Atmospheric Boundary Layer Measurements Research Articles

A meteorological dataset of the West African monsoon during the 2016 DACCIWA campaign

As part of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) project, extensive in-situ measurements of the southern West African atmospheric boundary layer (ABL) have been performed at three supersites Kumasi (Ghana), Savè (Benin) and Ile-Ife (Nigeria) during the 2016 monsoon period (June and July). The measurements were designed to provide data for advancing our understanding of the relevant processes governing the formation, persistence and dissolution of nocturnal low-level stratus clouds and their influence on the daytime ABL in southern West Africa. An extensive low-level cloud deck often forms during the night and persists long into the following day strongly influencing the ABL diurnal cycle. Although the clouds are of a high significance for the regional climate, the dearth of observations in this region has hindered process understanding. Here, an overview of the measurements ranging from near-surface observations, cloud characteristics, aerosol and precipitation to the dynamics and thermodynamics in the ABL and above, and data processing is given. So-far achieved scientific findings, based on the dataset analyses, are briefly overviewed.

Advancing Cross‐Disciplinary Understanding of Land‐Atmosphere Interactions

AbstractThe evolution of disciplinary silos and increasingly narrow disciplinary boundaries have together resulted in one‐sided approaches to the study of land‐atmosphere interactions—a field that requires a bi‐directional approach to understand the complex feedbacks and interactions that occur. The integration of surface flux and atmospheric boundary layer measurements is therefore essential to advancing our understanding. The Land‐Atmosphere 2021 workshop (held virtually, June 10‐11, 2021) involved almost 300 participants from around the world and promoted cross‐discipline collaboration by way of talks from invited speakers, moderated discussions, breakout sessions, and a virtual poster session. The workshop focused on five main theme areas: “big picture” overview, instrumentation and remote sensing, modeling, water, and aerosols and clouds. In talks and breakout groups, there were frequent calls for more AmeriFlux sites to be instrumented for boundary layer height measurements, and for the development of some “super sites” where profiling instruments would be deployed. There was further agreement on the need for the standardization of various datasets. There was also a consensus that funding agencies need to be willing to support the sorts of large projects (including associated instrumentation) which can drive interdisciplinary work. Early‐career scientists, in particular, expressed enthusiasm for working across disciplinary boundaries but noted that there need to be more financial support and training opportunities so they would be better prepared for interdisciplinary work. Investment in these career development opportunities would enable today's cohort of early‐career scientists to advance the frontiers of interdisciplinary work over the next couple of decades.

Nocturnal surface radiation cooling modulated by cloud cover change reinforces PM2.5 accumulation: Observational study of heavy air pollution in the Sichuan Basin, Southwest China

Surface radiation is crucial to atmospheric boundary layer development and air pollution formation. Several studies have revealed that surface radiation plays a vital role in developing the daytime convective boundary layer that controls the explosive growth of PM2.5 concentration; however, less attention has been paid to the effects of changing nighttime surface radiation on the near-surface temperature inversion layer and PM2.5 accumulation. In this study, we used long-term observations of meteorological and environmental data and atmospheric boundary layer measurements during a severe PM2.5 pollution event to investigate the effect of changes in nocturnal surface radiation on the increase in PM2.5 concentrations. The results showed that surface radiation cooling was enhanced (weakened) by decreased (increased) cloud cover fraction by changing longwave radiation at night; this strengthened (weakened) near-surface temperature inversion intensity and promoted (prevented) the accumulated increase in PM2.5. This observational study using 5-year data further confirmed the cloud radiative effect on the nighttime accumulation of PM2.5 with a significant negative correlation between nighttime averages of surface PM2.5 concentrations and cloud cover fractions. This reveals an important mechanism for the impact of surface radiation cooling modulated by cloud cover change on the nighttime accumulated increase in PM2.5. This finding extends our understanding of air pollutant accumulation at night with potential implications for atmospheric environment change.

An Analytically Derived Shear Stress and Kinetic Energy Equation for One-Equation Modelling of Complex Turbulent Flows

The Reynolds stress equations for two-dimensional and axisymmetric turbulent shear flows are simplified by invoking local equilibrium and boundary layer approximations in the near-wall region. These equations are made determinate by appropriately modelling the pressure–velocity correlation and dissipation rate terms and solved analytically to give a relation between the turbulent shear stress τ/ρ and the kinetic energy of turbulence (k=q2/2). This is derived without external body force present. The result is identical to that proposed by Nevzgljadov in A Phenomenological Theory of Turbulence, who formulated it through phenomenological arguments based on atmospheric boundary layer measurements. The analytical approach is extended to treat turbulent flows with external body forces. A general relation τ/ρ=a11−AFRiFq2/2 is obtained for these flows, where FRiF is a function of the gradient Richardson number RiF, and a1 is found to depend on turbulence models and their assumed constants. One set of constants yields a1= 0.378, while another gives a1= 0.328. With no body force, F ≡ 1 and the relation reduces to the Nevzgljadov equation with a1 determined to be either 0.378 or 0.328, depending on model constants set assumed. The present study suggests that 0.328 is in line with Nevzgljadov’s proposal. Thus, the present approach provides a theoretical base to evaluate the turbulent shear stress for flows with external body forces. The result is used to reduce the k–ε model to a one-equation model that solves the k-equation, the shear stress and kinetic energy equation, and an ε evaluated by assuming isotropic eddy viscosity behavior.

Comparison of existing and experimental weather models of the near surface atmospheric boundary layer

This work presents a comparison between existing weather models and experimental measurements of the lower atmospheric boundary layer. This work is part of a larger project developing a numerical model of long-range (~3 km) atmospheric acoustic propagation in littoral or riverine environments with a near-shore acoustic source and on-shore receivers. Temperature, humidity, and pressure data are collected with the use of iMet-XQ2 sensors mounted on an unmanned aerial vehicle. Vertical measured profiles that will then be compared to existing common models and assumptions (such as Monin-Obukhov similarity theory) to gain a better understanding of these mathematical models and how sufficient these models are at representing the meteorological parameters that will affect acoustic transmission.

The CopterSonde: an insight into the development of a smart unmanned aircraft system for atmospheric boundary layer research

Abstract. The CopterSonde is an unmanned aircraft system (UAS) developed in house by a team of engineers and meteorologists at the University of Oklahoma. The CopterSonde is an ambitious attempt by the Center for Autonomous Sensing and Sampling to address the challenge of filling the observational gap present in the lower atmosphere among the currently used meteorological instruments such as towers and radiosondes. The CopterSonde is a unique and highly flexible platform for in situ atmospheric boundary layer measurements with high spatial and temporal resolution, suitable for meteorological applications and research. Custom autopilot algorithms and hardware features were developed as solutions to problems identified throughout several field experiments carried out since 2017. In these field experiments, the CopterSonde has been proven capable of safely operating at wind speeds up to 22 m s−1, flying at 3050 m above mean sea level, and operating in extreme temperatures: nearly −20 ∘C in Finland and 40 ∘C in Oklahoma, United States. Leveraging the open-source ArduPilot autopilot code has allowed for seamless integration of custom functions and protocols for the acquisition, storage, and distribution of atmospheric data alongside the flight control data. This led to the development of features such as the “wind vane mode” algorithm, which commands the CopterSonde to always face into the wind. It also inspired the design of an asymmetric airframe for the CopterSonde, which is shown to provide more suitable locations for weather sensor placement, in addition to allowing for improvements in the overall aerodynamic characteristics of the CopterSonde. Moreover, it has also allowed the team to design and create a modular shell where the sensor package is attached and which can run independently of the CopterSonde's main body. The CopterSonde is on the trend towards becoming a smart UAS tool with a wide possibility of creating new adaptive and optimized atmospheric sampling strategies.

Physical simulations of neutral boundary layer in rotating tank

Laboratory measurements of neutral atmospheric boundary layer (ABL) are presented. The experiment was carried out in the large rotating tank of the Coriolis-LEGI laboratory in Grenoble. An ABL was created at reduced scale and measured. The mean flow was generated, both increasing (spin-up) and decreasing (spin-down) the rotation speed of the platform. Preliminary measurements by acoustic probes were used to assess the decay with time of the flow velocity at the position where turbulence measurements were subsequently performed. The mean velocity and the turbulence fields were then measured by using PIV (Particle Image Velocimetry) technique, which allows to obtain high-resolution measurements in the simulated ABL. For each rotation period and velocity conditions, two vertical cross-sections of the flow were measured at different times, then 3D velocity fields were reconstructed from the two planar fields under convenient geometric (orthogonal) and physical assumptions (reproducibility of the flow). The aim of this work is to collect a useful data set for testing and comparing turbulence models and parameterisations. For this reason, particular attention was paid to the turbulence statistics, turbulent fluxes and scales. The results of the data analysis are presented and discussed.

Surface and Aerodynamic Parameters Estimation for Urban and Rural Areas

Numerical weather prediction models require an accurate parametrization of the energy budget at the air-ground interface, that can be obtained only through long-term atmospheric boundary layer measurements at different spatial and temporal scales. Despite their importance, such measurements are still scarce even in well-characterized areas. In this paper, a three-year dataset from four micrometeorological stations run by the Regional Agency for Environmental Protection of Lazio was analyzed to estimate albedo, zero-displacement height, roughness length and surface properties over Rome and its suburbs, characterizing differences and interconnections between urban, suburban and rural areas of the same municipality. The integral albedo coefficient at the zenith for the urban station was found to be almost twice that for suburban and rural stations. The zero-displacement height of the urban site was strongly dependent on wind direction, with values varying between 12.0 and 17.8 m, while the roughness length (≈1.5 m) was almost independent of upwind direction, but it was significantly higher than the typical values calculated for rural stations (≈0.4 m). The apparent thermal capacities and thermal conductivity at all the non-urban sites were in fair agreement with each other and typical of soils with relatively low water content, as expected for a relatively dry Mediterranean area like Rome, while the apparent thermal diffusivity reflected the presence of different soil types.

Doppler Radar Measurements of Spatial Turbulence Intensity in the Atmospheric Boundary Layer

AbstractRemote sensing instruments that scan have the ability to provide high-resolution spatial measurements of atmospheric boundary layer winds across a region. However, the time required to collect the volume of measurements used to produce this spatial representation of atmospheric winds typically limits the extraction of atmospheric turbulence information using traditional temporal analysis techniques. To overcome this constraint, a spatial turbulence intensity (STI) metric was developed to quantify atmospheric turbulence intensity (TI) through analysis of spatial wind field variability. The methods used to determine STI can be applied throughout the measurement domain to transform the spatially distributed velocity fields to analogous measurement maps of STI. This method enables a comprehensive spatial characterization of atmospheric TI. STI efficacy was examined across a range of wind speeds and atmospheric stability regimes using both single- and dual-Doppler measurements. STI demonstrated the ability to capture rapid fluctuations in TI and was able to discern large-scale TI trends consistent with the early evening transition. The ability to spatially depict atmospheric TI could benefit a variety of research disciplines such as the wind energy industry, where an understanding of wind plant complex flow spatiotemporal variability is limited.

Enhanced estimation of boundary layer advective properties to improve space‐to‐time conversion processes for wind energy applications

AbstractRemote sensing instruments that scan have the ability to provide high‐resolution spatial measurements of atmospheric boundary layer winds across a region. However, the ability to use these spatially distributed measurements to extract temporal variations in the flow at time scales less than the measurement revisit period is historically limited. As part of this work, the framework for an enhanced space‐to‐time conversion technique is established, allowing for time histories of atmospheric boundary layer wind characteristics to be reliably extracted for locations within the measurement domain. This space‐to‐time conversion technique is made possible by quantifying momentum advection within the measurement domain, rather than simply assuming a uniform advection based on a singular mean wind speed and direction. The use of this technique enables the extraction of long lead‐time (ie, upwards of 60 seconds) forecasts of wind speed and direction at individual locations within the measurement domain, thereby expanding the application and potential benefits of scanning instruments. For example, these long lead‐time forecasts can be used to enhance proactive wind turbine control and more accurately define wind turbine wake statistics.

Characteristics of the Springtime Alpine Valley Atmospheric Boundary Layer Using Self-Organizing Maps

AbstractVertical profiles of wind velocity and air temperature from a sound detection and ranging (sodar) radio acoustic sounding system (RASS)-derived dataset within an alpine valley of the New Zealand Southern Alps were analyzed. The data covered the month of September 2013, and self-organizing maps (SOM; a data-clustering approach that is based on an unsupervised machine-learning algorithm) are used to detect topological relationships between profiles. The results of the SOM were shown to reflect the physical processes within the valley boundary layer by preserving valley boundary layer dynamics and its response to wind shear. By examining the temporal evolution of ridgetop wind speed and direction and SOM node transitions, the sensitivity of the valley boundary layer to ridgetop weather conditions was highlighted. The approach of using a composite variable (wind speed and potential temperature) with SOM was successful in revealing the coupling of dynamics and atmospheric stability. The results reveal the capabilities of SOM in analyzing large datasets of atmospheric boundary layer measurements and elucidating the connectivity of ridgetop wind speeds and valley boundary layers.

Generating an artificially thickened boundary layer to simulate the neutral atmospheric boundary layer

We aim to generate an artificially thickened boundary layer in a wind tunnel with properties similar to the neutral atmospheric boundary layer. We implement a variant of Counihan's technique which uses a combination of a castellated barrier, elliptical vortex generators, and surface roughness to create a thick boundary layer in a relatively short wind tunnel. We demonstrate an improved spanwise uniformity when compared to Counihan's original design by using a tighter vortex generator spacing with a smaller wedge angle which keeps the frontal area approximately constant while keeping the turbulence intensity and power spectral density unchanged. With this arrangement we were able to generate a 1:1000 scale atmospheric boundary layer at Reθ∼106 displaying logarithmic mean velocity behavior, a constant stress region, and turbulence intensities consistent with atmospheric boundary layer measurements and high Reynolds number laboratory rough-wall boundary layers. In addition, the power spectral density of the longitudinal velocity fluctuations agrees well with von Kármán's model spectrum, and the turbulence integral length scale agrees well with data from atmospheric boundary layer measurements.

Atmospheric boundary layer measurements at the 280 m high Hamburg weather mast 1995-2011: mean annual and diurnal cycles

Atmospheric boundary layer measurements at the 280 m high Hamburg weather mast 1995-2011: mean annual and diurnal cycles

Atmospheric boundary layer characteristics over the Pearl River Delta, China, during the summer of 2006: measurement and model results

Abstract. As part of the PRIDE-PRD2006 intensive campaign, atmospheric boundary layer (ABL) measurements were performed in Qingyuan, Panyu, and Xinken over the Pearl River Delta (PRD) on 1–30 July 2006. During the summer, the surface winds over the PRD are generally controlled by the south, usually with vertical wind shear at a height of approximately 800 m. Subsidence and precipitation from a tropical cyclone affects the air quality of the PRD. Under subsidence, wind speed in the ABL and the height of the ABL decrease and result in high-level concentrations. When the background wind speed is small or calm, the wind profile in Panyu and Xinken changes dramatically with height, which is perhaps caused by local circulation, such as sea-land breezes. To better understand the ABL of the PRD, simulations that used the Weather Research and Forecasting (WRF) mesoscale model were utilized to analyze the ABL characteristics over the PRD. Based on three types of weather condition simulations (i.e., subsidence days, rainy days, and sunny days), the WRF model revealed that the simulated temperature and wind fields in these three cases were moderately consistent with the measurements. The results showed that diurnal variations of the ABL height on subsidence days and sunny days were obvious, but diurnal variations of the ABL height on rainy days were not apparent. The ABL is obviously affected by local circulation, and the ABL features are different at various stations. A simulation focused on a high pollution episode during the subsidence days on 12–15 July 2006, occurred under high-pressure conditions, accompanied by the tropical cyclone "Bilis". A comparison of the simulated vertical wind fields and temperature structure with the ABL measurements at Xinken, Panyu, and Qingyuan stations found that the modeled and measured atmospheric fields revealed two different types of ABL characteristics over the PRD. When the surface winds over the PRD were light or nearly calm, the local circulation dominated, such as the sea-land breeze at Xinken station and the mountain-valley circulation at Qingyuan station. When the surface winds were strong, the stations were under the same background weather system, and the wind directions were almost the same. Furthermore, the modeled results also suggest that the subsidence by the typhoon "Bilis" had a great impact on the high Air Pollution Index (API).

Meteorological atmospheric boundary layer measurements and ECMWF analyses during summer at Dome C, Antarctica

Six levels of meteorological sensors have been deployed along a 45 m tower at the French‐Italian Concordia station, Dome C, Antarctic. We present measurements of vertical profiles, the diurnal cycle, and interdiurnal variability of temperature, humidity, and wind speed and direction for 3 weeks during the southern summer of 2008. These measurements are compared to 6‐hourly European Center for Medium‐Range Forecasts (ECMWF) analyses and daily radiosoundings. The ECMWF analyses show a 3–4°C warm bias relative to the tower observations. They reproduce the diurnal cycle of temperature with slightly weaker amplitude and weaker vertical gradients. The amplitude of the diurnal cycle of relative humidity is overestimated by ECMWF because the amplitude of the absolute humidity diurnal cycle is too small. The nighttime surface‐based wind shear and Ekman spiral is also not reproduced in the ECMWF analyses. Radiosonde temperatures are biased low relative to the tower observations in the lowest 30 m but approach agreement at the top of the tower. Prior to bias correction for age‐related contamination, radiosonde relative humidities are biased low relative to the tower observations in the lowest 10 m but agree with tower observations above this height. After correction for the age‐related bias, the radiosonde relative humidity agrees with tower observations below 10 m but is biased high above this height. Tower temperature observations may also be biased by solar heating, despite radiation shielding and natural ventilation.

Intermittency in the atmospheric surface layer: Unresolved or slowly varying?

We present streamwise velocity structure functions 〈 δ v L ( τ ) 〉 = 〈 | v ( t + τ ) − v ( t ) | p 〉 (with p = 1 : 5 ) obtained in the near neutral atmospheric surface layer at the Utah SLTEST site at the highest terrestrial Reynolds number R e τ = O ( 10 6 ) . We show that the occurrence of very large scale coherent oscillations in the streamwise velocity throughout the wall region, interpreted as genuine structural features of the canonical turbulent boundary layer, affects the scaling exponents of the p > 3 order structure functions. This results in a slight alteration of the intermittent behavior of the velocity field. It was found that for positive (fast) large scale oscillation of the low-pass filtered velocity signal, deviations from the Kolmogorov K41 prediction (absence of multiscaling) are more marked, as compared to negative (slow) excursion. The results are discussed in terms of convergence of statistics from atmospheric boundary layer measurements.

Wintertime aerosol characteristics over the Indo‐Gangetic Plain (IGP): Impacts of local boundary layer processes and long‐range transport

The Indo‐Gangetic Plain (IGP) encompasses a vast area, (accounting for ∼21% of the land area of India), which is densely populated (accommodating ∼40% of the Indian population). Highly growing economy and population over this region results in a wide range of anthropogenic activities. A large number of thermal power plants (most of them coal fed) are clustered along this region. Despite its importance, detailed investigation of aerosols over this region is sparse. During an intense field campaign of winter 2004, extensive aerosol and atmospheric boundary layer measurements were made from three locations: Kharagpur (KGP), Allahabad (ALB), and Kanpur (KNP), within the IGP. These data are used (1) to understand the regional features of aerosols and BC over the IGP and their interdependencies, (2) to compare it with features at locations lying at far away from the IGP where the conditions are totally different, (3) to delineate the effects of mesoscale processes associated with changes in the local atmospheric boundary layer (ABL), (4) to investigate the effects of long‐range transport or moving weather phenomena in modulating the aerosol properties as well as the ABL characteristics, and (5) to examine the changes as the season changes over to spring and summer. Our investigations have revealed very high concentrations of aerosols along the IGP, the average mass concentrations (MT) of total aerosols being in the range 260 to 300 μg m−3 and BC mass concentrations (MB) in the range 20 to 30 μg m−3 (both ∼5 to 8 times higher than the values observed at off‐IGP stations) during December 2004. Despite, BC constituted about 10% to the total aerosol mass concentration, a value quite comparable to those observed elsewhere over India for this season. The dynamics of the local atmospheric boundary layer (ABL) as well as changes in local emissions strongly influence the diurnal variations of MT and MB, both being inversely correlated with the mixed layer height (Zi) and the ventilation coefficient (Vc). The share of BC to total aerosols is highest (∼12%) during early night and lowest (∼4%) in the early morning hours. While an increase in the Vc results in a reduction in the concentration almost simultaneously, an increase in Zimax has its most impact on the concentration after ∼1 day. Accumulation mode aerosols contributed ∼90% to the aerosol concentration at ALB, ∼77 % at KGP and 74% at KNP. The BC mass mixing ratio was ∼10% over all three locations and is comparable to the value reported for Trivandrum, a tropical coastal location in southern India. This indicates presence of submicron aerosols species other than BC (such as sulfate) over KGP and KNP. A cross‐correlation analysis showed that the changes in MB at KGP is significantly correlated with those at KNP, located ∼850 km upwind, and ALB after a delay of ∼7 days, while no such delay was seen between ALB and KNP. Back trajectory analyses show an enhancement in MB associated with trajectories arriving from west, the farther from to the west they arrive, the more is the increase. This, along with the ABL characteristics, indicate two possibilities: (1) advection of aerosols from the west Asia and northwest India and (2) movement of a weather phenomena (such as cold air mass) conducive for build up of aerosols from the west to east. As the winter gives way to summer, the change in the wind direction and increased convective mixing lead to a rapid decrease in MB.

Atmospheric Boundary Layer Measurements in South-Central Kansas, 1997–2004

Atmospheric Boundary Layer Measurements in South-Central Kansas, 1997–2004

Synthetic turbulence, fractal interpolation, and large-eddy simulation.

Fractal interpolation has been proposed in the literature as an efficient way to construct closure models for the numerical solution of coarse-grained Navier-Stokes equations. It is based on synthetically generating a scale-invariant subgrid-scale field and analytically evaluating its effects on large resolved scales. In this paper, we propose an extension of previous work by developing a multiaffine fractal interpolation scheme and demonstrate that it preserves not only the fractal dimension but also the higher-order structure functions and the non-Gaussian probability density function of the velocity increments. Extensive a priori analyses of atmospheric boundary layer measurements further reveal that this multiaffine closure model has the potential for satisfactory performance in large-eddy simulations. The pertinence of this newly proposed methodology in the case of passive scalars is also discussed.

Identification of coherent structures of turbulence at the atmospheric surface layer

A parameter-free method based on orthonormal wavelet transforms is recommended for calculating the principal time scale of coherent structures in atmospheric boundary-layer measurements. First, the atmospheric turbulent signal is decomposed into the small scale vortex that has approximate isotropy and the large scale vortex with the digital filter. Then, the large scale vortex is used to detect coherent structures with this method. The principal time scale and profile of coherent structures for velocity components (u, v, w above rice fields are obtained. In order to testify the validity of this method, the correlation of coherent structures and non-coherent structures are also calculated.