Unstable Atmospheric Boundary Layer Research Articles

A thermal front within the marine atmospheric boundary layer over the Agulhas Current south of Africa: Composite aircraft observations

Aircraft cross‐section observations across the northern edge of the warm Agulhas Current to the south of Africa are used to form composite results of mean and turbulent meteorological parameters over a 4‐day period characterized by winter zonal westerlies. Across a sea surface temperature (SST) front of 8°C (40 km)−1 near 35°S, 23°E, surface heat fluxes and wind stress increase fivefold. The moist unstable marine atmospheric boundary layer is 400 m deeper on the warm side of the SST front, and turbulent variances increase with SST in a logarithmic correlation (r = +0.76). A jetlike core of accelerated winds (17 m s−1) over the axis of warmest water is found. Its existence is thought to be due to secondary circulations set up by enhanced pressure gradients and baroclinic effects. The sharp thermal front is quasi‐stationary and semipermanent during synoptic prefrontal conditions and zonal flows and lies approximately over the 200‐m isobath in association with the Agulhas Current.

Spray dispersal from aerial silvicultural glyphosate applications

Spray dispersal was measured from overlaid crosswind swaths applied using an aerial silvicultural herbiciding method under various meteorological conditions. Spray applications were made with D8-46 hollow-cone nozzles mounted on a C188 fixed-wing aircraft flying at 45 m s −1, from which a simulant tank mix was applied at 30 l ha −1 and an equivalent glyphosate application rate of 1.8 kg ha −1. Spray lines were released at 10 m above ground over a short, open forest canopy in neutrally-stable and unstable atmospheric boundary layers with average wind speeds and air temperatures of 1.5–3.1 m s −1 and 15–20°C at release height. Peak deposits on ground plates and artificial foliage at 2 m above ground averaged 57 and 36% of the glyphosate application rate, occurring at downwind distances of 0 and 10 m, respectively. Deposits decreased by 90% between 0 and 50 m downwind, but showed a more gradual decrease between 50 and 200 m downwind to an average of 0.3 and 0.2% of the application rate for ground plates and artificial foliage, respectively. Up to sevenfold differences in deposit were observed at a given downwind distance under different meteorological conditions. The volume median diameters on ground cards and artificial foliage averaged 425 and 419 μm respectively at 0 m, and 103 and 79 μm at 200 m. Number median diameters on ground cards and artificial foliage averaged 252 and 197 μm respectively at 0 m, and 55 and 45 μm at 200 m.

Regional shear stress of broken forest from radiosonde wind profiles in the unstable surface layer

Mean wind speed profiles were measured by tracking radiosondes in the unstable atmospheric boundary layer (ABL) over the forested Landes region in southwestern France. New Monin-Obukhov stability correction functions, recently proposed following an, analysis by Kader and Yaglom, as well as the Businger-Dyer stability formulation were tested, with wind speeds in the surface sublayer to calculate the regional shear stress. These profile-derived shear stresses were compared with eddy correlation measurements gathered above a mature forest stand, at a location roughly, 4.5 km from the radiosonde launch site. The shear stress values obtained by means of the newly proposed stability function were in slightly better agreement with the eddy correlation values than those obtained by means of a Businger-Dyer type stability function. The general robustness of the profile method can be attributed in part to prior knowledge of the regional surface roughness (z 0=1.2 m) and the momentum displacement height (d 0=6.0 m), which were determined from neutral wind profile analysis. The 100 m drag coefficient for the unstable conditions above this broken forest surface was found to beu 2* /V 2100 =0.0173.

Case studies of the atmospheric boundary layer structure over sea ice for weakly unstable conditions

The results of the approach in Myrhaug and Rue, where a simplified model for a weakly unstable atmospheric boundary layer capped by a stable inversion layer at the top was presented, are compared with the observed boundary layer flow structure as obtained from aircraft measurements over Arctic sea ice.

Off‐target glyphosate deposits from aerial silvicultural applications under various meteorological conditions

AbstractAerial spray applications of the herbicide glyphosate were made over a forest canopy under various meteorological conditions. A ‘Thru Valve Boom’ dispersal system carried by a Cessna 188 fixed‐wing aircraft flying at 49 m s−1 was used to generate an aqueous spray cloud with a volume median diameter of 150 μm. Glyphosate deposits from multiple overlaid crosswind line sources released at 10 m above ground level were measured on ground sheets and artificial foliage at downwind distances between 50 and 400 m. Trials were conducted in stable, neutral and unstable atmospheric boundary layers with average wind speeds between 2·2 and 5·7 m s−1 and vertical intensities of turbulence between 0·07 and 0·16. Linear regression lines fitted to logarithmically transformed measurements and downwind distances (x) gave statistically significant correlation coefficients (P = 0·01), and were compared by ANOVA. Glyphosate deposits on ground sheets and artificial foliage were attenuated at rates inversely proportional to x to the power 1·7‐4·3. Regression line comparison showed that, in general, deposits on ground sheets decreased with increasing wind speed and intensity of turbulence, and some statistically significant differences were found in slopes and elevations of regression lines from different trials. However, deposits at the 50‐m station increased with wind speed due to the large‐drop cloud component. Regression line comparison for deposits on artificial foliage showed that, in general, they were highest in the intermediate wind speed‐neutral stability case and similar in the high wind speed‐unstable and low wind speed‐stable boundary layers, although deposits at the 50‐m station also increased with wind speed.

Simplified model for a weakly unstable atmospheric boundary layer capped by an inversion layer

A simplified model for a weakly unstable atmospheric boundary layer capped by a stable inversion layer at the top is presented. This type of boundary layer flow might occur frequently over high-latitude ice fields for sufficiently strong winds. The shear stress is modelled by using a linear eddy viscosity representation. The vertical boundary layer structure, the associated surface shear stress and surface shear stress direction are presented together with comparison with a data set from field observations.

Regional Surface Fluxes From Remotely Sensed Skin Temperature and Lower Boundary Layer Measurements

During First International Satellite Land Surface Climatology Project Field Experiment in north‐eastern Kansas, surface temperature was measured by infrared radiation thermometers at some 12 stations spread over the 15 × 15 km experimental area. These data, together with wind and temperature profiles in the unstable atmospheric boundary layer measured by means of radiosondes, were analyzed within the framework of Monin‐Obukhov similarity. The radiometric scalar roughness corresponding to the radiometric surface temperature was found to increase as the season progressed; for the spring campaign the mean value was zoh,r = 4.56 × 10−7 m and for the fall zoh, r = 1.01 × 10 −2 m. The radiometric scalar roughness could also be expressed as a function of solar elevation and to a lesser extent, of canopy height or leaf area index. For an elevation range 10° ≤ α ≤ 75° the regression equation is zoh,r = exp [−0.735 ‐ 3.61 tan (α)]. With this function good agreement (r = 0.87) was obtained between the profile‐derived regional surface flux of sensible heat and the mean flux measured independently at ground‐based stations under unstable conditions. Similarly, regional values of evaporation, obtained by means of the energy budget method from these sensible heat flux estimates, were in good agreement (r = 0.96).

Surface Water Vapor and Momentum Fluxes under Unstable Conditions from a Rugged-Complex Area

Abstract Measurements were made of the profiles of mean wind velocity, of temperature, and of specific humidity in the unstable atmospheric boundary layer over macro-rough terrain; these data were obtained from radiosonde observations in a calibrated watershed of 3.2 km2 in the hilly Pre-Alps of Switzerland during the summer of 1982. The regional evaporation was reasonably well correlated (R = 0.7) with these profile measurements through a logarithmic height dependency between roughly 2h0 (where h0 ≈ 100 m is the mean height of the roughness obstacles) and 0.6h (where h is the height of the boundary layer above the mean valley level). The shapes of the profiles appear to be essentially independent of the Monin–Obukhov parameter (z− d0)/L, but they display a dependency on the wind shear aloft and on the value of (z0/h) (where z0 is the roughness height). Over this rugged surface the relative importance of mechanical turbulence, as compared to convective turbulence, is larger than over terrain with smaller ...

Surface influence upon vertical profiles in the nocturnal boundary layer

Near-surface wind profiles in the nocturnal boundary layer, depth h, above relatively flat, tree-covered terrain are described in the context of the analysis of Garratt (1980) for the unstable atmospheric boundary layer. The observations at two sites imply a surface-based transition layer, of depth z *, within which the observed non-dimensional profiles Φ M 0 are a modified form of the inertial sub-layer relation $$\Phi _M \left( {{z \mathord{\left/ {\vphantom {z L}} \right. \kern-\nulldelimiterspace} L}} \right) = \left( {{{1 + 5_Z } \mathord{\left/ {\vphantom {{1 + 5_Z } L}} \right. \kern-\nulldelimiterspace} L}} \right)$$ according to $$\Phi _M^{\text{0}} \simeq \left( {{{1 + 5z} \mathord{\left/ {\vphantom {{1 + 5z} L}} \right. \kern-\nulldelimiterspace} L}} \right)\exp \left[ { - 0.7\left( {{{1 - z} \mathord{\left/ {\vphantom {{1 - z} z}} \right. \kern-\nulldelimiterspace} z}_ * } \right)} \right]$$ , where z is height above the zero-plane displacement and L is the Monin-Obukhov length. At both sites the depth z * is significantly smaller than the appropriate neutral value (z *N ) found from the previous analysis, as might be expected in the presence of a buoyant sink for turbulent kinetic energy.

On the similarity of the characteristics of turbulence in an unstable boundary layer

The statistics of turbulence, such as the standard deviation of fluctuating velocities, in an unstable atmospheric boundary layer are assumed to be characterized by the combination of three specific lengths, Monin-Obukhov length L, observation height z and the height of mixing layer h. Unlike Monin-Obukhov similarity, even near the ground the effect of h is taken into account. According to observation, the length scale of the vertical velocity is proportional to z at least near the ground, but the lateral component depends mostly on h alone. The length scale of the longitudinal component depends on z and h.

Winds in the Atmospheric Boundary Layer-Prediction and Observation

Abstract A simple, but realistic, three-layer model of the unstable atmospheric boundary layer (ABL) comprising surface layer, deep mixed layer and transition layer, within which geostrophic adjustment takes place, is utilized. Relations for the mixed-layer wind components and velocity defects are derived, with the latter depending upon entrainment, baroclinity, advection and local acceleration. These predicted quantities are compared with observations from three field experiments in the unstable ABL, including conditions of moderate baroclinity, strong horizontal advection and rapid growth of the mixed layer. The observations cover a range in scale-height ratio fh/u*, of approximately 0.025–0.5, and in normalized height h/z0 of 103 to 107. Within the limits of experimental errors in wind speed measurements the observed and predicted wind components are in good agreement, implying an internal equilibrium with the turbulent field and no significant influence from, what are observed to be, strong entrainmen...

An aircraft study of turbulence dissipation rate and temperature structure function in the unstable marine atmospheric boundary layer

The dissipation rate of turbulent kinetic energy, e, and the temperature structure function parameter, C T 2, have been measured over water from the near surface (Z = 3 m) to the top of the boundary layer. The near surface values of e and C T 2 were used to calculate the velocity and temperature Monin-Obukhov scaling parameters u * and T *. The data collected during unstable lapse rates were used to evaluate the feasibility of extrapolating the values of e and C T 2 as a function of height with empirical scaling formulae. The dissipation rate scaling formula of Wyngaard et al. (l971 a) gave a good fit to an average of the e data for Z < 0.8 Z i. In the surface layer the scaling formula of Wyngaard et al. (1971b) disagreed with the C T 2 values by as much as 50%. This disagreement is due to an unexpected reduction in the measured values of C T 2 forZ < 30 m. At this point it is not clear if the discrepancy is a unique property of the marine boundary layer or if it is simply some unknown instrumental or analytical problem. The mixed layer scaling results were similar to the overland results of Kaimal et al. (1976).

Statistical Properties of Vertical Shear of Lateral Velocity

Abstract The statistical properties of two-point differences, Δv′, along the vertical of the lateral component of turbulence (horizontal component perpendicular to mean wind vector) in the first 150 m of the unstable atmospheric boundary layer are analyzed with experimental data. The results of the analysis show that Δv′ is distributed according to the Pearson Type IV distribution. The standard deviation of Δv′ is proportional to |(z2−z1)/z1|0.11, where z2 and z1 are elevations above natural grade of the points of concern. The skewness appears to be independent of z1l and z2 and the kurtosis is a monotonically decreasing function of (z2−z1)/z1. The analysis also shows no systematic dependence of the above noted moments on the Monin-Obukhoy stability parameter.

A note on free convection

The limiting condition of µ*, →0 in the unstable atmospheric boundary layer is usually referred to as ‘free convection’. Some of the similarity laws that are proposed for this condition do not agree with experiment. A mechanism is proposed in this paper to show why the asymptotic free convection condition is never completely reached near the surface. It is shown that local turbulent shear production plays an important role in shaping the temperature profile even when the average velocity is zero.