共查询到20条相似文献，搜索用时 31 毫秒
1.
Analysis of wind profiles at the Boulder Tower (BAO) leads to these conclusions:
- The variation of roughness with wind direction found earlier is confirmed. Roughness lengths measured on the tower are larger than those measured close to the surface.
- The profiles and measurements of Reynolds stress are consistent with a von-Karman constant of 0.35.
- The form φm=(1?15z/L)-1/3 fits best in the range -0.6 < z/L < 0. In the range 0 < z/L < 0.5, θ m ～ 1 + 4.7z/L provides a good fit to the observations. For z/L < 0.1, φ m also depends on h, the thickness of the PBL. For z/L < -0.6, Φ m approaches the constant 0.5, in contrast to all previous suggestions. For larger stabilities, the upper level is usually not in the surface layer, and wind ratios become independent of z/L.
- With snow cover, the effective roughness diminishes to about 1 cm, even for directions for which the roughness length without snow is large.
- Estimation of winds at 100 or 150 m from information near the surface is best for similarity theory provided that the ratio of height to Monin-Obukhov L is less than 0.1. For larger z/L, simple power laws seem more appropriate.
2.
Atmospheric boundary layer research at Cabauw 总被引：1，自引：1，他引：0
At Cabauw, The Netherlands, a 213 m high mast specifically built for meteorological research has been operational since 1973. Its site, construction, instrumentation and observation programs are reviewed. Regarding analysis of the boundary layer at Cabauw, the following subjects are discussed: - terrain roughness; - Monin-Obukhov theory in practice; - the structure of stable boundary layers; - observed evolution of fog layers; - inversion rise and early morning entrainment; - use of the geostrophic wind as a predictor for wind profiles; - height variation of wind climate statistics; - air pollution applications: long range transport and short range dispersion; - dependence of sound wave propagation on boundary-layer structure; - testing of weather and climate models. 相似文献
3.
Arieh Bitan 《Boundary-Layer Meteorology》1981,21(4):477-487
From 1973–1976, research was performed around the Sea of Galilee, aimed at examining the wind regime in the area and whether the area develops a land-sea breeze despite its particular topographical location.
- The main conclusions were:
- During the summer mornings a lake breeze develops, blowing towards the shores of the lake. It ceases at the peak of its development when a westerly wind, originating in the development of a breeze along the Israeli Mediterranean coast, plunges towards the lake.
- Late at night, a wind flow develops from the land towards the lake, which combines with the katabatic winds that blow along the steep slopes surrounding the Kinneret.
- The stations at the upper level, at a height of 400–500 m above the Kinneret, are not affected by the lake breeze during the day or by the land breeze at night.
- In winter, the Kinneret lake breeze is almost as developed as in summer, because the westerly winds, originating in the Mediterranean sea breeze which hardly develops in this season, do not plunge into the Kinneret.
4.
Cross-spectra between horizontal wind components at different levels of the Boulder Atmospheric Observatory (BAO) tower lead to the following conclusions:
- Davenport's hypothesis is satisfied that coherence decays exponentially with the ratio of vertical separation to horizontal wave length, at least to very small values of coherence.
- The decay coefficients increase with z/L for z/L < 0.5. For larger stabilities, irregular fluctuations with periods of order 10–20 min have considerable vertical coherence. Results at BAO are quite consistent with those elsewhere.
- Eddy slopes in vertical planes increase with wind shear up to a point where the slope (horizontal delay over vertical separation) is just above 2. Beyond that point, the systematic increase of slopes with shear ceases. Since wind shear decreases upward, slopes tend to decrease upward. Slopes for lateral components are significantly larger than those for u-components.
5.
The internal boundary layer — A review 总被引：2，自引：2，他引：0
J. R. Garratt 《Boundary-Layer Meteorology》1990,50(1-4):171-203
A review is given of relevant work on the internal boundary layer (IBL) associated with:
- Small-scale flow in neutral conditions across an abrupt change in surface roughness,
- Small-scale flow in non-neutral conditions across an abrupt change in surface roughness, temperature or heat/moisture flux,
- Mesoscale flow, with emphasis on flow across the coastline for both convective and stably stratified conditions.
6.
Nicholas G. Prezerakos 《Boundary-Layer Meteorology》1986,36(3):245-266
The characteristics of the sea breeze in the Attica region of Greece, in which Athens is located, have been studied for occasions of weak synoptic-scale pressure gradient. The analysis is based on synoptic observations from six meteorological stations, three on the coast and three inland. The three inland stations and one of the coastal stations lie almost in a straight line at different distances from the coast. For each meteorological station, the basic characteristics of the sea breeze were determined, i.e.,
- The mean number of sea-breeze days for each calendar month.
- The monthly mean wind speed for each synoptic hour.
- The times of onset and cessation of the sea breeze.
- The monthly vector mean wind, and its constancy ‘Constancy’ is defined as 100{itV{inr}/V{ins}}, where {itV{inr}} is the magnitude of the vector mean wind, and {itV{ins}} is the scalar mean wind speed. See Brooks and Carruthers (1953). (In this paper, the factor 100 is not used.) for each synoptic hour.
- For days on which there was a sea breeze at Helliniko (the coastal reference station), the percentage number of days on which there was also a sea breeze at the given station.
7.
Shear flows generated by movement of the atmosphere near the earth's surface are accompanied by complexities not ordinarily encountered in the treatment of turbulent boundary layers. Problems arising from the following physical features are considered:
- thermal stratification;
- surface roughness in the form of forests and cities;
- non-uniformity of surface roughness and/or temperature (leading to 3-dimensional turbulent boundary layers);
- surface irregularities in the form of hilly and mountainous topography.
8.
Bare-ground surface heat and water exchanges under dry conditions: Observations and parameterization
I. Braud J. Noilhan P. Bessemoulin P. Mascart R. Haverkamp M. Vauclin 《Boundary-Layer Meteorology》1993,66(1-2):173-200
A simplified land-surface parameterization is tested against bare-soil data collected during the EFEDA experiment conducted in Spain in June 1991. A complete data set, made up of soil properties as well as hydrological and atmospheric measurements, is described and discussed. The 11-day data set is characterized by very dry conditions and high surface temperatures during the day. Large values of sensible and soil heat fluxes and small values of surface evaporation (≈1 mm/day) were observed. This data set was modelled, leading to the following conclusions:
- In the model, the parameterization provides values of the soil thermal properties and subsequently of the predicted soil heat fluxes which are overestimated when compared with the observations.
- Following the literature, a value of the ratio between the roughness lengths for momentumZ oand heatZ ohof close to 10 for fairly homogeneous areas of bare soil and vegetation is used. This value leads to a fair prediction of the surface temperature. If the roughness lengths were taken to be equal, as is often assumed in atmospheric modelling, a poorer prediction results.
- Finally, the vapor phase transfer mode is found dominant close to the surface and a modified parameterization including this effect is proposed. It allows a fair prediction of both surface evaporation and near-surface water content.
9.
Based on the 1979 FGGE Level III b data, calculation is made of the transfer of sensible and latent heat and momentum due to a quasi-40-day periodic oscillation (QDPO) on a cross-equatorial meridional ver-tical cross-section, and analysis is done of the characteristics of the transfer at all phases of QDPO, with the following results obtained:1) During the monsoon’s QDPO activation and break phases, a strong transfer of sensible heat to the SH is felt in the upper troposphere over the Asian monsoon region; the conversion of perturbation effective potential into its kinetic energy attains its maximum at 500-300 hPa (15oN), serving as the source of kinetic energy for the quasi-40-day periodic perturbation; an intense transfer of potential energy is found above 200 hPa from the monsoon area to the SH to maintain the QDPO at the tropical latitudes;2) During the QDPO activation-break (and reverse) transitional phase the conversion of perturbation effective potential into kinetic energy reaches its maximum in the middle and lower troposphere over the SH middle latitudes and an appreciable lower transfer of potential energy occurs towards the SH tropical latitudes and the NH.3) The upper-tropospheric powerful transfer of westerly momentum caused by QDPO is discovered from the SH tropical latitudes to the NH, and the resulting momentum divergence and convergence are unfavorable for the maintenance of the seasonal mean fields of the NH tropical easterly and SH subtropical westerly winds.Finally possible synoptical processes responsible for QDPO are discussed together with its relation to the interaction between the circulations of both the hemispheres. It is found that QDPO is both the result of and medium for the interaction. 相似文献
10.
Multiple windbreaks: An aeolean ensemble 总被引：1，自引：0，他引：1
Near-neutral measurements of the turbulent wind field within and above a sequence of 15 parallel windbreaks on a flat pastoral site are presented. The windbreak fences each had a porosity of 60% and were equally-spaced at 6 times their height (h = 2 m). The following conclusions seem justified for wind directions within 10 ° of the normal to the array:
- Above the windbreaks (2h), mean windspeeds first decreased and then increased asymptotically to a value in equilibrium with the new surface roughness. At 0.5h, windspeeds exhibited a slow increase down the entire array.
- Reflecting differences in approach flows, the drag on the initial fence was almost twice that on barriers farther downstream. This reduction in momentum extraction per windbreak was associated with an elevation in the zero-plane displacement to a level equal to 0.8h.
- At positions well-removed from the initial fences, mean windspeeds were reduced throughout the entire region below shelter height. In this region, the flow became increasingly dominated by downward moving air with velocities much greater than the local average. The zone of reduced turbulence was small, extending only 2h downstream of a barrier at a height of 0.25h. This corresponded with the region excluded from smoke trails released at the top of windbreaks.
- An approximate TKE budget mid-way between windbreaks 7 and 8 suggests that shear and wake production peak near z = h and that production is balanced by dissipation and vertical transport components. Advective and inertial interaction terms are negligible at this midway position but are likely to be major sources of TKE closer to the windbreak. Local equilibrium is attained above z = 1.5h implying the existence of a constant-stress layer.
11.
Ground based measurements which were carried out in the Northern Sahel in southern Tunisia showed the following results:
- The albedo difference between ground and protected land is about 10%, half of the amount Charney (1975) used in his model.
- Bare soil is always warmer during times of bright sunshine than vegetated soil, which is in agreement with Jackson and Idso (1975). Temperature differences in excess of the 10 °C were observed between plants and the surrounding soil.
- For bare soil, the surface temperature increases with declining albedo. However the opposite holds true for plants. Here, when lowering the albedo, a decrease in temperature was found.
- In a sand dune field, the surface temperature depends strongly on the exposure. Surface temperature differences of 8 °C were observed for slopes of different exposures for measurements carried out around noon.
12.
A modified infrared CO2 gas analyzer, a small thermocouple assembly, a heated-thermocouple anemometer for horizontal wind, and a propeller-type vertical wind sensor were used to measure the eddy fluxes of heat and CO2 above a corn crop. Experimental results of these fluxes are discussed. The main sources of errors of the eddy fluxes using these instruments were estimated:
- Sensors with a time constant of 0.5 s appear to be fast enough to detect most of the vertical CO2 transfer as long as the sensors are located at least one meter above the crop surface.
- The deviation from steady-state conditions for 10-min periods was found to have a significant effect on the eddy flux estimates.
- Temperature fluctuations of the air sample passing through the CO2 infrared gas analyzer were found to be non-negligible but could be easily corrected.
- A 1° misalignment of the vertical anemometer affected these eddy fluxes by less than 10% under all circumstances studied.
13.
Assessing the Consequences of Climate Change for Food and forest Resources: A View from the IPCC 总被引：1，自引：0，他引：1
Important findings on the consequences of climate change for agriculture and forestry from the recently completed Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC) are reviewed, with emphasis on new knowledge that emerged since the Second Assessment Report (SAR). The State-Pressure-Response-Adaptation model is used to organize the review. The major findings are:
- Constant or declining food prices are expected for at least the next 25 yr, although food security problems will persist in many developing countries as those countries deal with population increases, political crisis, poor resource endowments, and steady environmental degradation. Most economic model projections suggest that low relative food prices will extend beyond the next 25 yr, although our confidence in these projections erodes farther out into the 21st century.
- Although deforestation rates may have decreased since the early 1990s, degradation with a loss of forest productivity and biomass has occurred at large spatial scales as a result of fragmentation, non-sustainable practices and infrastructure development.
- According to United Nations estimates, approximately 23% of all forest and agricultural lands were classified as degraded over the period since World War II.
- At a worldwide scale, global change pressures (climate change, land-use practices and changes in atmospheric chemistry) are increasingly affecting the supply of goods and services from forests.
- The most realistic experiments to date – free air experiments in an irrigated environment – indicate that C3 agricultural crops in particular respond favorably to gradually increasing atmospheric CO2 concentrations (e.g., wheat yield increases by an average of 28%), although extrapolation of experimental results to real world production where several factors (e.g., nutrients, temperature, precipitation, and others) are likely to be limiting at one time or another remains problematic. Moreover, little is known of crop response to elevated CO2 in the tropics, as most of the research has been conducted in the mid-latitudes.
- Research suggests that for some crops, for example rice, CO2 benefits may decline quickly as temperatures warm beyond optimum photosynthetic levels. However, crop plant growth may benefit relatively more from CO2 enrichment in drought conditions than in wet conditions.
- The unambiguous separation of the relative influences of elevated ambient CO2 levels, climate change responses, and direct human influences (such as present and historical land-use change) on trees at the global and regional scales is still problematic. In some regions such as the temperate and boreal forests, climate change impacts, direct human interventions (including nitrogen-bearing pollution), and the legacy of past human activities (land-use change) appear to be more significant than CO2 fertilization effects. This subject is, however an area of continuing scientific debate, although there does appear to be consensus that any CO2 fertilization effect will saturate (disappear) in the coming century.
- Modeling studies suggest that any warming above current temperatures will diminish crop yields in the tropics while up to 2–3 °C of warming in the mid-latitudes may be tolerated by crops, especially if accompanied by increasing precipitation. The preponderance of developing countries lies in or near the tropics; this finding does not bode well for food production in those countries.
- Where direct human pressures do not mask them, there is increasing evidence of the impacts of climate change on forests associated with changes in natural disturbance regimes, growing season length, and local climatic extremes.
- Recent advances in modeling of vegetation response suggest that transient effects associated with dynamically responding ecosystems to climate change will increasingly dominate over the next century and that during these changes the global forest resource is likely to be adversely affected.
- The ability of livestock producers to adapt their herds to the physiological stress of climate change appears encouraging due to a variety of techniques for dealing with climate stress, but this issue is not well constrained, in part because of the general lack of experimentation and simulations of livestock adaptation to climate change.
- Crop and livestock farmers who have sufficient access to capital and technologies should be able to adapt their farming systems to climate change. Substantial changes in their mix of crops and livestock production may be necessary, however, as considerable costs could be involved in this process because investments in learning and gaining experience with different crops or irrigation.
- Impacts of climate change on agriculture after adaptation are estimated to result in small percentage changes in overall global income. Nations with large resource endowments (i.e., developed countries) will fare better in adapting to climate change than those with poor resource endowments (i.e., developing countries and countries in transition, especially in the tropics and subtropics) which will fare worse. This, in turn, could worsen income disparities between developed and developing countries.
- Although local forest ecosystems will be highly affected, with potentially significant local economic impacts, it is believed that, at regional and global scales, the global supply of timber and non-wood goods and services will adapt through changes in the global market place. However, there will be regional shifts in market share associated with changes in forest productivity with climate change: in contrast to the findings of the SAR, recent studies suggest that the changes will favor producers in developing countries, possibly at the expense of temperate and boreal suppliers.
- Global agricultural vulnerability is assessed by the anticipated effects of climate change on food prices. Based on the accumulated evidence of modeling studies, a global temperature rise of greater than 2.5 °C is likely to reverse the trend of falling real food prices. This would greatly stress food security in many developing countries.
14.
Two aspects of convection over oceans are discussed and the following conclusions are derived from theoretical considerations.
- The air layer over the sea will usually convect even when the water surface is ten degrees or more colder than the initial air temperature.
- An inversion at stratus cloud tops is created by the stratus, and is not a necessary preexisting condition. Such inversions persist after subsidence evaporates the cloud.
- Radiation heat exchange does not play an essential role in stratus formation or maintenance, and can either heat or cool the cloud.
- Dry air convection does not erode inversions at the top of the convecting layer. Examples of soundings are discussed.
- Fogs are most likely to form at sea where the water is coolest, and need no radiation effects to initiate cooling, or a boost from patches of warmer water, to begin convection.
- Both stratus cloud growth, and the evaporation of clouds by cloud top entrainment, readjust the vertical structure of the air to leave a constant wet-bulb potential temperature with height.
15.
The most promising response strategies are -sustainable practices in agriculture to improve productivity on existing arable land especially in developing countries to meet the food requirements of a still rising population -sustainable practices in forestry both in tropical forests as well as in forests of temperate and boreal zones, in the latter case to achieve sufficient fast adaption to climate change. 相似文献
16.
Surface Heterogeneity Effects on Regional-Scale Fluxes in the Stable Boundary Layer: Aerodynamic Roughness Length Transitions 总被引：1，自引：1，他引：0
The effects of abrupt streamwise transitions of the aerodynamic roughness length ( $z_\mathrm{o}$ z o ) on the stable atmospheric boundary layer are evaluated using a series of large-eddy simulations based on the first Global Energy and Water Cycle Experiment Atmospheric Boundary Layer intercomparison study (GABLS1). Four $z_\mathrm{o}$ z o values spanning three orders of magnitude are used to create all possible binary distributions with each arranged into patches of characteristic length scales equal to roughly one-half, one, and two times the equivalent homogeneous boundary-layer height. The impact of the heterogeneity on mean profiles of wind speed and temperature, on surface fluxes of heat and momentum, and on internal boundary-layer dynamics are considered. It is found that $z_\mathrm{o}$ z o transitions do not significantly alter the functional relationship between the average surface fluxes and the mean profiles of wind speed and potential temperature. Although this suggests that bulk similarity theory is applicable for modelling the stable boundary layer over $z_\mathrm{o}$ z o heterogeneity, effective surface parameters must still be specified. Existing models that solve for effective roughness lengths of momentum and heat are evaluated and compared to values derived from the simulation data. The existing models are unable to accurately reproduce both the values of the effective aerodynamic roughness lengths and their trends as functions of patch length scale and stability. A new model for the effective aerodynamic roughness length is developed to exploit the benefits of the other models tested. It accurately accounts for the effects of the heterogeneity and stratification on the blending height and effective aerodynamic roughness length. The new model provides improved average surface fluxes when used with bulk similarity. 相似文献
17.
A. Weill C. Mazaudier F. Baudin C. Klapisz F. Leca M. Masmoudi D. Vidal Madjar R. Bernard O. Taconet B. S. Gera A. Sauvaget A. Druilhet P. Durand J. Y. Caneil P. Mery G. Dubosclard A. C. M. Beljaars W. A. A. Monna J. G. Van Der Vliet M. Crochet D. Thomson T. Carlson 《Boundary-Layer Meteorology》1988,42(3):251-264
18.
Field data for the unstable, baroclinic, atmospheric boundary layer over land and over the sea are considered in the context of a general similarity theory of vertical heat transfer. The dependence of δθ/θ* upon logarithmic functions of h c z T and stability (through the similarity function C) is clearly demonstrated in the data. The combined data support the conventional formulation for the heat transfer coefficient δθ/θ* when,
- the surface scaling length is z T (« z 0), the height at which the surface temperature over land is obtained by extrapolation of the temperature profile
- the height scale is taken as the depth of convective mixing h c
- the temperature profile equivalent of the von Karman constant is taken as 0.41
- areal average, rather than single point, values of δθ are employed in strongly baroclinic conditions. No significant effect of baroclinity or the height scale ratio as proposed in the general theory is found. Variations in C about a linear regression relation against stability are most probably due to uncertainties in the areal surface temperature and to experimental errors in general temperature measurements.
19.
This is the first of a series of three papers describing experiments on the dispersion of trace heat from elevated line and plane sources within a model plant canopy in a wind tunnel. Here we consider the wind field and turbulence structure. The model canopy consisted of bluff elements 60 mm high and 10 mm wide in a diamond array with frontal area index 0.23; streamwise and vertical velocity components were measured with a special three-hot-wire anemometer designed for optimum performance in flows of high turbulence intensity. We found that:
- The momentum flux due to spatial correlations between time-averaged streamwise and vertical velocity components (the dispersive flux) was negligible, at heights near and above the top of the canopy.
- In the turbulent energy budget, turbulent transport was a major loss (of about one-third of local production) near the top of the canopy, and was the principal gain mechanism lower down. Wake production was greater than shear production throughout the canopy. Pressure transport just above the canopy, inferred by difference, appeared to be a gain in approximate balance with the turbulent transport loss.
- In the shear stress budget, wake production was negligible. The role of turbulent transport was equivalent to that in the turbulent energy budget, though smaller.
- Velocity spectra above and within the canopy showed the dominance of large eddies occupying much of the boundary layer and moving downstream with a height-independent convection velocity. Within the canopy, much of the vertical but relatively little of the streamwise variance occurred at frequencies characteristic of wake turbulence.
- Quadrant analysis of the shear stress showed only a slight excess of sweeps over ejections near the top of the canopy, in contrast with previous studies. This is a result of improved measurement techniques; it suggests some reappraisal of inferences previously drawn from quadrant analysis.