Mixed Boundary Layer Research Articles

Error induced by neglecting subgrid chemical segregation due to inefficient turbulent mixing in regional chemical-transport models in urban environments

Abstract. We employed direct numerical simulations to estimate the error on chemical calculation in simulations with regional chemical-transport models induced by neglecting subgrid chemical segregation due to inefficient turbulent mixing in an urban boundary layer with strong and heterogeneously distributed surface emissions. In simulations of initially segregated reactive species with an entrainment-emission configuration with an A–B–C second-order chemical scheme, urban surface emission fluxes of the homogeneously emitted tracer A result in a very large segregation between the tracers and hence a very large overestimation of the effective chemical reaction rate in a complete-mixing model. This large effect can be indicated by a large Damköhler number (Da) of the limiting reactant. With heterogeneous surface emissions of the two reactants, the resultant normalised boundary-layer-averaged effective chemical reaction rate is found to be in a Gaussian function of Da, and it is increasingly overestimated by the imposed rate with an increased horizontal scale of emission heterogeneity. Coarse-grid models with resolutions commensurable to regional models give reduced yet still significant errors for all simulations with homogeneous emissions. Such model improvement is more sensitive to the increased vertical resolution. However, such improvement cannot be seen for simulations with heterogeneous emissions when the horizontal resolution of the model cannot resolve emission heterogeneity. This work highlights particular conditions in which the ability to resolve chemical segregation is especially important when modelling urban environments.

Process-based assessment of the impact of reduced turbulent mixing on Congo Basin precipitation in the RCA4 Regional Climate Model

In regions featuring strong convective activity (such as the Congo Basin, CB), turbulent mixing in the planetary boundary layer strongly affects the water budget. In this study, we use a process-based evaluation to assess the performance of the Rossby Centre Regional Climate Model (RCM) RCA4 in simulating the September–November CB rainfall, under conditions of strong and weak turbulent mixing. To this regard, results from two different versions of model are analysed: the version used in the COordinated Regional climate Downscaling EXperiment framework (RCA4-v1), and a modified version (RCA4-v4), in which turbulent mixing is reduced. Experiments are driven with boundary conditions from the ERA-Interim reanalysis. Results show that RCA4-v4 improves the CB rainfall climatology compared to RCA4-v1. This result is further related to the models’ different representations of the relevant driving mechanisms and processes. The model version with a reduced turbulent mixing (RCA4-v4) shifts less moisture from the lower troposphere towards the free troposphere. As the shallow convective mixing is reduced (owing to the reduction of the turbulent mixing), lower layers are moistened, and low level cloud fraction increases over Equatorial Africa. This increase is stronger over the West Equatorial African (WEA) coast than over the CB. The result is that surface solar radiation decreases more over the WEA coast than over the CB, which would result in a lower surface temperature over WEA coast than over the CB. An enhanced pressure gradient between the WEA and the CB is created as a result, thus enhancing the Congo low level cell, and low level westerlies (LLWs). LLWs are faster, meaning that more moisture flows through the CB Cell, is uplifted in eastern up-branch, and enters African Easterly Jets (AEJs), which, in turn, are intensified due to the increase in the surface temperature gradient. Intensification of the CB cell and mesoscale convective systems (MCSs) is the cause of the higher rainfall and is what improves the CB rainfall climatology in RCA4-v4. In addition, the increase in rainfall causes an increase in soil moisture in RCA4-v4 in both the north and south of the CB. Higher soil moisture does not affect evaporation in the north as soils are already saturated in RCA4-v1. However, the increase in rainfall increases soil moisture in the south in RCA4-v4, which increases evaporation as soils were initially unsaturated. This higher evaporation is exported out of the basin towards Southern Africa, does not recirculate through the Cell, and does not therefore contribute to further improving the rainfall bias over the Congo. Our results show that reducing turbulent mixing results in a better representation of the dynamics of the climate system over the CB and, in turn, improved precipitation.

Performance Augmentation of Solar Air Heater for Space Heating Using a Flexible Flapping Guide Winglet

A new idea is presented in this paper for improving the performance of solar air heater (SAH) designed for space heating by employing a thin flexible guide winglet. In addition to the role of winglet in pushing the convective airflow toward the heated surface, it behaves as a vortex generator (VG) due to its vibration by fluid-solid interaction (FSI) that causes flow mixing and breaking thermal boundary layer. In flow simulation, the finite element method (FEM) is employed with considering a two-way strongly-coupled FSI approach at transient condition. Numerical solution of the governing equations, including the continuity, momentum and energy for convective flow and the equation of motion for VG are obtained by COMSOL multi-physics. The well-known model is employed for computation of turbulent stress and heat flux. The present numerical results are validated against the most recent relevant literature. To provide a clear and deep understanding of the proposed concept, extensive comparisons are made between different test cases. Results reveal considerable performance enhancement of SAH with elastic guide winglet compared with clean solar air heater (CSAH), such that 56% increase in the natural airflow rate and 9% decrease in the average absorber temperature is seen because of the flapping winglet. But, the air outlet temperature decreases about 14% due to flapping VG. This study aims to make the proposed SAH as an essential renewable thermal-solar system more efficient and attractive so that this improvement pushes the industrial society toward more sustainable infrastructure.

Study on the impact of three Asian industrial regions on PM<sub>2.5</sub> in Taiwan and the process analysis during transport

Abstract. The outflow of the East Asian haze (EAH) has attracted much attention in recent years. For downstream areas, it is meaningful to understand the impact of crucial upstream sources and the process analysis during transport. This study evaluated the impact of PM2.5 from the three largest industrial regions on the Asian continent, namely the Bohai Rim industrial region (BRIR), Yangtze River Delta industrial region (YRDIR), and Pearl River Delta industrial region (PRDIR), in Taiwan and discussed the processes during transport with the help of air quality modeling. The simulation results revealed that the contributions of monthly average PM2.5 from BRIR and YRDIR were 0.7–1.1 and 1.2–1.9 µg m−3 (∼ 5 % and 7.5 % of the total concentration) in Taiwan, respectively, in January 2017. When the Asian anticyclone moved from the Asian continent to the western Pacific, e.g., on 9 January 2017, the contributions from BRIR and YRDIR to northern Taiwan could reach daily averages of 8 and 11 µg m−3. The transport of EAH from BRIR and YRDIR to low-latitude regions was horizontal advection (HADV), vertical advection (ZADV), and vertical diffusion (VDIF) over the Bohai Sea and East China Sea. Over the Taiwan Strait and the northern South China Sea, cloud processes (CLDS) were the major contribution to PM2.5 due to a high relative humidity environment. Along the transport from high-latitude regions to low-latitude regions, aerosol chemistry (AERO) and dry deposition (DDEP) were the major removal processes. When the EAH intruded into northern Taiwan, the major processes for the gains of PM2.5 in northern Taiwan were HADV and AERO. The stronger the EAH, the more the EAH could influence central and southern Taiwan. Although PRDIR is located downstream of Taiwan under northeasterly wind, the PM2.5 from PRDIR could be lifted upward above the boundary layer, allowing it to move eastwards. When the PM2.5 plume moved over Taiwan and was blocked by mountains, PM2.5 could be transported downward, via boundary layer mixing (VDIF), as it was further enhanced by the passing cold surge. In contrast, for the simulation of July 2017, the influence from the three industrial regions was almost negligible unless there was a special weather system, such as thermal lows which may have carried pollutants from PRDIR to Taiwan, but this occurrence was rare.

Using Tracer Variance Decay to Quantify Variability of Salinity Mixing in the Hudson River Estuary

AbstractThe salinity structure in an estuary is controlled by time‐dependent mixing processes. However, the locations and temporal variability of where significant mixing occurs is not well‐understood. Here we utilize a tracer variance approach to demonstrate the spatial and temporal structure of salinity mixing in the Hudson River Estuary. We run a 4‐month hydrodynamic simulation of the tides, currents, and salinity that captures the spring‐neap tidal variability as well as wind‐driven and freshwater flow events. On a spring‐neap time scale, salinity variance dissipation (mixing) occurs predominantly during the transition from neap to spring tides. On a tidal time scale, 60% of the salinity variance dissipation occurs during ebb tides and 40% during flood tides. Spatially, mixing during ebbs occurs primarily where lateral bottom salinity fronts intersect the bed at the transition from the main channel to adjacent shoals. During ebbs, these lateral fronts form seaward of constrictions located at multiple locations along the estuary. During floods, mixing is generated by a shear layer elevated in the water column at the top of the mixed bottom boundary layer, where variations in the along channel density gradients locally enhance the baroclinic pressure gradient leading to stronger vertical shear and more mixing. For both ebb and flood, the mixing occurs at the location of overlap of strong vertical stratification and eddy diffusivity, not at the maximum of either of those quantities. This understanding lends a new insight to the spatial and time dependence of the estuarine salinity structure.

A high-resolution prediction system for birch pollen in Sapporo

This study aims to develop a high-resolution forecast model for airborne birch pollen concentration in Sapporo. Using results that shows that the bulk of pollen grains was deposited near the source plant, we created a high-resolution birch tree density map from a field survey along public roads and footpaths and Street View on Google Maps. The map was linearly extrapolated to cover an area 10 km from the city centre, including forests in the south and southwest. A new atmospheric dispersion deposition model for birch pollen was developed to represent wind advection, turbulent mixing, and convection in the atmospheric boundary layer, and gravitational settling and scavenging with precipitation. Pollen emission was also modelled as a function of the birch tree density and surface air temperatures. The onset of the pollen season was based on the pollen observations by a sampler at Hokkaido Institute of Public Health in Sapporo, and the end of pollen season was assumed to be 40 days after its onset. Comparison with observations suggested that the hindcast simulation with the model reproduced the daily variations in the dry deposition well in most years from 2001 to 2011. The geographical distribution of birch pollen deposition was highly correlated with the birch tree density map, even on windy days.

Numerical analysis of the flow and heat transfer characteristics in serpentine microchannel with variable bend amplitude

PurposeThis study aims to propose the increase of heat dissipation requirements of modern electronic equipment and the fast development of micro-scale manufacturing technologies. The heat transfer mechanism is studied in-depth, especially for its pattern of secondary flow caused by the repeated inversion of centrifugal force. Effects of η on the frictional pressure drop and average Nusselt number are studied and the performance of such microchannel heat sink with various bend amplitudes is comprehensively evaluated. These results can provide important insight into the optimal design of this novel design configuration for microelectronics cooling.Design/methodology/approachA three-dimensional model based on the finite volume approach and SIMPLEC algorithm is performed to test an innovative serpentine microchannel, which behaves differently from conventional serpentine microchannel due to the significant effect of centrifugal force inversion.FindingsThe effect of centrifugal force significantly influences the flow and thermal fields which are responsible for the enhancement in heat transfer coefficient. The number, size and intensity of vortices increase with increasing Re, and the vortices are reformed at every change of the geometry in a periodic fashion. The serpentine microchannel studies more effectively at larger bend amplitude. Pressure fluctuations and temperature variation are greater with increasing bend amplitude.Practical implicationsSeveral techniques have been developed to augment single-phase convective heat transfer in channels. One technique is to use a serpentine channel that enhances the heat transfer due to flow mixing and periodic interruption of thermal boundary layers. This technique has been applied to micro-heat exchangers, thermal regenerators and mini/microreactors.Social implicationsThe optimal design of this novel design configuration for microelectronics cooling can be attained. It will become an effective cooling technology for solving the increasing of heat dissipation requirements of modern electronic equipment.Originality/valueThe flow and heat transfer characteristics are first presented for the circular serpentine microchannel made up of alternate U-bends without interposed straight segments. The present study first examines the effect of such centrifugal force inversion on velocity contour, pressure distribution and temperature distribution. The patterns of secondary flow along the flow passage caused by the repeated inversion of centrifugal force are further studied in depth. The effect of bend amplitude on the flow and heat transfer is explored and the performance of such microchannel heat sink has been comprehensively evaluated.

Исследование сезонной и суточной изменчивости концентраций приземного озона

The results of the comparison of seasonal and daily variability of surface ozone (О3) and nitrogen dioxide (NO2) at urban monitoring stations in Moscow (Mosekomonitoring), Berlin, and Warsaw (European Environment Agency) for 2017-2018 are presented. Observational data indicate a leading role of vertical mixing in the atmospheric boundary layer in the formation of seasonal and daily regimes of surface ozone. Both seasonal and diurnal variations in О3 and NO2 are in good agreement at all stations. In the warm season, the anthropogenic factor becomes a key one for the formation of the daily maximum and increased levels of surface ozone under favorable meteorological conditions for active photochemical processes. The frequency of occurrence of high ozone values is determined, and an episode of atmospheric ozone values dangerous for public health is analyzed. Keywords: meteorological conditions, surface ozone, seasonal variation, diurnal variation, episodes of high ozone values Tab. 1. Fig. 3. Ref. 19.

Performance enhancement of a two-phase closed thermosyphon with a vortex generator

This study focuses on improving the heat transfer performance of a two-phase closed thermosyphon by altering the geometry of a typical straight pipe thermosyphon body. Here, a vortex generating obstacle positioned in the pipe body augments the flow path of the liquid-vapour fluid mixture. This flow augmentation induces a thermal boundary layer mixing effect, which enhances the convection heat transfer within the system under thermal loading. A multiphase numerical simulation model is developed to simulate the fluid phase change and wall temperature distribution of a two-phase closed thermosyphon. The Lee model is adapted to calculate mass and energy transfer source terms during the condensation and evaporation phase-change processes, while the Volume of Fluid method is employed to track liquid-vapour interface movement. The analysis of the new design reveals that the vortex generator has the most significant impact on average wall temperature distribution and overall thermal resistance when it is positioned in the adiabatic section and condenser section of the thermosyphon body. The reduction in thermal resistance is 3.5% and 3.3% when positioned in these two sections, respectively. This reduction corresponds to a higher velocity of the liquid-vapour mixture flow in these regions, indicating that thermal boundary layer mixing can be most enhanced in regions where the liquid-vapour mixture flow velocity has a higher magnitude.

Visualization and characterization of thermals in a heated turbulent boundary layer

This research study investigated the three-dimensional nature and characteristics of buoyant thermals in the mixed convection turbulent boundary layer. Thermals were visualized using a multi-plane flow visualization technique. Experiments were performed in a variety of conditions where the bulk Richardson number was varied to cover a range describing both inertia dominant flow (RiL=0.05) and buoyancy dominant flow (RiL=2.0). Qualitative results define key interactions between thermals and the adjacent turbulent boundary layer flow. A robust algorithm for automatic thermal detection was developed and validated. This algorithm was then applied to experimental images to detect thermals and facilitate a quantitative analysis on the size and spatial distribution of buoyant thermals. Based on these findings an overall flow structure is proposed featuring convective cell like patterns at high Richardson numbers. A first approximation on the shape of thermals and velocity of key thermofluid interactions between thermals and turbulent boundary layer is made. Results indicate these thermal interactions can exhibit motion substantially different from the surrounding turbulent boundary layer flow.

Radiation and Mixed Convection Magnetohydrodynamics Boundary Layer of Hybrid Cu–Al2O3/Water Nanofluid Flow Over a Wall Jet

The mixed convection MHD boundary layer flow of the hybrid nanofluid (copper–titanium dioxide/water) over a permeable wall jet surface subject to thermal radiation was studied with the effects of suction/injection and thermal energy. The governing PDEs were converted into non-linear ODEs by using the proper similarity transformations. Then, these equations were numerically solved applying the method of Chebyshev pseudo spectral differentiation matrix (ChPDM). The results were deduced for the flow field, temperature distribution, reduced skin friction coefficient and reduced Nusselt number. It was found that the hydrodynamic and thermal boundary layers decreases and increases, respectively, as the strength of the magnetic field increases. Further, in the industrial applications, the normal angle is to be applied if low velocity and high temperature is needed. Moreover, as the mixed convection parameter rises, the velocity and temperature increases and decreases, respectively. In addition, it was mentioned that the radiation parameter can control the thermal boundary layer. Finally, the Cu-nanoparticles volume fraction plays a very important role in the velocity variation.

The sensitivity of a depth-coordinate model to diapycnal mixing induced by practical implementations of the isopycnal tracer diffusion scheme

The isopycnal tracer diffusion scheme is commonly used in a coarse-resolution global ocean model to parameterize the diffusive effect of mesoscale eddy stirring. This scheme implemented in a depth-coordinate model often accompanies a fail-safe system to prevent numerical instability caused by large isopycnal diffusion around steeply sloped isopycnal surfaces. Its practical implementation artificially induces spurious diapycnal mixing across an isopycnal surface whose slope exceeds a prescribed slope maximum. This slope maximum widely ranges from 1/1000 to 3/10 in a recent global ocean model intercomparison project of Coordinated Ocean-ice Reference Experiments phase II. This study quantitatively investigates the effects of isopycnal tracer diffusion on meridional overturning circulation by calculating water mass transformation rate due to isopycnal diffusion with the use of Walin’s (1982) methodology. We focus on the following three effects of isopycnal diffusion in a depth-coordinate model: densification induced by nonlinearity of the equation of state, spurious diapycnal mixing in the interior ocean induced by practical implementation of the isopycnal diffusion scheme, and diapycnal mixing in the surface diabatic layer. It is revealed that a slope maximum that is too small (1/1000) leads to large spurious diapycnal mixing in the interior ocean. This diapycnal mixing supplies much buoyancy to the bottom water around Antarctica and makes it locally upwell within the Southern Ocean. It results in less northward export of bottom waters from the Southern Ocean into the Atlantic and the Indo-Pacific oceans and weakening of the bottom cell of meridional overturning circulation. This spurious diapycnal mixing is significantly suppressed if we use the slope maximum of more than 1/100 or tapering isopycnal diffusivity around steep isopycnal slopes.This study also shows that these changes in the implementation of the isopycnal diffusion scheme also affect the various aspects of the simulated Southern Ocean. The experiment tapering isopycnal diffusivity suppresses unrealistic open ocean deep convections and polynyas in the Weddell Sea by both diminishing spurious diapycnal mixing in the interior ocean and applying the strong diapycnal mixing in the surface boundary layer associated with the isopycnal diffusion scheme. It results in the smallest biases of the potential temperature distribution, the winter sea ice concentration and mixed layer depth in the Weddell Sea and the ACC transport in our coarse resolution model.

Mixing Characteristics and Enhancement of Sonic Elliptic Jets from a Twin Elliptic Orifice

The knowledge of jet mixing and its enhancement of elliptic jet are important in a propulsion system of aircraft, rocket, and missile’s system design for advancement of combustion via fuel-air mixture increment, lowering the jet noise and reduction of the plume infrared (IR) signature. The jet issuing from a twin elliptic orifice is non-uniform in shape that promotes the faster mixing and it influences by orifice exit conditions, so knowledge of absence of boundary layer and jet mixing characteristics is important. Hence, an experimental work helps to study the jet mixing for a twin elliptic orifice of aspect ratio two at nozzle pressure ratios of one, two, and three. The proximity between the orifices kept as one to 3mm in steps of one. The experimental readings were taken using pitot probe. The results revealed that jet mixing is faster and effective when the proximity between the orifices is closer to each other than the faraway distances at measured nozzle pressure ratios. Difference in orifice jet core exerted a noticeable influence at high proximity levels of nozzle pressure ratio of three and four for elliptic orifice.

Meteorological Downscaling with WRF Model, Version 4.0, and Comparative Evaluation of Planetary Boundary Layer Schemes over a Complex Coastal Airshed

AbstractEvaluation of downscaled meteorological information is crucial to identifying model behaviors that may propagate to end applications such as the simulation of local air quality. This study conducted and assessed yearlong simulations of hourly meteorological conditions over the Terrace–Kitimat Valley of northwestern British Columbia, Canada, at 1-km horizontal gridding for six PBL schemes in the Weather and Forecasting (WRF) Model, version 4.0. In terms of key surface meteorological variables that affect air quality, simulations over land demonstrated better skill for specific humidity and wind direction than for air temperature and wind speed. Spatial differences in modeled atmospheric properties and vertical profiles, especially for moisture content, were used to diagnose the relative capacity of each PBL scheme to represent pollutant dispersion and dilution. Stable conditions at night increased suppression of boundary layer mixing by the nonlocal Yonsei University (YSU) scheme when compared with suppression by the local eddy-diffusion component of the Asymmetric Convective Model, version 2 (ACM2), scheme, resulting in decreased wind speed and ambient temperature but moister air with the YSU scheme. The weakening of mixing by the Mellor–Yamada–Nakanishi–Niino (MYNN3) scheme with inland distance suggested that higher-order, nonlocal transport is sensitive to increasing topographic steepness toward the northern part of the valley. Disparities in mixing strengths among PBL schemes were greater in the summer when conditions were generally less stable with moist, warm air blowing inland than in winter when the valley channels cold, stable air from the interior. Increased convection in daytime led to greater entrainment of air from aloft and a thicker PBL with the YSU scheme than with the ACM2 scheme in summer while increasing countergradient transport in the MYNN3 scheme that reduces dilution.

On the characteristics and climate effects of HV-WCP events over the Kuroshio SST front during wintertime

Neglected High-frequency Variability events of the sea surface Wind Coupled with Precipitation (HV-WCP) events are revealed over the Kuroshio Sea Surface Temperature (SST) front in the East China Sea (ECS) during wintertime. This phenomenon is characterized as modulation events of the surface wind directions, whereby the southerly anomaly changes into a northwest anomaly, accompanied with increasing wind speed, and coupled with decreasing precipitation from morning to night on the day with the peak variability. The events are associated with significant climate effects on the rainfall anomaly over southeastern China and adjacent ocean areas of the ECS. To sustain these events, the large-scale circulation needs to exhibit declines in the upstream monsoon and weakening vertical mixing in the planetary boundary layer (PBL). The Kuroshio thermal effect on the underlying atmosphere becomes prominent due to weakening vertical mixing, which results in pronounced PBL height gradient over the Kuroshio SST front. More plentiful moisture transported from PBL to the free atmosphere, generates the initialization of rainfall and the cyclone anomaly. Meanwhile, the latent heat releasing caused by this precipitation forces positive vorticity anomaly within the PBL, which again strengthens the cyclone and the upward moisture transport. Therefore, a complement between local circulation and rainfall is developed over the Kuroshio region. Due to the significant effect of the Kuroshio SST front on the PBL structure, the HV-WCP intensity is able to maintain its strength over the Kuroshio region. However, because of the northwestern background monsoon, the HV-WCP events vanish quickly when the coupled system moves out of Kuroshio SST frontal region.

COEFFICIENTS OF EDDY DIFFUSION OF MOMENTUM AND HEAT IN ATMOSPHERIC BOUNDARY LAYER: NUMERICAL STUDY

The changes in turbulent eddy mixing in the atmospheric boundary layer (ABL) are investigated with the use of the mesoscale RANS turbulence model with account for effects of internal gravitational waves, which support momentum transfer under condition of very stable stratification. A focus was put on analysis of behavior of the coefficients of vertical eddy diffusion of momentum and heat. The behavior of the turbulent eddy mixing parameters was found to be consistent with measurements in the laboratory and atmosphere. In particular, the flow Richardson number () during the transient flow to a strongly stable state can behave nonmonotonically, growing with increasing gradient Richardson number () to the state of saturation at a certain gradient Richardson number ( Ri@ 1 ), which separates two different turbulent regimes: strong mixing and weak mixing.

Impact of Improved Mellor–Yamada Turbulence Model on Tropical Cyclone-Induced Vertical Mixing in the Oceanic Boundary Layer

It has been identified that there are several limitations in the Mellor–Yamada (MY) turbulence model applied to the atmospheric mixed layer, and Nakanishi and Niino proposed an improved MY model using a database for large-eddy simulations. The improved MY model (Mellor–Yamada–Nakanishi–Niino model; MYNN model) is popular in atmospheric applications; however, it is rarely used in oceanic applications. In this study, the MY model and the MYNN model are compared to identify the efficiency of the MYNN model incorporated into an ocean general circulation model. To investigate the impact of the improved MY model on the vertical mixing in the oceanic boundary layer, the response of the East/Japan Sea to Typhoon Maemi in 2003 was simulated. After the typhoon event, the sea surface temperature obtained from the MYNN model showed better agreement with the satellite measurements than those obtained from the MY model. The MY model produced an extremely shallow mixed layer, and consequently, the surface temperatures were excessively warm. Furthermore, the near-inertial component of the velocity simulated using the MY model was larger than that simulated using the MYNN model at the surface layer. However, in the MYNN model, the near-inertial waves became larger than those simulated by the MY model at all depths except the surface layer. Comparatively, the MYNN model showed enhanced vertical propagation of the near-inertial activity from the mixed layer into the deep ocean, which results in a temperature decrease at the sea surface and a deepening of the mixed layer.

Role of Horizontal Eddy Diffusivity within the Canopy on Fire Spread

Wind profile observations are used to estimate turbulent mixing in the atmospheric boundary layer from 1 m up to 300 m height in two locations of pine forests characteristic of the southeast US region, and to 30 m height at one location in the northeast. Basic turbulence characteristics of the boundary layers above and within the canopy were measured near prescribed fires for time periods spanning the burns. Together with theoretical models for the mean horizontal velocity and empirical relations between mean flow and variance, we derive the lateral diffusivity using Taylor’s frozen turbulence hypothesis in the thin surface-fuel layer. This parameter is used in a simple 1D model to predict the spread of surface fires in different wind conditions. Initial assessments of sensitivity of the fire spread rates to the lateral diffusivity are made. The lateral diffusivity with and without fire-induced wind is estimated and associated fire spread rates are explored. Our results support the conceptual framework that eddy dynamics in the fuel layer is set by larger eddies developed in the canopy layer aloft. The presence of fire modifies the wind, hence spread rate, depending on the fire intensity.

Causes of the diurnal variation observed in gamma-ray spectrum using NaI (Tl) detector

Present paper studies the γ-ray spectrum observed using NaI (Tl) scintillation detector over a year. The paper reports the presence of a distinct diurnal pattern in the total number of γ-ray counts detected by the NaI detector. The counts start decreasing after sunrise and show gradual recovery after sunset. The amplitude of this variation is quite significant ( >10%) at the observation site Tirunelveli, South India. However, further investigation based on different energy ranges reveals that the mentioned diurnal pattern is actually present only in the energies related to the terrestrial background radioactivity. The study demonstrates that the pattern is associated with the radioactivity of isotopes of radon (222Rn and 220Rn) and their daughter radionuclides. The amplitude of the diurnal variation is found to have seasonal dependence, with the lowest amplitude during June–October (∼2%), and highest in April–May months (∼14%). The dependence of the amplitude of the diurnal variation on various atmospheric parameters, namely temperature, relative humidity, and the vertical wind is examined. The observed diurnal pattern is attributed to the generation of the inversion layer. The distribution of concentration of radon and thoron progenies in an air column changes with the vertical mixing and atmospheric boundary layer (ABL), resulting in the diurnal variation of γ-rays. The observed lower amplitudes in the months of June–October may be due to the presence of downward winds restricting the radon movement to a thin layer, giving smaller amplitudes of the diurnal variation in total γ-ray flux resulting in seasonal variation.

Verifying Operational Forecasts of Land–Sea-Breeze and Boundary Layer Mixing Processes

AbstractForecasters working for Australia’s Bureau of Meteorology (BoM) produce a 7-day forecast in two key steps: first they choose a model guidance dataset to base the forecast on, and then they use graphical software to manually edit these data. Two types of edits are commonly made to the wind fields that aim to improve how the influences of boundary layer mixing and land–sea-breeze processes are represented in the forecast. In this study the diurnally varying component of the BoM’s official wind forecast is compared with that of station observations and unedited model guidance datasets. Coastal locations across Australia over June, July, and August 2018 are considered, with data aggregated over three spatial scales. The edited forecast produces a lower mean absolute error than model guidance at the coarsest spatial scale (over 50 000 km2), and achieves lower seasonal biases over all spatial scales. However, the edited forecast only reduces errors or biases at particular times and locations, and rarely produces lower errors or biases than all model guidance products simultaneously. To better understand physical reasons for biases in the mean diurnal wind cycles, modified ellipses are fitted to the seasonally averaged diurnal wind temporal hodographs. Biases in the official forecast diurnal cycle vary with location for multiple reasons, including biases in the directions that sea breezes approach coastlines, amplitude biases, and disagreement in the relative contribution of sea-breeze and boundary layer mixing processes to the mean diurnal cycle.