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Lower tropospheric thermal structure greatly affects atmospheric boundary-layer (ABL) stability and mixing processes with the free troposphere. In particular, in polluted urban zones, ABL stratification becomes a key variable in air quality research. This study focuses on generating a climatology (1990–2017) of the seasonal variability of ABL thermal structure in Mexico City by way of radiosonde analysis. Thermal inversion intensity and frequency are shown to be greater during winter and spring, a behaviour which coincides with greater pollutant concentrations. Higher concentrations are found during the dry season (November to May) than during the rainy months. In addition, significantly higher than normal surface pollutant concentrations are found on days with simple thermal inversion layers as well as during multilayer inversion days. Furthermore, stable layers, determined by potential temperature, are found throughout the year but more frequently during winter, whereas stable layers based on the virtual potential temperature prevail all year. In regions of complex terrain, such multiple stable layers have also been identified by previous authors. Additionally, the most unstable surface layers (in which the bulk Richardson number ( $${Ri}_{\mathrm {B}}$$ ) is small) develop during the rainy season, whereas during winter there are more levels in the vertical column with higher $${Ri}_{\mathrm {B}}$$ values. Although the Mexico City ABL and pollution episodes have been widely studied, this represents the first long-term investigation to consider the thermal stability of the ABL. Therefore, the present study provides a baseline for further research employing different observational techniques and high-resolution numerical models.

We evaluated the performance of the Weather Research and Forecasting Model in simulating the boundary-layer structure at an urban site in Seoul on two clear summer days against observations made using a ceilometer, a Doppler wind lidar, and a microwave radiometer. The planetary boundary-layer height (PBLH) was estimated from two different methods using observations: the ceilometer-based method (CBM) and bulk Richardson number method (BRM). The maximum PBLH was well captured by the model but PBLH was underestimated in the late afternoon and evening. To examine the cause for underestimation of PBLH in the evening, we compared the wind and virtual potential temperature structure between the simulation and observations. The model captured the timing of the sea breeze well, but it considerably overestimated the intensity of the simulated sea breeze, resulting in an overestimated cold advection by a sea breeze. The effect of the overestimated cold advection on evening PBLH was greater when the sea breeze arrived in the late afternoon and hence the vertical extent of the sea breeze was shallow. This study underscores the importance of accurately simulating the sea breeze for PBLH prediction in the evening, over the inland urban areas under sea breeze penetration.

Entrainment of density flows is a key parameter in constructing predictive models of global climate, ocean flows, and turbidity currents. Our numerical experiment, which included 10 subaqueous dilu...

An open-pool type research reactor is designed and operated considering the accessibility around the pool top area to enhance the reactor utilization. The reactor structure assembly is placed at the bottom of the pool and filled with water as a primary coolant for the core cooling and radiation shielding. Most radioactive materials are generated from the fuel assemblies in the reactor core and circulated with the primary coolant. If the primary coolant goes up to the pool surface, the radiation level increases around the working area near the top of the pool. Hence, the hot water layer is designed and formed at the upper part of the pool to suppress the rising of the primary coolant to the pool surface. The temperature gradient is established from the hot water layer to the primary coolant. As this temperature gradient suppresses the circulation of the primary coolant at the upper region of the pool, the radioactive primary coolant rising up directly to the pool surface is minimized. Water mixing between these layers is reduced because the hot water layer is formed above the primary coolant with a higher temperature. The radiation level above the pool surface area is maintained as low as reasonably achievable since the radioactive materials in the primary coolant are trapped under the hot water layer. The key to maintaining the stable hot water layer and keeping the radiation level low on the pool surface is to have a stable flow of the primary coolant. In the research reactor with a downward core flow, the primary coolant is dumped into the reactor pool and goes to the reactor core through the flow guide structure. Flow fields of the primary coolant at the lower region of the reactor pool are largely affected by the dumped primary coolant. Simple, circular, and duct type discharge headers are designed to control the flow fields and make the primary coolant flow stable in the reactor pool. In this research, flow fields of the primary coolant and hot water layer are numerically simulated in the reactor pool. The heat transfer rate, temperature, and velocity fields are taken into consideration to determine the formation of the stable hot water layer and primary coolant flow. The bulk Richardson number is used to evaluate the stability of the flow field. A duct type discharge header is finally chosen to dump the primary coolant into the reactor pool. The bulk Richardson number should be higher than 2.7 and the temperature of the hot water layer should be 1 °C higher than the temperature of the primary coolant to maintain the stability of the stratified thermal layer.

An internal wave is a propagating disturbance within a stable density-stratified fluid. The internal seiche amplitude is often estimated through theories that describe the amplitude growth based on the Bulk Richardson number (Ri). However, most theoretical formulations neglect secondary effects that may influence the evolution of internal seiches. Since these waves have been pointed out as the most important process of vertical mixing, influencing the biogeochemical fluxes in stratified basins, the wrong estimation may have several impacts on the prediction of the system dynamics. This research paid particular attention to the importance of secondary effects that may play a major role on the basin-scale internal wave amplitude, especially related to the interaction between internal waves and lake boundaries, internal wave depth, and mixing processes due to turbulence. Based on a set of methods, which include auto- and cross-correlations, spectral analysis, and mathematical models, we analyzed the effect of total water depth, wind-resonance, and higher vertical modes on the amplitude growth. We based our analysis on underwater temperature measurements and meteorological data obtained from two small thermally-stratified basins, complemented with numerical simulations. We introduce here a new parametrization which takes into account the total water depth (H), lake length (L), epilimnion thickness ( $$h_e$$ ), as well as the resonance effect. We observed that the rate of amplitude growth decreases compared to linear theory when $$Ri~h_e/L\le 1$$ . In these cases, we suggests that previous theories overestimate the internal seiche amplitude, neglecting the instabilities generated near the wave crest due to weak stability and wave interactions. However, under shallow thermocline conditions, due to extra pressure in the upper layer, the vertical displacement may be higher than that predicted by the linear theory.

Climate models still have deficits in reproducing the surface energy and momentum budgets in Arctic regions. One of the reasons is that currently used transfer coefficients occurring in parameterizations of the turbulent fluxes are based on stability functions derived from measurements over land and not over sea ice. An improved parameterization is developed using the Monin–Obukhov similarity theory (MOST) and corresponding stability functions that reproduce measurements over sea ice obtained during the Surface Heat Budget of the Arctic Ocean (SHEBA) campaign. The new stability functions for the stable surface layer represent a modification of earlier ones also based on SHEBA measurements. It is shown that the new functions are superior to the former ones with respect to the representation of the measured relationship between the MOST stability parameter and the bulk Richardson number. Nevertheless, the functions fulfill the same criteria of applicability as the earlier functions and contain, as an extension, a dependence on the neutral-limit turbulent Prandtl number. Applying the new functions we develop an efficient noniterative parameterization of the near-surface turbulent fluxes of momentum and heat with transfer coefficients as a function of the bulk Richardson number (Rib) and roughness parameters. A hierarchy of transfer coefficients is recommended for weather and climate models. They agree better with SHEBA data for strong stability (Rib > 0.1) than previous parameterizations and they agree well with those based on the Businger–Dyer functions in the range Rib ≤ 0.1.

Thunderstorm overshooting is rare but not an unusual phenomenon in a metropolitan of India, Kolkata (22.57° N; 88.36° E) during the pre-monsoon months (April–May). An attempt is made in this study to identify the important parameters differentiating the thunderstorms in overshooting and non-overshooting categories through data analytics from 2000 to 2015. The present investigation on parametric classification would facilitate in estimating the predictability of thunderstorms with overshooting which subsequently might assist in operational forecast of thunderstorm severity over Kolkata. The altitudes of lifting condensation level (LCL), wind shear, bulk Richardson number (BRN), gust speed, boundary layer characteristics and their correlation with thunderstorm cloud top height (CTH) and also their variation and distribution during overshooting (OTS) and non-overshooting (TS) thunderstorms are analyzed in this study. The result depicts that over Kolkata the intensity of storms during OTS is higher than during TS though the frequency of OTS is less than that of TS. The results further show that the potential temperature (θ), equivalent potential temperature (θe), mixing ratio (es) in the boundary layer, convective available potential energy, convective inhibition energy, BRN and gust speed play significant roles in regulating the CTH during OTS and TS thunderstorms over Kolkata.

Four cases of an overlying inversion imposed on a stable boundary layer are investigated, extending the earlier work of Hancock and Hayden (Boundary-Layer Meteorol 168:29–57, 2018), where no inversion was imposed. The inversion is imposed to one or other of two depths within the layer: midway or deep. Four cases of changed surface condition are also investigated, and it is seen that the surface and imposed conditions behave independently. A change of imposed inversion condition leaves the bottom 1/3 of the layer almost completely unaffected; a change of the surface condition leaves the top 2/3 unaffected. Comparisons are made against two sets of local-scaling systems over the full height of the boundary layer. Both show some influence of the inversion condition. The surface heat flux and the reduction in surface shear stress, and hence the ratio of the boundary-layer height to surface Obukhov length, are determined by the temperature difference across the surface layer (not the whole layer), bringing all cases together in single correlations as functions of a surface-layer bulk Richardson number.

Diurnal variations in the planetary boundary layer height (PBLH) at different latitudes over different surface characteristics are described, based on 45 years (1973−2017) of radiosonde observations. The PBLH is determined from the radiosonde data by the bulk Richardson number (BRN) method and verified by the parcel method and the potential temperature gradient method. In general, the BRN method is able to represent the height of the convective boundary layer (BL) and neutral residual layer cases but has relatively large uncertainty in the stable BL cases. The diurnal cycle of the PBLH over land is quite different from the cycle over ocean, as are their seasonal variations. For stations over land, the PBLH shows an apparent diurnal cycle, with a distinct maximum around 15:00 LT, and seasonal variation, with higher values in summer. Compared with the PBLH over land, over oceans the PBLH diurnal cycles are quite mild, the PBLHs are much lower, and the seasonal changes are less pronounced. The seasonal variations in the median PBLH diurnal cycle are positively correlated with the near-surface temperature and negatively correlated with the near-surface relative humidity. Finally, although at most latitudes the daytime PBLH exhibits, over these 45 years, a statistically significant increasing trend at most hours between 12:00 LT and 18:00 LT over both land and ocean, there is no significant trend over either land or ocean in the nighttime PBLH for almost all the studied latitudes.

Ilustraciones, mapas Esta investigación caracterizó la estructura térmica y dinámica de la capa límite nocturna para el año 2017 en el Valle de Aburrá–Colombia, un área urbana tropical con topografía compleja. Se utilizaron registros provenientes de un radiómetro de microondas, un radar perfilador de vientos, un ceilómetro, estaciones meteorológicas y el modelo WRF-ARW acoplado al modelo de parametrización urbana SLUCM. Este último fue ejecutado con seis esquemas de parametrización distintos de la capa límite atmosférica, para 33 noches distribuidas en el periodo de estudio. Un análisis exploratorio fue ejecutado para identificar procesos espacio-temporales usando variables de estado como los vientos, el número de Richardson aproximado, la temperatura potencial virtual y la intensidad de retrodispersión. Mediante un análisis de sensibilidad de los registros se encontró que el espesor de la capa límite nocturna corresponde a un número de Richardson crítico de 0,5. Además se evaluó el modelo para las horas de la noche y se encontró un desempeño aceptable del esquema de parametrización MYNN. También se identificaron patrones de circulación asociados a un jet de bajo nivel, inversiones térmicas, vientos catabáticos y acoplamiento de los vientos alisios con los vientos orográficos. Se observó que los trimestres junio-julio-agosto y septiembre-octubre-noviembre son más estables dinámicamente, mientras que los trimestres de diciembre-enero-febrero y marzo-abril-mayo lo son más desde el punto de vista estático. Finalmente, se concluye que los espesores de la capa límite nocturna en el Valle de Aburrá son relativamente bajos, con condiciones de velocidades significantes al principio de la noche, pero al final de la noche con velocidades muy cercanas a cero y con una estabilidad atmosférica cada vez fortaleciéndose más por el enfriamiento radiativo. (texto tomado de la fuente) This research characterized the thermal structure and dynamics of the nocturnal boundary layer for the year 2017 in the Aburrá Valley-Colombia, a tropical urban area with complex topography. Records from a microwave radiometer, a wind profiler radar, a ceilometer, meteorological stations, and the WRF-ARW model coupled to the SLUCM urban parameterization model were used. This last one was run with six different atmospheric boundary layer parameterization schemes, for 33 nights distributed in the study period. An exploratory analysis was performed to detect spatio-temporal processes using state variables such as winds, bulk Richardson number, virtual potential temperature and backscatter. Through a sensitivity analysis of the records, it was found that the thickness of the nocturnal boundary layer corresponds to a critical Richardson number of 0,5. In addition, the model was evaluated during night hours and an acceptable performance of the MYNN parameterization scheme was found. Circulation patterns associated with a low-level jet, thermal inversions, katabatic winds and coupling of trade winds with orographic winds were also identified. It was observed that the quarters of june-july-august and september-october-november are more dynamically stable, and whereas those of the december-january-february and march-april-may are more statically stable. Finally, it is concluded that the thicknesses of the nocturnal boundary layer in the Aburrá Valley are relatively low, with significant velocities at the beginning of the night, but at the end of the night with velocities very close to zero and with atmospheric stability becoming increasingly stronger due to radiative cooling. Maestría Magíster en Ingeniería - Recursos Hidráulicos Meteorología urbana y de montañas Área Curricular de Medio Ambiente

Atmospheric stability has a significant effect on wind shear and turbulence intensity, and these variables, in turn, have a direct impact on wind power production and loads on wind turbines. It is therefore important to know how to characterise atmospheric stability in order to make better energy yield estimation in a wind farm. Based on the research-grade meteorological mast at Alaiz (CENER's test site in Navarre, Spain) named MP5, this work compares and evaluates different instrument set-ups and methodologies for stability characterisation, namely the Obukhov parameter, measured with a sonic anemometer, and the bulk Richardson number based on two temperature and one wind speed measurement. The methods are examined considering their theoretical background, implementation complexity, instrumentation requirements, and practical use in connection to wind energy applications. The sonic method provides a more precise local measurement of stability while the bulk Richardson is a simpler, robust and cost-effective technique to implement in wind assessment campaigns. Using the sonic method as a benchmark, it is shown that to obtain reliable bulk Richardson measurements in onshore sites it is necessary to install one of the temperature sensors close to the ground where the temperature gradient is stronger.

Ceilometer measurements of aerosol backscatter profiles have been widely used to provide continuous PBLHT estimations. To investigate the robustness of ceilometer-estimated PBLHT under different atmospheric conditions, we compared ceilometer- and radiosonde-estimated PBLHTs using long term U.S. Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) ceilometer and balloon-borne sounding data at three ARM fixed-location atmospheric observatories and from three ARM mobile observatories deployed around the world for various field campaigns, which cover from Tropics to Polar regions and over both ocean and land surfaces. Statistical comparisons of ceilometer-estimated PBLHTs from the Vaisala CL31 ceilometer data with radiosonde-estimated PBLHTs from the ARM PBLHT-SONDE Value-added Product (VAP) are performed under different atmospheric conditions including stable and unstable atmospheric boundary layer, low-level cloud-free, and cloudy conditions at these ARM observatories. Under unstable atmospheric boundary layer conditions, good comparisons are found between ceilometer- and radiosonde-estimated PBLHTs at ARM low- and mid-latitude land observatories. However, it is still challenging to obtain reliable PBLHT estimations over ocean surfaces even using radiosonde data. Under stable atmospheric boundary layer conditions, ceilometer- and radiosonde-estimated PBLHTs have weak correlations. Among different PBLHT estimations utilizing the Heffter, the Liu-Liang, and the bulk Richardson number methods in the ARM PBLHT-SONDE VAP, ceilometer-estimated PBLHTs have better comparisons with the Liu-Liang method under unstable and with the bulk Richardson number method under stable atmospheric boundary layer conditions.

Publisher Copyright: © Copyright © 2021 Huo, Wang, Paasonen, Liu, Tang, Ma, Petaja, Kerminen and Kulmala. Boundary layer height (BLH) is an important parameter in climatology and air pollution research, especially in urban city. We calculated the BLH with a bulk Richardson number (Ri) method over urban cities of China during 1980–2018 using European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-interim data after carefully validation with sounding data obtained from two meteorology stations in eastern China during 2010–2018. The values of BLH between these two types of data have correlation coefficients in the range of 0.65–0.87, which indicates that it is reasonable to analyze long-term trends of the BLH from ERA data sets. Using ERA-interim calculated BLH, we found that there is an increasing trend in the daytime BLH in most cities of eastern China, particularly during the spring season. A correlation analysis between the BLH and temperature, wind speed, relative humidity and visibility revealed that the variability in meteorological parameters, as well as in aerosol concentrations over highly polluted eastern China, play important roles in the development of the BLH.

We currently have a limited understanding of the spatial and temporal variability in vertically stratified atmospheric layers over Namibia and the southeast Atlantic (SEA) Ocean. Stratified layers are relevant to the transport and dilution of local and long-range transported atmospheric constituents. This study used eleven years of global positioning system radio occultation (GPS-RO) signal refractivity data (2007–2017) over Namibia and the adjacent ocean surfaces, and three years of radiosonde data from Walvis Bay, Namibia, to study the character and variability in stratified layers. From the GPS-RO data and up to a height of 10 km, we studied the spatial and temporal variability in the point of minimum gradient in refractivity, and the temperature inversion height, depth and strength. We also present the temporal variability of temperature inversions and the boundary layer height (BLH) from radiosondes. The BLH was estimated by the parcel method, the top of a surface-based inversion, the top of a stable layer identified by the bulk Richardson number (RN), and the point of minimum gradient in the refractivity (for comparison with GPS-RO data). A comparison between co-located GPS-RO to radiosonde temperature profiles found good agreement between the two, and an average underestimation of GPS-RO to radiosonde temperatures of −0.45 ± 1.25 °C, with smaller differences further from the surface and with decreasing atmospheric moisture content. The minimum gradient (MG) of refractivity, calculated from these two datasets were generally in good agreement (230 ± 180 m), with an exeption of a few cases when differences exceeded 1000 m. The surface of MG across the region of interest was largely affected by macroscale circulation and changes in atmospheric moisture and cloud, and was not consistent with BLH(RN). We found correlations in the character of low-level inversions with macroscale circulation, radiation interactions with the surface, cloud cover over the ocean and the seasonal maximum in biomass burning over southern Africa. Radiative cooling on diurnal scales also affected elevated inversions between 2.5 and 10 km, with more co-occurring inversions observed at night and in the morning. Elevated inversions formed most frequently over the subcontinent and under subsidence by high-pressure systems in the colder months. Despite this macroscale influence peaking in the winter, the springtime inversions, like those at low levels, were strongest.

Recent research has demonstrated that hidden Markov model (HMM) analysis is an effective tool to classify atmospheric observations of the stably stratified nocturnal boundary layer (SBL) into weakly stable (wSBL) and very stable (vSBL) regimes. Here we consider the development of explicitly stochastic representations of SBL regime dynamics. First, we analyze whether HMM-based SBL regime statistics (the occurrence of regime transitions, subsequent transitions after the first, and very persistent nights) can be accurately represented by “freely running” stationary Markov chains (FSMCs). Our results show that despite the HMM-estimated regime statistics being relatively insensitive to the HMM transition probabilities, these statistics cannot all simultaneously be captured by a FSMC. Furthermore, by construction a FSMC cannot capture the observed non-Markov regime duration distributions. Using the HMM classification of data into wSBL and vSBL regimes, state-dependent transition probabilities conditioned on the bulk Richardson number (RiB) or the stratification are investigated. We find that conditioning on stratification produces more robust results than conditioning on RiB. A prototype explicitly stochastic parameterization is developed based on stratification-dependent transition probabilities, in which turbulence pulses (representing intermittent turbulence events) are added during vSBL conditions. Experiments using an idealized single-column model demonstrate that such an approach can simulate realistic-looking SBL regime dynamics.

An inflow generation method for large-eddy simulations of wall-bounded turbulent flows is presented. The method builds upon the work of Munoz-Esparza et al. (“A Stochastic Perturbation Method to Ge...

To investigate the evolution of the nocturnal boundary layer (NBL) and its impacts on the vertical distributions of pollutant particulates, a combination of in situ observations from a large tethered balloon, remote sensing instruments (aerosol lidar and Doppler wind lidar) and an atmospheric environment-monitoring vehicle were utilized. The observation site was approximately 100 km southwest of Beijing, the capital of China. Results show that a considerable proportion of pollutant particulates were still suspended in the residual layer (RL) (e.g., the nitrate concentration reached 30 μg m−3) after sunset. The NBL height calculated by the aerosol lidar was closer to the top of the RL before midnight because of the pollutants stored aloft in the RL and the shallow surface inversion layer, after midnight, the NBL height was more consistent with the top of the surface inversion layer. As the convective mixing layer gradually became established after sunrise the following day, the pollutants stored in the nocturnal RL of the preceding night were entrained downward into the mixing layer. The early morning PM2.5 concentration near 700 m in the RL on 20 December decreased by 83% compared with the concentration at 13:34 on 20 December at the same height. The nitrate concentration also decreased significantly in the RL, and the mixing down of nitrate from the RL could contribute about 37% to the nitrate in the mixing layer. Turbulence activities still existed in the RL with the bulk Richardson number (Rb) below the threshold value. The corresponding increase in PM2.5 was likely to be correlated with the weak turbulence in the RL in the early morning.

The behaviour of the turbulent Prandtl number ( Pr t ) for buoyancy-affected flows near a vertical surface is investigated as an extension study of Gibson & Leslie, Int. Comm. Heat Mass Transfer, Vol. 11, pp. 73–84 (1984). By analysing the location of mean velocity maxima in a differentially heated vertical planar channel, we identify an infinity anomaly for the eddy viscosity ν t and the turbulent Prandtl number Pr t , as both terms are divided by the mean velocity gradient according to the standard definition, in vertical buoyant flow. To predict the quantities of interest, e.g. the Nusselt number, a machine learning framework via symbolic regression is used with various cost functions, e.g. the mean velocity gradient, with the aid of the latest direct numerical simulation (DNS) dataset for vertical natural and mixed convection. The study has yielded two key outcomes: ( i ) the new machine learnt algebraic models, as the reciprocal of Pr t , successfully handle the infinity issue for both vertical natural and mixed convection; and ( i i ) the proposed models with embedded coordinate frame invariance can be conveniently implemented in the Reynolds-averaged scalar equation and are proven to be robust and accurate in the current parameter space, where the Rayleigh number spans from 10 5 to 10 9 for vertical natural convection and the bulk Richardson number R i b is in the range of 0 and 0.1 for vertical mixed convection.

Gustiness is examined for the wind speed, fluctuations, turbulence intensities and fluxes for a real urban topography. Using large-eddy simulation (LES) and an ensemble sampling approach, which allows a more comprehensive characterisation of the urban morphological features, a wide range of boundary-layer stabilities is considered: the bulk Richardson number, Rb ​∈ ​[−0.41, 0.82]. Ratios of the proposed gustiness statistics, G , over the conventional time-averaged flow and turbulence statistics are maximised for z/Have ​≲ ​1 (where Have is the mean building height). The strong linear scaling of G with the plan-area index (λp) for neutral stratification is found to persist for stably- and unstably-stratified flows (R2 ~ 0.8). By contrast, the non-dimensionalised building-height variability, σH/Have, and the effective frontal-area index, λ f ^ ≡ λ f H a v e / σ H , are argued to be of more appropriateness as scaling parameters for G compared to their original forms, σH and λf. While the sensitivity of G to Rb is well defined at greater heights, the influence of surface inhomogeneity may be strong enough to oppose the effect of thermal stratification in the lower surface layers. Qualitative differences in the sensitivities to the boundary-layer stability are narrowly distinguishable amongst the zeroth-, first- and second-order gustiness statistics. The results are relevant to the understanding of urban wind hazards in thermally-stratified boundary layers.

In this study, a climatology of two key boundary layer features, the Planetary Boundary Layer Height (PBLH) and the wind field over Greece is derived. The climatology is based on daily soundings collected in Athens, Thessaloniki and Heraklion and spanning a 32-year period. The PBLH is estimated using a method based on the gradient of potential temperature and a method based on the bulk Richardson number. The wind field is analyzed by calculating the wind shear and the turning angle of the wind vector between the surface and the top of the boundary layer. The PBLH of the daytime boundary layer over Athens and Thessaloniki is found to exhibit seasonal variability with summer maxima and winter minima and has annual median values in the range of 1.4&ndash, 1.7 km estimated using the gradient method. The PBLH over Heraklion is found to exhibit weak seasonal variability with a lower median value of 1.2 km. The nighttime boundary layer over all three sites is found to be much shallower with PBLH values in the range of 150&ndash, 200 m with no seasonal variations. In addition, the bulk Richardson number method is found to systematically underestimate the PBLH compared to the gradient method. The wind field in the daytime boundary layer at all three sites is found to have small shear of the order of 1 ms&minus, 1 and wind turning angles that are lower than 15 degrees, while in the nocturnal boundary layer it has larger shear of the order of 5&ndash, 10 ms&minus, 1 with turning angles lower than 20 degrees. In addition, for both the daytime and the nighttime boundary layer there is no general preference for veering or backing.

The accurate prediction of wildfire behavior and spread is possible only when fire and atmosphere simulations are coupled. In this work, we present a mechanism that causes a small fire to intensify by altering the atmosphere. These alterations are caused by fire-related fluxes at the surface. The fire plume and fluxes increase the convective available potential energy (CAPE) and the chance of the development of a strong pyroconvection system. To study this possible mechanism, we used WRF-Fire to capture fire line propagation as the result of interactions between heat and moisture fluxes, pressure perturbations, wind shear development and dry air downdraft. The wind patterns and dynamics of the pyroconvection system are simulated for the Horse River wildfire at Fort McMurray, Canada. The results revealed that the updraft speed reached up to 12 m/s. The entrainment mixed the mid and upper-level dry air and lowered the atmospheric moisture. The mid-level and upper-level dew point temperature changed by 5&ndash, 10 ∘ C in a short period of time. The buoyant air strengthened the ascent as soon as the nocturnal inversion was eliminated by daytime heating. The 887 J/kg total increase of CAPE in less than 5 h and the high bulk Richardson number (BRN) of 93 were indicators of the growing pyro-cumulus cell. The presented simulation has not improved the original model or supported leading-edge numerical weather prediction (NWP) achievements, except for adapting WRF-Fire for Canadian biomass fuel. However, we were able to present a great deal of improvements in wildfire nowcasting and short-term forecasting to save lives and costs associated with wildfires. The simulation is sufficiently fast and efficient to be considered for a real-time operational model. While the project was designed and succeeded as an NWP application, we are still searching for a solution for the intractable problems associated with political borders and the current liable authorities for the further development of a new generation of national atmosphere&ndash, wildfire forecasting systems.

The calculation of the near-surface turbulent fluxes of energy and momentum in climate and weather prediction models requires transfer coefficients. Currently used parametrizations of these coefficients are based on stability functions derived from measurements over land and not over sea ice. However, recently, a non-iterative parametrization has been proposed by Gryanik and Lüpkes (2018), which can be applied to climate and weather prediction models as well but uses stability functions of Grachev et al. (2007). These functions had been obtained from measurements during the Surface Heat Budget over the Arctic Ocean campaign (SHEBA) and thus from measurements over sea ice. A drawback of the scheme of Gryanik and Lüpkes (2018) is that there is still some complexity due to the complexity of the SHEBA based functions.Thus new stability functions are proposed for the stable boundary layer, which are also based on the SHEBA measurements but avoid the complexity. It is shown that the new functions are superior to the former ones with respect to the representation of the measured relationship between the Obukhov length and the bulk Richardson number. Moreover, the resulting transfer coefficients agree slightly better with the SHEBA observations in the very stable range. Nevertheless, the functions fulfill the same criteria of applicability as the earlier functions and contain furthermore as an extension a dependence on the neutral Prandtl number. Applying the new functions, an efficient non-iterative parametrization of the near-surface turbulent fluxes of momentum and heat is developed where transfer coefficients result as a function of the bulk Richardson number (Rib) and roughness parameters. The new transfer coefficients, which are recommended for weather and climate models, agree well with the SHEBA data in a large range of stability (0< Ribb ReferencesGrachev A.A., Andreas E.L, Fairall C.W., Guest P.S., Persson POG (2007) Boundary-Layer Meteorol., 124, 315–333.Gryanik, V.M. and Lüpkes C. (2018) An Efficient Non-iterative Bulk Parametrization of Surface Fluxes for Stable Atmospheric Conditions Over Polar Sea-Ice,Boundary-Layer Meteorol 166:301-325

Atmospheric stability has been studied for decades. There are several methodologies that evolved over the years. In this study, a special experimental meteorological mast that has been erected to a complex site has been used to calculate dimensionless Obukhov length ( &zeta, = z L ) , dimensionless momentum ( &phi, m ), and heat coefficients ( &phi, h ). The results are compared with the ones from average value approaches: Richardson number, flux-profile (F-P) relations, and wind shear exponent methods. The results show that the estimated &zeta, values, using the bulk Richardson number, get along well with the reference &zeta, within the neutral and stable regimes. F-P relations and wind shear exponent methods result in the best agreement for stable and neutral regimes. Nevertheless, average oriented methods are not reliable for the other regimes.

Micrometeorological observations from a tower, an eddy-covariance (EC) station and an unmanned aircraft system (UAS) at the WINSENT test-site are used to validate a computational fluid dynamics (CFD) model, driven by a mesoscale model. The observation site is characterised by a forested escarpment in a complex terrain. A two-day measurement campaign with a flow almost perpendicular to the escarpment is analysed. The first day is dominated by high wind speeds, while, on the second one, calm wind conditions are present. Despite some minor differences, the flow structure, analysed in terms of horizontal wind speeds, wind direction and inclination angles shows similarities for both days. A real-time strategy is used for the CFD validation with the UAS measurement, where the model follows spatially and temporally the aircraft. This strategy has proved to be successful. Stability indices such as the potential temperature and the bulk Richardson number are calculated to diagnose atmospheric boundary layer (ABL) characteristics up to the highest flight level. The calculated bulk Richardson values indicate a dynamically unstable region behind the escarpment and near the ground for both days. At higher altitudes, the ABL is returning to a near neutral state. The same characteristics are found in the model but only for the first day. The second day, where shear instabilities are more dominant, is not well simulated. UAS proves its great value for sensing the flow over complex terrains at high altitudes and we demonstrate the usefulness of UAS for validating and improving models.

s The release of airborne hazardous substances in the urban roughness sublayer (RSL) is a risk to human health. The atmospheric dispersion of these materials should be contained to mitigate their adverse consequences. This study investigates the effects of atmospheric stability on the air, heat, and pollutant transport within RSL by a validated 3-D RANS code. A building morphology with height variance is proposed for better representing the real urban environment and characterizing the complex interactions between the roughness elements and the flow regimes. The thermal stratification stability test subject to the ground-inflow temperature difference is extended to span from strongly unstable to moderately stable. The quantitative indicators, including air exchange rate, heat removal rate, pollutant removal rate, heat transfer coefficient, and pollutant transfer coefficient, are introduced and analyzed. The correlations between indicators and thermal stabilities are established, which provides explicit expressions to describe the influence of the changing bulk Richardson number (Rb). Results show that the design of height-variance elements enables a stronger lateral momentum towards the target street canyon, which suppresses the spanwise dispersion of pollutants. The discussions upon heat and pollutant transport based on stability categories show a high Rb-dependence of heat/mass transfer efficiency. The stability threshold is demonstrated to be Rb ≈ 0.7, where the flow speed near the ground approaches zero. As a result, the high temperature gradient is formed and acts as positive feedback to facilitate a more stratified condition. The accurate and rational correlations obtained in this study will save the computational cost considerably for further research. Also, the available results will be a reference for environmentally friendly designs.

The objective of this chapter is to perform laboratory and direct numerical modeling of turbulent wind over water surface under stable stratification conditions. Laboratory and numerical experiments are performed under the same bulk Reynolds and Richardson numbers which allow a direct comparison between the measurements and calculations. The laboratory experiments are performed in a wind-wave flume on the basis of a thermostratified tank facility at IAP RAS. A sufficiently strong stable stratification (with the air–water temperature difference of up to 18 K) and a comparatively large bulk Richardson number (up to Ri ≈ 0.04) in the experiment are created by heating the incoming air flow while maintaining a relatively low wind speed (up to 3 m/s) and the corresponding bulk Reynolds number up to Re ≈ 60000. The air velocity field is retrieved by employing both contact (Pitot tube) and PIV methods, and the air temperature profile is measured simultaneously by a set of contact probes. The same bulk Ri and Re are prescribed in direct numerical simulation where turbulent Couette flow is considered as a model of the near water constant stress atmospheric boundary layer. The mean velocity and temperature profiles obtained in our laboratory and numerical experiments agree well and also are well predicted by the Monin–Obukhov similarity theory. The results show that sufficiently strong stratification, although allowing a statistically stationary turbulent regime, leads to a drastic reduction of both turbulent momentum and heat fluxes. Under this regime, the flow turbulent Reynolds number (based on the Obukhov length scale and friction velocity) is found to be in agreement with known criteria characterizing stationary strongly stratified turbulence.