Similarity Relations for C_T^2 in the Unstable Atmospheric Surface Layer: Dependence on Regression Approach, Observation Height and Stability Range
Braam, M. ; Moene, A.F. ; Beyrich, F. ; Holtslag, A.A.M. - \ 2014
Boundary-Layer Meteorology 153 (2014)1. - ISSN 0006-8314 - p. 63 - 87.
nocturnal boundary-layer - aperture scintillometer - structure parameters - sonic anemometer - sensible heat - fluxes - temperature - scintillation - cases-99 - momentum
A great variety of similarity functions for the structure parameter of temperature ( C 2 T ) have been proposed in the literature. They differ in the way they were derived from the data and in the characteristics of the dataset used for their derivation (surface type, observation level, stability range). In this study, we use one single dataset (CASES-99 experiment) and investigate the impact on the similarity functions of applying various regression approaches, and measuring at different heights and within different stability ranges. We limit ourselves to similarity functions under unstable conditions, and evaluate only the most common shape that describes the relation with two coefficients ( f(z/L)=c 1 (1-c 2 z/L) -2/3 , where z is the height, and L is the Obukhov length and a measure of the stability, and c 1 and c 2 are the regression coefficients). The results show that applying various regression approaches has an impact on the regression coefficients c 1 and c 2 . Thus studies should always specify the regression approach when presenting similarity relations. We suggest use of an orthogonal distance regression method such that uncertainties in -z/L are also taken into account, to apply this to the logarithmic transformation of both dimensionless groups, and to use a weighted dataset such that unreliable data points have a smaller influence on the fit. Dividing the dataset into eight height ( z ) and eight stability ( -1/L classes) classes, we show that the observation height and the stability range has an impact on the coefficients too. This implies that variations in c 1 and c 2 found in the literature may result from variations in the height and stability ranges among the datasets. Furthermore, application of the coefficients on a dataset obtained at a different height or within a different stability range has to be made with care. Finally, the variation in the coefficients between the classes indicates that the Monin–Obukhov similarity function for C 2 T is not sufficiently described by the two-coefficient function used here.
Stable atmospheric boundary layers and diurnal Cycles-Challenges for Weather and Climate Models
Holtslag, A.A.M. ; Svensson, G. ; Baas, P. ; Basu, S. ; Beare, B. ; Beljaars, A.C.M. ; Bosveld, F.C. ; Cuxart, J. ; Lindvall, J. ; Steeneveld, G.J. ; Tjernstrom, M. ; Wiel, B.J.H. van de - \ 2013
Bulletin of the American Meteorological Society 94 (2013). - ISSN 0003-0007 - p. 1691 - 1706.
low-level jets - land-surface - contrasting nights - soil-moisture - ecmwf model - sea-ice - turbulence - cases-99 - parameterization - fluxes
The representation of the atmospheric boundary layer is an important part of weather and climate models and impacts many applications such as air quality and wind energy. Over the years, the performance in modeling 2 m temperature and 10 m wind speed has improved but errors are still significant. This is in particular the case under clear skies and low wind-speed conditions at night as well as during winter in stably stratified conditions over land and ice. In this paper, we review these issues and provide an overview of the current understanding and model performance. Results from weather forecast and climate models are used to illustrate the state of the art, as well as findings and recommendations from three inter-comparison studies held within the “Global Energy and Water Exchanges (GEWEX)” Atmospheric Boundary Layer Study (GABLS). Within GABLS, the focus has been on the examination of the representation of the stable boundary layer and the diurnal cycle over land in clear sky conditions. For this purpose, single-column versions of weather and climate models have been compared with observations, research models and Large Eddy Simulations. The intercomparison cases are based on observations taken in the Arctic, Kansas and at Cabauw in the Netherlands. From these studies, we find that even for the non-cloudy boundary layer important parameterization challenges remain.
The Minimum Wind Speed for Sustainable Turbulence in the Nocturnal Boundary Layer
Wiel, B.J.H. van de; Moene, A.F. ; Jonker, H.J.J. ; Baas, P. ; Basu, S. ; Donda, J.M.M. ; Sun, J. ; Holtslag, A.A.M. - \ 2012
Journal of the Atmospheric Sciences 69 (2012)1. - ISSN 0022-4928 - p. 3116 - 3127.
large-eddy simulations - temperature-fluctuation method - surface-energy balance - low-level jet - stable conditions - intermittent turbulence - land-surface - heat-flux - regimes - cases-99
The collapse of turbulence in the nocturnal boundary layer is studied by means of a simple bulk model that describes the basic physical interactions in the surface energy balance. It is shown that for a given mechanical forcing, the amount of turbulent heat that can be transported downward is limited to a certain maximum. In the case of weak winds and clear skies, this maximum can be significantly smaller than the net radiative loss minus soil heat transport. In the case when the surface has low heat capacity, this imbalance generates rapid surface cooling that further suppresses the turbulent heat transport, so that eventually turbulence largely ceases (positive feedback mechanism). The model predicts the minimum wind speed for sustainable turbulence for the so-called crossing level. At this level, some decameters above the surface, the wind is relatively stationary compared to lower and higher levels. The critical speed is predicted in the range of about 5–7 m s-1, depending on radiative forcing and surface properties, and is in agreement with observations at Cabauw. The critical value appears not very sensitive to model details or to the exact values of the input parameters. Finally, results are interpreted in terms of external forcings, such as geostrophic wind. As it is generally larger than the speed at crossing height, a 5 m s-1 geostrophic wind may be considered as the typical limit below which sustainable, continuous turbulence under clear-sky conditions is unlikely to exist. Below this threshold emergence of the very stable nocturnal boundary layer is anticipated.
The Cessation of Continuous Turbulence as Precursor of the Very Stable Nocturnal Boundary Layer
Wiel, B.J.H. van de; Moene, A.F. ; Jonker, H.J.J. - \ 2012
Journal of the Atmospheric Sciences 69 (2012)1. - ISSN 0022-4928 - p. 3097 - 3115.
large-eddy simulations - temperature-fluctuation method - low-level jet - intermittent turbulence - atmospheric models - surface-layer - land-surface - part i - regimes - cases-99
The mechanism behind the collapse of turbulence in the evening as a precursor to the onset of the very stable boundary layer is investigated. To this end a cooled, pressure-driven flow is investigated by means of a local similarity model. Simulations reveal a temporary collapse of turbulence whenever the surface heat extraction, expressed in its nondimensional form h/L, exceeds a critical value. As any temporary reduction of turbulent friction is followed by flow acceleration, the long-term state is unconditionally turbulent. In contrast, the temporary cessation of turbulence, which may actually last for several hours in the nocturnal boundary layer, can be understood from the fact that the time scale for boundary layer diffusion is much smaller than the time scale for flow acceleration. This limits the available momentum that can be used for downward heat transport. In case the surface heat extraction exceeds the so-called maximum sustainable heat flux (MSHF), the near-surface inversion rapidly increases. Finally, turbulent activity is largely suppressed by the intense density stratification that supports the emergence of a different, calmer boundary layer regime.
Composite hodographs and inertial oscillations in the nocturnal boundary layer
Baas, P. ; Wiel, B.J.H. van de; Brink, L. van den; Holtslag, A.A.M. - \ 2012
Quarterly Journal of the Royal Meteorological Society 138 (2012)663. - ISSN 0035-9009 - p. 528 - 535.
low-level jet - wind-structure variations - southern great-plains - climatology - cabauw - intermittent - turbulence - cases-99 - model - land
In this work the dynamic behaviour of the wind in the nocturnal boundary layer is studied, with a particular focus on systematic behaviour of the near-surface wind. Recently, an extension of the well-known Blackadar model for frictionless inertial oscillations above the nocturnal boundary layer was proposed by Van deWiel et al., which accounts for frictional effects within the nocturnal boundary layer. It appears that the nocturnal wind velocity profile tends to perform an inertial oscillation around an equilibrium wind profile, rather than around the geostrophic wind vector (as in the Blackadar model). In the present studywe propose the concept of ‘composite hodographs’ to evaluate the ideas and assumptions of the aforementioned analytical model. Composite hodographs are constructed based on a large observational dataset from the Cabauw observatory. For comparison and deeper analysis, this method is also applied to single-column model simulations that represent the same dataset. From this, it is shown that winds in the middle and upper part of the nocturnal boundary layer closely follow the dynamics predicted by the model by Van de Wiel et al. In contrast, the near-surface wind shows more complex behaviour that can be described by two different stages: (1) a decelerating phase where the wind decreases rapidly in magnitude due to enlarged stress divergence in the transition period near sunset (an aspect not included in the analytical model), and (2) a regular type of inertial oscillation, but with relatively small amplitude as compared to the oscillations in the middle and upper parts of the nocturnal boundary layer
Evaluation of the Diurnal Cycle in the Atmospheric Boundary Layer Over Land as Represented by a Variety of Single-Column Models: The Second GABLS Experiment
Svensson, G. ; Holtslag, A.A.M. ; Kumar, V. ; Mauritsen, T. ; Steeneveld, G.J. ; Angevine, W.M. ; Bazile, E. ; Beljaars, A. ; Bruijn, E.I.F. de; Cheng, A. - \ 2011
Boundary-Layer Meteorology 140 (2011)2. - ISSN 0006-8314 - p. 177 - 206.
turbulence closure scheme - large-eddy simulations - part i - contrasting nights - morning transition - parameterization - mesoscale - cases-99 - surface - system
We present the main results from the second model intercomparison within the GEWEX (Global Energy andWater cycle EXperiment) Atmospheric Boundary Layer Study (GABLS). The target is to examine the diurnal cycle over land in today’s numerical weather prediction and climate models for operational and research purposes. The set-up of the case is based on observations taken during the Cooperative Atmosphere-Surface Exchange Study-1999 (CASES-99), which was held in Kansas, USA in the early autumn with a strong diurnal cycle with no clouds present. The models are forced with a constant geostrophic wind, prescribed surface temperature and large-scale divergence. Results from 30 different model simulations and one large-eddy simulation (LES) are analyzed and compared with observations. Even though the surface temperature is prescribed, the models give variable near-surface air temperatures. This, in turn, gives rise to differences in low-level stability affecting the turbulence and the turbulent heat fluxes. The increase in modelled upward sensible heat flux during the morning transition is typically too weak and the growth of the convective boundary layer before noon is too slow. This is related to weak modelled nearsurface winds during the morning hours. The agreement between the models, the LES and observations is the best during the late afternoon. From this intercomparison study, we find that modelling the diurnal cycle is still a big challenge. For the convective part of the diurnal cycle, some of the first-order schemes perform somewhat better while the turbulent kinetic energy (TKE) schemes tend to be slightly better during nighttime conditions. Finer vertical resolution tends to improve results to some extent, but is certainly not the solution to all the deficiencies identified
Screen level temperature increase due to higher atmospheric carbon dioxide in calm and windy nights revisited
Steeneveld, G.J. ; Holtslag, A.A.M. ; McNider, R.T. ; Pielke sr., R.A. - \ 2011
Journal of Geophysical Research: Atmospheres 116 (2011). - ISSN 2169-897X - 7 p.
nocturnal boundary-layer - land-surface - simulation - scale - urban - model - cases-99 - terrain
Long-term surface observations over land have shown temperature increases during the last century, especially during nighttime. Observations analyzed by Parker  show similar long-term trends for calm and windy conditions at night, and on basis of this it was suggested that the possible effect of urban heat effects on long-term temperature trends are small. On the other hand, a simplified analytic model study by Pielke and Matsui [2005, henceforth PM05] suggests that at night the resultant long-term temperature trends over land should depend on height and strongly on wind speed (mostly due to alterations in the rate of nocturnal cooling in the stable boundary layer (SBL)). In this paper we expand the PM05 study by using a validated atmospheric boundary-layer model with elaborated atmospheric physics compared to PM05, in order to explore the response of the SBL over land to a change in radiative forcing. We find that the screen level temperature response is surprisingly constant for a rather broad range of both geostrophic wind speed (5-15 ms-1) and 10 meter wind (2-4.0 ms-1). This is mostly due to land surface-vegetation-atmosphere feedbacks taken into account in the present study which were not considered by PM05.
Impact of Surface Flux Formulations and Geostrophic Forcing on Large-Eddy Simulations of Diurnal Atmospheric Boundary Layer Flow
Kumar, V. ; Svensson, G. ; Holtslag, A.A.M. ; Meneveau, C. ; Parlange, M.B. - \ 2010
Journal of Applied Meteorology and Climatology 49 (2010)7. - ISSN 1558-8424 - p. 1496 - 1516.
profile relationships - complex terrain - land-surface - model - wind - temperature - turbulence - cases-99
The impact of surface flux boundary conditions and geostrophic forcing on multiday evolution of flow in the atmospheric boundary layer (ABL) was assessed using large-eddy simulations (LES). The LES investigations included several combinations of surface boundary conditions (temperature and heat flux) and geostrophic forcing (constant, time varying, time and height varying). The setup was based on ABL characteristics observed during a selected period of the Cooperative Atmosphere–Surface Exchange Study—1999 (CASES-99) campaign. The LES cases driven by a constant geostrophic wind achieved the best agreement with the CASES-99 observations specifically in terms of daytime surface fluxes and daytime and nighttime profiles. However, the nighttime fluxes were significantly overestimated. The LES cases with the surface temperature boundary condition and driven by a time- and height-varying geostrophic forcing showed improved agreement with the observed nighttime fluxes, but there was less agreement with other observations (e.g., daytime profiles). In terms of the surface boundary condition, the LES cases driven by either surface temperature or heat fluxes produced similar trends in terms of the daytime profiles and comparisons with data from soundings. However, in reproducing the fluxes and nighttime profiles, the agreement was better with imposed temperature because of its ability to interact dynamically with the air temperature field. Therefore, it is concluded that surface temperature boundary condition is better suited for simulations of temporally evolving ABL flow as in the diurnal evolution of the ABL
A conceptual view on inertial oscillations and nocturnal low-level jets
Wiel, B.J.H. van de; Moene, A.F. ; Steeneveld, G.J. ; Baas, P. ; Bosveld, F.C. ; Holtslag, A.A.M. - \ 2010
Journal of the Atmospheric Sciences 67 (2010)8. - ISSN 0022-4928 - p. 2679 - 2689.
oscillatie - wind - frictie - meteorologische waarnemingen - oscillation - wind - friction - meteorological observations - stable-boundary-layer - tropospheric wind maxima - intermittent turbulence - resistance laws - cabauw - model - cases-99 - energy - land - climatology
In the present work Blackadar's concept on nocturnal inertial oscillations is extended. Blackadar's concept describes frictionless inertial oscillations above the nocturnal inversion layer. The current work includes frictional effects within the nocturnal boundary layer. As a result it is shown that the nocturnal wind speed profile describes an oscillation around the nocturnal equilibrium wind vector, rather than around the geostrophic wind vector (as in the Blackadar case). By using this perspective continuous time-dependent wind profiles are predicted. As such, information on both the height and the magnitude of the nocturnal low-level jet is available as a function of time. Preliminary analysis shows that the proposed extension performs well in comparison with observations, when a simple Ekman model is used to represent the equilibrium state in combination with a realistic initial velocity profile. In addition to jet dynamics, backward inertial oscillations are predicted at lower levels close to the surface, which also appear to be present in observations. The backward oscillation forms an important mechanism behind weakening low-level winds during the afternoon transition. Both observational and theoretical modeling studies are needed to explore this phenomenon further
Observations of the radiation divergence in the surface layer and its implication for its parameterization in numerical weather prediction models
Steeneveld, G.J. ; Wokke, M.J.J. ; Groot Zwaaftink, C.D. ; Pijlman, S. ; Jacobs, A.F.G. ; Holtslag, A.A.M. - \ 2010
Journal of Geophysical Research: Atmospheres 115 (2010). - ISSN 2169-897X - 13 p.
nocturnal boundary-layer - flux divergence - climate model - contrasting nights - transport models - turbulence - greenland - cases-99 - simulations - sensitivity
This paper presents the results of 5 months of in situ observations of the diurnal cycle of longwave radiative heating rate in the lower part of the atmospheric boundary layer over grassland, with a particular focus on nighttime conditions. The observed longwave radiative heating is minimal at the evening transition, with a median value of -1.8 K h-1 between 1.3 and 10 m and -0.5 K h-1 between 10 and 20 m, respectively. After the transition, its magnitude gradually decreases during the night. For individual clear calm nights, a minimal radiative heating rate of -3.5 and -2.0 K h-1 was found for the two indicated layers. The total radiative heating rate appears dominantly controlled by the upward longwave flux divergence. Surprisingly, at noon a radiative heating rate of ~1 K h-1 was found between 1.3 and 10 m for clear calm days. The availability of these radiation divergence measurements enables evaluation of the model performance for the temperature tendency caused by radiation divergence. The mesoscale model MM5 performs poorly for the stable boundary layer, because it overestimates the surface temperature and wind speed, while it underestimates the magnitude of radiative cooling. Some computationally efficient methods based on physical modeling, statistical modeling, and dimensional analysis are proposed by examining the gathered data set. The physical modeling approach appears to perform best.
Towards closing the surface energy budget of a mid-latitude grassland
Jacobs, A.F.G. ; Heusinkveld, B.G. ; Holtslag, A.A.M. - \ 2008
Boundary-Layer Meteorology 126 (2008)1. - ISSN 0006-8314 - p. 125 - 136.
energiebalans - graslanden - fotosynthese - aardoppervlak - warmtestroming - energy balance - grasslands - photosynthesis - land surface - heat flow - balance closure - heat-flux - eddy-correlation - boundary-layer - storage terms - forest - land - cases-99 - carbon - area
Observations for May and August, 2005, from a long-term grassland meteorological station situated in central Netherlands were used to evaluate the closure of the surface energy budget. We compute all possible enthalpy changes, such as the grass cover heat storage, dew water heat storage, air mass heat storage and the photosynthesis energy flux, over an averaging time interval. In addition, the soil heat flux was estimated using a harmonic analysis technique to obtain a more accurate assessment of the surface soil heat flux. By doing so, a closure of 96% was obtained. The harmonic analysis technique appears to improve closure by 9%, the photosynthesis for 3% and the rest of the storage terms for a 3% improvement of the energy budget closure. For calm nights (friction velocity u * <0.1 m s¿1) when the eddy covariance technique is unreliable for measurement of the vertical turbulent fluxes, the inclusion of a scheme that calculates dew fluxes improves the energy budget closure significantly.
Role of land-surface temperature feedback on model performance for the stable boundary layer
Holtslag, A.A.M. ; Steeneveld, G.J. ; Wiel, B.J.H. van de - \ 2007
Boundary-Layer Meteorology 125 (2007)2. - ISSN 0006-8314 - p. 361 - 376.
large-eddy simulations - radiation fog - turbulence - cases-99 - transport - wind
At present a variety of boundary-layer schemes is in use in numerical models and often a large variation of model results is found. This is clear from model intercomparisons, such as organized within the GEWEX Atmospheric Boundary Layer Study (GABLS). In this paper we analyze how the specification of the land-surface temperature affects the results of a boundary-layer scheme, in particular for stable conditions. As such we use a well established column model of the boundary layer and we vary relevant parameters in the turbulence scheme for stable conditions. By doing so, we can reproduce the outcome for a variety of boundary-layer models. This is illustrated with the original set-up of the second GABLS intercomparison study using prescribed geostrophic winds and land-surface temperatures as inspired by (but not identical to) observations of CASES-99 for a period of more than two diurnal cycles. The model runs are repeated using a surface temperature that is calculated with a simple land-surface scheme. In the latter case, it is found that the range of model results in stable conditions is reduced for the sensible heat fluxes, and the profiles of potential temperature and wind speed. However, in the latter case the modelled surface temperatures are rather different than with the original set-up, which also impacts on near-surface air temperature and wind speed. As such it appears that the model results in stable conditions are strongly influenced by non-linear feedbacks in which the magnitude of the geostrophic wind speed and the related land-surface temperature play an important role.
Predicting the collapse of turbulence in stably stratified boundary layers
Wiel, B.J.H. van de; Moene, A.F. ; Steeneveld, G.J. ; Hartogensis, O.K. ; Holtslag, A.A.M. - \ 2007
Flow, Turbulence and Combustion 79 (2007)3. - ISSN 1386-6184 - p. 251 - 274.
temperature-fluctuation method - intermittent turbulence - part ii - cases-99 - land - oscillations - stability - dynamics - surface - fluxes
The collapse of turbulence in a plane channel flow is studied, as a simple analogy of stably stratified atmospheric flow. Turbulence is parameterized by first-order closure and the surface heat flux is prescribed, together with the wind speed and temperature at the model top. To study the collapse phenomenon both numerical simulations and linear stability analysis are used. The stability analysis is nonclassical in a sense that the stability of a parameterized set of equations of a turbulent flow is analyzed instead of a particular laminar flow solution. The analytical theory predicts a collapse of turbulence when a certain critical value of the stability parameter ¿/L (typically O(0.5¿1)) is exceeded, with ¿ the depth of the channel and L the Obukhov length. The exact critical value depends on channel roughness to depth ratio z 0/¿. The analytical predictions are validated by the numerical simulations, and good agreement is found. As such, for the flow configuration considered, the present framework provides both a tool and a physical explanation for the collapse phenomenon.
Exploring Self-Correlation in Flux-Gradient Relationships for Stably Stratified Conditions
Baas, P. ; Steeneveld, G.J. ; Wiel, B.J.H. van de; Holtslag, A.A.M. - \ 2006
Journal of the Atmospheric Sciences 63 (2006). - ISSN 0022-4928 - p. 3045 - 3054.
atmospheric boundary-layer - monin-obukhov similarity - surface-layer - convective conditions - turbulence - temperature - land - profiles - cases-99 - validity
In this paper, the degree of scatter in flux-gradient relationships for stably stratified conditions is analyzed. It is generally found that scatter in the dimensionless lapse rate phi(n) is larger than in the dimensionless shear phi(m) when plotted versus the stability parameter z/Lambda (where Lambda is the local Obukhov length). Here, this phenomenon is explained to be a result of self-correlation due to the occurrence of the momentum and the heat flux on both axes, measurement uncertainties, and other possibly relevant physical processes left aside. It is shown that the ratio between relative errors in the turbulent fluxes influences the orientation of self-correlation in the flux-gradient relationships. In stable conditions, the scatter in phi(m) is largely suppressed by self-correlation while for phi(n) this is not the case (vice versa for unstable stratification). An alternative way of plotting is discussed for determining the slope of the linear phi(m) function.
An Intercomparison of Large-Eddy Simulations of the Stable Boundary Layer
Beare, R.J. ; MacVean, M.K. ; Holtslag, A.A.M. ; Cuxart, J. ; Esau, I. ; Golaz, J.C. ; Jimenez, M.A. ; Khairoutdinov, M. ; Kosovic, B. ; Lewellen, D. ; Lund, T.S. ; Lundquist, J.K. ; McCabe, A. ; Moene, A.F. ; Noh, Y. ; Raasch, S. ; Sullivan, P. - \ 2006
Boundary-Layer Meteorology 118 (2006)2. - ISSN 0006-8314 - p. 247 - 272.
shallow cumulus convection - subgrid model - sensitivity - surface - transition - resolution - turbulence - mesoscale - cases-99 - fluxes
Results are presented from the first intercomparison of large-eddy simulation (LES) models for the stable boundary layer (SBL), as part of the Global Energy and Water Cycle Experiment Atmospheric Boundary Layer Study initiative. A moderately stable case is used, based on Arctic observations. All models produce successful simulations, in as much as they generate resolved turbulence and reflect many of the results from local scaling theory and observations. Simulations performed at 1-m and 2-m resolution show only small changes in the mean profiles compared to coarser resolutions. Also, sensitivity to subgrid models for individual models highlights their importance in SBL simulation at moderate resolution (6.25 m). Stability functions are derived from the LES using typical mixing lengths used in numerical weather prediction (NWP) and climate models. The functions have smaller values than those used in NWP. There is also support for the use of K-profile similarity in parametrizations. Thus, the results provide improved understanding and motivate future developments of the parametrization of the SBL. Keywords Large-eddy simulation
Variance Method to Determine Turbulent Fluxes of Momentum And Sensible Heat in The Stable Atmospheric Surface Layer
Debruin, H.A.R. ; Hartogensis, O.K. - \ 2005
Boundary-Layer Meteorology 116 (2005)2. - ISSN 0006-8314 - p. 385 - 392.
nocturnal boundary-layer - sonic anemometer - eddy-correlation - temperature - similarity - cases-99
Evidence is presented that in the stable atmospheric surface layer turbulent fluxes of heat and momentum can be determined from the standard deviations of longitudinal wind velocity and temperature, ¿u and ¿T respectively, measured at a single level. An attractive aspect of this method is that it yields fluxes from measurements that can be obtained with two-dimensional sonic anemometers. These instruments are increasingly being used at official weather stations, where they replace the standard cup anemometer-wind vane system. With methods such as the one described in this note, a widespread, good quality, flux network can be established, which would greatly benefit the modelling community. It is shown that a 'variance' dimensionless height (¿¿) defined from ¿uand ¿Tis highly related to the 'conventional' dimensionless stability parameter ¿ = z/L, where is height and L is the Obukhov length. Empirical functions for ¿¿ are proposed that allow direct calculation of heat and momentum fluxes from ¿u and ¿F. The method performs fairly well also during a night of intermittent turbulence.
Monin-Obukhov Similarity Functions of the Structure Parameter of Temperature and Turbulent Kinetic Energy Dissipation Rate in the Stable Boundary Layer
Hartogensis, O.K. ; Debruin, H.A.R. - \ 2005
Boundary-Layer Meteorology 116 (2005)2. - ISSN 0006-8314 - p. 253 - 276.
atmospheric surface-layer - aperture scintillometer test - convective conditions - sonic anemometer - fluxes - scintillation - algorithms - cases-99 - validity - models
The Monin-Obukhov similarity theory (MOST) functions fepsi; and fT, of the dissipation rate of turbulent kinetic energy (TKE), ¿, and the structure parameter of temperature, CT2, were determined for the stable atmospheric surface layer using data gathered in the context of CASES-99. These data cover a relatively wide stability range, i.e. ¿ = z/L of up to 10, where z is the height and L the Obukhov length. The best fits were given by f¿ = 0.8 + 2.5¿ and fT= 4.7[1 + 1.6(¿)2/3], which differ somewhat from previously published functions. ¿ was obtained from spectra of the longitudinal wind velocity using a time series model (ARMA) method instead of the traditional Fourier transform. The neutral limit f¿ = 0.8 implies that there is an imbalance between TKE production and dissipation in the simplified TKE budget equation. Similarly, we found a production-dissipation imbalance for the temperature fluctuation budget equation. Correcting for the production-dissipation imbalance, the 'standard' MOST functions for dimensionless wind speed and temperature gradients (Øm and Øh) were determined from f¿ and compared with the Øm and Øh formulations of Businger and others. We found good agreement with the Beljaars and Holtslag [J. Appl. Meteorol. 30, 327-341 (1991)] relations. Lastly, the flux and gradient Richardson numbers are discussed also in terms of f¿ and fT.