Boundary-Layer Decoupling over Cold Surfaces as a Physical Boundary-Instability

In the stable atmospheric boundary layer (SBL), the interaction of processes in numerical weather prediction (NWP) models seems to test their representation more stringently than their separate 'validation.Some SBL schemes derived from micrometorological research seem to allow a 'decoupling behaviour when implemented in NWP. That is, turbulence dies out from the ground upwards. Such 'decoupling of the surface from atmospheric fluxes can permit dramatic and possibly unrealistic falls in surface temperature. This study traces the mechanism of model decoupling, asks whether this behaviour is in any sense correct and considers the implications.It is shown that decoupling can occur in idealized single-column models, originating from an unstable boundary-mode. This behaviour can depend critically on parameters such as surface roughness and soil thermal diffusivity as well as turbulence. But the turbulence dependence arises through the response of the boundary layer as a whole, and not just the surface-layer scheme. Such decoupling arises from the 'physics, rather than the finite-difference schemes, and appears to occur sometimes in the real atmosphere.     

Landscape Roughness Parameters for Sherwood Forest – Experimental Results

The aim of this work is to present experimentally evaluated effective roughnesses (zoe) of a partly forested landscape. Although the ratio of boundary-layer height to obstacle size was only of the order of 50, there still seemed to exist a height range of 75–200 m where surface-layer similarity was approximately valid. Attempts were made to use conventional wind profile analysis to evaluate zoe, but the small height range and the large number of variables initially led to unacceptable uncertainties. Fixing the displacement height zd, rather than fitting it, reduced the data scatter to an acceptable level. The profile-derived roughness lengths zop obtained in this way were in good agreement with previous work, and with an alternative roughness length estimate zof for which flux-derived profile parameters u* and * were used. This implies that the profile-derived roughnesses were consistent with the measured surface-layer momentum flux. Comparison of both roughness estimates also yielded an improved estimate of the displacement height. Besides this, the authors tested a landscape roughness evaluation method which makes use of the gustiness parameter Tu = u/U in the surface layer. The results obtained by this method were in fair agreement with the profile-derived data. In previous work, the gustiness method was advocated because it could be used at relatively low levels, perhaps even within the roughness sub-layer. At the present measuring site, this was not the case as the gustiness method was only valid in an approximate way, and for a limited height range.     

Stable boundary layers: Observations,models and variability part I: Modelling and measurements

Selected field measurements of evening stable boundary layers are presented in detail comparable with published Large Eddy Simulation results. Such models appear to match idealized theories more closely than do some boundary-layer observations. Any attempt to compare detailed observations with idealized models therefore highlights the variability of the real boundary layer.Here direct turbulence measurements across the stable boundary layer from a heterogeneous and an ideal site are contrasted. Recommendations are made for the information needed to distinguish heterogeneous and ideal cases.The companion paper (Part II) discusses further the issues of data, analysis in the presence of variability, and the effects of averaging over heterogeneous terrain.Part of UK Meteorological Office Atmospheric Process Research Division.     

Estimates of diabatic wind speed profiles from near-surface weather observations

In this paper we analyse diabatic wind profiles observed at the 213 m meteorological tower at Cabauw, the Netherlands. It is shown that the wind speed profiles agree with the well-known similarity functions of the atmospheric surface layer, when we substitute an effective roughness length. For very unstable conditions, the agreement is good up to at least 200 m or z/L–7(z is height, L is Obukhov length scale). For stable conditions, the agreement is good up to z/L1. For stronger stability, a semi-empirical extension is given of the log-linear profile, which gives acceptable estimates up to ~ 100 m. A scheme is used for the derivation of the Obukhov length scale from single wind speed, total cloud cover and air temperature. With the latter scheme and the similarity functions, wind speed profiles can be estimated from near-surface weather data only. The results for wind speed depend on height and stability. Up to 80 m, the rms difference with observations is on average 1.1 m s^–1. At 200 m, 0.8 m s^–1 for very unstable conditions increasing to 2.1 m s^–1 for very stable conditions. The proposed methods simulate the diurnal variation of the 80 m wind speed very well. Also the simulated frequency distribution of the 80 m wind speed agrees well with the observed one. It is concluded that the proposed methods are applicable up to at least 100 m in generally level terrain.     

A radiation and energy balance study of mature forest and clear-cut sites

Components of the radiation and energy balances were measured over a clear-cut area and a mature, mixed forest during the summer of 1981 at the Petawawa National Forestry Institute, Chalk River, Ontario. The work concentrated on the clear-cut site which supported a canopy layer composed primarily of bracken fern and logging remnants.Forty days of radiation data were collected at the clear-cut site. After the first four weeks of measurements (the green season), most of the ferns quickly died, and their foliage changed appearance from a green to brownish colour (the brown season). The daily mean reflection coefficient of solar radiation determined over the green season was 0.20 and decreased to 0.13 for the brown season. The corresponding value for the forest was 0.13, based on a limited amount of data. The clear-cut site received 11% and 21% less net radiation than the forest on a 24-hr and daylight-hours basis, respectively, as a consequence of the higher reflection coefficient and larger daytime longwave radiation emission.A reversing temperature difference measurement system (RTDMS), incorporating ten-junction thermopiles was employed at each site in order to determine Bowen ratios () via differential psychrometry. Both systems performed well, especially the RTDMS over the forest which was capable of resolving very small differences of temperature, typically less than 0.2 °C over a height of 3 m. The mean hourly Bowen ratio, calculated from values from 0800 to 1600 hr, varied from 0.2 to 1.0 for the forest and from 0.4 to 0.8 for the clear-cut site in the green season.A significant canopy heat storage component of the energy balance, Q _S , was found at the clear-cut site. In the early morning, a portion of the available energy was used to heat the biomass materials and air within the canopy layer. The stored heat within the canopy was released later in the day, increasing the available energy total.The daily mean value of the Priestley-Taylor coefficient (Priestley and Taylor, 1972) for the green season at the clear-cut site was 1.14, and individual values tended to increase during wet surface conditions and decrease when the surface dried. The daylight mean value during dry canopy conditions at the forest was 1.05, and much higher values occurred when the canopy was wet. The enhancement of for the wet forest was a result of the evaporation of intercepted rain (which is not limited by stomatal resistance) and the concomitant transfer of sensible heat to the forest.     

An observation of waves and turbulence in the earth's boundary layer

An account is given of an observation of a wave-like phenomenon obtained during a study of nocturnal inversions. Associated bursts of turbulent activity are also described.     

A method for determining flux parameters in the absence of a temperature profile

A numerical technique based on the least-square error method for evaluating fluxes and other surface-layer parameters is proposed. The special feature of this method is that it does not require a prior knowledge of the temperature profile. It can, in fact, be used to predict the temperature profile. The only other input parameter is the value of the roughness length z ₀ which has to be specified beforehand.This method has been tested on both the Kansas and the Wangara experimental data. Good agreement on fluxes and the Monin-Obukhov length compared with those obtained from direct measurements is obtained.     

One-dimensional checkerboards and blending heights

We analyze the checkerboard problem of many alternating surfaces with different properties, on scales up to (say) 3,000 m. Power-law representations of the vertical profiles of mean wind speed and eddy diffusivity lead to solutions in terms of Kelvin and trigonometric functions.These solutions are used to determine blending heights (_*), where deviations from the mean of concentration, or of vertical flux density, fall to some small fraction, , of their value at the surface. Values of _*are important for regional and larger-scale meteorological models. In smaller scale micrometeorological studies, they may serve also as the top levels of surface boundary layers.An important result for both theoretical and experimental contexts is that deviations of flux persist with elevation much more strongly than those of concentration, so that, in general, _* should be based on flux rather than concentration. Representative values of _*, for = 0.05, are of order 5 and 30 m for surface pattern wavelengths of 10² and 10³ m, respectively. Values of _* are robust to changes in adopted power-law indices, and are independent of wind speed. Surface roughness has a mild but calculable effect.     

A SIMPLE FORMULA FOR ATTENUATION OF EDDY FLUXES MEASURED WITH FIRST-ORDER-RESPONSE SCALAR SENSORS

A simple formula, (1 + (2fmc))-1,is proposed to estimate the attenuation of a scalar flux measurement made by eddy-correlation using a fast-response anemometer and a linear, first-order-response scalar sensor with a characteristic time constant c.In this formula, =7/8 for neutral and unstable stratification within the surface-flux layer and =1 both within the convective boundary layer (CBL) and for stable stratification in the surface layer.fm is the frequency of the peak of the logarithmic cospectrum and can be estimated from fm = nm /z, where z is the measurement height and is thewind speed at that height. The dimensionless frequency at the cospectral maximum nm is estimated here from observations of its behavioras a function of atmospheric stability, z/L within the surface layeror z/zi within the CBL, where L is the Obukhov stability length and zi is the depth of the CBL. The predicted dependence of flux attenuation on measurement height is discussed.     

The Effect of Atmospheric Stability on the Surface-Layer Characteristics in a Low-Wind Area of Tropical West Africa

This study details the observed effects ofatmospheric stability on characteristics of thesurface layer in a low wind speed (U = 1.5 m s-1)regime of tropical West Africa. Theaerodynamic roughness length, z0, anddisplacement height, d, obtained from profilewind-speed data at our bush land site (height 2 m)have values of 0.24 ± 0.10 m and 1.54 ± 0.04 mrespectively. In the unstable range (-2.5 < Ri < -0.1; Riis gradient Richardson number), thestandard deviation in wind speed fluctuations, u, increased from 0.57 ± 0.19 m s-1 toa maximum of 0.7 ± 0.2 m s-1 in near-neutralconditions, and in the stable range, the parameterdecreased rapidly to 0.41 ± 0.15 m s-1 at Ri 0.2.In the same stability range, the horizontal winddispersion, , decreased withincreasing stability from 19 ± 8 deg. to 13 ± 5 deg.The surface-layer integral quantity, u/u*, when plottedas a function of stability, is in agreement with theempirical results. The ratio ofsensible heat flux (estimated) to the net radiationranged between 0.1 and 0.2 at nighttime,increasing to about 0.5 during the daytime, and showeda strong dependency on season.     

Regular and seasonal behaviour of atmospheric radio noise field strength (ARNFS) over low latitude station calcutta

Summary The electromagnetic radiation of cloud discharge known as atmospheric radio noise field strength (ARNFS) shows a gradual fall from a frequency of 9 kHz to 81 kHz as studied over a period of two years at Calcutta, very close to Bay of Bengal. The main characteristic features of ARNFS at Calcutta are that-(i) ARNFS shows that midday median value is smaller than midnight median value in all months, (ii) level of daily minimum is higher in February and monsoon compared to other seasons, (iii) sunrise effect and sunset effect are well correlated with local sunrise and sunset times, (iv) the magnitude of sunrise fade and sunrise fade rate are maximum in April and lowest during winter period, (v) the magnitude of sunset fade is higher in premonsoon and postmonsoon while it is lowest in monsoon, (vi) number of occurrence of both sunrise effect and sunset effect is remark-ably smaller in monsoon. The positions of the sun and of atmospheric sources are jointly the causes of seasonal and diurnal variations. The missing of sunrise effect and sunset effect are due to local cloud activity and variation of electron density during geomagnetic storms.With 7 Figures     

Near-field dispersion from instantaneous sources in the surface layer

This paper considers the near-field dispersion of an ensemble of tracer particles released instantaneously from an elevated source into an adiabatic surface layer. By modelling the Lagrangian vertical velocity as a Markov process which obeys the Langevin equation, we show analytically that the mean vertical drift velocity w(t) is w()=bu _*(1–e ^–(1+)), where is time since release (nondimensionalized with the Lagrangian time scale at the source), b Batchelor's constant, and u _*, the friction velocity. Hence, the mean height and mean depth of the ensemble are calculated. Although the derivation is formally valid only when 1, the predictions for w, mean height and mean depth are consistent in the downstream limit ( 1) with surface-layer Lagrangian similarity theory and with the diffusion equation. By comparing the analytical predictions with numerical, randomflight solutions of the Langevin equation, the analytical predictions are shown to be good approximations at all times, both near-field and far-field.     

A Comparative Analysis of Transpiration and Bare Soil Evaporation

Transpiration E^v and bare soil evaporation E^b processes are comparatively analysed assuming homogeneous and inhomogeneous areal distributions of volumetric soil moisture content . For a homogeneous areal distribution of we use a deterministic model, while for inhomogeneous distributions a statistical-deterministic diagnostic surface energy balance model is applied. The areal variations of are simulated by Monte-Carlo runs assuming normal distributions of .The numerical experiments are performed for loam. In the experiments we used different parameterizations for vegetation and bare soil surface resistances and strong atmospheric forcing. According to the results theE^v()-^Eb() differences are great, especially in dry conditions. In spite of this, the available energy flux curves of vegetation A^v() and bare soil A^b() surfaces differ much less than the E^v() and ^Eb() curves. The results suggest that E^v is much more non-linearly related to environmental conditions than ^Eb. Both E^v and ^Eb depend on the distribution of , the wetness regime and the parameterization used. With the parameterizations, ^Eb showed greater variations than E^v. These results are valid when there are no advective effects or mesoscale circulation patterns and the stratification is unstable.     

Averaging Intervals For Spectral Analysis Of Nonstationary Turbulence

We formulate a method for determining the smallest time interval Tover which a turbulence time series can be averaged to decompose it intoinstantaneous mean and random components. From the random part the method defines the optimal interval (or averaging window) AW over which this part should be averaged to obtain the instantaneous spectrum. Both T and AW vary randomly with time and depend on physical properties of the turbulence. T also depends on the accuracy of the measurements and is thus independent of AW. Interesting features of the method are its real-time capability and the non-equality between AW and T.     

Determination of similarity functions of the resistance laws for the planetary boundary layer using surface-layer similarity functions

Functional forms of the universal similarity functions A, B (for wind components parallel and normal to the surface stress), and C (for potential temperature difference) are determined based on the generalized theory of the resistance laws for the Planetary Boundary Layer (PBL). The similarity-profile functions for the surface layer are matched with the velocity and temperature-defect profiles that are assumed to have shapes modified by certain powers of nondimensional height z/h, where h is the PBL height. The powers of the outer-layer profile functions are determined, so that the functions become negligible in the surface layer. To close the temperature defect law, an assumption that the temperature gradient across the top of the PBL is continuous with the stratification of the overlying atmosphere is used. The result of this assumption is that nondimensional momentum and temperature profiles in the PBL can be described in terms of four basic ratios: (1) roughness ratio = /h (2) scale-height ratio =|f|h/u_*, (3) ambient stratification parameter =h/_*, and (4) stability parameter =h/L, where L is the Monin-Obukhov length, z₀ is the surface roughness, is the upper-air stratification, u _* is the friction velocity, and _* is the temperature scale at the surface. For stable conditions, the scale-height ratio can be related to the atmospheric stability and the upperair stratification, and the generalized similarity and Rossby number similarity theories become identical. Under appropriate boundary conditions, function A is explicitly dependent on the stability parameter , while B is a function of scale-height ratio , which in turn depends on the stability. Function C is shown to be dependent on the stability and the upper-air stratification, due to the closure assumption used for the temperature profile.The suggested functional forms are compared with other empirical approximations by several authors. The general framework used to determine the functional forms needs to be tested against good boundary-layer measurements.     

Local similarity of spectral and cospectral characteristics in the stable-continuous boundary layer

The local similarity theory, presented in the recent papers of Sorbjan (1986a, b), is extended by taking into consideration spectral (u, v, w, ) and cospectral (uw, w, u) densities in the stable-continuous boundary layer. The resulting universal expressions for spectra, cospectra and the reduced frequencies of their peaks are in agreement with empirical data from the Kansas 1968 surface-layer and Minnesota 1973 boundary-layer experiments. In addition, the universal functions for the structure parameters and the dissipation rates are also derived and shown to fit the empirical data well.On leave from Institute of Environmental Engineering, Warsaw Polytechnic University, 00653 Warsaw, Poland.     

Reactions of alkoxy radicals under atmospheric conditions: The relative importance of decomposition versus reaction with O2

The reactions of alkoxy radicals determine to a large extent the products formed during the atmospheric degradations of emitted organic compounds. Experimental data concerning the decompositions, 1,5-H shift isomerizations and reactions with O₂ of several classes of alkoxy radicals are inconsistent with literature estimations of their absolute or relative rate constants. An alternative, although empirical, method for assessing the relative importance under atmospheric conditions of the reactions of alkoxy radicals with O₂ versus decomposition was derived. This estimation method utilizes the differences in the heats of reaction, (H)=(H_{decomposition}–H_{O 2 reaction}), between these two reactions pathways. For (H)[22–0.5(H_{O 2 reaction})], alkoxy radical decomposition dominates over the reaction with O₂ at room temperature and atmospheric pressure of air, while for (H)[25-0.5(H_{O 2 reaction})], the O₂ reaction dominates over decomposition (where the units of H are in kcal mol^–1). The utility and shortcomings of this approach are discussed. It is concluded that further studies concerning the reactions of alkoxy radicals are needed.     

On Trajectory Curvature as a Selection Criterion for Valid Lagrangian Stochastic Dispersion Models

Recently Wilson and Flesch (Boundary-Layer Meteorology, 84, 411-426, 1997) suggested that the average increment d z to the orientation = arctan(w/u) of the Lagrangian velocity-fluctuation vector can be used to distinguish the better Lagrangian stochastic models within the well-mixed class. Here it is demonstrated that the specification of d z constitutes neither a sufficient or universally applicable criterion to distinguish the better Lagrangian stochastic models within the well-mixed class. The hypothesis made by Wilson and Flesch that Lagrangian stochastic models with /P_E irrotational are zero-spin models, having d z=0, is proven     

Modelling of turbulence over low hills and varying surface roughness

Neutral surface layer flow over low hills and varying surface roughness is considered with emphasis on closure schemes in relation to the prediction of turbulence quantities. The equations are linearised, Fourier transformed in the two horizontal directions and solved by means of a finite difference method in the vertical. Three closure schemes are. employed, namely mixing length, E- and e-- closure where E, and indicate that differential equations are used for turbulent kinetic energy, dissipation rate and shear stress. Model calculations are compared with experimental data for the step in roughness problem and for the Askervein hill. The mean flow results turn out to be relatively insensitive to the closure scheme. The shear stress and the dimensionless shear, however, are much better predicted with the E- equations than with mixing length closure. In the outer layer of the hill problem, advection of shear stress becomes important. An equation for is needed here.