Turbulent Exchange Research Articles

The Namib Turbulence Experiment: Investigating Surface–Atmosphere Heat Transfer in Three Dimensions

Abstract The Namib Turbulence Experiment (NamTEX) was a multinational micrometeorological campaign conducted in the central Namib Desert to investigate three-dimensional surface layer turbulence and the spatiotemporal patterns of heat transfer between the subsurface, surface, and atmosphere. The Namib provides an ideal location for fundamental research that revisits some key assumptions in micrometeorology that are implicitly included in the parameterizations describing energy exchange in weather forecasting and climate models: homogenous flat surfaces, no vegetation, little moisture, and cloud-free skies create a strong and consistent diurnal forcing, resulting in a wide range of atmospheric stabilities. A novel combination of instruments was used to simultaneously measure variables and processes relevant to heat transfer: a 3-km fiber-optic distributed temperature sensor (DTS) was suspended in a pseudo-three-dimensional array within a 300 m × 300 m domain to provide vertical cross sections of air temperature fluctuations. Aerial and ground-based thermal imagers recorded high-resolution surface temperature fluctuations within the domain and revealed the spatial thermal imprint of atmospheric structures responsible for heat exchange. High-resolution soil temperature and moisture profiles together with heat flux plates provided information on near-surface soil dynamics. Turbulent heat exchange was measured with a vertical array of five eddy-covariance point measurements on a 21-m mast, as well as by collocated small- and large-aperture scintillometers. This contribution first details the scientific goals and experimental setup of the NamTEX campaign. Then, using a typical day, we demonstrate (i) the coupling of surface layer, surface, and soil temperatures using high-frequency temperature measurements, (ii) differences in spatial and temporal standard deviations of the horizontal temperature field using spatially distributed measurements, and (iii) horizontal anisotropy of the turbulent temperature field.

Time‐lapse photogrammetry reveals hydrological controls of fine‐scale High‐Arctic glacier surface roughness evolution

AbstractIn a warming Arctic, as glacier snowlines rise, short‐ to medium‐term increases in seasonal bare‐ice extent are forecast for the next few decades. These changes will enhance the importance of turbulent energy fluxes for surface ablation and glacier mass balance. Turbulent energy exchanges at the ice surface are conditioned by its topography, or roughness, which has been hypothesized to be controlled by supraglacial hydrology at the glacier scale. However, current understanding of the dynamics in surface topography, and the role of drainage development, remains incomplete, particularly for the transition between seasonal snow cover and well‐developed, weathered bare‐ice. Using time‐lapse photogrammetry, we report a daily timeseries of fine (millimetre)‐scale supraglacial topography at a 2 m2 plot on the Lower Foxfonna glacier, Svalbard, over two 9‐day periods in 2011. We show traditional kernel‐based morphometric descriptions of roughness were ineffective in describing temporal change, but indicated fine‐scale albedo feedbacks at depths of ~60 mm contributed to conditioning surface topography. We found profile‐based and two‐dimensional estimates of roughness revealed temporal change, and the aerodynamic roughness parameter, z0, showed a 22–32% decrease from ~1 mm following the exposure of bare‐ice, and a subsequent 72–77% increase. Using geostatistical techniques, we identified ‘hole effect’ properties in the surface elevation semivariograms, and demonstrated that hydrological drivers control the plot‐scale topography: degradation of superimposed ice reduces roughness while the inception of braided rills initiates a subsequent development and amplification of topography. Our study presents an analytical framework for future studies that interrogate the coupling between ice surface roughness and hydro‐meteorological variables and seek to improve parameterizations of topographically evolving bare‐ice areas.

Realistic Forests and the Modeling of Forest‐Atmosphere Exchange

AbstractForests cover nearly a third of the Earth's land area and exchange mass, momentum, and energy with the atmosphere. Most studies of these exchanges, particularly using numerical models, consider forests whose structure has been heavily simplified. In many landscapes, these simplifications are unrealistic. Inhomogeneous landscapes and unsteady weather conditions generate fluid dynamical features that cause observations to be inaccurately interpreted, biased, or over‐generalized. In Part I, we discuss experimental, theoretical, and numerical progress in the understanding of turbulent exchange over realistic forests. Scalar transport does not necessarily follow the flow in realistic settings, meaning scalar quantities are rarely at equilibrium around patchy forests, and significant scalar fluxes may form in the lee of forested hills. Gaps and patchiness generate significant spatial fluxes that current models and observations neglect. Atmospheric instability increases the distance over which fluxes adjust at forest edges. In deciduous forests, the effects of patchiness differ between seasons; counter intuitively, eddies reach further into leafy canopies (because they are rougher aerodynamically). Air parcel residence times are likely much lower in patchy forests than homogeneous ones, especially around edges. In Part II, we set out practical ways to make numerical models of forest‐atmosphere more realistic, including by accounting for reconfiguration and realistic canopy structure and beginning to include more chemical and physical processes in turbulence resolving models. Future challenges include: (a) customizing numerical models to real study sites, (b) connecting space and time scales, and (c) incorporating a greater range of weather conditions in numerical models.

The importance of turbulent ocean–sea ice nutrient exchanges for simulation of ice algal biomass and production with CICE6.1 and Icepack 1.2

Abstract. Different sea ice models apply unique approaches in the computation of nutrient diffusion between the ocean and the ice bottom, which are generally decoupled from the calculation of turbulent heat flux. A simple molecular diffusion formulation is often used. We argue that nutrient transfer from the ocean to sea ice should be as consistent as possible with heat transfer, since all of these fluxes respond to varying forcing in a similar fashion. We hypothesize that biogeochemical models that do not consider such turbulent nutrient exchanges between the ocean and the sea ice, despite considering brine drainage and bulk exchanges through ice freezing and melting, may underestimate bottom-ice algal production. The Los Alamos Sea Ice Model (CICE + Icepack) was used to test this hypothesis by comparing simulations without and with diffusion of nutrients across the sea ice bottom that are dependent on velocity shear, implemented in a way that is consistent with turbulent heat exchanges. Simulation results support the hypothesis, showing a significant enhancement of ice algal production and biomass when nutrient limitation was relieved by bottom-ice turbulent exchange. Our results emphasize the potentially critical role of turbulent exchanges to sea ice algal blooms and thus the importance of properly representing them in biogeochemical models. The relevance of this becomes even more apparent considering ongoing trends in the Arctic Ocean, with a predictable shift from light-limited to nutrient-limited growth of ice algae earlier in the spring, as the sea ice becomes more fractured and thinner with a larger fraction of young ice with thin snow cover.

Vertical dispersion mechanism of long-range transported dust in Beijing: Effects of atmospheric turbulence

An extreme dust storm hit North China from March 14 to 16, 2021. With the enhanced observations in Beijing, the vertical dispersion of long-range transported dust was investigated from the perspective of atmospheric turbulence. A framework was developed by combining two types of turbulence intermittency parameter to distinguish intermittent or continuous turbulence; thus, the evolution of turbulence and its roles in dust dispersion were clearly illustrated on March 15, 2021. Initially, turbulence remained in the long quiescent period and turbulent exchange was suppressed significantly, which negligibly increased the dust mass concentration in the lower atmosphere, despite the dust mass being located at 300 to 800 m. Then, a short turbulence burst occurred as the cold front approached Beijing, and the elevated dust was mixed downward intensely and rapidly, leading to a sharp increase in the near-surface dust mass concentration. Finally, continuous turbulence dominated and diffused dust upward when the cold front fully occupied Beijing. Furthermore, the dissimilarity between the turbulent transport of dust and momentum-heat depends on the presence of dust sources (e.g., elevated dust mass and advection). When dust sources exist, there is efficient dust transport by large-scale turbulence, and thus the transport of dust and momentum-heat is similar. However, when there is no dust source, dust particles do not follow the air mass for small-scale turbulence and exhibit strong random movements, which confounds downgradient transport by large-scale turbulence and leads to low dust transport efficiency. In this condition, the conventional flux-gradient method is not applicable to estimate the dust flux especially by replacing the diffusion coefficient or universal function of dust mass concentration with that of momentum or heat.

Tall tower eddy covariance measurements of CO2 fluxes in Vienna, Austria

This study reports on three years (2018–2020) of turbulent CO2 fluxes measured at 144 m above the city of Vienna, Austria using an eddy covariance system installed on the A1 Arsenal radio tower. Overall CO2 flux data availability after quality control filtering (49%) was comparable to that observed by other studies of urban and suburban eddy covariance measurements. Average monthly CO2 fluxes followed seasonal trends in temperature-based proxies of heating demand, with the highest fluxes observed in the winter months. When analysing separate trends for the two dominating wind directions, this seasonality was most pronounced for the flows from the north-west when flux footprints overlapped with Vienna's most populated and urbanised districts and fluxes. For flows from the south-east, when flux footprints covered areas of built-up and industrial land yet were dominated by green urban spaces and arable land, the seasonality was much less marked and average monthly CO2 fluxes were generally lower, particularly in winter. Weekday-weekend patterns as well as weekday diurnal variations in CO2 generally followed respective trends in local traffic counts; however, flux correlations with hourly traffic counts on weekends and public holidays were much weaker. Weekend/public holiday diurnal trends in CO2 fluxes indicated an influence of vegetation fluxes on net CO2 flux variations between midday and late afternoon in spring and summer. Furthermore, variations in weekend/public holiday CO2 fluxes from night-time to morning periods suggested a partial night-time decoupling between turbulent exchange at 144 m and the net fluxes at the surface. While the influence of CO2 advection could not be quantified, the pronounced morning flux peaks indicated that CO2 released to the atmosphere during these partially decoupled, night-time periods accumulated to some extent and was subsequently mixed vertically with the morning increase in boundary layer depth. Indeed, the calculated annual fluxes suggested that positive and negative 30-min biases were largely offset when averaging over longer time intervals. The 2018 flux of 10.89 kt CO2 km−2 was in close agreement with the annual CO2 emissions estimate of 10.19 kt CO2 km−2 derived from an official emissions inventory (excluding large point source emissions). A similar annual flux was observed in 2019; however, the substantially lower flux in 2020 likely reflected emissions reductions associated with the COVID-19 pandemic. Comparison of normalised daytime CO2 fluxes between years indicated that the initial lockdown led to a ca. 50–60% reduction in net CO2 emissions from Vienna's most populated and urbanised districts. This study thus provides further evidence that defensible estimates of urban CO2 fluxes can be achieved with tall tower eddy covariance.

Influence of the Local Urban Environment on the Thermoradiative and Hydrological Behavior of a Garden Lawn

Abstract Several urban canopy models now incorporate urban vegetation to represent local urban cooling related to natural soil and plant evapotranspiration. Nevertheless, little is known about the realism of simulating these processes and turbulent exchanges within the urban canopy. Here, the coupled modeling of thermal and hydrological exchanges was investigated for a lawn located in an urban environment and for which soil temperature and water content measurements were available. The ISBA diffusive (ISBA-DF) surface–vegetation–atmosphere transfer model is inline coupled to the Town Energy Balance urban canopy model to model mixed urban environments. For the present case study, ISBA-DF was applied to the lawn and first evaluated in its default configuration. Particular attention was then paid to the parameterization of turbulent exchanges above the lawn and to the description of soil characteristics. The results highlighted the importance of taking into account local roughness related to surrounding obstacles for computing the turbulent exchanges over the lawn and simulating realistic surface and soil temperatures. The soil nature and texture vertical heterogeneity are also key properties for simulating the soil water content evolution and water exchanges.

Algorithms for calculating the microturbulent exchange coefficient for a three-dimensional mathematical model of hydrodynamics of coastal systems

The work is devoted to the construction of a model of turbulent exchange in shallow water bodies using the results of expeditionary measurements. The Kalman filter algorithm was used to filter field data. An algorithm for calculating the coefficient of vertical turbulent exchange, which decisively affects the heat and gas exchange of the coastal system, has been developed, which has made it possible to significantly reduce the effect f noise in the experimental data used. As a result of the numerical implementation of the developed algorithm, the coefficient of microturbulent exchange in the vertical direction was obtained for a three-dimensional mathematical model of hydrodynamics of coastal systems.

Mathematical modeling of hydrodynamic processes in shallow waters in the presence of pollutants of various origin, as well as areas covered with plastic waste

The paper covers the modeling of hydrodynamic processes in shallow waters in the presence of pollutants of various types, as well as areas covered with plastic waste, including nano- and microplastics. The simulation performed using new and effective mathematical instruments, including assimilation methods of measurement data, restoration of the state function, diagnostics of quality of the developed models, the research the sensitivity of models to variations in input data, the integration of models with various scales. Interrelated non-stationary, spatially inhomogeneous mathematical models of hydrodynamics and biological kinetics of shallow waters were developed and numerically implemented. They describe non-linear processes of hydrophysics, biogeochemical cycles, transport and transformation of suspensions and sediments with microplastic particles and other pollutants, absorption of it by zooplankton and fish, movement of plastic particles through food, the forecast of their impact on the main hydrobionts of shallow waters of the Azov-Black Sea basin, including the Azov Sea, Taganrog Bay, Gelendzhik Bay. The models take into account the heat and salt transport; the complex, dynamically changing water geometry; friction on the bottom and wind stresses; turbulent and advective heat and mass exchange in three coordinate directions; the Coriolis force; river flows; evaporation, temperature and oxygen regimes. They make it possible to improve the accuracy of hydrophysical processes modeling in shallow waters, to detect vortex structures of currents that can be a natural pollution traps, including microplastic. Based on a supercomputer, a complex of application has been designed and adapted for solving predictive problems of water ecology and water resources management.

Математическое моделирование гидродинамических процессов в прибрежной акватории Японского моря

A three-dimensional mathematical model of coastal water dynamics in the Peter the Great Bay of the Sea of Japan is presented. The system of Navier-Stokes equations is written under the assumption of a hydrodynamic approximation of an inhomogeneous fluid and takes into account turbulent exchange, wind stresses, friction forces, complex topography of the bottom and coastline. On the basis of hydrographic information, a numerical method is used to reconstruct the bottom topography and coastline. The free water surface equation is replaced by a singularly perturbed problem. In the FreeFem++ application software package, a code was written for calculating the hydrodynamic characteristics of coastal waters. During the solution, the finite element mesh is built automatically, the mesh parameters are set by the user. To solve the singular problem, at each time layer, the grid is adapted around the numerical solution with the grid adaptation parameters.

3D turbulent currents simulation in the coastal zone using the LES approach based on filtered ADCP data

The interest in turbulence in shallow waters, such as the Azov Sea, is caused by the fact that in the places of its existence there is an intensive transfer of the amount of motion and heat, the spread of passive impurities, the transfer of suspended particles. These processes significantly affect the formation and spatial structure of physical, chemical and biological fields of reservoirs and their spatial and temporal changes. Depending on the smoothing scale of the series of initial readings of the flow meters, the pulsation characteristics of the flow velocity were determined, which were processed in order to obtain data on turbulence and its scales. The research results provided empirical data on the conditions of generation and existence of small-scale turbulence. The collected empirical material is currently being processed to study the internal structure of the recorded disturbances of small-scale turbulence. In this paper, small-scale motion is excluded from the Navier-Stokes equations by applying the filtration operation and is modeled using subgrid models. To do this, in two-dimensional and three-dimensional cases, various types of filters are used: a box filter, a Gaussian filter and a Fourier filter, with a gradual decrease in the filter width, which allows you to reproduce a wider frequency range of fluctuations of the solution. The obtained data are planned to be used for numerical simulation of three-dimensional turbulent flows using the LES approach and comparison with the results of averaging by RANS. The article considers the possibilities of using various types of approximations for parametrization of vertical turbulent exchange. Algebraic models for calculating the coefficient of vertical turbulent exchange and semi-empirical turbulence models are compared.

Simulation of the Scalar Transport above and within the Amazon Forest Canopy

The parallelized large-eddy simulation model (PALM) was used to understand better the turbulent exchanges of a passive scalar above and within a forested region located in the central Amazon. Weak (2 ms−1) and strong (6 ms−1) wind conditions were simulated. A passive scalar source was introduced to the forest floor for both simulations. The simulations reproduced the main characteristics of the turbulent flow and of the passive scalar transport between the forest and the atmosphere. Noteworthily, strong and weak wind conditions presented different turbulence structures that drove different patterns of scalar exchange both within and above the forest. These results show how passive scalar concentration is influenced by the wind speed at the canopy top. Additionally, higher wind speeds are related to stronger sweep and ejection regimes, generating more intense plumes that are able to reduce the passive scalar concentration inside the forest canopy. This work was the first that used PALM to investigate scalar transport between the Amazon rainforest and the atmosphere.

Vertical turbulent exchange structure and parametrization for 3D shallow water hydrodynamics models

The work describes research of vertical turbulent exchange structure and parametrization for 3D shallow water hydrodynamics models. In this paper, the coefficients of horizontal turbulent exchange are calculated using a whole set of averaging periods of turbulent velocity pulsations. Using experimental data on the pulsations of the velocity components, the coefficient of vertical turbulent exchange was calculated on the basis of various approaches to its parameterization, based on the analysis of the obtained coefficient distributions, the most adequate parameterization of the coefficient was selected, which is used in the software package. The distribution of the vertical turbulent exchange coefficient obtained in a numerical experiment was compared with the results of full-scale measurements, and the calculation results obtained using the mathematical statistics apparatus were analysed.

Wake scaling of actuator discs in different aspect ratios

The wake recovery from planar porous actuators that surrogate the effect of wind turbines is investigated, focusing on rectangular shapes for vertical axis wind turbines (VAWTs). We proposed an effective mixing diameter D∗ to scale the streamwise momentum recovery for actuators of arbitrary shape. The length-scale D∗ is given by the ratio between frontal area and disc perimeter characterising the wake-freestream interface, whereby the momentum loss and the turbulent exchange of momentum take place. Wind tunnel experiments of planar actuators from porous plates are presented. The three-dimensional development of the wake is surveyed up to six widths/diameters downstream of the actuators making use of robotic particle image velocimetry with helium-filled soap bubbles as flow tracers. The recovery rate analysis is performed using D∗ for wake normalisation. The scaled wake data agrees well among actuators in different shapes. And it is significantly improved for rectangular actuators, comparing with existing scaling lengths. The flow behaviour is confirmed with numerical simulations of VAWT wakes with different aspect ratios, indicating the validity of this scaling concept for wind turbine wake modelling.

On the Wind Turbine Wake and Forest Terrain Interaction

Future wind power developments may be located in complex topographic and harsh environments; forests are one type of complex terrain that offers untapped potential for wind energy. A detailed analysis of the unsteady interaction between wind turbines and the distinct boundary layers from those terrains is necessary to ensure optimized design, operation, and life span of wind turbines and wind farms. Here, laboratory experiments were carried to explore the interaction between the wake of a horizontal-axis model wind turbine and the boundary layer flow over forest-like canopies and the modulation of forest density in the turbulent exchange. The case of the turbine in a canonical boundary layer is included for selected comparison. The experiments were performed in a wind tunnel fully covered with tree models of height H/zhub≈0.36, where zhub is the turbine hub height, which were placed in a staggered pattern sharing streamwise and transverse spacing of Δx/dc=1.3 and 2.7, where dc is the mean crown diameter of the trees. Particle image velocimetry is used to characterize the incoming flow and three fields of view in the turbine wake within x/dT∈(2,7) and covering the vertical extent of the wake. The results show a significant modulation of the forest-like canopies on the wake statistics relative to a case without forest canopies. Forest density did not induce dominant effects on the bulk features of the wake; however, a faster flow recovery, particularly in the intermediate wake, occurred with the case with less dense forest. Decomposition of the kinematic shear stress using a hyperbolic hole in the quadrant analysis reveals a substantial effect sufficiently away from the canopy top with sweep-dominated events that differentiate from ejection-dominated observed in canonical boundary layers. The comparatively high background turbulence induced by the forest reduced the modulation of the rotor in the wake; the quadrant fraction distribution in the intermediate wake exhibited similar features of the associated incoming flow.

Double Averaging Analysis Applied to a Large Eddy Simulation of Coupled Turbulent Overlying and Porewater Flow

AbstractFreestream turbulence in rivers is a key contributor to the flux of dissolved nutrients, carbon, and other ecologically important solutes into porewater. To advance understanding of turbulent hyporheic exchange and porewater transport, we investigate flow over and through a rough bed of spheres using large eddy simulation (LES). We apply double averaging (combined space and time averaging) to the LES results to determine the mean velocity distribution, momentum balance, and drag forces. Our simulations show large‐scale freestream structures interacting strongly with vortices generated at the surfaces of individual spheres to control turbulent momentum fluxes into the bed. The transition between turbulent flow and Darcy flow occurs over the first row of spheres, where turbulence decays rapidly and turbulent kinetic energy, Reynolds stress, and drag forces peak. Below this region, turbulence is only present in the high‐velocity flow in open pore throats. Experimental observations suggest that minimum mean porewater velocity occurs in the first open pore space below the transition region, but our results show that the minimum occurs between the first and second pore spaces. The simulation mean porewater velocities are approximately half those captured in measurements because the model resolves the entire flow continuum while measurements can access high‐velocity fluid in open pores. The high‐resolution dual time‐space averaging of the LES resolves both turbulent and mean flow features that are important to interfacial solute and particle fluxes, providing a means to include turbulent hyporheic exchange in upscaled river models, which has not been achieved to date.

A nonlocal three‐dimensional turbulence parameterization (NLT3D) for numerical weather prediction models

AbstractWith increasing resolution of numerical weather prediction (NWP) models, classical subgrid‐scale processes become increasingly resolved on the model grid. In particular, turbulence in the planetary boundary layer (PBL) is vertically already partially resolved in contemporary models. For classical local PBL schemes, resulting up‐gradient heat transports cannot be treated correctly. Thus, nonlocal turbulence schemes have been developed in the past. As the horizontal grid sizes of NWP models become smaller than a few kilometers, the large turbulence eddies in the PBL will also start to become partially resolved in the horizontal direction. A very flexible way to formulate nonlocal turbulent exchange is the transilient matrix method, which is used here to develop a new turbulence parameterization. The resulting NLT scheme applies transilient mixing matrices to subgrid‐scale transports in all three dimensions. We compare results of WRF real‐case simulations including our scheme, a classical local turbulence scheme (MYNN), and an existing nonlocal one‐dimensional scheme (ACM2) with observations from field campaigns over homogeneous terrain (CASES‐99) and complex terrain (CAPTEX). Over homogeneous terrain, all three schemes similarly well capture the observed surface fluxes and radiosonde profiles, whereas over complex terrain more differences become obvious. During a tracer release experiment (CAPTEX) over the Appalachian mountain region, the mixing and vertical extent of the PBL turn out to be decisive to reproduce the observed advection speed of the tracer‐marked air mass. Deeper mixing not only accelerates surface winds but also enables tracer to travel faster at higher altitudes and then mix back to the ground. As results from a version of NLT with only standard horizontal Smagorinsky diffusion (NLT) demonstrate, simulating three‐dimensional turbulence can be beneficial already at horizontal grid sizes of a few kilometers.

Investigation of the Mixing Height in the Planetary Boundary Layer by Using Sodar and Microwave Radiometer Data

The height of the mixing layer is a significant parameter for describing the dynamics of the planetary boundary layer (PBL), especially for air quality control and for the parametrizations in numerical modeling. The problem is that the heights of the mixing layer cannot be measured directly. The values of this parameter are depending both on the applied algorithms for calculation and on the measuring instruments which have been used by the data source. To determine the height of a layer of intense turbulent heat exchange, data were used from acoustic meteorological locator (sodar) and from a passive single-channel scanning microwave radiometer MTP-5 (MWR) to measure the temperature profile in a layer of up to 1 km. Sodar can provide information on the structure of temperature turbulence in the PBL directly. These data have been compared with the mixing layer height calculated with the Parcel method by using the MTP-5 data. For the analysis, July and September 2020 were selected in the city of Tomsk in Siberia as characteristic periods of mid-summer and the transition period to autumn. The measurement results, calculations and inter-comparisons are shown and discussed in this work. During temperature inversions in the boundary layer, it was observed that turbulent heat transfer (increased dispersion of air temperature) is covering the inversion layers and the overlying ones. Moreover, this phenomenon is not only occurring during the morning destruction of inversions, but also in the process of their formation and development.

Stream Transport and Substrate Controls on Nitrous Oxide Yields From Hyporheic Zone Denitrification

AbstractRivers and streams act as globally significant sources of nitrous oxide (N2O) to the atmosphere, in part through denitrification reactions that will increase in response to ongoing anthropogenic nitrogen loading. While many factors that contribute to the release of N2O relative to inert dinitrogen (N2) are well described, the ability to predict N2O yields from streams remains a fundamental challenge. Here, I revisit results from the second Lotic Intersite Nitrogen eXperiments (LINX II) in the context of turbulent hyporheic exchange. Denitrification efficiency, or the fraction of nitrate delivered to the streambed by stream turbulence that is chemically reduced, emerges as the single best predictor of N2O yields and underpins the first statistically significant models of inter‐site N2O yields. This mechanistic connection is supported by reactive transport modeling of hyporheic zone denitrification representing advective flowpaths, flowpath mixing, and diffusion‐dominated anoxic microzones. Simulated N2O yields are inversely correlated with denitrification efficiency; however, advective models are unable to capture low LINX II N2O yields at low denitrification efficiencies. Hyporheic zone mixing exacerbates this inability to capture observed N2O yields via the promotion of N2O release from fast, oxic flowpaths. Instead, anoxic microzones are required to account for LINX II observations through consistently low N2O yields and the consumption of upstream‐produced N2O. Together, these results provide a framework for controls on stream N2O yields and suggest that stream corridor restoration designs aimed at increasing the capacity of hyporheic zones to remediate nitrate loading, as opposed to increasing hyporheic exchange, will also reduce proportional N2O emissions.

Feasibility study on using asphalt pavements as heat absorbers and sensible heat storage materials in solar air heaters: An experimental study

Asphalt pavements are affordable infrastructures exposed to solar irradiation for a long time a day, and they experience considerable high temperatures. Due to high thermal capacity, asphalt pavements can store high sensible thermal energy during daytime hours and release it when solar irradiation is negligible or absent. The current study investigates the heat dynamics and thermal efficiency of asphalt solar air heaters (SAHs) utilizing asphalt materials that function as both heat absorbers and sensible heat storage units. To this aim, a 1.2 m2 single-glazed baffled SAH prototype was constructed, and its heat dynamics were monitored on cold and mild winter days at two different air mass flow rates of 0.02 kg/s and 0.03 kg/s. The constructed SAH uses black stainless-steel baffles to produce a serpentine air passageway and improve turbulence and heat exchange rates. The acquired data illustrates that the maximum intake and exhaust air temperature difference reaches up to 14 °C and 24 °C on cold and mild winter days, respectively. When the solar irradiation was negligible in the evening, the asphalt pavement serves as sensible heat storage and heats the passing airflow up to 8 °C and 12 °C at sunset on cold and mild winter days, respectively. This issue causes the collector thermal efficiency to reach as high as 80%. Therefore, using asphalt materials is a feasible approach to simultaneously serve as heat absorber and sensible heat storage.