Stable Planetary Boundary Layer Research Articles

Impact of aerosol–meteorology interactions on fine particle pollution during China’s severe haze episode in January 2013

In January 2013, a severe regional haze occurred over the North China Plain. An online-coupled meteorology-chemistry model was employed to simulate the impacts of aerosol–meteorology interactions on fine particles (PM2.5) pollution during this haze episode. The response of PM2.5 to meteorology change constituted a feedback loop whereby planetary boundary layer (PBL) dynamics amplified the initial perturbation of PM2.5. High PM2.5 concentrations caused a decrease of surface solar radiation. The maximal decrease in daily average solar radiation reached 53% in Beijing, thereby leading to a more stable PBL. The peak PBL height in Beijing decreased from 690 m to 590 m when the aerosol extinction was considered. Enhanced PBL stability suppressed the dispersion of air pollutants, and resulted in higher PM2.5 concentrations. The maximal increase of PM2.5 concentrations reached 140 μg m−3 in Beijing. During most PM2.5 episodes, primary and secondary particles increased simultaneously. These results imply that the aerosol–radiation interactions played an important role in the haze episode in January 2013.

The Influence of an SST Front on a Heavy Rainfall Event over Coastal Taiwan during TiMREX

Abstract A long-lived heavy precipitation area was observed along the southwest coast of Taiwan from 13 to 18 June 2008 during the Terrain-Influenced Monsoon Rainfall Experiment (TiMREX). Rainfall amounts exceeded 500 mm along portions of the coast, and the coastal plains experienced severe flooding. The precipitation systems were influenced by blocking effects, as the southerly moist monsoon flow impinged on the island. A relatively strong gradient in the sea surface temperature (SST) off the southwest coast of Taiwan existed during the rainfall event. Mesoscale SST fronts are known to influence the planetary boundary layer (PBL) such that low-level convergence and precipitation are enhanced under certain circumstances. In this study, the authors investigate the role of the SST front in enhancing the 13–18 June 2008 precipitation event over Taiwan using the Weather Research and Forecasting (WRF) Model. In control simulations with the observed SST, there is a transition from a well-mixed to a stable PBL across the front, causing the low-level flow to decelerate, resulting in an enhancement of horizontal convergence. Such a transition in the PBL and the associated convergence is greatly reduced in smoothed SST gradient model simulations, which produce over 20% less precipitation over southwest Taiwan. Sensitivity tests show that, qualitatively, the results are independent of the existence of the island of Taiwan. These findings indicate that the SST gradient over the northern South China Sea during the early summer monsoon can have a significant impact on the intensity of rainfall over Taiwan.

The Dispersion of Silver Iodide Particles from Ground-Based Generators over Complex Terrain. Part II: WRF Large-Eddy Simulations versus Observations

AbstractA numerical modeling study has been conducted to explore the ability of the Weather Research and Forecasting (WRF) model-based large-eddy simulation (LES) with 100-m grid spacing to reproduce silver iodide (AgI) particle dispersion by comparing the model results with measurements made on 16 February 2011 over the Medicine Bow Mountains in Wyoming. Xue et al.'s recently developed AgI cloud-seeding parameterization was applied in this study to simulate AgI release from ground-based generators. Qualitative and quantitative comparisons between the LES results and observed AgI concentrations were conducted. Analyses of turbulent kinetic energy (TKE) features within the planetary boundary layer (PBL) and comparisons between the 100-m LES and simulations with 500-m grid spacing were performed as well. The results showed the following: 1) Despite the moist bias close to the ground and above 4 km AGL, the LES with 100-m grid spacing captured the essential environmental conditions except for a slightly more stable PBL relative to the observed soundings. 2) Wind shear is the dominant TKE production mechanism in wintertime PBL over complex terrain and generates a PBL of about 1000-m depth. The terrain-induced turbulent eddies are primarily responsible for the vertical dispersion of AgI particles. 3) The LES-simulated AgI plumes were shallow and narrow, in agreement with observations. The LES overestimated AgI concentrations close to the ground, which is consistent with the higher static stability in the model than is observed. 4) Non-LES simulations using PBL schemes had difficulty in capturing the shear-dominant turbulent PBL structure over complex terrain in wintertime. Therefore, LES of wintertime orographic clouds with grid spacing close to 500 m or finer are recommended.

An Experimental Study of the Statistics of Temperature Fluctuations in the Atmospheric Boundary Layer

A statistical characterization for two-point temperature fluctuations in the planetary boundary layer (PBL) is analyzed and its implications on the long-standing closure problem discussed. Despite the non-triviality of the dynamics of temperature fluctuations, our analysis supports the idea that the most relevant statistical properties can be captured solely in terms of two scaling exponents. They turned out to be weakly dependent on the stability properties of the PBL. Its statistics have been investigated by collecting data from a field experiment carried out in the urban area of Turin (Italy) from January 2007 to March 2008. Our results confirm those from a large-eddy simulation (LES) analysis carried out for the convective PBL with different level of convection. We extend the scenario to the stable PBL, a regime much more difficult to simulate when exploiting LES.

Connecting Turbulence and Meandering Parameterization to Describe Passive Scalars Dispersion in Low Wind Speed Conditions

The following study deals with meandering of the horizontal mean wind. The main motivation for such investigation came from the difficulty in describing contaminant dispersion in meandering conditions. Observational field measurements point out that the autocorrelation function of the horizontal wind components, obtained for the meandering cases, displays an oscillating behavior with the presence of large negative lobes. Such negative lobes are described by an equation containing functions that represent patterns of movement associated to meandering and turbulence. As a consequence, this mathematical formulation connects the turbulence and meandering phenomenon establishing the employment of hybrid parameters in models that describe the meandering dispersion. Therefore, considering this dualistic aspect between meandering and turbulence manifestations, a new set of relations for the turbulence parameterization joined with the meandering of the wind have been developed and are available. This new turbulence parameterization for a stable shear forcing planetary boundary layer, united with a meandering mean time scale is able to describe contaminant meandering enhanced spread in a low wind speed stable planetary boundary layer.

Light non-methane hydrocarbons at two sites in the Indo-Gangetic Plain

Measurements of light (C(2)-C(5)) non-methane hydrocarbons (NMHCs) were made along with ozone (O(3)), oxides of nitrogen (NO(x)), carbon monoxide (CO) and methane (CH(4)) at Hissar and Kanpur in the Indo-Gangetic Plain (IGP) in India during the month of December, 2004. Air samplings during noon and evening hours provided an opportunity to study the emission characteristics and changes during this period at these sites. The mixing ratio of O(3) was higher during noon hours due to photochemical formation, while the levels of precursor gases showed elevated values during the evening hours on a clear sky day. On foggy days there is no such variation. The lower mixing ratios of O(3) observed on foggy days could be due to the slower rate of photochemical formation caused by a reduction in solar flux and surface deposition caused by the presence of a stable planetary boundary layer. Propene and ethene show the highest evening to noon ratio due to their faster reactivities with OH radicals. Correlations among different species of the measured gases indicate contributions of emissions from biomass and biofuel burning as well as fossil fuel combustion. Although qualitatively in relation to O(3), the propylene (propene) equivalents of NMHCs have been calculated to investigate their roles in O(3) photochemistry and compared with the data from Ahmedabad, an urban site in western India. The important result, which has emerged from the analysis of the observed data, is that while the total amount of these NMHCs is least at Hissar and highest at Ahmedabad, the total propylene-equivalent is highest at Hissar and lowest at Ahmedabad. Further, these two sites in the IGP show significant contributions, almost 72-77%, by propene and ethene while the contribution by these two gases at Ahmedabad is only about 47%. The surface level mixing ratios of O(3) could be treated as representative for the chemical characterization of air mass at a regional scale over the IGP as the month long trends of O(3) show significant similarity compared to the trends in precursors at the two sites.

Improving the horizontal transport in the lower troposphere with four dimensional data assimilation

The physical processes involved in air quality modeling are governed by dynamically-generated meteorological model fields. This research focuses on reducing the uncertainty in the horizontal transport in the lower troposphere by improving the four dimensional data assimilation (FDDA) strategy in retrospective meteorological modeling. In particular, characterization of winds in the nocturnal low-level jet and overlying residual layer is crucial to accurately model regional-scale ozone transport in the key airsheds of the US. Since model errors in wind speed and direction lead to spatial displacements of pollution plumes, observations not routinely used in previous retrospective modeling are introduced through FDDA in an effort to help reduce this transport uncertainty. Prior to the main modeling sensitivity, an observational uncertainty analysis was pursued to identify uncertainties in the wind speed and direction in the lower 1-km of the troposphere that are inherent in the observational data sets used in FDDA. Comparisons of observations among various platforms (radar wind profilers, radiosonde soundings and weather radar profiles) taken in close proximity revealed that an uncertainty of approximately 1.8 m s �1 for wind speed and about 20� for wind direction was intrinsic to the measurements. In the modeling sensitivities, some minimal improvement of modeled winds within the convective planetary boundary layer (PBL) was found when surface analysis nudging of wind was eliminated. Improvements in the nocturnal jet and residual layer winds at night are demonstrated as a reaction to the use of new observations in the data assimilation in layers above the stable PBL. There is also evidence that the assimilated observations above the convective PBL during the day led to improvements of winds within the PBL, which may relieve the need of nudging within the PBL, including surface analysis nudging. Published by Elsevier Ltd.

A Note on Turbulence Stationarity and Wind Persistence Within the Stable Planetary Boundary Layer

This note presents initial results of an analysis of the stationarity of turbulence kinetic energy and the persistence of winds within the stable boundary layer (SBL). Measurements were made at 1.5 and 11 m above ground level from 0100 to 0600 local time on five nights during the JORNADA field experiment. The average stationarity ranged from about 160 to about 570 s. Wind persistence ranged from about ± 40° (3-min average) to about ± 36° (30-min average) on a weakly stable night, and from about ± 40° (3-min average) to about ± 27° (30-min average) on an strongly stable night. It is shown that, at 1.5 m, which we take to be within the surface layer, the average duration of stationarity of turbulent kinetic energy tends to correlate with the kurtosis of the heat flux; however, at 11 m, which we take to be outside of the surface layer, this correlation is poorly approximated.

Wave-Modified Flux and Plume Dispersion in the Stable Boundary Layer

The effects of a pressure jump and a following internal gravity wave on turbulence and plume diffusion in the stable planetary boundary layer are examined. The pressure jump was accompanied by a sudden increase in turbulence and plume dispersion. The effects of wave perturbations on turbulence statistics are analysed by calculating fluxes and variances with and without the wave signal for averaging times ranging from 1 to 30 min. The wave signals are obtained using a band-pass filter. It is shown that second-order turbulence quantities calculated without first subtracting the wave perturbations from the time are greater than those calculated when the wave signal is separated from the turbulence. Estimates of the vertical dispersion of an elevated tracer plume in the stable boundary layer are made using an elastic backscatter lidar. Plume dispersion observed 25 m downwind of the source increases rapidly with the arrival of the flow disturbances. Measured plume dispersion and plume centreline height correlate with the standard deviation of the vertical velocity but not with the wave signal.

Turbulence dissipation rate derivation for meandering occurrences in a stable planetary boundary layer

Abstract. A new formulation for the turbulence dissipation rate ε occurring in meandering conditions has been presented. The derivation consists of a MacLaurin series expansion of a lateral dispersion parameter that represents cases in which turbulence and oscillatory movements associated to the meandering events coexist. The new formulation presents the identical physical premises contained in the classical and largely used one, but the new formulation derived from meandering situations is expressed in terms of the loop parameter m that controls the absolute value of the negative lobe in the meandering autocorrelation function. Therefore, the m magnitude regulates the turbulence dissipation rate. This dissipation rate decreases for cases in which turbulence and low frequency horizontal wind oscillations coexist and increases for a fully developed turbulence. Furthermore, a statistical comparison to observed concentration data shows that the alternative relation for the turbulent dissipation rate occurring in situations of meandering enhanced dispersion is suitable for applications in Lagrangian Stochastic dispersion models.

Application of a Large-Eddy Simulation Database to Optimisation of First-Order Closures for Neutral and Stably Stratified Boundary Layers

Large-eddy simulation (LES) is a well-established numerical technique, resolving the most energetic turbulent fluctuations in the planetary boundary layer. By averaging these fluctuations, high-quality profiles of mean quantities and turbulence statistics can be obtained in experiments with well-defined initial and boundary conditions. Hence, LES data can be beneficial for assessment and optimisation of turbulence closure schemes. A database of 80 LES runs (DATABASE64) for neutral and stably stratified planetary boundary layers (PBLs) is applied in this study to optimize first-order turbulence closure (FOC). Approximations for the mixing length scale and stability correction functions have been made to minimise a relative root-mean-square error over the entire database. New stability functions have correct asymptotes describing regimes of strong and weak mixing found in theoretical approaches, atmospheric observations and LES. The correct asymptotes exclude the need for a critical Richardson number in the FOC formulation. Further, we analysed the FOC quality as functions of the integral PBL stability and the vertical model resolution. We show that the FOC is never perfect because the turbulence in the upper half of the PBL is not generated by the local vertical gradients. Accordingly, the parameterised and LES-based fluxes decorrelate in the upper PBL. With this imperfection in mind, we show that there is no systematic quality deterioration of the FOC in the strongly stable PBL provided that the vertical model resolution is better than 10 levels within the PBL. In agreement with previous studies, we found that the quality improves slowly with the vertical resolution refinement, though it is generally wise not to overstretch the mesh in the lowest 500 m of the atmosphere where the observed, simulated and theoretically predicted stably stratified PBL is mostly located.

Further comments on the equilibrium height of neutral and stable planetary boundary layers

AbstractThe height of the neutrally and stably stratified turbulent planetary boundary layers (PBLs) in the atmosphere and hydrosphere is controlled by numerous factors of different natures. The PBL deepening is facilitated by turbulent mixing caused, first of all, by the velocity shear at the surface, and prevented by the Earth's rotation and the negative buoyancy forces caused by the flow–surface interaction and the stable static stability in the free atmosphere above the PBL. The balance between these factors is often complicated by the baroclinic shears and large‐scale vertical motions (usually disregarded in field experiments), not to mention temporal and horizontal variability of natural PBL flow. It is not surprising that until now many alternative (sometimes contradictory) formulations for the stable PBL height have been proposed and no consensus achieved. To the authors' opinion, the most natural way to investigate this complex problem is to consider step by step the PBL height in comparatively simple, basic equilibrium regimes (controlled by limited factors), to develop and to validate theoretical models for these regimes, and only afterwards to switch to general diagnostic (equilibrium) and prognostic (non‐steady) equations. In the present paper, we briefly summarize recent investigations following the above strategy, and present new data supporting our earlier theoretical conclusions and recommendations. We would not recommend using PBL height equations inconsistent with firmly established basic‐regime formulations. Copyright © 2007 Royal Meteorological Society

Dispersion of radionuclides released into a stable planetary boundary layer using a MonteCarlo model

In this paper a Monte Carlo model for describing the atmospheric dispersion ofradionuclides (represented by Lagrangian particles/neutral tracers) continuously releasedinto a stable planetary boundary layer is presented. The effect of variation in release heightand wind directional shear on plume dispersion is studied. The resultant plumeconcentration and dose rate at the ground is also calculated. The turbulent atmosphericparameters, like vertical profiles of fluctuating wind velocity components andeddy lifetime, were calculated using empirical relations for a stable atmosphere.The horizontal and vertical dispersion coefficients calculated by a numerical Lagrangianmodel are compared with the original and modified Pasquill–Gifford and Briggs empiricalσs. The comparison shows that the Monte Carlo model can successfully predict dispersion ina stable atmosphere using the empirical turbulent parameters. The predicted groundconcentration and dose rate contours indicate a significant increase in the affected areawhen wind shear is accounted for in the calculations.

Comments on: ‘The Neutral, Barotropic Planetary Boundary Layer, Capped by a Low-Level Inversion’ by G. D. Hess, Boundary-Layer Meteorology (2004) 110, pp. 319–355

Precision measurements indicate that the stability capping of the neutral planetary boundary layer (PBL) that leads to a reduced PBL height is caused by the very stable upper part of the PBL, rather than by an overlying inversion. Radiative processes related to liquid water in boundary-layer clouds seem to play the key role for the formation of the stable upper PBL. The famous Leipzig Profile – generally considered as an example of a neutral PBL – has been included in Hess’s analysis because its PBL height is considerably lower than the ca. 3000 m to be expected by numerical models in truly neutral conditions. An analysis of the original observations reveals that the Leipzig PBL was stable and that it can be consistently treated as a ‘normal’ stable PBL with a height of ca. 700 m. A further finding is that the super-geostrophic PBL wind speed maxima predicted by almost all models are not observed in near-steady-state conditions. For the ‘ranking’ of analytical models versus numerical models, the comparisons with measurements show that the analytical models perform comparably well and even partially better than the numerical models.

Atmospheric boundary layers in storms: advanced theory and modelling applications

Abstract. Turbulent planetary boundary layers (PBLs) control the exchange processes between the atmosphere and the ocean/land. The key problems of PBL physics are to determine the PBL height, the momentum, energy and matter fluxes at the surface and the mean wind and scalar profiles throughout the layer in a range of regimes from stable and neutral to convective. Until present, the PBLs typical of stormy weather were always considered as neutrally stratified. Recent works have disclosed that such PBLs are in fact very strongly affected by the static stability of the free atmosphere and must be treated as factually stable (we call this type of the PBL "conventionally neutral" in contract to the "truly neutral" PBLs developed against the neutrally stratified free flow). It is common knowledge that basic features of PBLs exhibit a noticeable dependence on the free-flow static stability and baroclinicity. However, the concern of the traditional theory of neural and stable PBLs was almost without exception the barotropic nocturnal PBL, which develops at mid latitudes during a few hours in the night, on the background of a neutral or slightly stable residual layer. The latter separates this type of the PBL from the free atmosphere. It is not surprising that the nature of turbulence in such regimes is basically local and does not depend on the properties of the free atmosphere. Alternatively, long-lived neutral (in fact only conditionally neutral) or stable PBLs, which have much more time to grow up, are placed immediately below the stably stratified free flow. Under these conditions, the turbulent transports of momentum and scalars even in the surface layer - far away from the PBL outer boundary - depend on the free-flow Brunt-Väisälä frequency, N. Furthermore, integral measures of the long-lived PBLs (their depths and the resistance law functions) depend on N and also on the baroclinic shear, S. In the traditional PBL models both non-local parameters N and S were overlooked. One of possible mechanisms responsible for non-local features of the long-lived PBLs could be the radiation of internal gravity waves (IGW) from the PBL upper boundary to the free atmosphere and the IGW-induced transport of the squared fluctuations of velocity and potential temperature. The free-flow stability plays an especially important role in is the conventionally neutral PBLs (those with the zero potential-temperature flux at the surface: Fθ=0 at z=0, developed against non-zero static stability in the free atmosphere: N>0). The above reasoning obviously calls for a comprehensive revision of the traditional theory. In a series of papers (quoted below in References) an advanced theory has been proposed. It includes the following developments.

Modelling of air pollution dispersion in Santiago de Chile

In Santiago de Chile air quality standards are frequently exceeded during both summer and winter time. The evaluation of the temporal and spatial evolution of the chemical and physical characteristics of these air pollution problems requires the application of three-dimensional models. In this work, the Chilean Air Pollution Dispersion Model (CADM), a three-dimensional Eulerian chemical transport model is presented. The model is applied for the simulation of air pollution dispersion during summer. Carbon monoxide (CO) dispersion is studied in order to evaluate advection and diffusion as simulated by CADM and to characterise dispersion patterns in the Santiago basin. Horizontal and vertical dispersion patterns are presented, explaining how pollutants are ventilated out of the Santiago basin. Also, the role of turbulence on the pollutant distribution is discussed. Furthermore, an attempt is made to explain how pollutants are accumulated in the basin during night in the nocturnal stable planetary boundary layer and what mechanisms affect the transport of pollutants under these conditions. Comparing results from model simulations with observations, it can be shown that the model is able to represent the diurnal cycles of CO reasonably well. It can be concluded that CADM is suited for the application of air pollution dispersion in the Santiago basin in particular and complex terrain in general.

The effect of baroclinicity on the equilibrium depth of neutral and stable planetary boundary layers

AbstractEarlier studies of the depths of the geophysical planetary boundary layers (PBLs) basically focused on the effects of the earth's rotation and density stratification. One more inherent feature of geophysical flows, namely their baroclinicity, has not been considered in that context. The present paper makes up for this deficiency. Diagnostic equations for the depths of the equilibrium, baroclinic, neutral or stable PBLs are derived and verified against data from large‐eddy simulations (LES). These latter represents the most convenient empirical‐validation tool, which allows considering separately different types of PBL: barotropic and baroclinic; truly neutral and conventionally neutral; nocturnal stable and long‐lived stable. The LES are performed using a new code specifically designed for modelling the neutral and stable PBL. Results from the comparison show an excellent correspondence between the proposed equations and LES data. Copyright © 2003 Royal Meteorological Society

On Internal Gravity Waves Associated with the Stable Boundary Layer

The stable planetary boundary layer at the baseof the residual layer supports internalwaves that are unambiguously ‘boundary layer’ incharacter. Some of these wavesare instabilities and some are neutrally stable modes, but they all have critical levelsin the residual layer. These waves exist for a broad range of conditions and should bea major component of any ducted disturbance that propagates within ninety degreesof the wind direction. The wave properties can be computed without the numericaldifficulties usually associated with critical-level systems.

Sensitivity of Arctic climate simulations to different boundary-layer parameterizations in a regional climate model

Arctic climate simulations with the high resolution regional climate model HIRHAM showsome deviations from station data in the planetary boundary layer (PBL) during winter, whichindicates the necessity of improvements in the atmospheric PBL parameterization for a betterdescription of the vertical stratification and atmosphere–surface energy exchange. A1-dimensional single column model scheme has been used to investigate the influence of twodifferent PBL parameterizations in monthly integrations for January 1991 and July 1990. Thefirst scheme uses the boundary layer parameterization of the atmospheric circulation modelECHAM3, including the Monin–Obukhov similarity theory in the surface layer and a mixinglength approach above. The second scheme applies the Rossby-number similarity theory forthe whole PBL, connecting external parameters with turbulent fluxes and with universal functionsdetermined on the basis of Arctic data. For both schemes the heat and humidity advectionhas been determined as residual term of the PBL balance equations. Diabatic sources havebeen computed from the current model solution and local temperature and humidity changesare estimated from radiosonde data. The simulated vertical structure and the atmosphere–surface energy exchange during January strongly depends on the used PBL parameterizationscheme. These different PBL parameterization schemes were then applied for simulations of theArctic climate in the 3-dimensional regional atmospheric climate model HIRHAM, usingECHAM3 with Monin–Obukhov similarity theory, ECHAM3 with Rossby-number similaritytheory and ECHAM4 parameterizations with a turbulent kinetic energy closure. The nearsurface temperature, the large-scale fields of geopotential and horizontal wind are simulatedsatisfactorily by all three schemes, but strong regional differences occur. The results show asensitivity to the type of turbulence exchange scheme used. The comparison with ECMWFanalyses and with radiosonde data reveals that during January ECHAM3 with Rossby numbersimilarity theory more succesfully simulates the cold and stable PBL over land surfaces, whereasover the open ocean ECHAM3 with Monin Obukhov similarity works better. ECHAM3 withRossby-number similarity theory delivers a better adapted vertical heat exchange under stableArctic conditions and reduces the cold bias at the surface. The monthly mean surface turbulentheat flux distribution strongly depends on the use of different PBL parameterizations and leadsto different Arctic climate structures throughout the atmosphere with the strongest changes atthe ice edge for January.

A Terminal Area PBL Prediction System at Dallas–Fort Worth and Its Application in Simulating Diurnal PBL Jets

Abstract A state–of–the–science meso–b–scale numerical weather prediction model is being employed in a prototype forecast system for potential operational use at the Dallas–Fort Worth International Airport (DFW). The numerical model is part of a unique operational forecasting system being developed to support the National Aeronautics and Space Administration's (NASA) Terminal Area Productivity Program. This operational forecasting system will focus on meso–b–scale aviation weather problems involving planetary boundary layer (PBL) turbulence, and is named the Terminal Area PBL Prediction System (TAPPS). TAPPS (version 1) is being tested and developed for NASA in an effort to improve 1–6–h terminal area forecasts of wind, vertical wind shear, temperature, and turbulence within both stable and convective PBLs at major airport terminal areas. This is being done to enhance terminal area productivity, that is, aircraft arrival and departure throughput, by using the weather forecasts as part of the Aircraft Vort...