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Shallow cold fronts with a north-west/south-east orientation sometimes experience blocking by the coastal mountain range of south-eastern Australia and greatly increase their propagation speed on the eastern (seaward) side of the range while they advance more slowly to the west of the range. The violent behaviour of some cold-frontal passages, or southerly busters, is found to be at least orographically initiated, but the phenomenon does not exhibit the characteristics of a coastally-trapped density current. The head of the front has the character of an evolving density current and its propagation is well predicted by density current theory over more than half of its lifetime. Nevertheless, it differs from steady laboratory-simulated examples in that a horizontal roll-vortex just behind the front is found to be accelerating relative to the rate of advection of cold air behind the front. This implies that its evolution will be governed by warm air entrainment, a fact which is confirmed by the observations. General agreement between extensive airborne observations over the ocean and measurements made at selected points along the coast indicates the usefulness of these basic studies for future improvement to forecasting in the near-coastal region. 相似文献
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C. E. Coulman 《Boundary-Layer Meteorology》1978,14(4):493-513
Observations from an instrumented aircraft are used to study the small-scale structure of turbulence and convection in well-mixed boundary layers and the erosion processes in the nocturnally-formed inversions above them. The ways in which turbulence statistics for temperature, humidity and vertical velocity scale with height in the mixed layer are compared with the results of a three-dimensional model by Deardorff (1974a, b), and agreement is found in many aspects. Conditional sampling enables the statistics of thermals and their environment to be considered separately and, in particular, shows that the mode of the vertical velocity in thermals markedly decreases with height in the upper half of the mixed layer. Thermals may be recognized equally readily by either their excess of temperature or humidity. Transfers of heat and moisture through the nocturnal inversions influence the structure of the upper region of the mixed layer and there is strong evidence that these transfer processes are turbulent and not organized on scales similar to convective thermals. 相似文献
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A previously described microthermal measurement technique is applied to diagnose the causes and locations of sources of harmful ‘seeing’ effects in solar telescopes. Examples of the investigation of a compact 30-cm refractor at Culgoora Observatory and the massive 60-in. McMath reflector at Kitt Peak Observatory are described. The separate contributions from various parts of the telescopes and their immediate environments to the overall modulation transfer function and to the rms wavefront deformation are determined. The effectiveness of ventilation equipment fitted to these telescopes is assessed. In particular, it is shown that rms wavefront deformations ranging from λ/8 to 2λ may originate within the immediate atmospheric environments of these telescopes. Since the locations of these disturbances are determined there is a possibility of effecting improvements for example by mounting a telescope adequately high above the floor of its supporting tower. In the McMath reflector the major contribution to bad ‘seeing’ generally arises in the upper part of the instrument in the vicinity of the heliostat.
相似文献4.
Turbulence characteristics of convectively mixed layers over rugged and homogenous terrain are presented, and their differences are sought. The data used in the study were obtained from aircraftborne instruments. Apart from statistics related to the mixing ratio q, there does not seem to be a marked difference between the two sets of data. This lack of difference is attributed to the degree of ruggedness of the terrain and the dominant wind direction. The behaviour of some statistics related to q is attributed to the entrainment effect of the unstable layer of the moist, cool air above the mixed layer.This work was initiated and a large part of it was completed while all three authors were with CSIRO (Commonwealth Scientific Industrial Research Organisation), Australia.Formerly at: Division of Atmospheric Research, CSIRO, Epping, NSW, Australia.Formerly at: Division of Mathematics and Statistics, CSIRO, Lindfield, NSW, Australia. 相似文献
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C. E. Coulman 《Boundary-Layer Meteorology》1973,4(1-4):169-177
Measurements of air temperature fluctuations at frequencies from 1–100 Hz have been made with instruments carried by free-flying balloons. Structure functions and spectra of the temperature field have been calculated for various heights within the range 0.1–8 km above ground. The frequent occurrence of shallow regions, possibly layers, which exhibit locally enhanced values of the structure coefficient has been confirmed. 相似文献
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C. E. Coulman 《Boundary-Layer Meteorology》1978,14(4):471-491
The evolution of boundary layers capped by nocturnal inversions has been studied with an instrumented aircraft. A large sample of the original data obtained on two of the four observation days is presented. Profiles of temperature, humidity and sensible and latent heat flux are compared with the results of numerical models. The sensitivity of simple models for the prediction of nocturnal inversion dispersal is discussed in relation to certain measured input parameters. 相似文献
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C. E. Coulman 《Boundary-Layer Meteorology》1980,19(4):403-420
Observations made over land and ocean show that the structure of convection in the lowest one-third of a well-mixed layer may be studied in terms of a density-related variable (e.g.,
v
) without regard to whether structural elements are ascending or descending. This no longer holds in the remainder of the layer. Further, the temperature-humidity correlation is generally not the same for ascending and descending elements in any part of the layer. The consequences for short-path optical propagation are considered. 相似文献
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C. E. Coulman 《Boundary-Layer Meteorology》1980,19(1):31-50
Observations of temperature, humidity and velocity structure beneath fields of cumulus clouds are analysed with the needs of cloud modelling and cloud field parameterization in mind. Sub-cloud fluxes of sensible and latent heat and their diurnal variations are determined from layer-mean profiles and from eddy-correlation measurements; the latter are especially sensitive to sampling on cloud-sized scales. The characteristic fluctuations in velocity temperature and humidity beneath clouds have time scales like the lifetime typical of small cumulus clouds; there is no evidence that such fluctuations occur only as brief pulses at cloud initiation. The magnitudes of these fluctuations exhibit well-marked diurnal patterns of change. 相似文献
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C. E. Coulman 《Solar physics》1969,7(1):122-143
Measurements of temperature-fluctuation statistics in a locally homogeneous, isotropic turbulent atmosphere above level land are shown to enable the quality of telescopic images of celestial objects to be calculated. Data obtained with captive-balloon-borne apparatus are used, together with other existing data, to calculate the modulation transfer function and Strehl Definition for both long- and short-exposure images formed by typical solar telescopes operating under a variety of meteorological conditions. The size of telescope may thus be matched to the expected atmospheric conditions; the improvement in performance and utilization of the instrument, obtainable by raising it on a tower, may be calculated. An 11-cm aperture telescope, for example, is shown to be atmosphere-limited for long exposures in average daytime conditions; raising it from 2 to 30 m above the ground makes an almost threefold improvement in definition. If observations are confined to intervals of good seeing the full resolution of such a telescope may be realized. The existence of such intervals is attributed to convective thermal structure in the atmosphere and observational statistics of periods of below-average temperature fluctuation are given for a range of general meteorological conditions. 相似文献
10.
Observations of temperature, pressure and humidity have been made from an aircraft beneath cumulus clouds which formed over extensive flat country. In fair weather over land, cumulus cloud base is generally above the average top of the well-mixed convection layer so that penetrative convection is necessary to initiate cloud formation. The convective layer does not evolve and deepen uniformly over large areas (say greater than 100-km radius). Rather, it develops a patchy structure at 1–10 km scales. Such patches, close beneath cloud base, have thermodynamic properties very like those of the convection layer and in such regions that layer effectively extends right up to cloud base. Meso-scale effects (e.g., 50 km) seem to be important in determining where clear and cloudy areas occur, and although it appears reasonable to attribute this to local dynamic effects (e.g., subsidence), it is not possible to eliminate other possibilities on the basis of the present data. 相似文献
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