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991.
Quantifying aeolian sand transport rates relies upon the computation of the near-surface shear velocity (u*) determined from velocity profiles of the wind. While it has been recognized that various conditions, such as saltation, surface roughness, surface slope and atmospheric conditions, have an effect on the velocity profile, it is commonly assumed that measurements made above the surface will be representative of the near-surface shear velocity. Airflow and temperature data collected over a flat substrate at White Sands National Monument in New Mexico, however, show the significant effects that atmospheric conditions have on velocity profiles. During the day, when solar insolation is heating the surface, atmospheric conditions in the lowest several metres become unstable, resulting in enhanced convection and vertical mixing so that the velocity gradient changes little with height. As a result, the shear stress in this region of vertical mixing lessens, while the near-surface shear stress is increased because the higher wind speeds are now nearer the surface. At night, the near-surface atmospheric conditions are stable, thereby reducing convection and vertical mixing, resulting in stratified airflow and increased shear velocity away from the surface. Unless this atmospheric effect is accounted for, estimates of sediment transport rates may be in error by as much as a factor of 15 times when wind speeds are near threshold velocity. At wind speeds approaching 10 ms1, at 5m above the surface, this error in computing sediment transport is reduced to a factor of only two to three times, and may be within the range of measurement error. 相似文献
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Gary L. Achtemeier 《Boundary-Layer Meteorology》1991,55(4):309-324
Laboratory and numerical model simulations of turbulent circulations within the wake regions of thunderstorm outflows have been done with the assumption that there is no turbulence within the ambient airmass. Furthermore, many observational studies have used Doppler radar data that have been filtered so that turbulent structures are reduced in amplitude or eliminated altogether. This study presents unique Doppler radar observations of the collision of a roll-like boundary-layer circulation with a gust flow. The boundary-layer circulation is seen to interact with the circulation within the gust flow head and to reappear within the wake region. It is suggested that the ambient boundary layer may be an energy source for the generation and/or maintenance of turbulence in the wake region. 相似文献
995.
Water-vapor transport from low to high latitudes in a given ocean and from one ocean to another must be compensated by a net flow of salt through the sea. A comparison is presented which shows that water-vapor fluxes derived from meteorological information, from an atmospheric general circulation model and from a radiocarbon-calibrated ocean box model are in first-order agreement.
Offprint requests to: WS Broecker 相似文献
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It is shown that the autocorrelation function of a telescope diffraction-limited image is closely approximated by a subtraction of the mean cross correlation of pairs of speckle photographs from the mean autocorrelation of the same set of data. This fact is used to derive the angular diameter of the asteroid Vesta from a series of speckle interferometry data. The resultant apparent angular diameter of 0″.40 ± 0″.04 corresponds to an absolute diameter of 513 ± 51 km. 相似文献
999.
Images of Mars were obtained at 20 wavelengths between 0.35 and 1.10 μm during the 1973 opposition using the MIT MKIII silicon vidicon imaging system at the 88-in. telescope of the Mauna Kea Observatory, Hawaii. The observations covered a 3-hr interval on Day 2 of the 1973 Solis Lacus dust storm. At blue wavelengths two huge crescent-shaped anomalous brightenings surrounded the central dust storm in the early morning but faded during the morning; they were probably deposits of H2O frost or low level haze that was degassed from the regolith in Solis Lacus. The north polar hood, which was nonsymmetrically oriented about the north pole, faded and receded during the observing period. Regions of known dust storm activity could be clearly distinguished, and the images revealed dust cloud activity in Chryse as well. Relative reflectance spectra generated from the images showed compositional variations within the Erythraeum M. dark area complex: Margaritifer S. and Eastern Erythraeum M. have similar ferrosilicate mineralogy, but have stronger ferrosilicate absorption bands than Pyrrhae R. and Eos. Western Erythraeum M. and northern Eos were contaminated by high albedo dust. Reflectance spectra obtained from orbit around Mars could provide higher spatial resolution and more complete coverage of the dark areas, yielding an opportunity to correlate geologic features with crustal petrology. An orbiting spectrometer could also provide spectra in the compositionally diagnostic wavelength range 1–4 μm, where the Earth's atmosphere contains intense absorption bands. 相似文献
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