Observational studies in the atmospheric boundary layer

Abstract

SUMMARYWinds and temperatures in the boundary layer measured during two Australian expeditions are analysed according to the similarity scheme, with the use of four stability classifications.Under conditions of deep convection there is a minimum potential temperature and a maximum velocity component in the direction of the surface wind, at a height of about 0.12 u . / $In the very stable cases, the temperature gradient follows rather closely a z-* law for a considerable height range from 0.08 u./f upwards. An expression closely fitting the mean data in stable conditions is suggested for the vertical temperature structure at all levels in the boundary layer.Wind data processed in this way show, for all four stability classes, a rudimentary Ekman spiral. With deep convection the spiral is found to be reversed in sense, but the Bow in the convecting layer is down the gradient of pressure. If the convective limit lies broadly within the Ekman layer, a spiral of the expected sense is found. The upper limit of the Ekman layer (as defined by the ' spiral ') is found to lie at a height of 0.17 to 0.24 (increasing with stability) in units of u./j. Stress and heat-flux are apparently considerable above this level, with sub-geostrophic wind, when deep convection is occurring.Approximations to the universal distributions of stress, eddy coefficients, mixing length and rate of degradation of mean flow kinetic energy are computed for the various stabilities. The mixing length in unstable conditions increases almost as height up to a level of about 0.08 &/f, and then decreases, but in general appears not to vanish in stable layers above the boundary layer. In the unstable boundary layer with deep convection, the eddy transfer coefficient for heat exceeds that for momentum up to 0.12 uJf, where it becomes infinite, and is negative at higher levels. In stable conditions the transfer coefficients for a small sample of soundings were estimated to be closely similar.The universal functions of stability, A, B and C, which enable one to compute free atmosphere wind vector and temperature, given surface conditions, have been evaluated with moderate success, although B, which essentially describes the change of wind direction with height, exhibits excessive variability. A method is suggested for computing horizontal advection in the boundary layer when this is to be ' parameterized ' in mathematical models.The drag coefficient, in termsof freeatmosphere wind, has almost a 50-fold range, due to stability variation only. Most of the variation occurs relatively close to neutal, so warning against too ready an assumption of neutrality in practical applications.It is suggested that, for modelling purposes, it is preferable to adopt boundary layer formulations which are not too sensitive to departures from ideal conditions, and eddy coefficients, perhaps based on mixing lengths, may well provide the best approach currently available.

Analytical description of a nocturnal low‐level jet

“…The eddy-viscosity concept has led to useful qualitative descriptions of a variety of turbulent geophysical flows including atmospheric Ekman layers (Clarke, 1970;Houghton, 1977;Lettau, 1983), oceanic Ekman layers (Price et al, 1987;Chereskin, 1995;Ralph and Niiler, 1999), sea breezes (Walsh, 1974;Sun and Orlanski, 1981), and anabatic and katabatic flows (Defant, 1949;Egger, 1981;Kondo, 1984;Papadopoulos et al, 1997;Oerlemans, 1998). However, despite the successes of some appropriately tuned models based on the eddy-viscosity concept, it should always be borne in mind that eddy viscosity is a rather ad hoc concept.…”

Fiber optic distributed temperature sensing for the determination of the nocturnal atmospheric boundary layer height

“…An important tool for studying the boundary layer height has been measurements on tall towers and tethered balloons (e.g. Clarke, 1972;Haugen et al, 1971;Soilemes et al, 1993;Van Ulden and Wieringa, 1996;Schnitzhofer et al, 2009). While high spatial resolution can be achieved with a tall tower (although associated with high costs), it is impossible to obtain an instantaneous high resolution temperature profile from a tethered balloon system which typically carries only one sensor.…”

A Numerical Model Study of Turbulent Airflow in and Above a Forest Canopy

“…Limited data appear to suggest a maximum at between 150 and 200m above the ground followed by either a more or less constant value (Blackadar, 1962) or a decrease with height (Clarke, 1970 1975) have been successfully used to simulate some observational data. However, direct application of the interpolation formulas for a canopy case appears to be unphysical since the interpolation formulas mentioned above consider no effects of mechanical and thermal turbulence produced by trees.…”