Abstract: Known as the heat-mitigation effect, irrigated rice-paddy fields distribute a large fraction of their received energy to the latent heat during the growing season. The present hypothesis is that increased atmospheric CO2 concentration decreases the stomatal conductance of rice plants and increases the air temperature by means of an increased sensible heat flux. To test this hypothesis, a coupled regional atmospheric and crop energy-balance model is developed and applied to a 300 × 300 km2 region in Japan. Downscaling meteorological variables from grid-mean values of mixed land use (3 × 3 km2) generates realistic typical diurnal cycles of air temperature in rice paddies and adjacent residential areas. The model simulation shows that, on a typical sunny day in summer, doubling the CO2 concentration increases the daily maximum grid-mean air temperature, particularly where rice paddies are present, by up to 0.7 °C. This CO2 effect on the grid-mean air temperature is approximately half the effect of the reduction in rice-paddy area that is postulated to occur on a time scale similar to that of the atmospheric CO2 change. However, within the internal atmospheric boundary layer of the rice paddies, the CO2 effect on the air temperature (+ 0.44 °C) still exceeds the effects of the land-use change (+ 0.11 °C). These results show a potentially important interplay of plant physiological responses regarding atmospheric CO2 in the heat-mitigation effect of rice-paddy fields under a changing climate. PubDate: 2021-03-05

Abstract: We consider the Janjic (NCEP Office Note 437:61, 2001) boundary-layer model, which is one of the most widely used in numerical weather prediction models. This boundary-layer model is based on a number of length scales that are, in turn, obtained from a master length multiplied by constants. We analyze the simulation results obtained using different sets of constants with respect to measurements using sonic anemometers, and interpret these results in terms of the turbulence processes in the atmosphere and of the role played by the different length scales. The simulations are run on a virtual machine on the Chameleon cloud for low-wind-speed, unstable, and stable conditions. PubDate: 2021-02-11

Abstract: An analysis based on the law of linear momentum conservation demonstrates unequivocally that the mass fraction is the scalar whose gradient determines gas diffusion, both molecular and turbulent. It illustrates sizeable errors in previous micrometeorological definitions of the turbulent gas flux based on fluctuations in other scalars such as the mixing ratio or density. In deference to conservation law, we put forth a new definition for the turbulent gas flux. Net gas transport is then defined as the sum of this turbulent flux with systematic transport by the mean flow. This latter, non-diffusive flux is due to the net upward boundary-layer momentum, a Stefan flow forced by evaporation, which is the dominant surface gas exchange. A comparison with the traditional methodology shows exact agreement between the two methods regarding the net flux, but with the novelty of partitioning gas transport according to distinct physical mechanisms. The non-diffusive flux is seen to be non-negligible in general, and to dominate turbulent transport under certain conditions, with broad implications for boundary-layer meteorology. PubDate: 2021-02-11

Abstract: In the original publication, the same figure was illustrated twice as figures 10 and 11. Consequently, figure 11 was incorrect. The correct version of figure 11 is provided in this correction. PubDate: 2021-02-07

Abstract: Exploration of the flow inside the roughness sublayer often suffers from sub-sampling of its complex three-dimensional and non-homogeneous flow fields. Based on detailed particle image velocimetry within a randomly-ordered canopy model, we analyze the potential differences between single-location flow statistics and their spatially-averaged values. Overall, higher variability exists inside the canopy than above it, and is two to four times higher than found inside similar, however ordered, canopy arrangements. The local mean absolute percentage error (MAPE), vertically averaged within three different regions (below, above, and at canopy height), provides a measure for quantifying and characterizing the spatial distribution of errors for various flow properties (mean velocity and stresses). We calculated the value of MAPE at predefined farthest-locations based only on geometric considerations (i.e., farther away from surrounding roughness elements), as commonly done in the field. Interestingly, most of the vertical profiles at the farthest locations lie within the interquartile range of the measured spatial variability for all studied flow and turbulent properties. Additionally, our results show that, for at least 23% of the total canopy plan area, the double-averaged streamwise velocity component and its variance inside the canopy can be reproduced from a single measured profile for which the value of MAPE does not exceed 25%. These regions also constitute most of the farthest locations. The property that exhibits the highest MAPE value inside the canopy is the Reynolds stress (up to \(130\%\) ); however, these errors are dramatically reduced in the upper half of the canopy. Furthermore, at the canopy interface and above it, the errors rarely exceed \(20\%\) . The variability is also manifested in the computed integral length scales. The single-point velocity autocorrelation always underestimates the length scales obtained from the two-point statistics. These findings have implications for canopy flow and transport modelling inside the roughness sublayer and can help explain and evaluate the source of discrepancies between measurements and transport models. PubDate: 2021-02-07

Abstract: Inland freshwater bodies form the largest natural source of carbon to the atmosphere. To study this contribution to the atmospheric carbon cycle, eddy-covariance flux measurements at lake sites have become increasingly popular. The eddy-covariance method is derived for solely local processes from the surface (lake). Non-local processes, such as entrainment or advection, would add erroneous contributions to the eddy-covariance flux estimations. Here, we use four years of eddy-covariance measurements of carbon dioxide from Lake Erken, a freshwater lake in mid-Sweden. When the lake is covered with ice, unexpected lake fluxes were still observed. A statistical approach using only surface-layer data reveals that non-local processes produce these erroneous fluxes. The occurrence and strength of non-local processes depend on a combination of wind speed and distance between the instrumented tower and upwind shore (fetch), which we here define as the time over water. The greater the wind speed and the shorter the fetch, the higher the contribution of non-local processes to the eddy-covariance fluxes. A correction approach for the measured scalar fluxes due to the non-local processes is proposed and also applied to open-water time periods. The gas transfer velocity determined from the corrected fluxes is close to commonly used wind-speed based parametrizations. PubDate: 2021-02-01

Abstract: Peatlands often experience turbulent sheltering from their surrounding upland forests, which results in spatially variable surface–atmosphere exchanges of momentum, heat, and moisture produced by flow-separation dynamics, which suppresses the transport of such scalars in the sheltered region while promoting transport in the reattachment zone. With evapotranspiration being the dominant source of water loss in the Boreal Plains, it is necessary to understand the dynamics and controls on evapotranspiration within these peatlands. We used the regional atmospheric forest large-eddy simulation (RAFLES) model to study the impact of flow separation and surface roughness on microclimates leeward of a forest-to-peatland roughness transition. We parametrized our simulation with observed vegetation characteristics and meteorological data from three natural peatlands to accurately estimate natural ranges of peatland roughnesses and energy dynamics. Our simulations show that changes to peatland roughness do not affect the distances required for flow reattachment, and therefore the size of the sheltered region. However, increasing the surface roughness produces greater surface turbulence, quicker flow recovery, and decreased flow reversals within the sheltered region. Further, substantial microclimatic differences are observed throughout the flow regions of the roughness transition. Our results show that turbulence, aerodynamic resistance, and the microclimate vary throughout the backward-facing step transition and should be taken into account when estimating the spatial dynamics of evaporative demand leeward of a roughness transition. Furthermore, increasing the surface roughness of a peatland minimizes the spatial variability of turbulent drivers of evapotranspiration across a roughness transition. That is, flow separation and the surface roughness of the peatland should be accounted for when estimating the spatial variability and total evaporative potential across a peatland. PubDate: 2021-02-01

Abstract: We use a database of direct numerical simulations to evaluate parametrizations for energy dissipation rate in stably stratified flows. We show that shear-based formulations are more appropriate for stable boundary layers than commonly used buoyancy-based formulations. As part of the derivations, we explore several length scales of turbulence and investigate their dependence on local stability. PubDate: 2021-02-01

Abstract: Polarization lidar observations were made to study the transport of an elevated aerosol layer over Gadanki, India (13.45° N, 79.17° E) during the pre-monsoon period of the year 2009. Observations show significant aerosol layering within and above the boundary layer. Coordinated observations with radiosondes were carried out from 2 to 10 April 2009. Temporal and spatial variations of the parameters are studied for the boundary layer (≈ 2.5 km) and up to 5 km. The backscattering coefficient and the depolarization ratio are observed to increase and decrease with an increase in humidity, respectively. Clouds are not formed, indicating less efficiency of the aerosol in acting as condensation nuclei. The transport of the elevated aerosol layer is investigated using a back-trajectory analysis, revealing that the transported layer originating from the central Indian region has a depolarization ratio of at least 0.05. From model analysis and satellite fire-count data, it is inferred that the source of the aerosol layer is wildfire events over the central Indian region. The elevated smoke-aerosol layer (not mixing with the boundary layer) has implications for the altering of the temperature profile of the atmosphere and the suppression of cloud formation. PubDate: 2021-02-01

Abstract: We report the ability of an urban canopy model, coupled with a regional climate model, to simulate energy fluxes, the intra-urban variability of air temperature, urban-heat-island characteristics, indoor temperature variation, as well as anthropogenic heat emissions, in Berlin, Germany. A building energy model is implemented into the Double Canyon Effect Parametrization, which is coupled with the mesoscale climate model COSMO-CLM (COnsortium for Small-scale MOdelling in CLimate Mode) and takes into account heat generation within buildings and calculates the heat transfer between buildings and the urban atmosphere. The enhanced coupled urban model is applied in two simulations of 24-day duration for a winter and a summer period in 2018 in Berlin, using downscaled reanalysis data to a final grid spacing of 1 km. Model results are evaluated with observations of radiative and turbulent energy fluxes, 2-m air temperature, and indoor air temperature. The evaluation indicates that the improved model reproduces the diurnal characteristics of the observed turbulent heat fluxes, and considerably improves the simulated 2-m air temperature and urban heat island in winter, compared with the simulation without the building energy model. Our set-up also estimates the spatio–temporal variation of wintertime energy consumption due to heating with canyon geometry. The potential to save energy due to the urban heat island only becomes evident when comparing a suburban site with an urban site after applying the same grid-cell values for building and street widths. In summer, the model realistically reproduces the indoor air temperature and its temporal variation. PubDate: 2021-02-01

Abstract: The shear-stress cospectrum and the horizontal and vertical temperature-flux cospectra in the convective boundary layer (CBL) are predicted using the multi-point Monin–Obukhov similarity (MMO theory). MMO theory was recently proposed and then derived from first principles by Tong and Nguyen (Journal of the Atmospheric Sciences, 2015, Vol. 72, 4337 – 4348) and Tong and Ding (Journal of Fluid Mechanics, 2019, Vol. 864, 640 – 669) to address the issue of the incomplete similarity in the Monin–Obukhov similarity theory. According to MMO theory, the CBL has a two-layer structure: the convective layer ( \(z \gg -L\) ) and the convective–dynamic layer ( \(z \ll -L\) ). The former consists of the convective range ( \(k \ll -1/L\) ) and the inertial range ( \(k \gg 1/z\) ), while the latter consists of the convective range, the dynamic range ( \(-1/L \ll k\ll 1/z\) ), and the inertial range, where z, k, and L are the height from the ground, the horizontal wavenumber, and the Obukhov length, respectively. We use MMO theory to predict the cospectra for the convective range and the dynamic range. They have the same scaling in the convective range for both \(z \ll -L\) and \(z \gg -L\) . The shear-stress cospectrum and the vertical temperature-flux cospectrum have \(k^0\) scaling in both the convective and dynamic ranges. The horizontal temperature-flux cospectrum has \(k^{-1/3}\) and \(k^{-1}\) scaling in the convective and dynamic ranges respectively. The predicted scaling exponents are in general agreement with high-resolution large-eddy-simulation results. However, the horizontal temperature-flux cospectrum is found to change sign from the dynamic range (negative) to the convective range (positive), which is shown to be caused by the temperature–pressure-gradient interaction. PubDate: 2021-02-01

Abstract: Large-eddy simulations of nine idealized heterogeneous urban morphologies with identical building density and frontal area index are used to explore the impact of heterogeneity on urban airflow. The fractal-like urban morphologies were generated with a new open-source Urban Landscape Generator tool (doi:10.5281/zenodo.3747475). The vertical structure of mean flow and the dispersive vertical momentum transport within the roughness sublayer are shown to be strongly influenced by the building morphologies. The friction velocity and displacement height show high correlations with the maximum building height rather than the average height. Well-known roughness parametrizations of the logarithmic layer cannot adequately capture the large spread observed in the large-eddy simulation data. A generalized frontal area index \({\Lambda }_f\) is introduced that characterizes the vertical distribution of the frontal area in the urban canopy. The vertically distributed stress profiles, which differ significantly per simulation, are shown to roughly collapse upon plotting them against \({\Lambda }_f\) . The stress distribution representing urban drag can be fitted with a third degree polynomial. The results can be used for more detailed and robust representations of building effects in the development of urban canopy models. PubDate: 2021-02-01 DOI: 10.5281/zenodo.3747475).

Abstract: We describe and explain the turbulent processes at play in the lower part of the urban boundary layer through performing a large-eddy simulation of the flow over an urban-like canopy composed of a staggered array of cubes with a packing density of 25%. The simulation models neutral thermal conditions at a Reynolds number (based on both velocity at the top of the domain and the domain height) of \(Re = 50{,}000\) . A dynamic Smagorinsky model is implemented in order to allow for energy backscattering from subgrid scales. A wall refinement of the grid allows resolving the viscous sublayer. Turbulent statistics up to the third order, as well as each term of the turbulence-kinetic-energy budget, are computed individually everywhere in the domain. Results are discussed in relation to experimental and numerical data from the literature in order to describe turbulent energy transfers occurring in the roughness sublayer. The fine grid resolution close to surfaces serves to analyze in depth the three-dimensional distribution of turbulence production inside the urban canopy layer. This analysis in turn leads to discovering areas, never previously documented in an urban-like canopy, of highly positive and highly negative production close to the surface, away from the well-known high production area in the shear layer. Furthermore, evidence of a close link between high and low production areas near the surfaces and singular points in the mean flow is presented, thus laying the groundwork for a simple pre-diagnostic tool to detect turbulence-kinetic-energy production areas near surfaces. PubDate: 2021-02-01

Abstract: The propagation of a pollutant emitted from localized sources both within and above a regular street network is studied by analyzing data from direct numerical simulations of passive scalar dispersion. Two wind directions are considered, corresponding to aligned and oblique flow with respect to the street axes. Particular attention is paid to the role of entrainment of the scalar into the urban canopy from an elevated source and re-entrainment of material originally released further upstream from a ground source. The variation of concentration differences and vertical fluxes between the streets and the air above as a function of distance reveals important differences between the rate of lateral and vertical mixing for the two sources. Detrainment and entrainment need a longer fetch to equilibrate for the elevated source than for the ground source. There are large differences between the advection and detrainment velocities for the aligned and oblique cases, so that a change in wind direction could affect ventilation efficiency considerably. Time scales associated with different dispersion processes are computed and the time of first appearance of the scalar from the onset of release in different streets is mapped. It is shown that re-entrainment can provide a shortcut dispersion pathway for reaching certain parts of the network. This is particularly striking in the case of oblique flow, when material can be transferred by entrainment up to twice as rapidly as by advection. Taken together, these results highlight the overall message that vertical exchange is a two-way process and that entrainment needs to be considered in the context of emergency response as well as urban ventilation. PubDate: 2021-01-21

Abstract: In most land-surface models, the evolution of soil moisture is governed by soil-hydraulic processes. In hyper-arid soils, these processes break down, but soil moisture continues to show clear temporal variations, suggesting that other processes may be at work. We hypothesize that moisture in such soils varies due to evaporation in the soil and to vapour fluxes at the air–soil interface. To test this, we include vapour exchange between the air and soil in a land-surface model, apply the model to a desert site, and compare the simulated and observed soil moisture. The good agreement between the simulations and observations confirms our hypothesis. Using the model results, we examine the interactions between the soil-moisture and soil-vapour phases and influences of the soil-vapour phase on the surface energy balance. PubDate: 2021-01-21

Abstract: The correct simulation of pollutant dispersion in coastal regions demands understanding the turbulence structure of the thermal internal boundary layer (TIBL), which typically occurs in daytime when maritime air is advected over the continent. Such a structure is investigated using 10 levels of turbulence observations made at a 140-m micrometeorological mast installed at 3500 m from the shoreline in south-eastern Brazil, with TIBL dimensionless vertical profiles of the turbulence parameters commonly used in Lagrangian and Eulerian dispersion models determined. To accomplish that, the TIBL height \(z_i\) is estimated using a vertical-flux-convergence approach, developed here. The values experimentally obtained for \(z_i\) agree with those predicted by a widely-used model for TIBL growth. In general, the normalized turbulent profiles evaluated for the TIBL differ from those previously obtained in the convective boundary layer (CBL). The horizontal eddy diffusivities evaluated for the TIBL are larger than those typically observed in the CBL, while the vertical ones are similar for both boundary layers. Finally, it is shown that CBL similarity relationships can be used to describe turbulent parameters as long as the proper empirical constants are used. PubDate: 2021-01-21

Abstract: We present ensemble-based large-eddy simulations based on a lattice Boltzmann method for a realistic urban area. A plume-dispersion model enables a real-time simulation over several kilometres by applying a local mesh-refinement method. We assess plume-dispersion problems in the complex urban environment of Oklahoma City on 16 July using realistic mesoscale velocity boundary conditions produced by the Weather Research and Forecasting model, as well as building structures and a plant-canopy model introduced into the plume-dispersion model. Ensemble calculations are performed to reduce uncertainties in the macroscale boundary conditions due to turbulence, which cannot be determined by the mesoscale model. The statistics of the plume-dispersion field, as well as mean and maximum concentrations, show that ensemble calculations improve the accuracy of the simulations. Factor-of-2 agreement is found between the ensemble-averaged concentrations based on the simulations over a 4.2 × 4.2 × 2.5 km2 area with 2-m resolution with the plume-dispersion model and the observations. PubDate: 2021-01-21

Abstract: Over the past few years large-eddy simulation (LES) has demonstrated success in modelling continental radiation fog, and several recent studies have used LES to investigate the sensitivity of fog formation to physical processes such as turbulent mixing and surface heat and moisture exchange, as well as to the parametrization of microphysical processes such as cloud droplet activation. Here we extend these sensitivity studies to marine fog. There are several important differences in the formation of marine and continental fog, however moisture availability is no longer a decisive factor, and surface temperature changes over a much longer time scale. Here LES is used to examine the sensitivity of simulated marine-fog formation and maintenance to the cloud-droplet number concentration, turbulent mixing, and air–sea temperature difference. The strength of the fog (in terms of liquid water content) is found to be highly sensitive to all three factors. Varying only the cloud-droplet number concentration, even within a range of physically realistic values for marine regions, can mean the difference between fog halving or doubling in liquid water content. The sensitivities demonstrated herein indicate the great need and challenge for constraining these parameters in numerical weather prediction. Similarities and differences to the findings for continental radiation fog are examined, and important considerations for future improvements in marine-fog forecasting are discussed. PubDate: 2021-01-17 DOI: 10.1007/s10546-020-00599-6

Abstract: Open cellular convection (OCC) over, for example, the North Sea is often observed in connection with cold-air outbreaks. It is accompanied by large temporal and spatial variability in wind speed, which affects offshore wind energy in the area. This study uses the global Model for Prediction Across Scales (MPAS), with regional mesh refinement down to convection-permitting scales of 2 km, to simulate an OCC episode in the North Sea, with a focus on wind-speed variability. Modelled data are combined with wind speeds retrieved from satellite data and in situ measurements to investigate the spatial and temporal variability of offshore wind speeds under OCC conditions from a synoptic to mesoscale perspective, and to examine the model’s ability to represent the OCC structures and wind-speed variability. The model can simulate realistic OCC structures and mesoscale wind-speed variability within the limits set by the effective model resolution. Under OCC conditions, significant differences from climatological conditions are found in the spatial wind-speed power spectrum and in 10-min wind-speed step changes. The very high horizontal mesh-cell spacing in the refinement region of 2 km, and the focus on OCC wind-speed variability, makes this the first investigation of this kind using the MPAS modelling framework with mesh refinement. PubDate: 2021-01-14 DOI: 10.1007/s10546-020-00591-0

Abstract: To simulate the airflow through a wind farm across a wide range of atmospheric conditions, microscale models (e.g., large-eddy simulation, LES, models) have to be coupled with mesoscale models, because microscale models lack the atmospheric physical processes to represent time-varying local forcing. Here we couple mesoscale model outputs to a LES solver by applying mesoscale momentum- and temperature-budget components from the Weather Research and Forecasting model to the governing equations of the Simulator fOr Wind Farm Applications model. We test whether averaging the budget components affects the LES results with regard to quantities of interest to wind energy. Our study focuses on flat terrain during a quiescent diurnal cycle. The simulation results are compared with observations from a 200-m tall meteorological tower and a wind-profiling radar, by analyzing time series, profiles, rotor-averaged quantities, and spectra. While results show that averaging reduces the spatio-temporal variability of the mesoscale momentum-budget components, when coupled with the LES model, the mesoscale bias (in comparison with observations of wind speed and direction, and potential temperature) is not reduced. In contrast, the LES technique can correct for shear and veer. In both cases, however, averaging the budget components shows no significant impact on the mean flow quantities in the microscale and is not necessary when coupling mesocale budget components to the LES model. PubDate: 2021-01-14 DOI: 10.1007/s10546-020-00584-z