Influence of the atmospheric conditions on PM<Subscript>10</Subscript> concentrations in Poznań, Poland

This study investigates atmospheric conditions’ influence on the mean and extreme characteristics of PM₁₀ concentrations in Poznań during the period 2006–2013. A correlation analysis was carried out to identify the most important meteorological variables influencing the seasonal dynamics of PM₁₀ concentrations. The highest absolute correlation values were obtained for planetary boundary layer height (r = ?0.57), thermal (daily minimum air temperature: r = ?0.51), anemological (average daily wind speed: r = ?0.37), and pluvial (precipitation occurrence: r = ?0.36) conditions, however the highest correlations were observed for temporal autocorrelations (1 day lag: r = 0.70). As regulated by law, extreme events were identified on the basis of daily threshold value i.e. 50 μg m^?3. On average, annually there are approximately 71.3 days anywhere in the city when the threshold value is exceeded, 46.6 % of those occur in winter. Additionally, 83.7 % of these cases have been found to be continuous episodes of a few days, with the longest one persisting for 22 days. The analysis of the macro-scale circulation patterns led to the identification of an easy-to-perceive seasonal relations between atmospheric fields that favour the occurrence of high PM₁₀ concentration, as well as synoptic situations contributing to the rapid air quality improvement. The highest PM₁₀ concentrations are a clear reaction to a decrease in air temperature by over 3 °C, with simultaneous lowering of PBL height, mean wind speed (by around 1 m s^?1) and changing dominant wind directions from western to eastern sectors. In most cases, such a situation is related to the expansion of a high pressure system over eastern Europe and weakening of the Icelandic Low. Usually, air quality conditions improve along with an intensification of westerlies associated with the occurrence of low pressure systems over western and central Europe. Opposite relations are distinguishable in summer, when air quality deterioration is related to the inflow of tropical air masses originating over the Sahara desert.     

精细PBL模式及其诊断应用

在已建立的一种三维非静力细网格Ｅ－ε湍能闭合的动力学框架基础上,作了一些改进的技术处理和试验,包括：（１）利用ＧＩＳ提供的地形资料,较精确地计算太阳辐射,进而诊断地面温度;（２）引进位势流概念,处理稳定层结条件下初始内插风场的风向变化;（３）引入动力学调整项Ｇ（αｏｂｓ－α）。在此基础上,就地理信息系统（ＧＩＳ）提供的一个 ６０ｋｍ×４８ｋｍ区域作了边界层结构和湍流特征的数值模拟试验,并与实测作了比较,讨论了精细ＰＢＬ模式的模拟效果,结果表明,新建的ＰＢＬ模式能较好地模拟表征出复杂下垫面地域的陡峭地形和不规则海岸线对局地风场和湍流场的动力和热力学作用。     

Models for estimating offshore winds from onshore meteorological measurements

To understand and estimate wind speed differences across the coastal zone, two models, one theoretical and another semi-empirical, have been developed and verified by available data sets. Assuming that: (1) mean horizontal motion exists across the coastal zone; and (2) the geostrophic wind does not change appreciably at the top of the planetary boundary layer (PBL), the equation of motion in the direction of the wind can be reduced so that 341-01, where U, H, and C _D are wind speed, height of PBL, and drag coefficient over the sea and land, respectively. For practice, C _D SEA has been modified from a formula with U _LAND as the only input. H _SEA may be estimated routinely from known H _{D LAND LAND} and the temperature difference between land and sea, which can be provided by such means as remote sensing from meteorological satellites. For a given coast, Cmay be estimated also. This formula is recommended for weather forecasters. The semiempirical formula is based mainly on the power law wind distribution with height in the PBL. The formula states that 341-02. Simultaneous offshore and onshore wind measurements made at stations ranging from Somalia, near the equator, to the Gulf of Alaska indicated that values of a and b are 2.98 and 0.34 with a correlation coefficient of -0.95. For oceanographic applications, a simplified equation, i.e., 341-03, is also proposed.     

多普勒雷达资料同化在“7.21”北京特大暴雨个例中的应用

基于WRF(Weather Research Forecast)模式和GSI(Gridpoint Statistical Interpolation)同化系统,研究了同化4部多普勒雷达探测资料对"7.21"北京特大暴雨过程中降水预报的改善作用。GSI系统直接同化径向风,而采用云分析的方式间接同化反射率。2012年7月20日21时—21日00时(世界时)雷达探测资料同化试验采用30 min循环同化径向风和反射率资料。结果表明,循环同化雷达探测资料改善了短时(0—6 h)和短期(0—24 h)降水预报,ETS评分提高了约0.2。同化反射率资料增加了初始场的水凝物,改善了温度场分布,直接影响了降水的形成,同时还使650—250 hPa位势高度的均方根误差平均降低了8 gpm。直接同化径向风资料对中尺度风场产生了一定影响。ETS评分结果表明:同化反射率资料的效果要优于同化径向风。     

The impact of the PBL scheme and the vertical distribution of model layers on simulations of Alpine foehn

Summary This paper investigates the influence of the planetary boundary-layer (PBL) parameterization and the vertical distribution of model layers on simulations of an Alpine foehn case that was observed during the Mesoscale Alpine Programme (MAP) in autumn 1999. The study is based on the PSU/NCAR MM5 modelling system and combines five different PBL schemes with three model layer settings, which mainly differ in the height above ground of the lowest model level (z ₁). Specifically, z ₁ takes values of about 7 m, 22 m and 36 m, and the experiments with z ₁ = 7 m are set up such that the second model level is located at z = 36 m. To assess if the different model setups have a systematic impact on the model performance, the simulation results are compared against wind lidar, radiosonde and surface measurements gathered along the Austrian Wipp Valley. Moreover, the dependence of the simulated wind and temperature fields at a given height (36 m above ground) on z ₁ is examined for several different regions. Our validation results show that at least over the Wipp Valley, the dependence of the model skill on z ₁ tends to be larger and more systematic than the impact of the PBL scheme. The agreement of the simulated wind field with observations tends to benefit from moving the lowest model layer closer to the ground, which appears to be related to the dependence of lee-side flow separation on z ₁. However, the simulated 2 m-temperatures are closest to observations for the intermediate z ₁ of 22 m. This is mainly related to the fact that the simulated low-level temperatures decrease systematically with decreasing z ₁ for all PBL schemes, turning a positive bias at z ₁ = 36 m into a negative bias at z ₁ = 7 m. The systematic z ₁-dependence is also observed for the temperatures at a fixed height of 36 m, indicating a deficiency in the self-consistency of the model results that is not related to a specific PBL formulation. Possible reasons for this deficiency are discussed in the paper. On the other hand, a systematic z ₁-dependence of the 36-m wind speed is encountered only for one out of the five PBL schemes. This turns out to be related to an unrealistic profile of the vertical mixing coefficient. Correspondence: Günther Z?ngl, Meteorologisches Institut der Universitat München, 80333 München, Germany     

Effect of wind vertical shear on diffusion characteristics in the mesoscale range

Using the turbulent statistical form of the vertical vortex diffusion coefficient K, in the planetary boundary layer (PBL) and Ekman spiral wind profile, the three-dimensional diffusion equation is solved by the numeri-cal method. The influences of vertical shear of both wind direction and wind speed on pollution trajectory and horizontal diffusion parameters σy are numerically analysed. The expressions of both pollution trajectory and σy, including the factor of wind shear, are obtained. The results show that the vertical shear of wind is important among all factors affecting the mesoscale dispersion. Specifically, from neutral to stable atmospheric conditions, vertical shear of wind makes greater contribution to σy than turbulence, thus it is the most important factor. In this paper, we have compared horizontal dispersion pattern with both Pasquill's dispersion pattern considering wind direction shear, and experimental data collected at 9 different sites rang-ing from 10 to 100 km, and the results show that our dispersion pattern is closer to the experimental values than Pasquill's results, and his correction to shear of wind direction is too large under the stable conditions.     

Wind profiles at the Boulder Tower

Analysis of wind profiles at the Boulder Tower (BAO) leads to these conclusions:

1. The variation of roughness with wind direction found earlier is confirmed. Roughness lengths measured on the tower are larger than those measured close to the surface.
2. The profiles and measurements of Reynolds stress are consistent with a von-Karman constant of 0.35.
3. The form φ_m=(1?15z/L)^-1/3 fits best in the range -0.6 < z/L < 0. In the range 0 < z/L < 0.5, θ _m ～ 1 + 4.7z/L provides a good fit to the observations. For z/L < 0.1, φ _m also depends on h, the thickness of the PBL. For z/L < -0.6, Φ _m approaches the constant 0.5, in contrast to all previous suggestions. For larger stabilities, the upper level is usually not in the surface layer, and wind ratios become independent of z/L.
4. With snow cover, the effective roughness diminishes to about 1 cm, even for directions for which the roughness length without snow is large.
5. Estimation of winds at 100 or 150 m from information near the surface is best for similarity theory provided that the ratio of height to Monin-Obukhov L is less than 0.1. For larger z/L, simple power laws seem more appropriate.

     

Influence of non-flat terrain and wind direction shear on canopy turbulence

We have analyzed eddy covariance data collected within open canopy to investigate the influence of non-flat terrain and wind direction shear on the canopy turbulence. The study site is located on non-flat terrain with slopes in both south-north and east-west directions. The surface elevation change is smaller than the height of roughness element such as building and tree at this site. A variety of turbulent statistics were examined as a function of wind direction in near-neutral conditions. Heterogeneous surface characteristics results in significant differences in measured turbulent statistics. Upwind trees on the flat and up-sloping terrains yield typical features of canopy turbulence while upwind elevated surface with trees yields significant wind direction shear, reduced u and w skewness, and negligible correlation between u and w. The directional dependence of turbulence statistics is due that strong wind blows more horizontally rather than following terrain, and hence combination of slope related momentum flux and canopy eddy motion decreases the magnitude of Sk _w and r _uw for the downslope flow while it enhances them for the upslope flow. Significant v skewness to the west indicates intermittent downdraft of northerly wind, possibly due to lateral shear of wind in the presence of significant wind direction shear. The effects of wind direction shear on turbulent statistics were also examined. The results showed that correlation coefficient between lateral velocities and vertical velocity show significant dependence on wind direction shear through change of lateral wind shear. Quadrant analysis shows increased outward interaction and reduced role of sweep motion for longitudinal momentum flux for the downslope flow. Multi-resolution analysis indicates that uw correlation shows peak at larger averaging time for the upslope flow than for the downslope flow, indicating that large eddy plays an active role in momentum transfer for the upslope flow. On the other hand, downslope flow shows larger velocity variances than other flows despite similar wind speed. These results suggest that non-flatness of terrain significantly influences on canopy-atmosphere exchange.     

A barotropic planetary boundary layer

The temperature and wind profiles in the planetary boundary layer (PBL) are investigated. Assuming stationary and homogeneous conditions, the turbulent state in the PBL is uniquely determined by the external Rossby number and the stratification parameters. In this study, a simple two-layer barotropic model is proposed. It consists of a surface (SL) and overlying Ekman-type layer. The system of dynamic and heat transfer equations is closed usingK theory. In the SL, the turbulent exchange coefficient is consistent with the results of similarity theory while in the Ekman layer, it is constant. Analytical solutions for the wind and temperature profiles in the PBL are obtained. The SL and thermal PBL heights are properly chosen functions of the stratification so that from the solutions for wind and temperature, the PBL resistance laws can be easily deduced. The internal PBL characteristics necessary for the calculation (friction velocity, angle between surface and geostrophic winds and internal stratification parameter) are presented in terms of the external parameters. Favorable agreement with experimental data and model results is demonstrated. The simplicity of the model allows it to be incorporated in large-scale weather prediction models as well as in the solution of various other meteorological problems.     

Sensitivity of PBL model predictions to model design and uncertainties in environmental inputs

A simple time-dependent one-dimensional model of the planetary boundary layer (PBL) is described and used to examine the degree to which model design decisions affect model output variables. The model's sensitivity to changes in the environmental conditions is also explored. Averages of the surface fluxes, near-ground wind speeds and other PBL properties from 48 h simulations are compared to control runs. The model-calculated surface fluxes are most sensitive, in decreasing order of importance, to the vertical grid spacing, the form of closure between the surface temperature and the atmosphere, the use of vertical diffusivity smoothing, the choice of maximum time step and choice of turbulence closure scheme. These fluxes are relatively insensitive to mixing-length scaling or choice of implicit time step weighting factor. Sensitivity to changes in soil type exceeds any of the design criteria tested. The modeled fluxes are moderately sensitive to small variations in the horizontal pressure gradient, to unsteadiness in the geostrophic wind and to variations in surface roughness. They are relatively insensitive to uncertainties in local vertical velocities and small (25%) variations applied separately to soil thermal diffusivity or heat capacity. The sensitivity of the average PBL depth (Z _i ) to model and environmental changes are similar to those of surface fluxes except thatZ _i is more sensitive to changes in mixing length, albedo and imposed vertical velocity then are the surface fluxes.     

非定常过程对大气边界层的内参数与风廓线的影响(二)

本文用大气边界层运动方程的数值积分研究了当边界层顶风向不变但风速变化时对大气边界层的内参数u_*/A(u_*为摩擦速度,A为上界风速)和α角(地面风与上界风向的交角)的影响。设上界处风速随时间指数增加及减少,最后趋于定常。在大气正、斜压时,u_*/A及α角的时间变化均是振幅衰减的振荡,最后趋于定常时相应的值。在时间变化过程中的任一时刻,内参数值与当上界条件取该时刻上界风时的定常解结果有一定的差别,严格说,定常时的内参数值并不能直接用于当上界风非定常时。本文还考虑了上界风速非定常对风廓线的影响。     

Mass transfer from discrete water drops on leaves in a cereal canopy

The boundary-layer resistance, r _d , for water vapour transfer from single drops on a wheat leaf was derived from field measurements of the evaporation rate, drop temperature and air humidity. Parameters are estimated in an equation to calculate r _d from drop diameter and wind speed. The relationship between resistance and wind speed is compared with that from other sources, and possible systematic errors in temperature measurements are examined using a model of the drop energy balance.On secondment from Department of Agricultural Sciences, University of Bristol, IACR, Long Ashton Research Station, Long Ashton, Bristol BS18 9AF, U.K.     

Sensitivity of MM5-simulated planetary boundary layer height to soil dataset: comparison of soil and atmospheric effects

The effects of two soil datasets on planetary boundary layer (PBL) height are analyzed, using model simulations. Simulations are performed with the MM5 weather prediction system over the Carpathian Basin, with 6?km horizontal resolution, investigating three summer days, two autumn, and one winter day of similar synoptic conditions. Two soil datasets include that of the United States Department of Agriculture, which is globally used, and a regional Hungarian called Hungarian unsaturated soil database. It is shown that some hydraulic parameter values between the two datasets can differ up to 5–50%. These differences resulted in 10% deviations in the space–time-averaged PBL height (averaged over Hungary and over 12?h in the daytime period). Over smaller areas, these relative deviations could reach 25%. Daytime course changes in the PBL height for reference run conditions were significant (p?<?0.01) in ≈70% of the grid points covering Hungary. Ensemble runs using different atmospheric parameterizations and soil moisture initialization setups are also performed to analyze the sensitivity under changed conditions. In these cases, the sensitivity test showed that irrespective of the radiation and PBL scheme, the effect of different soil datasets on PBL height is roughly the same. PBL height is also sensitive to field capacity (Θ_f) and wilting point (Θ_w) changes. Θ_f changes seem to be more important for loamy sand, while Θ_w changes for the clay soil textural class.     

Formulation of the thermal internal boundary layer in a mesoscale model

Mesoscale models using a non-local K-scheme for parameterization of boundary-layer processes require an estimate of the planetary boundary layer (PBL) height z _i at all times. In this paper, two-dimensional sea-breeze experiments are carried out to evaluate three different formulations for the advective contribution in the z _i prognostic equation of Deardorff (1974).Poor representation of the thermal internal boundary layer in the sea breeze is obtained when z _i is advected by the wind at level z _i . However, significantly better results are produced if the mean PBL wind is used for the advecting velocity, or if z _i is determined simply by checking for the first sufficiently stable layer above the ground.A Lagrangian particle model is used to demonstrate the effect of each formulation on plume dispersion by the sea breeze.     

Determination des echanges verticaux moyens dans la couche limite planetaire a partir des donnees statistiques obtenues par les radiosondages de temperature

We have studied the vertical structure of the planetary boundary layer (PBL) as well as the vertical exchanges between this layer and the free atmosphere, using average macroscopic temperature data obtained from radiosondes. For this study we have used, for seven months in 1972, twice-daily radiosondes (00 and 12 H) from Trappes (Paris area) and PointK (Atlantic Ocean). The vertical structure of the PBL is given in the first part of the present work in terms of monthly average statistical parameters (vertical temperature gradient, frequency and level of inversion layers, frequency and thickness of mixing layers). We have thus demonstrated for the continental station, the influence of the daily cycle on the vertical temperature gradient; we have determined the monthlyH _M level above which the daily variation is not noticed. However, for the oceanic station, the absence of a daily cycle makes the temperature gradients at 00 and 12 H identical. The study of temperature inversion layers clearly indicates a high probability of their existence between 1500 and 2000 m; this probability is more than 80% both in summer at PointK, and in winter at Trappes. Similarly, we have demonstrated the annual evolution of the level of these elevated inversions at the two stations. An identical process has been performed in the case of the mixing layers. In the second part of our study, we have used a relationship between the vertical temperature gradient and the coefficients of matter exchange (K _z ), obtained from natural radioactive tracers (Guedaliaet al., 1974). Statistics have thus been obtained on the values ofK _z in the various layers above the two stations. These statistics prove that for the two stations and above 1500 m, values of the coefficients between 1 and 5 m² s^?1 are the most frequent; on the other hand, below 1500 m, the distribution of the coefficientsK _z offers different characteristics according to the month and to the station considered. Finally, we have used the concept of ‘equivalent coefficient’ -K _e - to characterize the exchanges between two levels considered as a whole. We have made a comparison of the values ofK _e when in the 0–1000 m layer and when in the 0–2000 m layer. The equivalent coefficientK _e allows us to compare the average exchanges above the two areas; thus, in summer, between the 0 and 1000 m level, the exchanges are more important above Trappes than they are above Point K. On the other hand, whatever may be the vertical structure of the PBL below, the value ofK _e in the 0–2000 m-layer is always between 1 and 5 m² s^?1. A generalisation of such a study applied to better chosen continental and oceanic sites would allow a comprehensive view of the structure of the PBL as well as of the turbulent exchanges between the PBL and the free atmosphere.     

Estimation of atmospheric surface layer parameters and numerical simulation using MM5 at Mangalore, West Coast of India

Atmospheric surface layer meteorological observations obtained from 20-m-high meteorological tower at Mangalore, situated along the west coast of India are used to estimate the surface layer scaling parameters of roughness length (z _o) and drag coefficient (C _D), surface layer fluxes of sensible heat and momentum. These parameters are computed using the simple flux–profile relationships under the framework of Monin–Obukhov (M–O) similarity theory. The estimated values of z _o are higher (1.35–1.54 m) than the values reported in the literature (>0.4–0.9 m) probably due to the undulating topography surrounding the location. The magnitude of C _D is high for low wind speed (<1.5 m s^?1) and found to be in the range 0.005–0.03. The variations of sensible heat fluxes (SHF) and momentum fluxes are also discussed. Relatively high fluxes of heat and momentum are observed during typical days on 26–27 February 2004 and 10–11 April 2004 due to the daytime unstable atmospheric conditions. Stable or near neutral conditions prevail after 1700 h IST with negative SHF. A mesoscale model PSU/NCAR MM5 is run using a high-resolution (1 km) grid over the study region to examine the influence of complex topography on the surface layer parameters and the simulated fluxes are compared with estimated values. Spatial variations of the frictional velocity (u _*), C _D, surface fluxes, planetary boundary layer (PBL) height and surface winds are noticed according to the topographic variations in the simulation.     

The wind field in the nonlinear multilayer planetary boundary layer

By use of the small parameter expansion method, the nonlinear planetary boundary layer (PBL) is studied in this paper. The PBL is divided into the surface layer and the Ekman layer, which is divided into several sublayers. In the surface-layer, the eddy coefficient K is taken as a linear function of height; in the Ekman layer, different constant K values are taken within different sublayers: these values are determined from O'Brien's formula (O'Brien, 1970) approximately. Under the upper and lower boundary conditions and the continuity conditions of the wind velocities and turbulent stresses at each boundary between sublayers, analytical expressions for wind velocity in all sublayers and the vertical velocity at the top of the PBL are obtained. A specific example of steady axisymmetrical circular high and low pressure areas is analysed, and some new conclusions are obtained. The results are in better agreement with reality than previous results. This example also shows that the vertical velocity at the top of the PBL caused by friction approaches zero near the center of a high or low pressure system for this model, but attains its maximum absolute values near the center of the high or low pressure area for Wu's (1984) model. This is due to the fact that in our model, the geostrophic wind speed near the center of this specific vortex approaches zero, which causes the wind shear and the friction effect to be very weak. Therefore the wind distribution in the PBL is very sensitive to the type of eddy coefficient.     

Land-Surface Heterogeneity Effects in the Planetary Boundary Layer

We investigate the cumulative added value of assimilating temperature, moisture, and wind observations in the three-dimensional non-hydrostatic Fifth-Generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model MM5 and use these forecasts to analyze the relationship between surface forcing and planetary boundary-layer (PBL) depth. A data assimilation methodology focused on the surface and the PBL, previously tested in a one-dimensional version of MM5, is applied to 29 May, 6 June, and 7 June 2002 during the International $\hbox {H}_{2}\hbox {O}$ Project over the Southern Great Plains. Model-predicted PBL depth is evaluated against PBL depth diagnosed from data across 4,800 km of airborne lidar data (flight tracks 100–300 km long). The forecast with data assimilation verifies better against observations and is thus used to investigate the environmental conditions that govern PBL depth. The spatial structure in PBL depth is found to be most affected by spatial variations in surface buoyancy flux and capping inversion strength. The spatial scales of surface flux forcing reflected in the PBL depth are found through Fourier analysis and multiresolution decomposition. Correlations are ${<}0.50$ at scales of 64 km or less and increase at larger scales for 29 May and 6 June, but on 7 June low correlations are found at all scales, possibly due to greater within-PBL wind speeds, a stronger capping inversion on this day, and clouds. The results suggest a minimum scale, a function of wind speed, below which heterogeneity in surface buoyancy fluxes is not reflected directly in PBL depth.     

Measurements of Turbulence and Dispersion in Three Idealized Urban Canopies with Different Aspect Ratios and Comparisons with a Gaussian Plume Model

Water-tunnel measurements of velocity, turbulence and scalar concentration for three model urban canopies with aspect ratios A _r of building height-to-width of 0.25, 1 and 3 are presented. The measurements for the canopies with A _r = 1 and 3 are new, while the measurements for A _r = 0.25 were previously published. A passive scalar was continuously released from a near-ground point source, and the concentration was measured at several distances from the source and at different heights above the ground. Plume spreads, concentration and distance from the source were non-dimensionalized using length, time and velocity scales reflecting the geometry of the buildings. The scaling collapses the data for all aspect ratios and is valid when the vertical extent of the plume is smaller than the canopy height. The observed plume spreads are compared with analytical relations, which predict linear growth in both transverse and vertical directions. The observed mean concentration is compared with a Gaussian dispersion model that predicts a ?2 power-law decay with distance from the source.     

Evaluation of the boundary layer depth in semi-arid region of India

The annual variation in Planetary Boundary Layer (PBL) height is determined from the profiles of conserved thermodynamic variables, i.e. virtual potential temperature θ_v, equivalent potential temperature θ_e and saturated equivalent potential temperature θ_es, using radiosonde data at Anand (23°35′N, 72°55′E, 45.1 m a.s.l.), India. Out of all the variables, the θ_v profile seems to provide the most reasonable estimate of the PBL height. This has been supplemented by T–Phi gram analysis for specific days. The analysis has been done for 00, 03, 06, 09 and 12 GMT for the 14th and 15th day of each month in the year 1997 based on LASPEX-97 data. In winters the height of boundary layer is very low due to subsidence and radiation cooling while heights in pre-monsoon months exhibit large variations.