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Sensible heating as a potential mechanism for enhanced cloud formation over temperate forest
Bosman, Peter J.M. ; Heerwaarden, Chiel C. van; Teuling, Adriaan J. - \ 2019
Quarterly Journal of the Royal Meteorological Society 145 (2019)719. - ISSN 0035-9009 - p. 450 - 468.
cloud formation - flux partitioning - forest cloud cover - large-eddy simulation - sensible heat flux - sensible heating - surface heat fluxes
It has been recently shown for two forests in France (Les Landes and Sologne) that summer cloud cover over the forest is increased relative to its surroundings. This study aims to contribute to the elucidation of the physical mechanisms responsible for this increased cloud cover, focusing on surface flux partitioning. This was done by performing a case study for a heatwave day on which enhanced cloud cover over the forest of Les Landes was observed. Two numerical experiments (large-eddy simulations) with a homogeneous forest cover were performed, one in which the sensible heat flux was increased by approximately 5% of the total available energy and another one in which the same amount of energy was added to the latent heat flux. The addition of energy to the sensible heat flux led to a stronger increase in cloud cover than the same addition to the latent heat flux. The mean relative humidity at the boundary layer top showed only small differences, indicating it was not a sufficient indicator for cloud formation in this case. Important information, which immediately underlines the need for large-eddy simulations, is contained in modifications of the shape of the probability density functions of temperature and humidity. With enhanced sensible heating, the higher peak values of relative humidity contribute to an increased cloud cover. A crucial reason for the differences in cloud cover between the experiments is conjectured to be a decrease in the required amount of energy for air parcels to reach the lifting condensation level, indirectly caused by the boundary layer and near-surface warming associated with the stronger sensible heat flux. As forests in the region do have a higher sensible heat flux than their surroundings, we highlight one potential mechanism for enhanced cloud cover.
On jet instability modes of a subsonic Hartmann whistle
Varadharajan, Ramanathan ; Kamin, Manu ; Ganesh, Subramanian ; Mathew, Joseph - \ 2018
Sadhana 43 (2018)9. - ISSN 0256-2499
Hartmann whistle - jet-instability mode - large-eddy simulation - subsonic flow
Numerical experiments to understand the resonant acoustic response of a subsonic jet impinging on the mouth of a tube, known as the Hartmann whistle configuration, were performed as large-eddy simulations. The tube length was chosen so that its fundamental duct mode, for one end closed and one end open, would match the dominant mode in the exciting jet. When the tube mouth was placed in the path of a regular stream of vortex rings, formed by the instability of the jet’s bounding shear layer, a strong resonant, tonal response (whistling) was obtained. At three diameters from the jet, OASPL was 150–160 dB. A tube with a thicker lip generated a louder response. When the tube was held closer to the nozzle exit, the impinging unsteady shear layer could not provoke any significant resonance. The simulations reveal that the tonal response of a Hartmann whistle operating in subsonic mode is significant.
Flow structure caused by a local cross-sectional area increase and curvature in a sharp river bend
Vermeulen, B. ; Hoitink, A.J.F. ; Labeur, R.J. - \ 2015
Journal of Geophysical Research: Earth Surface 120 (2015)9. - ISSN 2169-9003 - p. 1771 - 1783.
channel width - large-eddy simulation - nonhydrostatic pressure - reverse flow - scour - sharp bends
Horizontal flow recirculation is often observed in sharp river bends, causing a complex three-dimensional flow structure with large implications for the morphological and planimetric development of meanders. Several field observations in small-scale systems show that sharp bends are often found in association with a strong increase in cross-sectional area, the deposition of outer bank benches, and reattachment bars near the inner bank. Recent studies show that these bends can also occur in large-scale systems. In this study, we present field measurements of a sharp bend in the Mahakam River, East Kalimantan, Indonesia. The cross-sectional area increases by a factor of 3 compared with the reach-averaged cross-sectional area. Along a river reach of about 150 km, cross-sectional area correlates strongly with curvature. The field measurements are analyzed together with the results from numerical simulation with a three-dimensional finite element model, which yields a comprehensive view of the intricate flow structure. In turn, the model is used to validate a new equation that captures the water surface topography dependence on cross-sectional area variation and curvature. The results show the importance of the increase in cross-sectional area in the development of horizontal recirculation. Vertical acceleration of the flow into the scour causes the pressure to deviate from a hydrostatic pressure distribution. Strong downflow (up to 12 cm s-1) advects longitudinal momentum toward the bed, causing the flow to concentrate in the lower part of the cross section. This increases the velocity magnitude throughout the cross section, which is expected to maintain the large scour depth found in several bends along the Mahakam River.
Study of a prototypical convective boundary layer observed during BLLAST: contributions by large-scale forcings
Pietersen, H.P. ; Vilà-Guerau De Arellano, J. ; Augustin, P. ; Boer, A. van de; Coster, O. de; Delbarre, H. ; Durand, P. ; Fourmentin, M. ; Gioli, B. ; Hartogensis, O.K. ; Lohou, F. ; Lothon, M. ; Ouwersloot, H.G. ; Pino, D. ; Reuder, J. - \ 2015
Atmospheric Chemistry and Physics 15 (2015). - ISSN 1680-7316 - p. 4241 - 4257.
large-eddy simulation - turbulence - entrainment - transition - radiation - decay
We study the influence of the large-scale atmospheric contribution to the dynamics of the convective boundary layer (CBL) in a situation observed during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign. We employ two modeling approaches, the mixed-layer theory and large-eddy simulation (LES), with a complete data set of surface and upper-air atmospheric observations, to quantify the contributions of the advection of heat and moisture, and subsidence. We find that by only taking surface and entrainment fluxes into account, the boundary-layer height is overestimated by 70 %. Constrained by surface and upper-air observations, we infer the large-scale vertical motions and horizontal advection of heat and moisture. Our findings show that subsidence has a clear diurnal pattern. Supported by the presence of a nearby mountain range, this pattern suggests that not only synoptic scales exert their influence on the boundary layer, but also mesoscale circulations. LES results show a satisfactory correspondence of the vertical structure of turbulent variables with observations. We also find that when large-scale advection and subsidence are included in the simulation, the values for turbulent kinetic energy are lower than without these large-scale forcings. We conclude that the prototypical CBL is a valid representation of the boundary-layer dynamics near regions characterized by complex topography and small-scale surface heterogeneity, provided that surface- and large-scale forcings are representative for the local boundary layer.
Subcloud-Layer Feedbacks Driven by the Mass Flux of Shallow Cumulus Convection over Land
Stratum, B.J.H. van; Vilà-Guerau De Arellano, J. ; Heerwaarden, C.C. van; Ouwersloot, H.G. - \ 2014
Journal of the Atmospheric Sciences 71 (2014)3. - ISSN 0022-4928 - p. 881 - 895.
large-eddy simulation - capped boundary-layer - topped mixed layers - relative-humidity - daytime evolution - soil-moisture - part i - cloud - model - equilibrium
The processes and feedbacks associated with the mass flux of shallow cumulus clouds over land are studied by analyzing the results from large-eddy simulations and a mixed-layer model. The primary focus is to study the development of the (well mixed) subcloud layer and understand the four primary feedbacks between the subcloud-layer dynamics and cumulus mass flux. Guided by numerical experiments in large-eddy simulations that show the transition from clear to cloudy boundary layers at midlatitudes over land, the feedbacks introduced by shallow cumuli are first conceptually described. To study the complex interplay between the subcloud and cloud layer, a mixed-layer model is proposed and validated with large-eddy simulations for the Atmospheric Radiation Measurement Southern Great Plains case. The mixed-layer model is shown to identify and reproduce the most relevant feedbacks in the transition from clear to cloudy boundary layers: a reduced mixed-layer growth and drying of the subcloud layer by enhanced entrainment and mass flux transport of moisture to the cloud layer. To complete the study, the strength of the different feedbacks is further quantified by an analysis of the individual contributions to the tendency of the relative humidity at the top of the mixed layer.
Quantifying the transport of subcloud layer reactants by shallow cimulus clouds over the Amazon
Ouwersloot, H.G. ; Vilà-Guerau De Arellano, J. ; Stratum, B.J.H. van; Krol, M.C. ; Lelieveld, J. - \ 2013
Journal of Geophysical Research: Atmospheres 118 (2013)23. - ISSN 2169-897X - p. 13041 - 13059.
large-eddy simulation - topped mixed layers - convective boundary-layer - diurnal cycle - surface - land - fluxes - model - chemistry - campaign
We investigate the vertical transport of atmospheric chemical reactants from the subcloud layer to the cumulus cloud layer driven by shallow convection over the Amazon during the dry season. The dynamical and chemical assumptions needed for mesoscale and global chemistry transport model parametrizations are systematically analyzed using a Large Eddy Simulation model. We quantify the mass flux transport contribution to the temporal evolution of reactants. Isoprene, a key atmospheric compound over the tropical rain forest, decreases by 8.5% h-1 on average and 15% h-1 at maximum due to mass¿flux¿induced removal. We apply mass flux parametrizations for the transport of chemical reactants and obtain satisfactory agreement with numerically resolved transport, except for some reactants like O3, NO, and NO2. The latter is caused by the local partitioning of reactants, influenced by UV radiation extinction by clouds and small¿scale variability of ambient atmospheric compounds. By considering the longer¿lived NOx (NO + NO2), the transport is well represented by the parametrization. Finally, by considering heterogeneous surface exchange conditions, it is demonstrated that the parametrizations are sensitive to boundary conditions due to changes in the boundary layer dynamics.
Surface and atmospheric controls on the onset of moist convection over land
Gentine, P. ; Holtslag, A.A.M. ; Andrea, F. D'; Ek, M. - \ 2013
Journal of Hydrometeorology 14 (2013). - ISSN 1525-755X - p. 1443 - 1462.
large-eddy simulation - fraction diurnal behavior - probabilistic bulk model - coupled mixed-layer - boundary-layer - soil-moisture - evaporative fraction - relative-humidity - hydrologic perspective - spatial variability
The onset of moist convection over land is investigated using a conceptual approach with a slab boundary layer model. We here determine the essential factors for the onset of boundary layer clouds over land, and study their relative importance. They are: 1) the ratio of the temperature to the moisture lapse rates of the free troposphere, i.e. the inversion Bowen ratio, 2) the mean-daily surface temperature, 3) the relative humidity of the free troposphere and 4) the surface evaporative fraction. A clear transition is observed between two regimes of moistening of the boundary layer as assessed by the relative humidity at the boundary layer top. In the first so-called wet soil advantage regime, the moistening results from the increase of the mixed-layer specific humidity, which linearly depends on the surface evaporative fraction and inversion Bowen ratio through a dynamic boundary layer factor. In the second so-called dry soil advantage regime, the relative humidity tendency at the boundary layer top is controlled by the thermodynamics and changes in the moist adiabatic induced by the decreased temperature at the boundary layer top and consequent reduction in saturation water vapor pressure. This regime pertains for very deep boundary layers under weakly stratified free troposphere over hot surface conditions. In the context of the conceptual model, a rise in free-tropospheric temperature (global warming) increases the occurrence of deep convection and reduces the cloud cover over moist surfaces. This study provides new intuition and predictive capacity on the mechanism controlling the occurrence of moist convection over land
Diagnosing evaporative fraction over land from boundary-layer clouds
Gentine, P. ; Ferguson, C.R. ; Holtslag, A.A.M. - \ 2013
Journal of Geophysical Research: Atmospheres 118 (2013)15. - ISSN 2169-897X - p. 8185 - 8196.
large-aperture scintillometer - large-eddy simulation - relative-humidity - mixed-layer - cumulus convection - diurnal behavior - soil-moisture - atmosphere interaction - surface-temperature - spatial variability
The potential use of continental fair-weather shallow cumuli as a way to retrieve the daily surface evaporative fraction over land is evaluated in convective conditions. The proposed method utilizes the fact that both the timing of cloud occurrence and the cloud base height at the time of occurrence provide strong constraints on the surface energy balance and evaporative fraction. The retrieval is especially reliable in the presence of relatively stable and humid free troposphere profiles. The advantage of the method is that it provides a more direct estimate of the surface evaporative fraction than indirect estimation based on inversion of a highly parameterized land surface model. In addition, the evaporative fraction is obtained at a scale of a few kilometers, which is more pertinent for weather and climate studies. The retrieval strategy is tested and validated for three contrasting climates: the U.S. southern Great Plains, West Africa, and the Netherlands. We suggest that the use of satellite observations of shallow cumuli can help constrain the retrieval of the surface evaporative fraction within a data assimilation scheme/reanalysis
Improving Stable Boundary-Layer Height Estimation Using a Stability-Dependent Critical Bulk Richardson Number
Richardson, H. ; Basu, S. ; Holtslag, A.A.M. - \ 2013
Boundary-Layer Meteorology 148 (2013)1. - ISSN 0006-8314 - p. 93 - 109.
large-eddy simulation - turbulence structure - resistance laws - climate model - surface - depth - formulations - evolution - profile - range
For many decades, attempts have been made to find the universal value of the critical bulk Richardson number (Ri Bc ; defined over the entire stable boundary layer). By analyzing an extensive large-eddy simulation database and various published wind-tunnel data, we show that Ri Bc is not a constant, rather it strongly depends on bulk atmospheric stability. A (qualitatively) similar dependency, based on the well-known resistance laws, was reported by Melgarejo and Deardorff (J Atmos Sci 31:1324–1333, 1974) about forty years ago. To the best of our knowledge, this result has largely been ignored. Based on data analysis, we find that the stability-dependent Ri Bc estimates boundary-layer height more accurately than the conventional constant Ri Bc approach. Furthermore, our results indicate that the common practice of setting Ri Bc as a constant in numerical modelling studies implicitly constrains the bulk stability of the simulated boundary layer. The proposed stability-dependent Ri Bc does not suffer from such an inappropriate constraint.
Characterization of a boreal convective boundary layer and its impact on atmospheric chemistry during HUMPPA-COPEC-2010
Ouwersloot, H.G. ; Vilà-Guerau de Arellano, J. ; Nölscher, A.C. ; Krol, M.C. ; Ganzeveld, L.N. ; Breitenberger, C. ; Mammarella, I. ; Williams, J. ; Lelieveld, J. - \ 2012
Atmospheric Chemistry and Physics 12 (2012). - ISSN 1680-7316 - p. 9335 - 9353.
large-eddy simulation - aerosol formation - forest - model - entrainment - turbulence - evolution - inversion - emissions - exchanges
We studied the atmospheric boundary layer (ABL) dynamics and the impact on atmospheric chemistry during the HUMPPA-COPEC-2010 campaign. We used vertical profiles of potential temperature and specific moisture, obtained from 132 radio soundings, to determine the main boundary layer characteristics during the campaign. We propose a classification according to several main ABL prototypes. Further, we performed a case study of a single day, focusing on the convective boundary layer, to analyse the influence of the dynamics on the chemical evolution of the ABL. We used a mixed layer model, initialized and constrained by observations. In particular, we investigated the role of large scale atmospheric dynamics (subsidence and advection) on the ABL development and the evolution of chemical species concentrations. We find that, if the large scale forcings are taken into account, the ABL dynamics are represented satisfactorily. Subsequently, we studied the impact of mixing with a residual layer aloft during the morning transition on atmospheric chemistry. The time evolution of NOx and O3 concentrations, including morning peaks, can be explained and accurately simulated by incorporating the transition of the ABL dynamics from night to day. We demonstrate the importance of the ABL height evolution for the representation of atmospheric chemistry. Our findings underscore the need to couple the dynamics and chemistry at different spatial scales (from turbulence to mesoscale) in chemistry-transport models and in the interpretation of observational data.
On the segregation of chemical species in a clear boundary layer over heterogeneous land surfaces
Ouwersloot, H.G. ; Vilà-Guerau de Arellano, J. ; Heerwaarden, C.C. van; Ganzeveld, L.N. ; Krol, M.C. ; Lelieveld, J. - \ 2011
Atmospheric Chemistry and Physics 11 (2011). - ISSN 1680-7316 - p. 10681 - 10704.
large-eddy simulation - volatile organic-compounds - tropical rain-forest - atmospheric chemistry - deciduous forest - heat-flux - emission - campaign - scale - turbulence
Using a Large-Eddy Simulation model, we have systematically studied the inability of boundary layer turbulence to efficiently mix reactive species. This creates regions where the species are accumulated in a correlated or anti-correlated way, thereby modifying the mean reactivity. We quantify this modification by the intensity of segregation, IS, and analyse the driving mechanisms: heterogeneity of the surface moisture and heat fluxes, various background wind patterns and non-uniform isoprene emissions. The heterogeneous surface conditions are characterized by cool and wet forested patches with high isoprene emissions, alternated with warm and dry patches that represents pasture with relatively low isoprene emissions. For typical conditions in the Amazon rain forest, applying homogeneous surface forcings and in the absence of free tropospheric NOx, the isoprene- OH reaction rate is altered by less than 10 %. This is substantially smaller than the previously assumed IS of 50% in recent large-scale model analyses of tropical rain forest chemistry. Spatial heterogeneous surface emissions enhance the segregation of species, leading to alterations of the chemical reaction rates up to 20 %. The intensities of segregation are enhanced when the background wind direction is parallel to the borders between the patches and reduced in the case of a perpendicular wind direction. The effects of segregation on trace gas concentrations vary per species. For the highly reactive OH, the differences in concentration averaged over the boundary layer are less than 2% compared to homogeneous surface conditions, while the isoprene concentration is increased by as much as 12% due to the reduced chemical reaction rates. These processes take place at the sub-grid scale of chemistry transport models and therefore need to be parameterized.
Boundary Layer Characteristics over Homogeneous and Heterogeneous Surfaces Simulated by MM5 and DALES
Braam, M. ; Vilà-Guerau de Arellano, J. ; Górska, M. - \ 2011
Journal of Applied Meteorology and Climatology 50 (2011)6. - ISSN 1558-8424 - p. 1372 - 1386.
large-eddy simulation - relative-humidity - diurnal cycle - model - land - entrainment - diffusion - evaporation - ihop-2002 - moisture
The multiple-single-column approach is proposed as a new concept to study the boundary layer parameterization scheme in the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). The results are compared with the Dutch Atmospheric Large-Eddy Simulation Model (DALES). Numerical experiments were performed over homogeneous and heterogeneous surfaces under clear convective boundary layer conditions. Identical simulations using MM5 and DALES were performed, which enabled an evaluation of theMM5boundary layer scheme with DALES results. From the experiment with a homogeneous surface, MM5 shows a slightly shallower, colder, and moister boundary layer than DALES. This result is produced by an underestimation of turbulent mixing near the surface and less-vigorous entrainment of heat and dry air in MM5. In the heterogeneous surface experiment, the domain is divided into dry and wet patches, with the result that both models produce a mesoscale circulation. However, relative to the homogeneous case, larger differences were found between the models in the representation of the boundary layer dynamics. In DALES, the surface heterogeneity influenced the turbulent motions, making the mesoscale circulation much stronger (wmax is 6 times as large) than in MM5. Because of this stronger circulation, the boundary layer height, bulk temperature, and humidity also displayed differences in time and spatial patterns. Because of the land–atmosphere coupling in MM5, the mesoscale circulation strengthened the surface flux heterogeneity. Cold and moist air advection close to the surface from the wet patch to the dry patch increased the sensible heat flux above the dry patch and thus the induced mesoscale flow
The role of boundary layer dynamics on the diurnal evolution of isoprene and the hydroxyl radical over tropical forests
Vilà-Guerau de Arellano, J. ; Patton, E.G. ; Karl, T. ; Dries, K. van den; Barth, M.C. ; Orlando, J.J. - \ 2011
Journal of Geophysical Research: Atmospheres 116 (2011)D7. - ISSN 2169-897X - 16 p.
large-eddy simulation - atmospheric chemistry - stiff odes - fluxes - entrainment - emissions - scalars - model - hydrocarbons - inversion
We investigate diurnal variability of isoprene and related chemical species in the Amazonian region. The dynamics and chemistry of an atmospheric boundary layer are studied with a large-eddy simulation code and a mixed-layer model which are guided by observations available for the same area. The main features of isoprene and related species are reproduced well, but their evolution raises questions regarding the physical and chemical processes responsible for the observed diurnal behaviors. To address these questions, we systematically examine the role of (1) the exchange of chemical species between the free troposphere and the atmospheric boundary layer (entrainment), (2) surface isoprene and nitric oxide emissions, and (3) new chemical pathways to recycle the hydroxyl radical. The entrainment flux of isoprene is shown to be equally important as surface isoprene emissions in determining the isoprene temporal evolution. Varying the relationship between the initial isoprene mixing ratio in the boundary layer and that in the overlying free troposphere in the early morning results in an 50% increase/decrease in isoprene mixing ratio or more within the atmospheric boundary layer at noon. Entrainment of free tropospheric nitrogen oxides creates changes of similar magnitude to the boundary layer isoprene mixing ratio. These effects of entrainment and surface emissions on isoprene are found for two different chemical regimes. The introduction of an OH recycling pathway in the chemical mechanism increases midday OH. Our findings show that atmospheric dynamics and chemistry are equally important for interpreting the diurnal observation of reactants and for including in regional-scale modeling efforts where turbulence is parameterized.
Analysis of Model Results for the Turning of the Wind and Related Momentum Fluxes in the Stable Boundary Layer
Svensson, G. ; Holtslag, A.A.M. - \ 2009
Boundary-Layer Meteorology 132 (2009)2. - ISSN 0006-8314 - p. 261 - 277.
large-eddy simulation - turbulence - scheme
The turning of wind with height and the related cross-isobaric (ageostrophic) flow in the thermally stable stratified boundary layer is analysed from a variety of model results acquired in the first Global Energy and Water Cycle Experiment (GEWEX) Atmospheric Boundary Layer Study (GABLS1). From the governing equations in this particular simple case it becomes clear that the cross-isobaric flow is solely determined by the surface turbulent stress in the direction of the geostrophic wind for the quasi-steady state conditions under consideration. Most models indeed seem to approach this relationship but for very different absolute values. Because turbulence closures used in operational models typically tend to give too deep a boundary layer, the integrated total cross-isobaric mass flux is up to three times that given by research numerical models and large-eddy simulation. In addition, the angle between the surface and the geostrophic wind is typically too low, which has important implications for the representation of the larger-scale flow. It appears that some models provide inconsistent results for the surface angle and the momentum flux profile, and when the results from these models are removed from the analysis, the remaining ten models do show a unique relationship between the boundary-layer depth and the surface angle, consistent with the theory given. The present results also imply that it is beneficial to locate the first model level rather close to the surface for a proper representation of the turning of wind with height in the stable boundary layer
Turbulent dispersion in cloud-topped boundary layers
Verzijlbergh, R.A. ; Jonker, H.J.J. ; Heus, T. ; Vilà-Guerau de Arellano, J. - \ 2009
Atmospheric Chemistry and Physics 9 (2009)4. - ISSN 1680-7316 - p. 1289 - 1302.
large-eddy simulation - shallow cumulus clouds - plume dispersion - relative dispersion - models - stratocumulus - statistics - transport - scheme
Compared to dry boundary layers, dispersion in cloud-topped boundary layers has received less attention. In this LES based numerical study we investigate the dispersion of a passive tracer in the form of Lagrangian particles for four kinds of atmospheric boundary layers: 1) a dry convective boundary layer (for reference), 2) a "smoke" cloud boundary layer in which the turbulence is driven by radiative cooling, 3) a stratocumulus topped boundary layer and 4) a shallow cumulus topped boundary layer. We show that the dispersion characteristics of the smoke cloud boundary layer as well as the stratocumulus situation can be well understood by borrowing concepts from previous studies of dispersion in the dry convective boundary layer. A general result is that the presence of clouds enhances mixing and dispersion ¿ a notion that is not always reflected well in traditional parameterization models, in which clouds usually suppress dispersion by diminishing solar irradiance. The dispersion characteristics of a cumulus cloud layer turn out to be markedly different from the other three cases and the results can not be explained by only considering the well-known top-hat velocity distribution. To understand the surprising characteristics in the shallow cumulus layer, this case has been examined in more detail by 1) determining the velocity distribution conditioned on the distance to the nearest cloud and 2) accounting for the wavelike behaviour associated with the stratified dry environment
Role of Shear and the Inversion Strength During Sunset Turbulence Over Land: Characteristic Length Scales, pre print
Pino, D. ; Jonker, H.J.J. ; Vilà-Guerau de Arellano, J. ; Dosio, A. - \ 2006
Boundary-Layer Meteorology 121 (2006)3. - ISSN 0006-8314 - p. 537 - 556.
convective boundary-layer - large-eddy simulation - homogeneous isotropic turbulence - decay - transition - transport
The role of shear and inversion strength on the decay of convective turbulence during sunset over land is systematically studied by means of large-eddy simulations. Different decay rates have been found for the vertical and horizontal velocity fluctuations, resulting in an increase of the anisotropy for all the studied cases. Entrainment, which persists during the decay process, favours the appearance of vertical upward movements associated with a conversion from kinetic to potential energy. Particular attention is paid to the evolution of the characteristic length scale of the various turbulent variables during this process. The length scale evolution is found to depend on the wind shear characteristics, but not on the strength of the inversion. In general the length scales of the variables grow during decay because small-scale fluctuations dissipate faster than large-scale fluctuations. Only the length scale of the vertical velocity component remains nearly constant during decay. Spectral analysis of the variance budgets shows that pressure correlations are responsible for fixing this length scale, effectively compensating the strong but oscillating influence of buoyancy. In the shear cases, after an initial period of growth, the length scales start to decrease once the buoyancy-generated variance has sufficiently subsided. Also here the effect of pressure redistribution is crucial, as it transfers the spectral influence of shear to the other velocity components
Modelling the Arctic Stable boundary layer and its coupling to the surface
Steeneveld, G.J. ; Wiel, B.J.H. van de; Holtslag, A.A.M. - \ 2006
Boundary-Layer Meteorology 118 (2006)2. - ISSN 0006-8314 - p. 357 - 378.
large-eddy simulation - intermittent turbulence - land-surface - atmospheric models - energy-balance - parameterization - fluxes - oscillations - temperature - validation
The impact of coupling the atmosphere to the surface energy balance is examined for the stable boundary layer, as an extension of the first GABLS (GEWEX Atmospheric Boundary-Layer Study) one-dimensional model intercomparison. This coupling is of major importance for the stable boundary-layer structure and its development because coupling enables a realistic physical description of the interdependence of the surface temperature and the surface sensible heat flux. In the present case, the incorporation of a surface energy budget results in stronger cooling (surface decoupling), and a more stable and less deep boundary layer. The proper representation of this is a problematic feature in large-scale numerical weather prediction and climate models. To account for the upward heat flux from the ice surface beneath, we solve the diffusion equation for heat in the underlying ice as a first alternative. In that case, we find a clear impact of the vertical resolution in the underlying ice on boundary-layer development: coarse vertical resolution in the ice results in stronger surface cooling than for fine resolution. Therefore, because of this impact on stable boundary-layer development, the discretization in the underlying medium needs special attention in numerical modelling studies of the nighttime boundary layer. As a second alternative, a bulk conductance layer with stagnant air near the surface is added. The stable boundary-layer development appears to depend heavily on the bulk conductance of the stagnant air layer. This result re-emphasizes the fact that the interaction with the surface needs special attention in stable boundary-layer studies. Furthermore, we perform sensitivity studies to atmospheric resolution, the length-scale formulation and the impact of radiation divergence on stable boundary-layer structure for weak windy conditions
Single-Column Model Intercomparison for a Stably Stratified Atmospheric Boundary Layer
Cuxart, J. ; Holtslag, A.A.M. ; Beare, R.J. ; Bazile, E. ; Beljaars, A. ; Cheng, A. ; Conangla, L. ; Ek, M.B. ; Freedman, F. ; Hamdi, R. ; Kerstein, A. ; Kitagawa, H. ; Lenderink, G. ; Lewellen, D. ; Mailhot, J. ; Mauritsen, T. ; Perov, V. ; Schayes, G. ; Steeneveld, G.J. ; Svensson, G. ; Taylor, P. ; Weng, W. ; Wunsch, S. ; Xu, K.M. - \ 2006
Boundary-Layer Meteorology 118 (2006)2. - ISSN 0006-8314 - p. 273 - 303.
turbulence closure-model - large-eddy simulation - part i - scheme - parameterization - formulation - diffusion - dynamics - surfaces - system
The parameterization of the stably stratified atmospheric boundary layer is a difficult issue, having a significant impact on medium-range weather forecasts and climate integrations. To pursue this further, a moderately stratified Arctic case is simulated by nineteen single-column turbulence schemes. Statistics from a large-eddy simulation intercomparison made for the same case by eleven different models are used as a guiding reference. The single-column parameterizations include research and operational schemes from major forecast and climate research centres. Results from first-order schemes, a large number of turbulence kinetic energy closures, and other models were used. There is a large spread in the results; in general, the operational schemes mix over a deeper layer than the research schemes, and the turbulence kinetic energy and other higher-order closures give results closer to the statistics obtained from the large-eddy simulations. The sensitivities of the schemes to the parameters of their turbulence closures are partially explored
Relating Eulerian and Lagrangian Statistics for the Turbulent Dispersion in the Atmospheric Convective Boundary Layer
Dosio, A. ; Vilà-Guerau de Arellano, J. ; Holtslag, A.A.M. ; Builtjes, P.J.H. - \ 2005
Journal of the Atmospheric Sciences 62 (2005)4. - ISSN 0022-4928 - p. 1175 - 1191.
large-eddy simulation - direct numerical simulations - laboratory model - time-scale - plume dispersion - buoyancy-driven - diffusion - shear - field - spectra
Eulerian and Lagrangian statistics in the atmospheric convective boundary layer (CBL) are studied by means of large eddy simulation (LES). Spectra analysis is performed in both the Eulerian and Lagrangian frameworks, autocorrelations are calculated, and the integral length and time scales are derived. Eulerian statistics are calculated by means of spatial and temporal analysis in order to derive characteristic length and time scales. Taylor's hypothesis of frozen turbulence is investigated, and it is found to be satisfied in the simulated flow. Lagrangian statistics are derived by tracking the trajectories of numerous particles released at different heights in the turbulent flow. The relationship between Lagrangian properties (autocorrelation functions) and dispersion characteristics (particles' displacement) is studied through Taylor's diffusion relationship, with special emphasis on the difference between horizontal and vertical motion. Results show that for the horizontal motion, Taylor's relationship is satisfied. The vertical motion, however, is influenced by the inhomogeneity of the flow and limited by the ground and the capping inversion at the top of the CBL. The Lagrangian autocorrelation function, therefore, does not have an exponential shape, and consequently, the integral time scale is zero. If distinction is made between free and bounded motion, a better agreement between Taylor's relationship and the particles' vertical displacement is found. Relationships between Eulerian and Lagrangian fr ameworks are analyzed by calculating the ratio ß between Lagrangian and Eulerian time scales. Results show that the integral time scales are mainly constant with height for z/zi <0.7. In the upper part of the CBL, the capping inversion transforms vertical motion into horizontal motion. As a result, the horizontal time scale increases with height, whereas the vertical one is reduced. Current parameterizations for the ratio between the Eulerian and Lagrangian time scales have been tested against the LES results showing satisfactory agreement at heights z/zi <0.7.
Introducing effective reaction rates to account for the inefficient mixing of the convective boundary layer
Vinuesa, J.F. ; Vilà-Guerau de Arellano, J. - \ 2005
Atmospheric Environment 39 (2005)3. - ISSN 1352-2310 - p. 445 - 461.
large-eddy simulation - chemical-reactions - turbulence - chemistry - diffusion - inversion - scalars - fluxes - models - time
The convective boundary layer (CBL) is characterised by narrow vigorous thermals (updraft motions) surrounded by relatively large subsidence motions. In such a flow. reactants are normally segregated and their chemical transformations depend on the ability of atmospheric turbulence to mix them. This process is particularly important when the time-scale of the chemical transformation is similar to the turbulent characteristic time scale. For large atmospheric models, the segregation occurs at scales smaller than the grid length. As a result. instantaneous and homogeneous mixing of the reactants is normally assumed. This paper is aimed at the study of this assumption and to apply a parameterisation of an effective reaction rate accounting for the inefficient mixing due to convective turbulence in the CBL. We simulate a growing CBL with two models that use different physical assumptions. The first one. the so-called mixed-layer model, assumes an instantaneous and homogeneous mixing of the reactants in the boundary layer (BL). The second one, a three-dimensional large eddy simulation (LES) model: explicitly solves atmospheric turbulent motions and describes the heterogeneity of the mixing due to the turbulent characteristics of the CBL. By comparing their results in a simple case, i.e. a second-order chemical reaction. we show that the heterogeneous mixing due to convective turbulence has an important impact on the chemical transformations by slowing down the reaction rate. By, introducing effective reaction rates through a parameterisation which accounts for this inefficient mixing. the mixed-layer model results improve significantly. We extend our study to a chemical mechanism that accounts for the ozone formation and depletion in the CBL. We show that the reaction rate can be slowed down or increased depending on whether the reactants are transported in opposite directions or not. We propose coefficients to be used to calculate the effective reaction rates in large-scale or mixed-layer models. (C) 2004 Elsevier Ltd. All rights reserved.