Nocturnal Boundary Layer Research Articles (Page 1)

Abstract This study extends the linear theory of Shapiro et al. (S18) for the onset of horizontal convergence and ascent in nocturnal boundary layers in baroclinic environments such as the U.S. Great Plains. In S18, the sudden decay of turbulence in a surface-based warm tongue at sunset triggers a surge of convergent inflow/ascent as well as a Blackadar-like nocturnal low-level jet. For conditions typical of broad warm-season surface-based baroclinic zones over the Great Plains, the S18 theory predicts that air parcels can rise 500 m–1 km before the onset of a descent phase. Such displacements may help sustain or initiate convection and play a role in the well-known nocturnal maximum in rainfall over the region. In this study, the Cloud Model 1 is used to examine the S18 predictions in a more realistic setting in which the nonlinear terms in the governing equations are retained, and the sudden shutdown of turbulence at sunset is replaced by a more gradual evening transition. A warm tongue arises in the simulated boundary layer over a 5-day period through a prescribed deficit in surface moisture which causes the greatest daytime heating in the domain center. As in S18, the simulations depict a surge of convergent flow, descent of the zone of peak ascent, replacement of the ascent zone by subsidence, peak vertical motion decreasing with latitude and warm tongue width, and the generation of free-atmosphere inertia–gravity waves. The divergence and vorticity fields are found to oscillate at the inertial frequency.

Clear Low-wind Nocturnal Boundary Layers Over Topography

Nitrous acid (HONO) is a key precursor of hydroxyl radicals (OH) in the urban atmospheric boundary layer. However, most HONO observations so far are on the ground level, while HONO chemistry at higher altitude remains largely unknown. Through one-month observations at a 450 m platform of Canton Tower in Guangzhou, China, we have identified two distinct regimes of nocturnal HONO chemistry. One is dominated by heterogeneous reactions on the ground surface, likely corresponding to the period when the platform was within the stable nocturnal boundary layer. Another regime, occurring in the residual layer, is dominated by in situ formation from oxidation of nitric oxide (NO) by OH. During the daytime, HONO from emissions and heterogeneous sources at the ground undergoes ∼60% loss through photolysis before reaching 450 m. A detailed HONO budget analysis considering chemistry and vertical transport suggests that on average 32% of the observed HONO at 450 m is from OH oxidation of NO, while there remains 51% unidentified. These findings emphasize the increased contribution of NO + OH to the overall HONO budget throughout the urban boundary layer, in contrast to the diminished role of ground-related processes, and warrant future continuous measurements at high altitudes to supplement data at the ground to develop a complete understanding of HONO chemistry in the urban boundary layer.

Abstract In this study, Large Eddy Simulations (LES) with realistic inflow conditions are conducted to investigate the dynamics of the nocturnal marine atmospheric boundary layer (MABL) (with approximately 8 m/s mean inflow speeds) around the South Fork Wind Farm, completed in March 2024 and located 35 miles offshore and east of Montauk Point, NY. The LES framework incorporates 12 turbines represented by actuator disk models. We analyze the turbulence and circulation dynamics in the wake regions, focusing on the effects of tilting and stretching on vorticity as they represent the biggest budget terms. Our findings reveal that these processes intensify circulation and induce convergent (upward) motions in the near wake, while acting as sinks of vorticity in the far wake, thereby weakening circulation and facilitating flow recovery mechanisms. Additionally, we identify shear production as the primary source of turbulent kinetic energy (TKE) downstream of the actuator discs, peaking above the hub height, particularly at the top tip. The summer nocturnal boundary layer typically has consistent upward buoyancy flux. Heating at the surface induces convection and produces TKE. Overall, TKE is primarily removed through dissipation and transport.

Effects of nocturnal boundary layer subsidence and long-distance transports on PM2.5 vertical profiles in the Yangtze River Delta of China measured by PM sensor on unmanned aerial vehicle and PM Lidar.

Abstract In a stable boundary layer (SBL), turbulence is generally weak and exhibits significant intermittent characteristics. Interactions among motions of different scales complicate its structural evolution, making prediction challenging. This study focuses on two typical processes in the SBL: low‐level jet (LLJ) and internal gravity waves (IGWs), investigating how their interactions influence the evolution of turbulence structures. Utilizing a full boundary layer turbulence observation network and data processing system at Zhongchuan International Airport, this study includes eddy covariance system, Doppler Lidar, and wind profiling radar. In strongly SBL, turbulence energy accumulates in higher layers and, during downward transfer, generates local LLJ and IGWs, triggering intermittent turbulence events. Internal factors of turbulence intermittency dominated the process. The interaction between LLJ and IGWs maintains intermittent turbulence bursts, accompanied by the conversion of sub‐mesoscale energy to turbulent energy. In weakly SBL, the conversion of sub‐mesoscale motion energy drives intermittent turbulence events, along with energy transfers between different scales of IGWs, resulting in weaker turbulence intermittency. External factors of turbulence intermittency dominated the process. In both cases, the interaction between LLJ and IGWs alters turbulence structure and atmospheric stability. Turbulent mixing changes the mean gradient field, further influencing the LLJ height. This study elucidates the mechanisms of interaction between internal and external factors in turbulence intermittency. It outlines energy transfer among different scales of motion and clarifies the mechanisms behind state transitions and structural evolution in strongly and weakly SBL. These findings are significant for advancing theoretical research and simulation developments of the SBL.

Abstract. As a component of the ICOS Cities project, a “mid-cost” NDIR (nondispersive infrared) CO2 sensor network was deployed across the city of Zurich (Switzerland), known as ZiCOS-M. The network was operational between July 2022 and July 2024 and consisted of 26 monitoring sites, 21 of which were located in or around the city of Zurich, with 5 sites outside the urban area. Daily calibrations using two reference gas cylinders and corrections of the sensors' spectroscopic response to water vapour were performed to reach a high level of measurement accuracy. The hourly mean root mean squared error (RMSE) was 0.98 ppm (range of 0.46 and 1.5 ppm) and the mean bias ranged between −0.72 and 0.66 ppm when undergoing parallel measurements with a high-precision reference gas analyser for a period of 2 weeks or more. CO2 concentrations (technically, dry-air mole fractions) were highly variable with site means in Zurich ranging from 434 to 460 ppm, and Zurich's mean urban CO2 dome was 15.4 ppm above the regional background. Some of the highest CO2 levels were found at two sites exposed to strong plant respiration in a very confined nocturnal boundary layer. High-CO2 episodes were detected outside Zurich's urban area, demonstrating that processes acting on a variety of scales drove CO2 levels. The ZiCOS-M network offered significant insights at a cost an order of magnitude lower compared to reference instruments, and the observations generated by ZiCOS-M will be used in additional ICOS Cities activities to conduct CO2 emission inventory validation with inversion modelling systems.

Abstract Recent years have witnessed a surge in nitrate‐driven aerosol pollution across China with N2O5 hydrolysis emerging as a critical formation pathway. Common surface measurements may misleadingly imply this process due to low nighttime ozone at surface level in winter. However, our study reveals a more complex picture by unveiling the vertical dynamics of nitrate formation through an integration of tethered airship campaign, long‐term ground measurements, and model simulations. Interestingly, we observed rapid nitrate growth at approximately 400 m altitude, where the box model revealed optimal conditions for sustained nocturnal nitrate production. The nitrate accumulated overnight in the residual layer (RL) is transported downward in the next morning as the nocturnal boundary layer breaks down, substantially increasing surface‐level nitrate and thus exacerbating pollution. Annual‐averaged diurnal patterns of nitrate measured at the ground station clearly confirm this morning increase. The vertical mixing from the RL contributes up to 80% of the total surface nitrate at 10:00 LT with its influence persisting at 31% even after sunset. Air mass trajectory analysis further confirms that emissions from city‐cluster significantly contribute to downwind pollution by transporting pollutants into the RL. This research underscores the important role of RL chemical processes, facilitated by elevated ozone, in shaping surface‐level pollution. It highlights the indispensability of vertical profiling in understanding aerosol pollution and advocates for regionally coordinated control strategies across eastern China.

Abstract A single-column model is used to investigate regime transitions within the stably stratified atmospheric boundary layer, focusing on the role of small-scale fluctuations in wind and temperature dynamics and of turbulence intermittency as triggers for these transitions. Previous studies revealed abrupt near-surface temperature inversion transitions within a limited wind speed range. However, representing these transitions in numerical weather prediction (NWP) and climate models is a known difficulty. To shed light on boundary layer processes that explain these abrupt transitions, the Ekman layer height and its correlation with regime shifts are analyzed. A sensitivity study is performed with several types of perturbations of the wind and temperature tendencies, as well as with the inclusion of intermittent turbulent mixing through a stochastic stability equation. The effect of small fluctuations of the dynamics on regime transitions is thereby quantified. The combined results for all tested perturbation types indicate that small-scale phenomena can drive persistent regime transitions from very to weakly stable regimes, but for the opposite direction, no evidence of persistent regime transitions was found. The inclusion of intermittency prevents the model from getting trapped in the very stable regime, thus preventing the so-called “runaway cooling,” an issue for commonly used short-tail stability functions. The findings suggest that using stochastic parameterizations of boundary layer processes, either through stochastically perturbed tendencies or parameters, is an effective approach to represent sharp transitions in the boundary layer regimes and is, therefore, a promising avenue to improve the representation of stably stratified atmospheric boundary layers in NWP and climate models.

Abstract We evaluate the use of clumped isotopes of methane (CH4) to fingerprint local atmospheric sources of methane. We focus on a regenerative stormwater conveyance (RSC) stream wetland site running through the University of Maryland campus, which emits methane due to its engineering. Air samples in the RSC were collected at different heights above the surface and at different times of the day including both early in the morning, after methane accumulated below the nocturnal boundary layer, and late in the afternoon when convection mixed air to the cloud layer. Measured Δ12CH2D2 values of air samples record mixing between locally produced methane with low D/H and ambient air. The Δ12CH2D2 of the near surface air collected at the RSC during the early morning ranges from ∼+23‰ to ∼+35‰ which is lower than the ∼+50‰ values of tropospheric air. Mixing between background air (with Δ12CH2D2 ∼+50‰) and methane captured from chamber and bubble samples, as well as produced in incubation (all with negative Δ12CH2D2), explains the observed values of Δ12CH2D2 and Δ13CH3D of near surface RSC air samples. The effect of mixing with biogenic sources on Δ13CH3D is much smaller. The findings demonstrate how methane isotopologues can be used as a tool not only to fingerprint local contributions to these greenhouse gas emissions but also to identify sources of near‐surface methane hot spots.

The nocturnal boundary layer (NBL) significantly influences the dispersion and fate of atmospheric species at night. Subtropical forests are crucial in carbon and water exchange between the biosphere and the atmosphere. However, the NBL characteristics and their impact on atmospheric species over these forests remain unknown. This study conducted vertical measurements of atmospheric species such as O3 and volatile organic compounds (VOCs), along with meteorological variables, over a national forest reserve in Southern China. Results reveal that the NBL height ranged from 180 to 300 m in the summer and from 80 to 160 m in the winter. The vertical distribution of chemical species varied by time and season, with greater concentration gradients observed in the summer. Over 90% of VOCs above the NBL were anthropogenic, while biogenic VOCs were mainly found within the NBL. Higher O3 concentration and VOC product-to-reactant ratios were observed in the residual layer, suggesting enhanced oxidation levels. This unique vertical distribution of atmospheric species at night is driven by factors, such as emission, deposition, turbulence, and atmospheric chemistry, potentially affecting ecosystem functions. Results from this study highlight the importance of incorporating NBL dynamics into atmospheric models to better understand the evolution of chemical species and their ecological effects over forests.

Abstract This study presents the development of a so‐called Turbulent Kinetic Energy (TKE)‐l, or TKE‐l, parameterization of the diffusion coefficients for the representation of turbulent diffusion in neutral and stable conditions in large‐scale atmospheric models. The parameterization has been carefully designed to be completely tunable in the sense that all adjustable parameters have been clearly identified and the number of parameters has been minimized as much as possible to help the calibration and to thoroughly assess the parametric sensitivity. We choose a mixing length formulation that depends on both static stability and wind shear to cover the different regimes of stable boundary layers. We follow a heuristic approach for expressing the stability functions and turbulent Prandlt number in order to guarantee the versatility of the scheme and its applicability for planetary atmospheres composed of an ideal and perfect gas such as that of Earth and Mars. Particular attention has been paid to the numerical stability and convergence of the TKE equation at large time steps, an essential prerequisite for capturing stable boundary layers in General Circulation Models (GCMs). Tests, parametric sensitivity assessments and preliminary tuning are performed on single‐column idealized simulations of the weakly stable boundary layer. The robustness and versatility of the scheme are assessed through its implementation in the Laboratoire de Météorologie Dynamique Zoom GCM and the Mars Planetary Climate Model and by running simulations of the Antarctic and Martian nocturnal boundary layers.

The Arabian Peninsula (AP) is notable for its unique meteorological and climatic patterns and plays a pivotal role in understanding regional climate dynamics and dust emissions. The scarcity of ground-based observations makes atmospheric data essential, rendering reanalysis and satellite products invaluable for understanding weather patterns and climate variability. However, the accuracy of these products in the AP’s desert environment has not been extensively evaluated. This study undertakes the first comprehensive validation of reanalysis products—the European Centre for Medium-Range Weather Forecasts’ European Reanalysis version 5 (ERA5) and ERA5 Land (ERA5L), along with Clouds and Earth’s Radiant Energy System (CERES) radiation fluxes—against measurements from the Liwa desert in the UAE. The data, collected during the Wind-blown Sand Experiment (WISE)–UAE field experiment from July 2022 to December 2023, includes air temperature and relative humidity at 2 m, 10 m wind speed, surface pressure, skin temperature, and net radiation fluxes. Our analysis reveals a strong agreement between ERA5/ERA5L and the observed diurnal T2m cycle, despite a warm night bias and cold day bias with a magnitude within 2 K. The wind speed analysis uncovered a bimodal distribution attributed to sea-breeze circulation and the nocturnal low-level jet, with the reanalysis overestimating the nighttime wind speeds by 2 m s−1. This is linked to biases in nighttime temperatures arising from an inaccurate representation of nocturnal boundary layer processes. The daytime cold bias contrasts with the excessive net radiation flux at the surface by about 50–100 W m−2, underscoring the challenges in the physical representation of land–atmosphere interactions.

The dependence of fluxes on the wind speed near the surface of the stable nocturnal boundary layer has been organized in terms of a threshold wind speed. When the wind speed is smaller than the threshold speed (near-calm conditions), the eddies at the observational level tend to be decoupled from the surface. When the wind speeds are larger than the threshold value, the main eddies are thought to directly interact with the underlying surface. As an example, the threshold wind speed at the 2-m observational level is typically 1 m s-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {s}^{-1}$$\\end{document}. Investigation of the heat flux as a function of the wind speed is relatively straightforward. The dependence of the friction velocity u∗\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$u_*$$\\end{document} on the wind speed requires somewhat different strategies. The friction velocity is derived from the momentum flux averaged in different ways. For near-calm conditions, the stress vector and wind vector are often not aligned. The wind direction frequently varies rapidly with height near the surface for near-calm conditions.

Nighttime oxidation of monoterpenes (MT) via the nitrate radical (NO3) and ozone (O3) contributes to the formation of secondary organic aerosol (SOA). This study uses observations in Atlanta, Georgia from 2011-2022 to quantify trends in nighttime production of NO3 (PNO3) and O3 concentrations and compare to model outputs from the EPA's Air QUAlity TimE Series Project (EQUATES). We present urban-suburban gradients in nighttime NO3 and O3 concentrations and quantify their fractional importance (F) for MT oxidation. Both observations and EQUATES show a decline in PNO3, with modeled PNO3 declining faster than observations. Despite decreasing PNO3, we find that NO3 continues to dominate nocturnal boundary layer (NBL) MT oxidation (FNO3 = 60%) in 2017, 2021, and 2022, which is consistent with EQUATES (FNO3 = 80%) from 2013-2019. This contrasts an anticipated decline in FNO3 based on prior observations in the nighttime residual layer, where O3 is the dominant oxidant. Using two case studies of heatwaves in summer 2022, we show that extreme heat events can increase NO3 concentrations and FNO3, leading to short MT lifetimes (<1 h) and high gas-phase organic nitrate production. Regardless of the presence of heatwaves, our findings suggest sustained organic nitrate aerosol formation in the urban SE US under declining NOx emissions, and highlight the need for improved representation of extreme heat events in chemistry-transport models and additional observations along urban to rural gradients.

Exploring the impact of nocturnal boundary layer stability on wintertime air pollution in a highly polluted basin city using unsupervised learning classification

A segregated approach with either synthetically generated velocity fluctuations or a precursor Large-Eddy Simulation (LES) of the atmospheric boundary layer (ABL) and a subsequent hybrid RANS/LES (HRLS) of the resolved rotor is presented in this paper. At first the approach is validated with synthetically generated velocity fluctuations that resemble neutrally stratified conditions. Subsequently the same turbine is artificially exposed to a stably stratified atmospheric inflow (nocturnal boundary layer) with a comparable mean velocity but different shear and veer characteristics. The HRLS simulations with the neutrally stratified and stably stratified inflow are compared to LESs with an implemented actuator disc (AD) model. The presented work is a first important step towards a digital twin for research wind parks.

Interpretable diurnal impacts on extreme urban PM2.5 concentrations of soil temperature, soil water content, humidity and temperature inversion

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 atmospheric processes that affect the Nocturnal Boundary Layer (NBL) raise unresolved questions, more critical in the case of urban, tropical, and mountainous areas. This research examines the structure of the NBL in the Aburrá Valley - Colombia, a tropical and urbanized region characterized by complex topography. Here six methods were used to estimate the thickness of the NBL, considering the minimum backscattering gradient method (based on ceilometer data) as a reference. Although all the methods contribute to the understanding of the NBL, it was found that the Critical Richardson Number equal to 0,5 fits the best to the reference method, at least for the year 2017, indicating that the tops of the NBL were below the peaks of the surrounding mountains. These results provide technical arguments to consider in managing urban air quality in Valle de Aburrá and other urban, tropical, and mountain areas