Stable Nocturnal Boundary Layer Research Articles

CALOTRITON: a convective boundary layer height estimation algorithm from ultra-high-frequency (UHF) wind profiler data

Abstract. Long time series of observations of atmospheric dynamics and composition are collected at the French Pyrenean Platform for Observation of the Atmosphere (P2OA). Planetary boundary layer depth is a key variable of the climate system, but it remains difficult to estimate and analyse statistically. In order to obtain reliable estimates of the convective boundary layer height (Zi) and to allow long-term series analyses, a new restitution algorithm, named CALOTRITON, has been developed. It is based on the observations of an ultra-high-frequency (UHF) radar wind profiler (RWP) from P2OA with the help of other instruments for evaluation. Estimates of Zi are based on the principle that the top of the convective boundary layer is associated with both a marked inversion and a decrease in turbulence. Those two criteria are respectively manifested by larger RWP reflectivity and smaller vertical-velocity Doppler spectral width. With this in mind, we introduce a new UHF-deduced dimensionless parameter which weighs the air refractive index structure coefficient with the inverse of vertical velocity standard deviation to the power of x. We then search for the most appropriate local maxima of this parameter for Zi estimates with defined criteria and constraints such as temporal continuity. Given that Zi should correspond to fair-weather cloud base height, we use ceilometer data to optimize our choice of the power x and find that x=3 provides the best comparisons. The estimates of Zi by CALOTRITON are evaluated using different Zi estimates deduced from radiosounding according to different definitions. The comparison shows excellent results with a regression coefficient of up to 0.96 and a root-mean-square error of 71 m, which is close to the vertical resolution of the UHF RWP of 75 m, when conditions are optimal. In more complex situations, that is when the atmospheric vertical structure is itself particularly ambiguous, secondary retrievals allow us to identify potential thermal internal boundary layers or residual layers and help to qualify the Zi estimations. Frequent estimate errors are observed nevertheless; for example, when Zi is below the UHF RWP first reliable gate or when the boundary layer begins its transition to a stable nocturnal boundary layer.

The Development of Atmospheric Bores in Non‐Uniform Baroclinic Environments and Their Roles in the Maintenance, Structure, and Evolution of an MCS

AbstractThis study used radar observations and a high‐resolution numerical simulation to explore the interactions between an mesoscale convective system (MCS), cold pool outflows, and atmospheric bores in a non‐uniform baroclinic environment. The bores were generated by a nocturnal MCS that occurred on 2–3 June 2017 over the southern North China Plain. The goal of this investigation is to determine how the structure of bores varied within this non‐uniform environment and whether and how the bores would maintain the MCS and alter its structure. To the southwest of the MCS, where there was large CAPE and a well‐mixed boundary layer, discrete convection initiation occurred behind a single radar fine line (RFL) maintaining the propagation of the MCS. To the southeast of the MCS, multiple RFLs were found suggesting the generation of an undular bore in an environment containing an intense nocturnal stable boundary layer with dry upper layers and little CAPE. Hydraulic and nonlinear theory were applied to the simulation of the MCS revealing that the differences in the bore evolution depended on both the characteristics of the cold pool and the variations in the ambient environment. Thus, the characteristics of the ambient environment and the associated differences in bore structure impacted the maintenance and organization of the MCS. This study implies the importance of an accurate representation of the low‐level ambient environment and the microphysics and kinematics within the MCS to accurately simulate and forecast cold pools, the generation and evolution of bores, and their impact on nocturnal MCSs.

Characteristics of particulate matter during New Year's eve fireworks and Taal volcano ashfall in Metro Manila on January 2020

In this study, we describe the chemical and physical characteristics of two air pollution events in the first half of January 2020 in Metro Manila, Philippines, namely the New Year's eve fireworks and the Taal volcano ashfall. During the New Year's eve fireworks, PM2.5 concentration and most of the chemical components increased three-fold. However, we found disproportionately higher increases for certain metallic components typically used as pyrotechnic propellant and/or colorant for blue, white, and green light: Fe(5-fold), Cr(5-fold), Cu(8-fold), Ni(9-fold), V(10-fold), Ti(11-fold), and Ba(34-fold). These results provide insights for health assessments and legislation related to fireworks emissions. On January 12, 2020, the Taal volcano eruption caused much concern in Metro Manila as visible tephra deposits were observed. There was only a slight increase (8%) in PM2.5 concentration and its components related to basaltic ash. The low PM2.5 concentration was mainly due to the inhibited intrusion of ash into the stable nocturnal boundary layer. In addition, the detection of lightning in the plume and umbrella region showed size segregation of ash particles. The WRF-Chem model was able to capture the distribution of both tephra advection and fallout, a first for Taal volcano, making it possible for use in the future ashfall forecasting.

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

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

The effects of coastal local circulations and their interactions on ozone pollution in the Hangzhou metropolitan area

Impacts of coastal local circulations and their interactions on ozone (O3) in Hangzhou on 5 June 2018 with relatively low temperatures were investigated using surface observations and three-dimensional air quality simulations. Local circulations and their interactions were primary causes of the episode instead of high temperatures. Northwesterly land breeze on the previous day transported O3 to sea, and sea breeze (SB) on the episode day blew pollution back to land. Inland penetration (east to west) of the combined SB front (SBF) was accelerated by urban heat island (UHI) induced easterly flow over coast and slowed by its westerly flow and surface roughness over inland urban areas. O3 accumulation at surface and in the elevated air over city was caused by mountain barrier effect and venting process. Subsidence heating associated with valley circulation accelerated UHI circulation (UHIC) before 1400 LT, indirectly strengthening the SB circulation (SBC). The UHIC and valley circulation decoupled with SBC at noon and in the late afternoon, respectively, leading to a stronger SBC. The stronger SBC brought additional O3 (>20 ppb) to further inland area, to elevated air with updrafts in SB head, and to nocturnal stable boundary layer and residual layer with its upper-level offshore flow.

Differences in Near-Storm Parameters Useful for Forecasting Intensity of Nocturnal and Diurnal Bow Echo Winds

Abstract Because bow echoes are often associated with damaging wind, accurate prediction of their severity is important. Recent work by Mauri and Gallus showed that despite increased challenges in forecasting nocturnal bows due to an incomplete understanding of how elevated convection interacts with the nocturnal stable boundary layer, several near-storm environmental parameters worked well to distinguish between bow echoes not producing severe winds (NS), those only producing low-intensity severe winds [LS; 50–55 kt (1 kt ≈ 0.51 m s−1)], and those associated with high-intensity (HS; >70 kt) severe winds. The present study performs a similar comparison for daytime warm-season bow echoes examining the same 43 SPC mesoanalysis parameters for 158 events occurring from 2010 to 2018. Although low-level shear and the meridional component of the wind discriminate well for nocturnal bow severity, they do not significantly differ in daytime bows. CAPE parameters discriminate well between daytime NS events and severe ones, but not between LS and HS, differing from nocturnal events where they discriminate between HS and the other types. The 500–850-hPa layer lapse rate works better to differentiate daytime bow severity, whereas the 500–700-hPa layer works better at night. Composite parameters work well to differentiate between all three severity types for daytime bow echoes, just as they do for nighttime ones, with the derecho composite parameter performing especially well. Heidke skill scores indicate that both individual and pairs of parameters generally are not as skillful at predicting daytime bow echo wind severity as they are at predicting nocturnal bow wind severity.

Observational study of the South American low‐level jet during the SALLJEX

AbstractThe South American low‐level jet (SALLJ) is a narrow northerly wind speed maximum present just above the boundary layer. It is an important component of the tropical–extratropical heat and moisture exchange in South America and can favour deep moist convection in southeastern South America. The main objective of this study is to analyse the SALLJ characteristics at 21 upper‐air stations deployed between the Tropics and the subtropics from just east of the Andes to the eastern plains during the SALLJ Experiment (SALLJEX). The greatest wind speed occurs between 300 and 2,000 m AGL between 0300 and 1200 UTC, mainly in connection with greater northerly winds during an anticlockwise rotation of the wind from sunset to sunrise, thus suggesting the important role of the inertial oscillation in the wind's diurnal cycle. The spatial variation of the LLJ throughout the SALLJEX network shows a weakening of the maximum wind speed from stations near the Andes towards the Plains suggesting the presence of the LLJ core just east of the Andes around Santa Cruz de la Sierra's latitude (17°48′S). Weak, moderate, and strong SALLJ categories defined from a local maximum northerly wind speed threshold at each station are defined to analyse the relationship between the SALLJ intensity and the thermodynamic properties of the lower layers of the atmosphere. Strong SALLJs are frequently observed at night‐time, while weak SALLJs are likely to occur at any time of the day. Strong cases have deeper and less stable nocturnal boundary layers, which could be due to the SALLJ warm advection near the time of wind speed maximum (0600 UTC). Deeper convective boundary layers and higher low‐level temperatures observed at 1800 UTC prior to strong nocturnal SALLJs can potentially lead to larger amplitudes of inertial oscillation and contribute to generating stronger SALLJs.

Measurement report: Characterization of the vertical distribution of airborne <i>Pinus</i> pollen in the atmosphere with lidar-derived profiles – a modeling case study in the region of Barcelona, NE Spain

Abstract. This paper investigates the mechanisms involved in the dispersion, structure, and mixing in the vertical column of atmospheric pollen. The methodology used employs observations of pollen concentration obtained from Hirst samplers (we will refer to this as surface pollen) and vertical distribution (polarization-sensitive lidar), as well as nested numerical simulations with an atmospheric transport model and a simplified pollen module developed especially for this study. The study focuses on the predominant pollen type, Pinus, of the intense pollination event which occurred in the region of Barcelona, Catalonia, NE Spain, during 27–31 March 2015. First, conversion formulas are expressed to convert lidar-derived total backscatter coefficient and model-derived mass concentration into pollen grains concentration, the magnitude measured at the surface by means of aerobiological methods, and, for the first time ever, a relationship between optical and mass properties of atmospheric pollen through the estimation of the so-called specific extinction cross section is quantified in ambient conditions. Second, the model horizontal representativeness is assessed through a comparison between nested pollen simulations at 9, 3, and 1 km horizontal resolution and observed meteorological and aerobiological variables at seven sites around Catalonia. Finally, hourly observations of surface and column concentration in Barcelona are analyzed with the different numerical simulations at increasing horizontal resolution and varying sedimentation/deposition parameters. We find that the 9 or 3 km simulations are less sensitive to the meteorology errors; hence, they should be preferred for specific forecasting applications. The largest discrepancies between measured surface (Hirst) and column (lidar) concentrations occur during nighttime, where only residual pollen is detected in the column, whereas it is also present at the surface. The main reason is related to the lidar characteristics which have the lowest useful range bin at ∼ 225 m, above the usually very thin nocturnal stable boundary layer. At the hour of the day of maximum insolation, the pollen layer does not extend up to the top of the planetary boundary layer, according to the observations (lidar), probably because of gravity effects; however, the model simulates the pollen plume up to the top of the planetary boundary layer, resulting in an overestimation of the pollen load. Besides the large size and weight of Pinus grains, sedimentation/deposition processes have only a limited impact on the model vertical concentration in contrast to the emission processes. For further modeling research, emphasis is put on the accurate knowledge of plant/tree spatial distribution, density, and type, as well as on the establishment of reliable phenology functions.

Revealing the Morning Transition in the Mountain Boundary Layer Using Fiber‐Optic Distributed Temperature Sensing

AbstractIn the morning, the nocturnal stable boundary layer, SBL, transitions into its daytime convective counterpart substantially impacting the distribution of temperature, humidity, and pollutants. Applying distributed temperature sensing (DTS) below a tethered balloon (2–200 m) and along a tower (0–11 m), for the first time we observed three morning transitions (MTs) in a mountain boundary layer with high temporal (<10 s) and spatial (<0.25 m) resolutions. We show that MTs are best derived from a change in static stability from synchronous DTS observations. Our findings confirm that the MT occurs at the SBL top and bottom simultaneously, and identify horizontal heat advection as a main driver aiding solar surface heating in this midrange mountain valley. We conclude that heterogenous land use and mountainous topography cause complex interactions between valley‐scale and local airflows leading to thermal signatures characterized by strong, small‐scale variability. Our study highlights DTS as a crucial tool for investigating complex thermodynamic processes.

Thermal Submeso Motions in the Nocturnal Stable Boundary Layer. Part 2: Generating Mechanisms and Implications

In the stable boundary layer, thermal submesofronts (TSFs) are detected during the Shallow Cold Pool experiment in the Colorado plains, Colorado, USA in 2012. The topography induces TSFs by forming two different air layers converging on the valley-side wall while being stacked vertically above the valley bottom. The warm-air layer is mechanically generated by lee turbulence that consistently elevates near-surface temperatures, while the cold-air layer is thermodynamically driven by radiative cooling and the corresponding cold-air drainage decreases near-surface temperatures. The semi-stationary TSFs can only be detected, tracked, and investigated in detail when using fibre-optic distributed sensing (FODS), as point observations miss TSFs most of the time. Neither the occurrence of TSFs nor the characteristics of each air layer are connected to a specific wind or thermal regime. However, each air layer is characterized by a specific relationship between the wind speed and the friction velocity. Accordingly, a single threshold separating different flow regimes within the boundary layer is an oversimplification, especially during the occurrence of TSFs. No local forcings or their combination could predict the occurrence of TSFs except that they are less likely to occur during stronger near-surface or synoptic-scale flow. While classical conceptualizations and techniques of the boundary layer fail in describing the formation of TSFs, the use of spatially continuous data obtained from FODS provide new insights. Future studies need to incorporate spatially continuous data in the horizontal and vertical planes, in addition to classic sensor networks of sonic anemometry and thermohygrometers to fully characterize and describe boundary-layer phenomena.

Haze pollution under a high atmospheric oxidization capacity in summer in Beijing: insights into formation mechanism of atmospheric physicochemical processes

Abstract. Under a high atmospheric oxidization capacity, the synergistic effect of the physicochemical processes in the atmospheric boundary layer (ABL) caused summer haze pollution in Beijing. The southern and southwestern areas, generally 60–300 km away from Beijing, were seriously polluted in contrast to Beijing, which remained clean. Southerly winds moving faster than 20–30 km h−1 since the early morning primarily caused haze pollution initiation. The PM2.5 (particulate matter with a dynamic equivalent diameter smaller than 2.5 µm) level increased to 75 µg m−3 over several hours during the daytime, which was simultaneously affected by the ABL structure. Additionally, the O3 concentration was quite high during the daytime (250 µg m−3), corresponding to a high atmospheric oxidation capacity. Much sulfate and nitrate were produced through active atmospheric chemical processes, with sulfur oxidation ratios (SORs) up to ∼0.76 and nitrogen oxidation ratios (NORs) increasing from 0.09 to 0.26, which further facilitated particulate matter (PM) level enhancement. However, the increase in sulfate was mainly linked to southerly transport. At midnight, the PM2.5 concentration sharply increased from 75 to 150 µg m−3 over 4 h and remained at its highest level until the next morning. Under an extremely stable ABL structure, secondary aerosol formation dominated by nitrate was quite intense, driving the haze pollution outbreak. The PM levels in the southern and southeastern areas of Beijing were significantly lower than those in Beijing at this time, even below air quality standards; thus, the contribution of pollution transport had almost disappeared. With the formation of a nocturnal stable boundary layer (NSBL) at an altitude ranging from 0–0.3 km, the extremely low turbulence kinetic energy (TKE) ranging from 0 to 0.05 m2 s−2 inhibited the spread of particles and moisture, ultimately resulting in elevated near-surface PM2.5 and relative humidity (∼90 %) levels. Due to the very high humidity and ambient oxidization capacity, NOR rapidly increased from 0.26 to 0.60, and heterogeneous hydrolysis reactions at the moist particle surface were very notable. The nitrate concentration steeply increased from 11.6 to 57.8 µg m−3, while the sulfate and organics concentrations slightly increased by 6.1 and 3.1 µg m−3, respectively. With clean and strong winds passing through Beijing, the stable ABL dissipated with the potential temperature gradient becoming negative and the ABL height (ABLH) increasing to ∼2.5 km. The high turbulence activity with a TKE ranging from 3 to 5 m2 s−2 notably promoted pollution diffusion. The self-cleaning capacity of the atmosphere is commonly responsible for air pollution dispersion. However, reducing the atmospheric oxidization capacity, through strengthening collaborative control of nitrogen oxide (NOx) and volatile organic compounds (VOCs), as well as continuously deepening regional joint air pollution control, is urgent.

Seasonal Variations of the Near-Surface Atmospheric Boundary Layer Structure in China’s Gurbantünggüt Desert

As the largest fixed and semifixed desert in China, the Gurbantünggüt Desert has a longperiod of snow in winter and the rapid growth of ephemeral plants in spring, presentingthe obvious seasonal changes in the underlying desert surface type, which could lead to the significantvariety in the near-surface boundary layer over this desert. To clarify the influence of the underlying surface change on the near-surface atmospheric boundary layer, gradient tower data and Eddy covariance data in 2017 were analyzed. The results were as follows: the wind profile can be divided into the nocturnal stable boundary layer and the daytime unstable boundary in spring, summer, and autumn, while the wind profile dominating nighttime stability in winter. During the study period, the four-season temperature profiles can be divided into four types: night radiation type, morning transition type, daylight solar radiation type, and evening transition type, and the temperature difference between spring and summer is more than that of autumn and winter. The vertical temperature lapse rate can reach 4.5°C/100 m in spring and summer, while the vertical temperature lapse rate is 0.5°C/100 m in winter. The special humidity value in summer and spring is greater than autumn and winter. The profile is almost in the inverse humidity state at almost all periods in winter. The inverse humidity phenomenon occurred on the autumn night. Besides, the specific humidity is closely related to the temperature and the near-surface wind speed. The “rapid change” of the underlying surface of the spring desert region affects the surface energy budget, which affects the turbulent energy and the stability of the near-surface layer, thus affecting the changes in temperature, humidity, and wind profile.

Linking Small-Scale Flight Manoeuvers and Density Profiles to the Vertical Movement of Insects in the Nocturnal Stable Boundary Layer

Huge numbers of insects migrate over considerable distances in the stably-stratified night-time atmosphere with great consequences for ecological processes, biodiversity, ecosystem services and pest management. We used a combination of meteorological radar and lidar instrumentation at a site in Oklahoma, USA, to take a new look at the general assistance migrants receive from both vertical and horizontal airstreams during their long-distance flights. Movement in the nocturnal boundary layer (NBL) presents very different challenges for migrants compared to those prevailing in the daytime convective boundary layer, but we found that Lagrangian stochastic modelling is effective at predicting flight manoeuvers in both cases. A key feature for insect transport in the NBL is the frequent formation of a thin layer of fast-moving air – the low-level jet. Modelling suggests that insects can react rapidly to counteract vertical air movements and this mechanism explains how migrants are retained in the jet for long periods (e.g. overnight, and perhaps for several hours early in the morning). This results in movements over much longer distances than are likely in convective conditions, and is particularly significant for the reintroduction of pests to northern regions where they are seasonally absent due to low winter temperatures.

Wave Disturbances and Their Role in the Maintenance, Structure, and Evolution of a Mesoscale Convection System

AbstractThis study investigates a nocturnal mesoscale convective system (MCS) observed during the Plains Elevated Convection At Night (PECAN) field campaign. A series of wavelike features were observed ahead of this MCS with extensive convective initiation (CI) taking place in the wake of one of these disturbances. Simulations with the WRF-ARW Model were utilized to understand the dynamics of these disturbances and their impact on the MCS. In these simulations, an “elevated bore” formed within an inversion layer aloft in response to the layer being lifted by air flowing up and over the cold pool. As the bore propagated ahead of the MCS, the lifting created an environment more conducive to deep convection allowing the MCS to discretely propagate due to CI in the bore’s wake. The Scorer parameter was somewhat favorable for trapping of this wave energy, although aspects of the environment evolved to be consistent with the expectations for an n = 2 mode deep tropospheric gravity wave. A bore within an inversion layer aloft is reminiscent of disturbances predicted by two-layer hydraulic theory, contrasting with recent studies that suggest bores are frequently initiated by the interaction between the flow within stable nocturnal boundary layer and convectively generated cold pools. Idealized simulations that expand upon this two-layer approach with orography and a well-mixed layer below the inversion suggest that elevated bores provide a possible mechanism for daytime squall lines to remove the capping inversion often found over the Great Plains, particularly in synoptically disturbed environments where vertical shear could create a favorable trapping of wave energy.

Influence of the geostrophic wind direction on the atmospheric boundary layer flow

Proper simulation and modelling of geophysical flows is crucial to the study of numerical weather prediction, wind energy and many other applications. When simulating the atmospheric boundary layer, Coriolis forces act as a result of Earth’s rotation. The horizontal component of Earth’s rotation, which is often neglected, influences the balance of vertical momentum. The horizontal component results in systematic differences in the structure and statistics of stratified atmospheric boundary layers as a function of the direction of the geostrophic velocity. These differences are particularly relevant to atmospheric flows which include inhomogeneous roughness elements such as drag disks or wind turbines since the presence of these drag elements alters the balance between turbulent stresses and the Coriolis contributions in Reynolds stress budgets. Even at latitudes as high as , changing the geostrophic wind velocity vector direction alone changes the magnitude of shear stress, and therefore vertical transport of kinetic energy, in the conventionally neutral atmospheric boundary layer up to . As such, the boundary layer height, shear and veer profiles, surface friction velocity and other key features are affected by the direction of the geostrophic wind. The influence of the horizontal component of Earth’s rotation in stable nocturnal boundary layers depends on the strength of the stratification as there is a strong influence in the present study and a weak influence in the GEWEX Atmospheric Boundary Layer Study (GABLS) case. A model of the effect of the horizontal component on the boundary layer shear stress is also proposed and verified with the present simulations. While not studied here, the present observations are also relevant to the oceanic Ekman boundary layer.

Climatological Features of the Weakly and Very Stably Stratified Nocturnal Boundary Layers. Part I: State Variables Containing Information about Regime Occupation

AbstractThe atmospheric nocturnal stable boundary layer (SBL) can be classified into two distinct regimes: the weakly SBL (wSBL) with sustained turbulence and the very SBL (vSBL) with weak and intermittent turbulence. A hidden Markov model (HMM) analysis of the three-dimensional state-variable space of Reynolds-averaged mean dry static stability, mean wind speed, and wind speed shear is used to classify the SBL into these two regimes at nine different tower sites, in order to study long-term regime occupation and transition statistics. Both Reynolds-averaged mean data and measures of turbulence intensity (eddy variances) are separated in a physically meaningful way. In particular, fluctuations of the vertical wind component are found to be much smaller in the vSBL than in the wSBL. HMM analyses of these data using more than two SBL regimes do not result in robust results across measurement locations. To identify which meteorological state variables carry the information about regime occupation, the HMM analyses are repeated using different state-variable subsets. Reynolds-averaged measures of turbulence intensity (such as turbulence kinetic energy) at any observed altitude hold almost the same information as the original set, without adding any additional information. In contrast, both stratification and shear depend on surface information to capture regime transitions accurately. Use of information only in the bottom 10 m of the atmosphere is sufficient for HMM analyses to capture important information about regime occupation and transition statistics. It follows that the commonly measured 10-m wind speed is potentially a good indicator of regime occupation.

Contrasting Characteristics and Evolution of Southerly Low-Level Jets During Different Boundary-Layer Regimes

Nocturnal low-level jets (LLJs) have been documented at many places around the world, particularly across the Great Plains of the United States. Nocturnal LLJs significantly affect the regional weather and climate, as well as several industrial activities. While the LLJ has been extensively studied, little attention has been paid to the relationship between the evolution of the LLJ, its properties, and the boundary-layer structure. Here, observations collected in northern Oklahoma, USA are used to analyze the characteristics and evolution of the LLJ during conditions of weak and strong turbulence, respectively, coinciding with strongly and weakly stable nocturnal boundary layers. Additionally, the relationship between the LLJ and the temperature profile under these different conditions is examined. Three case studies are used during conditions of weak and strong turbulence, followed by statistics computed from a larger dataset of LLJs. When turbulence is weak, the LLJ is shown to reach peak intensity later near sunrise, while a maximum in the intensity is reached about six hours after sunset in the highly turbulent boundary layer. Furthermore, the LLJ tends to stay constant with height when turbulence is maintained throughout the night, while the LLJ tends to follow isentropic surfaces when vertical mixing is suppressed. When the LLJ follows isentropic surfaces, changes in the wind speed are related to the stability modification.

Improved method for characterising temporal variability in urban air quality part II: Particulate matter and precursors in central Poland

Four years (2008–2011) of air quality observations are investigated near Łódź, central Poland. Despite reduced industrialisation in the region since around 1990, peak hourly PM10 (PM2.5) occasionally exceeded 400 (300) μg m−3 in the ‘heating season’ (Oct–Mar), attributable to a combination of ‘imported’ pollution and local traffic-related and domestic sources. High heating season emissions caused annual-mean PM10 and PM2.5 values of 33.6 and 21.1 μg m−3 (13.6 and 11.1 μg m−3, respectively, above the WHO guidelines), with daily mean PM10 exceeding the WHO guideline of 50 μg m−3 56 times a year. An approach is outlined by which urban air quality can be more directly related to local meteorology by separate consideration of atmospheric stability on diurnal, synoptic and seasonal timescales. Newly developed radon-based techniques for classification of diurnal and synoptic timescale changes in stability are adapted for use in this study. Daily PM10 and PM2.5 exceeded WHO guidelines for only two of six mixing state categories: (i) strong persistent synoptic inversion conditions, associated with lingering anti-cyclonic systems in the non-summer months (comprising ≤15% of heating season months), and (ii) fine-weather conditions conducive to the formation of strongly stable nocturnal boundary layers (15–20% of heating season months). Both mixing states were associated with low mean wind speeds (1–1.5 m s−1), near-surface temperature gradients of 1–2 °C m−1, shallow nocturnal boundary layers (45–55 m), and south-easterly winds. Radon-based stability “class-typing” is shown to constitute a convenient, consistent, objective and economical means by which to relate local meteorological conditions to air quality in complex urban environments. It also provides a statistically-robust method of retrospectively relating exposure to poor air quality to likely health implications.

Investigating vertical motion of small insects in atmospheric boundary layer using millimetre‐wavelength radar and Doppler LIDAR

It is well known in the meteorological community that millimetre-wavelength cloud radars contain considerable contamination from aerial biota, and much of this comprises small weakly flying insects. Several methods have been explored for removing insect contamination from cloud radar data yet using this data to study the behaviour of insects remains a relatively unexplored area. Here, the authors describe the use of a collocated Ka-band cloud radar and Doppler LIDAR to study the vertical motion of small insects and investigate how this varies depending on the surrounding vertical air motion. We find that in the convective boundary layer, insects are largely concentrated in updrafts. During the daytime, small insects in downdrafts are found to descend at an average rate of 0.25 m s−1, yet insects caught in updrafts showed a descent response that was dependent upon the strength of the updraft in which they were embedded. Although the downward motion of the insects increased with increasing updraft strength, it was insufficient to overcome the rising motion in the updraft, i.e. in updrafts the insects ascend but at a slower speed than the surrounding air. We also report an ongoing efforts to extend this research to the nocturnal stable boundary layer.

On wind turbine loads during the evening transition period

AbstractThe late afternoon hours in the diurnal cycle precede the development of the nocturnal stable boundary layer. This “evening transition” (ET) period is often when energy demand peaks. This period also corresponds to the time of day that is a precursor to late‐afternoon downbursts, a subject of separate interest. To capture physical characteristics of wind fields in the atmospheric boundary layer (ABL) during this ET period, particularly the interplay of shear and turbulence, stochastic simulation approaches, although more tractable, are not suitable. Large‐eddy simulation (LES), on the other hand, may be used to generate high‐resolution ABL turbulent flow fields. We present a suite of idealized LES four‐dimensional flow fields that define a database representing different combinations of large‐scale atmospheric conditions (characterized by associated geostrophic winds) and surface boundary conditions (characterized by surface heat fluxes). Our objective is to evaluate the performance of wind turbines during the ET period. Accordingly, we conduct a statistical analysis of turbine‐scale wind field variables. We then employ the database of these LES‐based inflow wind fields in aeroelastic simulations of a 5‐MW wind turbine. We discuss how turbine loads change as the ET period evolves. We also discuss maximum and fatigue loads on the rotor and tower resulting from different ABL conditions. Results of this study suggest that, during the ET period, the prevailing geostrophic wind speed affects the mean and variance of longitudinal winds greatly and thus has significant influence on all loads except the yaw moment which is less sensitive to uniform and symmetric incoming flow. On the other hand, surface heat flux levels affect vertical turbulence and wind shear more and, as a result, only affect maximum blade flapwise bending and tower fore‐aft bending loads.