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Low Wind Speed (low + wind_speed)
Selected AbstractsForm,flow interactions of an aeolian saucer blowoutEARTH SURFACE PROCESSES AND LANDFORMS, Issue 7 2009Chris H. Hugenholtz Abstract Airflow patterns through a saucer blowout are examined from wind speed and direction measurements made during a chinook wind event. The blowout long-axis is oriented east,west with a broad depositional apron on the east side. Wind directions during the event rotated from south-westerly to westerly, permitting an assessment of oblique and axis-parallel flows. Results show that airflow passing over the windward rim of the saucer blowout expands and decelerates, leading to flow separation and a small re-circulation zone on sheltered lee slopes. Near the deflation basin, airflow re-attaches to the blowout surface and accelerates up to a small opening in the east rim, where it can be up to 50% faster than on the windward edge. Beyond the downwind rim the airflow expands and decelerates and sand is deposited onto a broad apron. Similar to coastal trough blowouts, the degree of airflow steering and acceleration along the deflation basin is determined by the angle of incidence between the approach wind and the long-axis of the blowout. As the angle of incidence increases wind speed accelerates at 0·3 m above the surface of the deflation basin and the degree of airflow steering increases. Overall, a two-fold process is identified, where south-westerly flows have greater potential for eroding the deflation basin, while westerly flows have greater potential for evacuating sand from within the blowout. Visual observations indicate that sand eroded from the deflation basin during south-westerly flows is re-distributed to adjacent zones of low wind speed until axis-parallel winds evacuate the sand through the opening in the east rim. Morphometric changes since 1994 indicate that the blowout morphology has remained relatively constant, suggesting a persistent interplay between oblique and axis-parallel wind erosion events. Collectively, these findings indicate that the angle of approach winds is an important control on saucer blowout morphodynamics. Copyright © 2009 John Wiley & Sons, Ltd. [source] Causes of variability in the summertime Antarctic boundary-layer climateINTERNATIONAL JOURNAL OF CLIMATOLOGY, Issue 13 2007Dirk van As Abstract A high-resolution one-dimensional atmospheric model is used to assess the contribution of various surface characteristics and external forcings on the structure and dynamics of the atmospheric boundary layer (ABL) over the Antarctic Plateau in summer. The reference run simulates the boundary layer over a mildly sloping surface (1.5 m km,1) for a clear sky near the end of the Antarctic summer (31 January-3 February). The ABL depth is approximately 100 m. At night, a low-level jet forms due to the combined effect of katabatic forcing and an inertial oscillation. During the day a convective mixed layer is present. As expected, the ABL is very sensitive to surface slope; a larger slope forces higher wind speeds and a deeper boundary layer. Over a horizontal surface, a nocturnal jet is also found as a result of the inertial oscillation. A modest change in surface albedo alters the mixed-layer temperature and the height and strength of the nocturnal jet considerably. Rotating the large-scale wind relative to the slope direction also has a large impact on ABL depth and structure. The deepest boundary layer and largest wind speed over a northward down-sloping surface are found for an easterly (cross slope) large-scale wind, as is typical for Antarctica. A very shallow ABL with low wind speed is found for the opposite large-scale wind direction. ABL sensitivity to surface roughness was found to be small. For all experiments, the ABL sensitivity is enhanced due to the positive feedback between the cooling of the ABL and katabatic wind speed. Copyright © 2007 Royal Meteorological Society [source] Comparison of some sigma schemes for estimation of air pollutant dispersion in moderate and low windsATMOSPHERIC SCIENCE LETTERS, Issue 2 2005Khaled S. M. Essa Abstract One of the most important parameters in plume dispersion modeling is the plume growth (dispersion coefficients ,). Different models for estimating dispersion parameters are discussed to establish the relative importance of one over the others. Comparisons were made between power law functions, standard, split sigma and split sigma theta methods. We use the double Gaussian expression for calculating concentration in this comparison. The results show that, with low wind speed (<2 m/s), split sigma and split sigma theta methods give much better results than other methods, while, with wind speed greater than 2 m/s, the power law function methods give more plausible results. Copyright © 2005 Royal Meteorological Society [source] Ecophysiological significance of leaf size variation in Proteaceae from the Cape Floristic RegionFUNCTIONAL ECOLOGY, Issue 3 2010Megan J. Yates Summary 1.,Small leaves of species endemic to Mediterranean-type climate areas have been associated with both low rainfall and nutrient availability, but the physiological reasons for this association remain unknown. 2.,We postulated that small leaves have thin boundary layers that facilitate transpiration in winter and sensible heat loss in summer. High transpiration rates when water is available may facilitate nutrient acquisition in winter, whereas efficient sensible heat loss reduces the requirement for transpirational leaf cooling in summer. 3.,The consequences of varying leaf sizes for water and heat loss in Cape Proteaceae were examined at two scales. At the leaf level, gas exchange and thermoregulatory capacities of 15 Proteaceae species with varying leaf size were assessed under controlled conditions using phylogenetically independent contrasts. At an environmental level, leaf attributes of Proteaceae occurring in the winter-rainfall area of the Cape Floristic Region were correlated with climatic environments derived from distribution data for each species. 4.,Leaf temperature was positively correlated with leaf size when wind speed was negligible. However, transpiration decreased significantly with increasing leaf size when measured on individual leaves, detached branches and when expressed on a per stoma basis. 5.,From multiple stepwise regression analysis of climatic variables obtained from distribution data, leaf size was negatively correlated with A-Pan evaporation, mean annual temperatures and water stress in January. We conclude that leaf size is conservative for survival over relatively rare periods of hot dry conditions with low wind speeds. 6.,Narrow leaves enable plants to shed heat through sensible heat loss during summer droughts, without the need for transpirational cooling. Additionally, small leaf dimensions confer a capacity for high transpiration when evaporative demand is low and water is abundant (i.e. winter). This may be a particularly important strategy for driving nutrient mass-flow to the roots of plants that take up most of their nutrients in the wet winter/spring months from nutrient-poor soils. [source] Hunting strategies and foraging performance of the short-toed eagle in the Dadia-Lefkimi-Soufli National Park, north-east GreeceJOURNAL OF ZOOLOGY, Issue 3 2010D. E. Bakaloudis Abstract The foraging performance and the hunting strategies of foraging short-toed eagles Circaetus gallicus were studied in Dadia-Lefkimi-Soufli National Park during 1996,1998. A general linear model analysis showed that the eagle's hunting mode was related to wind velocity. At low wind speeds, the eagles more frequently soared and/or hovered, whereas on windy days, they hung more frequently than soared or hovered. Individuals appear to compensate for the high-cost foraging method (hovering) with a high capture rate or a low capture rate with low-cost foraging methods (soaring and hanging). In addition, their foraging activities exhibited two patterns. In the early (April) and late (September) breeding season, eagles foraged mainly during midday, while from May to August eagles foraged largely during the morning and a little during the afternoon, reflecting to some extent the diurnal activity of prey (reptiles) throughout the breeding season. Short-toed eagles tended to forage for longer as the breeding season progressed, peaking during August due to additional food requirements before autumn migration. Following a mixed foraging strategy throughout the breeding season, short-toed eagles increased their hunting efficiency, which may benefit increased breeding success and energy reserves for migration. [source] Wind direction azimuthal signature in the Stokes emission vector from the ocean surface at microwave frequenciesMICROWAVE AND OPTICAL TECHNOLOGY LETTERS, Issue 6 2001A. J. Camps Abstract An ocean polarimetric emission model is presented. It is found that skewness and upwind/cross-wind rms slopes are responsible for the first and second azimuthal harmonic, respectively. Atmospheric effects contribute significantly at low wind speeds, and at horizontal polarization at certain observation angles. Simulation results compare favorably with reported JPL,WINDRAD measurements. © 2001 John Wiley & Sons, Inc. Microwave Opt Technol Lett 29: 426,432, 2001. [source] | |||||||||||||