Constant Flux Layer Research Articles

乾燥表土層(DSL)法によるHEIFE沙漠観測点の砂地面からの蒸発量評価

Most methods of measuring soil-surface evaporation originate from those of measuring water-surface evaporation, and hence they are not necessarily suitable for making measurements of evaporation from dry land surfaces. The major reason is that, as observation shows, when the dry surface layer (DSL) forms on the soil, the constant-flux layer of water vapor is barely developed in the surface air layer. However, during most of the day, the upward water vapor flux in the DSL is nearly constant with depth. The principle of the DSL method is to estimate this upward near-constant flux in the DSL from the depth profiles of temperature and soil water content in the layer. The DSL method was applied to measurements of evaporation from a sand surface at the HEIFE desert station and they were compared to the decreases in soil moisture in the top 1 m of sand, from which the following results were obtained. (a) Estimates of daily evaporation about a week after rainfall of 15 mm or so, which were made on the basis of seven measurements taken by the DSL method during the day, were in good agreement with the daily decreases in soil moisture in the top 30 cm of sand. (b) Daily evaporation from dry soil can be approximately estimated from one measurement of evaporation rate made by the DSL method in the afternoon and the time lapsed since the last rainfall.

Response of a grassland ecosystem to air pollutants:—II the chemical climate: Fluxes of sedimenting airborne matter

Measurement of the deposition of sedimenting particles requires a sampling device, which avoids simultaneous deposition of gases and aerosols to the collection surface. A sampler constructed for the purpose of collecting rain and sedimenting particles is described and characterized in detail, in particular with regard to its collection efficiency for rain. Its collection properties for gases and aerosols are shown to be negligible. From two years of sampling at different heights it was found that resuspension of particles and co-condensation of gases near the plant canopy may lead to a major overestimation of bulk deposition. As a consequence, the extension towards the canopy of the constant flux layer for sedimenting particles has to be determined experimentally. Bulk deposition of sodium, potassium, magnesium, calcium, lead, copper, cadmium, manganese, iron, ammonium, nitrate, phosphate, sulfate, total sulfur and chloride at Braunschweig-Völkenrode, Southeast Lower Saxony, Germany, were recorded for six years. During this period a considerable decrease was observed in the deposition of lead, cadmium, nitrate, sulfate and total sulfur.

Atmospheric turbulence within and above a douglas-fir stand. Part II: Eddy fluxes of sensible heat and water vapour

This is the second paper describing a study of the turbulence regimes and exchange processes within and above an extensive Douglas-fir stand. The experiment was conducted on Vancouver Island during a two-week rainless period in July and August 1990. Two eddy correlation units were operated in the daytime to measure the fluxes of sensible heat and water vapour and other turbulence statistics at various heights within and above the stand. Net radiation was measured above the overstory using a stationary net radiometer and beneath the overstory using a tram system. Supplementary measurements included soil heat flux, humidity above and beneath the overstory, profiles of wind speed and air temperature, and the spatial variation of sensible heat flux near the forest floor. The sum of sensible and latent heat fluxes above the stand accounted for, on average, 83% of the available energy flux. On some days, energy budget closure was far better than on others. The average value of the Bowen ratio was 2.1 above the stand and 1.4 beneath the overstory. The mid-morning value of the canopy resistance was 150–450 s/m during the experiment and mid-day value of the Omega factor was about 0.20. The daytime mean canopy resistance showed a strong dependence on the mean saturation deficit during the two-week experimental period. The sum of sensible and latent heat fluxes beneath the overstory accounted for 74% of the available energy flux beneath the overstory. One of the reasons for this energy imbalance was that the small number of soil heat flux plates and the short pathway of the radiometer tram system was unable to account for the large horizontal heterogeneity in the available energy flux beneath the overstory. On the other hand, good agreement was obtained among the measurements of sensible heat flux made near the forest floor at four positions 15 m apart. There was a constant flux layer in the trunk space, a large flux divergence in the canopy layer, and a constant flux layer above the stand. Counter-gradient flux of sensible heat constantly occurred at the base of the canopy. The transfer of sensible heat and water vapour was dominated by intermittent cool downdraft and warm updraft events and dry downdraft and moist updraft events, respectively, at all levels. For sensible heat flux, the ratio of the contribution of cool downdrafts to that of warm updrafts was greater than one in the canopy layer and less than one above the stand and near the forest floor.

The effects of streamline curvature and spanwise rotation on near-surface, turbulent boundary layers

A second moment closure model is used to study the mean fields and turbulence structure of spanwise rotating flows and flows with streamline curvature. The effects of flow stabilization and destabilization by rotation and/or curvature and their interpretation in terms of a Rayleigh instability mechanism are discussed in the context of the present model. When applied to the constant flux layer adjacent to a bounding surface, the model provides a similarity theory for flows with spanwise rotation and streamline curvature like that of Monin-Obukhov in the case of density stratified flows. In particular, it is shown that Bradshaw's empirical length scale correction can be derived in terms of the basic constants of the model determined in the absence of rotation and curvature. Also, direct comparisons with experimental data confirm the model predictions. The definitions of strong and mild curvature are discussed and a distinguishing criterion derived.

Numerical Experiment on Convective Clouds Considering the Effect of Air-Soil Interactions With Large Eddy Simulation

A new numerical model of three dimensional hydro-metorological process has been developed for investigating the initiation and development of convective clouds considering the effect of air-soil interactions. This model consists of three parts, that is, atmosphere, constant flux layer and soil. For the subgrid turbulence process, ‘ Large Eddy simulation (LES)’ model is adopted with a timedependent turbulence energy equation. Soil temperature distribution is determined time-dependently and locally to satisfy both energy and moisture balances.Microphysical processes of cloud and rain generation are described by the Kessler parameterization.The results show that the initiation of convection is effected strongly by the soil moisture distribution and the air-soil interaction intensify the development of convective clouds.

Effect of rotation on vertical mixing and associated turbulence in stratified fluids

Combined effects of stratification and rotation on vertical mixing and the characteristics of associated small‐scale turbulence are explored using second‐moment closure methodology; the rotational terms in the equations for Reynolds stresses and turbulent heat fluxes are retained, not ignored as in earlier works. Semianalytical results valid for arbitrary values of rotation and stratification are derived by further invoking the local equilibrium limit of closure. Two cases are considered: nonzero vertical rotation and nonzero meridional rotation; the latter case is of more general interest in geophysics because of its potential application to equatorial mixed layers. In both cases the influence of rotation on mixing coefficients and Monin‐Obukhov constant flux layer similarity relations is investigated for arbitrary values of rotation and stratification. In both cases, turbulent mixing coefficients assume tensorial properties. However, meridional rotation appears to have a stronger influence on vertical mixing and turbulence characteristics than does vertical rotation. These results, along with perturbation expansions for weak rotation, suggest that for geophysical flows, in most cases, the direct effect of rotation on vertical turbulent mixing itself is but a small correction, a few tens of percent at best. It is seldom large, although it might not be negligible in some particular cases. Nevertheless, the study of rotational effects on small‐scale turbulence provides a fascinating insight into the direct impact of rotation on the characteristics of small‐scale turbulence and mixing in stratified fluids; the results are also of interest in other fields such as engineering.

Control of urban night temperature in semitropical regions during summer

In summer, weather in urban areas of the south-east region of Japan is humid and hot. It seems that the heat-island effect in urban areas on human life is greatest at night during summer, because hot and humid weather inhibits sleep thereby leading to poor health. A prediction model for urban air temperatures is needed in order to develop countermeasures for thermal environmental problems. The estimation of air temperature within the constant flux layer is based on a one-dimensional heat balance. Further, using the Monin-Obuklov length, momentum, heat and moisture fluxes are calculated to account for atmospheric stabilities. This paper discusses a study of the influence of atmospheric stabilities on the formation of nigth temperature within the basic concept of a ‘heat transfer coefficient of the surface boundary layer’. The results clearly indicated that the greatest causes of the increase of night temperatures in urban areas are the release of artificial heat and heat storage in soil. In conclusion, from the results of this investigation, a concept of the required green-area ratio needed to control urban night temperatures is proposed.

Observations and model calculations of aerodynamic drag on sea ice in the Fram Strait

Wind and temperature profiles in the constant flux layer obtained by tethersonde were used to compute the total aerodynamic drag on an area of 60% pack ice in the Fram Strait (79°20′N, 1−3°W). The boundary layer appeared adiabatic to heights greater than 150 m, and there were only minor air/water temperature differences. Drag coefficients of 4.9 and 5.1 × 10 -3 referred to 10 m above ground level were found. Eddy correlation measurements in the local constant flux layers over ice floes were used to estimate the skin drag of an area of 100% ice cover. This was less than 40% of the total drag on the actual area. The corresponding drag coefficient was 1.4×10 -3 . A drag partition model is proposed for computing the total drag over an area of pack ice as a function of ice concentration, mean freeboard and length of the ice floes, and typical roughness lengths of ice and sea surfaces. The model predicts maximum form drag at 73% ice concentration for floes of the type observed in the Fram Strait. DOI: 10.1111/j.1600-0870.1988.tb00413.x

A numerical investigation on the interaction of turbulent and long-wave radiative fluxes in the surface layer

The interaction of turbulent and radiative transfer applied to a number of plausible atmospheric situations in the surface layer under the stably stratified condition is discussed. The calculated results show that the long-wave radiative flux has a great influence upon the thermal structure of the surface layer, and that it usually acts in such a way as to weaken the thickness of the constant turbulent heat flux layer. In the case of low wind velocities and strongly stable stratifications, the thickness of the turbulent heat flux layer will become very thin and/or inexistent.

The vertical extent of the log-linear wind profile under stable stratification

The nondimensional wind shear is examined over a wide range of stable and highly stable stratifications. The velocity profiles from the Kansas, Moses Lake and Wangara experiments are analyzed using Webb's (1970) method. The results show that the log-linear function accurately represents the velocity profile close to the ground, up to z/L ≈ 1. The log-linear layer covers a significant part of the lower nocturnal boundary layer and extends above the constant flux layer.

The Effect of Line Averaging on Scalar Flux Measurements with a Sonic Anemometer near the Surface

Abstract We present a theoretical analysis of the effect of line averaging by a sonic anemometer on scalar fluxes and an observational study of this phenomenon in the atmospheric surface layer. The theoretical analysis rests on an axisymmetric model for the cross-spectral tensor of vertical velocity and scalar fluctuations, limiting the validity of the derived transfer function to a constant-flux layer. Observations of temperature flux in the unstable surface layer confirm the theoretical prediction that line averaging does not significantly affect the flux estimate down to heights only several times the sonic path length. However, the observations exhibit large scatter at small height, indicating that problems with the representativity of the measurement and not with line averaging may become a limiting factor. Based on the analysis of the Kansas data and the characteristics of the transfer function, we infer that temperature flux measurements will, in general, be affected by some line averaging under st...

A Heuristic Investigation to Evaluate the Feasibility of Developing a Desert Dust Prediction Model

A boundary layer model, to investigate various aspects of desert circulation, has been devised. The planetary boundary layer is divided into a surface (constant flux) layer, a transition layer and an inversion layer. The basic equations (motion, continuity, first law of thermodynamics, dust concentration equation)are integrated with respect to height from the bottom of the transition layer up to the top of the boundary layer. It is assumed that this height coincides with that of the inversion, which is allowed to vary in space and time. Interface conditions are derived by integration of the equations across the inversion layer. These may be used to eliminate the momentum, moisture and dust concentration turbulent fluxes from the vertically integrated equations. The turbulent beat flux is eliminated from the first law by a closure assumption, relating the turbulent flux at the base of the inversion to that at the bottom of the transition layer. Modelling assumptions, specifying the variation with height of wind, temperature and dust concentration, are used in the vertically integrated equations. The resulting equations predict the horizontal wind components at a “mean” level, and the temperature and dust concentration at the bottom of the transition layer. A simplified version of the model, in which advective terms are neglected, couples the dust concentration and inversion height equations. In the former, the time variation of the dust concentration depends upon the difference between sedimentation and turbulent diffusion of dust, and upon the time variation of the inversion height. The latter depends upon the turbulent transport of heat from the ground, upon the difference between the initial temperature and temperature at a later time, and upon the vertical velocity of the larger scale motions at interface height. A numerical integration of this system describes the evolution of the inversion height, its effect on the dust concentration evolution, as well as the evolution of the dust concentration itself.

Energy Balance at the Surface of the Ice Sheet in the Coastal Area of Adélie Land, Antarctica (Abstract only)

Hourly measurements of wind, temperature, and humidity were made between elevations of 1 and 20 m in the coastal area of Adélie Land over a period of 45 d in January and February 1978. About 1000 profiles are available.The height of the constant flux layer is >5 m despite the influence of katabatic winds, the wind profile is logarithmic with a mean standard deviation from the logarithmic law of 15 mm s−1. The potential temperature profile is also logarithmic with a mean standard deviation of 0.04°C. The most important deviation from the logarithmic law appears between -4 and 0°C.Humidity was measured at two points only, between 1 and 20 m; the mean gradient was about 0.01 mbar m−1 and the flux of latent heat was generally negligible.Net radiation was measured for only 10 d because of radiometer failure. All the fluxes are calculated using data for the 5 m layer. For the 10 d period, the heat loss was about 3 Wm−2 with a mean heat flux of -27.5 Wm−2 and mean net radiation of -10.5 Wm−2(the minus sign signifying the flux is towards the surface). In general, the heat flux is towards the surface (92% of the cases) but is away from the surface in the afternoon. The ablation during the 10 d period without snow-fall was about 200 mm of ice.

Energy Balance at the Surface of the Ice Sheet in the Coastal Area of Adélie Land, Antarctica (Abstract only)

Hourly measurements of wind, temperature, and humidity were made between elevations of 1 and 20 m in the coastal area of Adélie Land over a period of 45 d in January and February 1978. About 1000 profiles are available. The height of the constant flux layer is >5 m despite the influence of katabatic winds, the wind profile is logarithmic with a mean standard deviation from the logarithmic law of 15 mm s−1. The potential temperature profile is also logarithmic with a mean standard deviation of 0.04°C. The most important deviation from the logarithmic law appears between -4 and 0°C. Humidity was measured at two points only, between 1 and 20 m; the mean gradient was about 0.01 mbar m−1 and the flux of latent heat was generally negligible. Net radiation was measured for only 10 d because of radiometer failure. All the fluxes are calculated using data for the 5 m layer. For the 10 d period, the heat loss was about 3 Wm−2 with a mean heat flux of -27.5 Wm−2 and mean net radiation of -10.5 Wm−2(the minus sign signifying the flux is towards the surface). In general, the heat flux is towards the surface (92% of the cases) but is away from the surface in the afternoon. The ablation during the 10 d period without snow-fall was about 200 mm of ice.

A Study of the Early Winter Effects of the Great Lakes.I: Comparison of Very Fine Scale Numerical Simulations with Observed Data

Abstract Development of a framework for study of the Great Lakes' effects on late fall-early winter cyclones andArctic air masses has been initiated. The central theoretical component is a three-dimensional numericalprimitive equations model. The 40-45 km horizontal grid point spacing provides for resolution of the shapeand dimension of each lake, and 15 levels in the vertical resolve interaction of boundary layer and midtropospheric processes. Over land, locally computed vertical diffusion coefficients depending on static stabilityand Richardson number are used to parameterize subgrid-scale turbulent processes throughout the depthof the atmosphere. At over-lake grid points, the Businger et al. (1971) formulation of the Monin-Obukhov(1954) similarity equations is used in the constant flux layer, and the combined O'Brien (1971) K profileand Deardorff (1974) boundary layer depth prediction equation are used for turbulence parameterizationin the rest of the boundary layer. The prognostic variables, pressure,...

Observation of the bottom boundary layer on the continental shelf

Current meter data from various depths near the sea bottom collected for 31 days at time intervals of 10 minutes using a subsurface buoy system at a depth at 38 m on the continental shelf off Akita, Japan have been analyzed. The results show the existence of a stationary Ekman layer. The typical range of the characteristic parameters are estimated as follows; friction velocity: 0.38 cm s−1; Ekman layer thickness: 16 m; logarithmic layer thickness: 4 m–6 m; constant flux layer thickness: 0.4–0.6 m; Ekman veering: 28.7°; drag coefficient: 0.24×10−2−0.53×10−2. Veering was also observed in the logarithmic layer.

PBL similarity profiles determined from a level-2 turbulence-closure model

A diagnostic model for the determination of similarity profiles of turbulence and mean-wind gradient in the planetary boundary layer is developed. Vertical profiles of a turbulence length scale and the flux Richardson number are formulated through the extension of the relationships for the constant flux layer. These profiles together with a turbulence energy equation and a similarity profile empirically determined for heat or momentum flux are used to compute the turbulence energy. Relationships previously derived from a turbulence closure model are used to compute second moments of turbulence.

A similarity model for maximum ground-level concentration in a height-invariant, stably stratified atmospheric boundary layer

A model is presented for determining the location and magnitude of the maximum ground-level concentration arising from an elevated buoyant source in a very stable atmospheric boundary layer. The development combines the turbulent structure of such a boundary layer, Lagrangian similarity of the diffusion process, and similarity solutions of the conservation equations of the buoyant plume with mass conservation to produce a simple, experimentally verifiable formulation. Functional analogy with previous results for the constant flux layer and a deep convectively unstable layer suggest a heuristic model by which to visualize the process.

Upslope Enhanced Extreme Rainfall Events over the Canadian Western Plains: A Mesoscale Numerical Simulation

A limited-area numerical model, previously applied to the study of cold easterly circulations over the Canadian Western Plains, is used to simulate upslope enhanced extreme rainfall events over the Saskatchewan River and adjacent drainage basins. Following Lavoie (1972) the atmospheric structure was represented by three layers: a constant flux layer in contact with earth's surface, a well-mixed planetary boundary layer capped by an inversion, and a deep stratum of overlying stable air. The governing primitive equations were averaged through the depth of the mixed layer with interactions between the mixed layer and both the underlying and overlying air being suitably parameterized. A 47.6 km grid mesh of 1369 (37×37) points covering the Canadian Prairie Provinces was used to represent the variables. The governing equations were solved numerically for the mixed layer with terrain influences, surface roughness, temperature variations, moisture fluxes and the release of latent heat allowed to perturb the mixed layer from its initial conditions. The model successfully simulated mean persistent (3-day) extreme rainfall distributions associated with occluded cyclones over the Saskatchewan River Basin. A case study based on observed initial conditions showed that the model could reproduce mesoscale rainfall distributions associated with an individual storm. Implications of such a simulation for developing short-term streamflow forecasts over the Saskatchewan River and adjacent drainage basins are considered.

A theory for natural convection turbulent boundary layers next to heated vertical surfaces

The turbulent natural convection boundary layer next to a heated vertical surface is analyzed by classical scaling arguments. It is shown that the fully developed turbulent boundary layer must be treated in two parts: an outer region consisting of most of the boundary layer in which viscous and conduction terms are negligible and an inner region in which the mean convection terms are negligible. The inner layer is identified as a constant heat flux layer. A similarity analysis yields universal profiles for velocity and temperature in the outer and constant heat flux layers. An asymptotic matching of these profiles in an intermediate layer (the buoyant sublayer) as H δ &z.tbnd gβF 0δ 4 α 3 → ∞ yields analytical expressions for the buoyant sublayer profiles as (T−T w) T 1 = K 2( y n ) − 1 3 + A(pr) , U U 1 = K 1( y n ) 1 3 + B(pr) , where K 1, K 2 are universal constants and A( Pr), B( Pr) are universal functions of Prandtl number. Asymptotic heat transfer and friction laws are obtained as Nu x = C' H(Pr)H∗ x 1 4 , τ w grU 2 1 = C f(Pr) , where C' H ( Pr) is simply related to A( Pr). Finally, conductive and thermo-viscous sublayers characterized by a linear variation of velocity and temperature are shown to exist at the wall. All predictions are seen to be in excellent agreement with the abundant experimental data.