric system is demonstrated based on the Curier-Prigogine principle of cross coupling of linear thermodynamics. The vertical turbulent transportation of energy and substance in the atmospheric boundary layer is related not only to their macroscopic gradient but also to the convergence and the divergence movement. The transportation of the convergence or divergence movement is important for the atmospheric boundary layer of the heterogeneous underlying surface and the convection boundary layer. Based on this, the turbulent transportation in the atmospheric boundary layer, the energy budget of the heterogeneous underlying surface and the convection boundary layer, and the boundary layer parameterization of land surface processes over the heterogeneous underlying surface are studied. This research offers clues not only for establishing the atmospheric boundary layer theory about the heterogeneous underlying surface, but also for overcoming the difficulties encountered recently in the application of the atmospheric boundary layer theory.

The Influences of Boundary Layer Parameterization Schemes on Mesoscale Heavy Rain System

The mesoscale numerical weather prediction model (MM4) in which the computations of the turbulent exchange coefficient in the boundary layer and surface fluxes are improved, is used to study the influences of boundary layer parameterization schemes on the predictive results of the mesoscale model. Seven different experiment schemes (including the original MM4 model) designed in this paper are tested by the observational data of several heavy rain cases so as to find an improved boundary layer parameterization scheme in the mesoscale meteorological model. The results show that all the seven different boundary layer parameterization schemes have some influences on the forecasts of precipitation intensity, distribution of rain area, vertical velocity, vorticity and divergence fields, and the improved schemes in this paper can improve the precipitation forecast. Key words Boundary layer parameterization - Mesoscale numerical weather prediction (MNWP) - Turbulent exchange coefficient - Surface fluxes - Heavy rain This paper was supported by the National Natural Science Foundation of China (Grant No. 49875005 and No. 49735180).

Urban Heat Island and Boundary Layer Structures under Hot Weather Synoptic Conditions:A Case Study of Suzhou City,China

A strong urban heat island （UHI） appeared in a hot weather episode in Suzhou City during the period from 25 July to 1 August 2007. This paper analyzes the urban heat island characteristics of Suzhou City under this hot weather episode. Both meteorological station observations and MODIS satellite observations show a strong urban heat island in this area. The maximum UHI intensity in this hot weather episode is 2.2℃, which is much greater than the summer average of 1.0℃ in this year and the 37–year （from 1970 to 2006） average of 0.35℃. The Weather Research and Forecasting （WRF） model simulation results demonstrate that the rapid urbanization processes in this area will enhance the UHI in intensity, horizontal distribution, and vertical extension. The UHI spatial distribution expands as the urban size increases. The vertical extension of UHI in the afternoon increases about 50 m higher under the year 2006 urban land cover than that under the 1986 urban land cover. The conversion from rural land use to urban land type also strengthens the local lake-land breeze circulations in this area and modifies the vertical wind speed field.

Tomographic PIV investigation of coherent structures in a turbulent boundary layer flow

Tomographic particle image velocimetry was used to quantitatively visualize the three-dimensional co- herent structures in the logarithmic region of the turbulent boundary layer in a water tunnel. The Reynolds number based on momentum thickness is Reo = 2 460. The in- stantaneous velocity fields give evidence of hairpin vortices aligned in the streamwise direction forming very long zones of low speed fluid, which is flanked on either side by high- speed ones. Statistical support for the existence of hairpins is given by conditional averaged eddy within an increasing spanwise width as the distance from the wall increases, and the main vortex characteristic in different wall-normal re- gions can be reflected by comparing the proportion of ejec- tion and its contribution to Reynolds stress with that of sweep event. The pre-multiplied power spectra and two-point cor- relations indicate the presence of large-scale motions in the boundary layer, which are consistent with what have been termed very large scale motions （VLSMs）. The three dimen-sional spatial correlations of three components of veloc- ity further indicate that the elongated low-speed and high- speed regions will be accompanied by a counter-rotating roll modes, as the statistical imprint of hairpin packet structures, all of which together make up the characteristic of coherent structures in the logarithmic region of the turbulent boundary layer （TBL）.

Diurnal Variations of Air Pollution and Atmospheric Boundary Layer Structure in Beijing During Winter 2000/2001

The diurnal variations of gaseous pollutants and the dynamical and thermodynamic structures of the atmospheric boundary layer (ABL) in the Beijing area from January to March 2001 are analyzed in this study using data from the Beijing City Air Pollution Observation Field Experiment (BECAPEX). A heavy pollution day (22 February) and a good air quality day (24 February) are selected and individually analyzed and compared to reveal the relationships between gaseous pollutants and the diurnal variations of the ABL. The results show that gaseous pollutant concentrations exhibit a double-peak-double-valley-type diurnal variation and have similar trends but with different magnitudes at different sites in Beijing. The diurnal variation of the gaseous pollutant concentrations is closely related to (with a 1-2 hour delay of) changes in the atmospheric stability and the mean kinetic energy in the ABL.

Using a Modified Soil-Plant-Atmosphere Scheme(MSPAS)to Simulate the Interaction between Land Surface Processes and Atmospheric Boundary Layer in Semi-Arid Regions

This paper uses a Modified Soil-Plant-Atmosphere Scheme (MSPAS) to study the interaction between land surface and atmospheric boundary layer processes. The scheme is composed of two main parts: atmospheric boundary layer processes and land surface processes. Compared with SiB and BATS, which are famous for their detailed parameterizations of physical variables, this simplified model is more convenient and saves much more computation time. Though simple, the feasibility of the model is well proved in this paper. The numerical simulation results from MSPAS show good agreement with reality. The scheme is used to obtain reasonable simulations for diurnal variations of heat balance, potential temperature of boundary layer, and wind field, and spatial distributions of temperature, specific humidity, vertical velocity, turbulence kinetic energy, and turbulence exchange coefficient over desert and oasis. In addition, MSPAS is used to simulate the interaction between desert and oasis at night, and again it obtains reasonable results. This indicates that MSPAS can be used to study the interaction between land surface processes and the atmospheric boundary layer over various underlying surfaces and can be extended for regional climate and numerical weather prediction study.