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1.
An attempt has been made to investigate the role of vertical wind shear, corrective instability and the thermodynamic parameter (θes - θe) below the first lifting condensation level (FLCL) in the occurrence of instanta-neous premonsoon thunderstorm over Agartala (AGT) and Ranchi (RNC) at 12 GMT Radiosonde data of 1988 have been utilized here. The study has however been confined to 1000 hPa-500 hPa range at most Here the convectively unstable layers with positive vertical wind shear upto 500 hPa have been termed as ‘Fa?vourable Layers’ (FL) and the level at which an initially stable layer turns out to be convectively unstable for the first time has been termed as ‘Transition Level’ (TL). It is observed that the changes in vertical wind shear are positive at TL at the time of occurrence of thunderstorm (TS) and the corresponding change is negative on fair-weather situa?tion Moreover, the 90% confidence interval for (θes - θe) reveals that for AGT the upper layer thermodynamic characteristic is important at the time of occurrence of TS whereas for RNC, the value of (θes - θe) at the surface is much more effective 相似文献
2.
3.
M. G. Mennen H. J. M. A. Zwart J. E. M. Hogenkamp J. W. Erisman 《Boundary-Layer Meteorology》1996,81(3-4):353-371
Flow distortion errors on wind and friction velocity induced by a box simulating the housing of a gas analyzer used in dry
deposition eddy correlation measurements were determined in a field experiment. ‘Undisturbed’ and ‘disturbed’ wind and friction
velocities, measured with two dry deposition monitoring systems run simultaneously, were compared, one to the other. In the
‘disturbed’ case the box was mounted below the 3-component probe of the sonic anemometer of one of these systems, while in
the ‘undisturbed’ case the box was removed. When the probe was located on the upstream side of the box, the results showed
satisfactory agreement with theoretical estimates using Wyngaard’s potential-flow approach and a spherical model for the box.
This model can be applied to obtain first-order corrections for flow distortion errors induced by cubic-like (or spherical)
obstacles such as a gas analyzer housing used in dry deposition research systems, or to determine the optimal location of
this housing relative to the sonic probe in such systems. When the probe was located halfway downstream and halfway to the
side of the box, the experimental flow distortion errors did not exceed those for the upstream case. This implies that to
keep flow distortion errors in dry deposition systems as small as possible the sonic probe can be placed upstream but also
to the side of the gas analyzer housing. The results of our experiments also confirmed that correcting for flow distortion
with the commonly used tilt equations yields underestimated values. 相似文献
4.
Dispersion of a Point-Source Release of a Passive Scalar Through an Urban-Like Array for Different Wind Directions 总被引:1,自引:1,他引:0
The dispersion of a point-source release of a passive scalar in a regular array of cubical, urban-like, obstacles is investigated
by means of direct numerical simulations. The simulations are conducted under conditions of neutral stability and fully rough
turbulent flow, at a roughness Reynolds number of Re
τ
= 500. The Navier–Stokes and scalar equations are integrated assuming a constant rate release from a point source close to
the ground within the array. We focus on short-range dispersion, when most of the material is still within the building canopy.
Mean and fluctuating concentrations are computed for three different pressure gradient directions (0°, 30°, 45°). The results
agree well with available experimental data measured in a water channel for a flow angle of 0°. Profiles of mean concentration
and the three-dimensional structure of the dispersion pattern are compared for the different forcing angles. A number of processes
affecting the plume structure are identified and discussed, including: (i) advection or channelling of scalar down ‘streets’,
(ii) lateral dispersion by turbulent fluctuations and topological dispersion induced by dividing streamlines around buildings,
(iii) skewing of the plume due to flow turning with height, (iv) detrainment by turbulent dispersion or mean recirculation,
(v) entrainment and release of scalar in building wakes, giving rise to ‘secondary sources’, (vi) plume meandering due to
unsteady turbulent fluctuations. Finally, results on relative concentration fluctuations are presented and compared with the
literature for point source dispersion over flat terrain and urban arrays. 相似文献
5.
6.
Variance Method to Determine Turbulent Fluxes of Momentum And Sensible Heat in The Stable Atmospheric Surface Layer 总被引:1,自引:0,他引:1
Evidence is presented that in the stable atmospheric surface layer turbulent fluxes of heat and momentum can be determined
from the standard deviations of longitudinal wind velocity and temperature, σ u and σ T respectively, measured at a single level. An attractive aspect of this method is that it yields fluxes from measurements
that can be obtained with two-dimensional sonic anemometers. These instruments are increasingly being used at official weather
stations, where they replace the standard cup anemometer–wind vane system. With methods such as the one described in this
note, a widespread, good quality, flux network can be established, which would greatly benefit the modelling community. It
is shown that a ‘variance’ dimensionless height (ζ σ) defined from σ u and σ T is highly related to the ‘conventional’ dimensionless stability parameter ζ=z/L, where z is height and L is the Obukhov length. Empirical functions for ζ σ are proposed that allow direct calculation of heat and momentum fluxes from σ u and σ T. The method performs fairly well also during a night of intermittent turbulence. 相似文献
7.
Long-Term Mean Wind Profiles Based on Similarity Theory 总被引:1,自引:1,他引:0
We provide general forms for long-term mean wind profiles from similarity-based wind profiles, beginning with a probabilistic
adaptation of Monin–Obukhov similarity theory. We develop an analytical formulation for the stability distributions prevailing
in the atmospheric surface layer, which in turn facilitates the derivation of a long-term mean wind profile based on Monin–Obukhov
similarity theory. The modelled stability distributions exhibit good agreement with measurements from sites having different
local conditions. The long-term wind profile formulation is further extended to include the influence of the depth of the
atmospheric boundary layer (h), which becomes relevant for heights above h/3, and the resultant long-term ‘tall’ profile form also matches observations. 相似文献
8.
9.
Measurements of concentration fluctuation intensity, intermittency factor, and integral time scale were made in a water channel
for a plume dispersing in a well-developed, rough surface, neutrally stable, boundary layer, and in grid-generated turbulence
with no mean velocity shear. The water-channel simulations apply to full-scale atmospheric plumes with very short averaging
times, on the order of 1–4 min, because plume meandering was suppressed by the water-channel side walls. High spatial and
temporal resolution vertical and crosswind profiles of fluctuations in the plume were obtained using a linescan camera laser-induced
dye tracer fluorescence technique. A semi-empirical algebraic mean velocity shear history model was developed to predict these
concentration statistics. This shear history concentration fluctuation model requires only a minimal set of parameters to
be known: atmospheric stability, surface roughness, vertical velocity profile, and vertical and crosswind plume spreads. The
universal shear history parameter used was the mean velocity shear normalized by surface friction velocity, plume travel time,
and local mean wind speed. The reference height at which this non-dimensional shear history was calculated was important,
because both the source and the receptor positions influence the history of particles passing through the receptor position. 相似文献
10.
This paper discusses methodological issues relevant to the calculation of historical responsibility of countries for climate
change (‘The Brazilian Proposal’). Using a simple representation of the climate system, the paper compares contributions to
climate change using different indicators: current radiative forcing, current GWP-weighted emissions, radiative forcing from
increased concentrations, cumulative GWP-weighted emissions, global-average surface-air temperature increase and two new indicators:
weighted concentrations (analogue to GWP-weighted emissions) and integrated temperature increase. Only the last two indicators
are at the same time ‘backward looking’ (take into account historical emissions), ‘backward discounting’ (early emissions
weigh less, depending on the decay in the atmosphere) and ‘forward looking’ (future effects of the emissions are considered)
and are comparable for all gases. Cumulative GWP-weighted emissions are simple to calculate but are not ‘backward discounting’.
‘Radiative forcing’ and ‘temperature increase’ are not ‘forward looking’. ‘Temperature increase’ discounts the emissions of
the last decade due to the slow response of the climate system. It therefore gives low weight to regions that have recently
significantly increased emissions. Results of the five different indicators are quite similar for large groups (but possibly
not for individual countries): industrialized countries contributed around 60% to today’s climate change, developing countries
around 40% (using the available data for fossil, industrial and forestry CO2, CH4 and N2O). The paper further argues including non-linearities of the climate system or using a simplified linear system is a political
choice. The paper also notes that results of contributions to climate change need to be interpreted with care: Countries that
developed early benefited economically, but have high historical emission, and countries developing at a later period can
profit from developments in other countries and are therefore likely to have a lower contribution to climate change. 相似文献
11.
H. E. Fossum B. A. Pettersson Reif M. Tutkun T. Gjesdal 《Boundary-Layer Meteorology》2012,144(1):21-40
Aerosol dispersion in the area surrounding an existing biological treatment facility is investigated using large-eddy simulation,
with the objective to investigate the applicability of computational fluid dynamics to complex real-life problems. The aerosol
sources consist of two large aeration ponds that slowly diffuse aerosols into the atmosphere. These sources are modelled as
dilute concentrations of a non-buoyant non-reacting pollutant diffusing from two horizontal surfaces. The time frame of the
aerosol release is restricted to the order of minutes, justifying a statistically steady inlet boundary condition. The numerical
results are compared to wind-tunnel experiments for validation. The wind-tunnel flow characteristics resemble neutral atmospheric
conditions with a Reynolds number, based on the boundary-layer thickness, of Re
δ
≈ 2 × 105. The numerical inflow conditions are based upon the wind-tunnel flow field. The predicted decay of both the mean and root-mean-square
concentrations are in good agreement with experimental data; at 3 m from the ground, the plume mean concentration 200 m downwind
of the source is approximately 2% of the source strength. The numerical data in the near-surface layer (0–50 m from the ground)
correspond particularly well with the wind-tunnel data. Tentative deposition simulations suggest that there seems to be little
difference in the deposition rates of large (1.8 × 10−5 m) and small (3 × 10−6 m) particles in the near-field under the flow conditions considered. 相似文献
12.
The SF6 gas tracer observations for puffs released near the ground during the Joint Urban 2003 (JU2003) urban dispersion experiment
in Oklahoma City have been analysed. The JU2003 observations, at distances of about 100–1,100 m from the source, show that,
at small times, when the puff is still within the built-up downtown domain, the standard deviation of the concentration time
series, σt, is influenced by the initial puff spread due to buildings near the source and by hold-up in the wakes of large buildings
at the sampler locations. This effect is parameterised by assuming an initial σto of about 42 s, leading to a comprehensive similarity formula: σt = 42 + 0.1t. The second term, 0.1t, is consistent with an earlier similarity relation, σt = 0.1t, derived from puff observations in many experiments over rural terrain. The along-wind dispersion coefficient, σx, is assumed to equal σt
u, in which u is the puff speed calculated as the distance from the source to the sampler, x, divided by the time after the release that the maximum concentration is observed at the sampler. σx can be expressed as σx = σxo + 0.14x, with the initial σxo of 45 m. This initial σxo agrees with the suggestion of an initial plume spread of about 40 m, made by McElroy and Pooler from analysis of the 1960s’
St. Louis urban dispersion experiment. The puff speeds, u, are initially only about 20% of the observed wind speed, averaged over about 80 street-level and rooftop anemometers in
the city, but approach the mean observed wind speed as the puffs grow vertically. The scatter in the σt data is about ± a factor of two or three at any given travel time. The maximum σt is about 250 s, and the maximum duration of the puff over the sampler, Dt, sometimes called the retention time, is about
1,100 s or 18 min for these puffs and distances. 相似文献
13.
In order to estimate the impacts of buildings on air pollution dispersion, numerical simulations are performed over an idealized
urban area, modelled as regular rows of large rectangular obstacles. The simulations are evaluated with the results of the
Mock Urban Setting Test (MUST), which is a near full-scale experiment conducted in Utah’s West Desert area: it consists of
releases of a neutral gas in a field of regularly spaced shipping containers. The numerical simulations are performed with
the model Mercure_Saturne, which is a three-dimensional computational fluid dynamics code adapted to atmospheric flow and dispersion simulations. It
resolves complex geometries and uses, in this study, a k–∈ closure for the turbulence model. Sensitivity studies focus on how to prescribe the inflow conditions for turbulent kinetic
energy. Furthermore, different sets of coefficients available in the literature for the k–∈ closure model are tested. Twenty MUST trials with different meteorological conditions are simulated and detailed analyses
are performed for both the dynamical variables and average concentration. Our results show overall good agreement according
to statistical comparison parameters, with a fraction of predictions for average concentration within a factor of two of observations
of 67.1%. The set of simulations offers several inflow wind directions and allows us to emphasize the impact of elongated
buildings, which create a deflection of the plume centerline relative to the upstream wind direction. 相似文献
14.
边界层对流对示踪物抬升和传输影响的大涡模拟研究 总被引:3,自引:1,他引:2
利用"西北干旱区陆气相互作用野外观测实验"加密观测期间敦煌站的实测资料以及大涡模式, 通过一系列改变地表热通量和风切变的敏感性数值试验, 分析了地表热通量和风切变对边界层对流的强度、形式, 以及对对流边界层结构和发展的影响。模拟结果显示风切变一定, 增大地表热通量时, 由于近地层湍流运动增强, 向上输送的热量也较多, 使对流边界层变暖增厚, 而且边界层对流的强度明显增强, 对流泡发展的高度也较高。当地表热通量一定, 增大风切变时, 由于风切变使夹卷作用增强, 将逆温层中的暖空气向下卷入混合层中, 使对流边界层增暖增厚, 但是对流泡容易破碎, 对流的强度也较弱。另外通过在模式近地层释放绝对浓度为100的被动示踪物方法, 用最小二乘法定量地分析了地表热通量和风切变分别与示踪物抬升效率和传输高度的关系。分析结果表明, 风切变小于10.5×10-3 s-1时, 增大地表热通量加强了上层动量的下传, 使示踪物的抬升效率也线性增大;地表热通量小于462.5 W m-2时, 增大风切变减弱了边界层对流的强度, 从而使示踪物的抬升效率减弱。当风切变一定时, 示踪物的平均传输高度随地表热通量增加而增大, 而地表热通量一定, 只有风切变大于临界值时, 示踪物平均传输高度才随风切变的增加而增大, 而临界风速的大小由地表热通量决定。 相似文献
15.
Sheikh Ahmad Zaki Aya Hagishima Jun Tanimoto Naoki Ikegaya 《Boundary-Layer Meteorology》2011,138(1):99-120
It is difficult to describe the flow characteristics within and above urban canopies using only geometrical parameters such
as plan area index (λ
p
) and frontal area index (λ
f
) because urban surfaces comprise buildings with random layouts, shapes, and heights. Furthermore, two types of ‘randomness’
are associated with the geometry of building arrays: the randomness of element heights (vertical) and that of the rotation
angles of each block (horizontal). In this study, wind-tunnel experiments were conducted on seven types of urban building
arrays with various roughness packing densities to measure the bulk drag coefficient (C
d
) and mean wind profile; aerodynamic parameters such as roughness length (z
o
) and displacement height (d) were also estimated. The results are compared with previous results from regular arrays having neither ‘vertical’ nor ‘horizontal’
randomness. In vertical random arrays, the plot of C
d
and z
o
versus λ
f
exhibited a monotonic increase, and z
o
increased by a factor of almost two for λ
f
= 48–70%. C
d
was strongly influenced by the standard deviation of the height of blocks (σ) when λ
p
≥ 17%, whereas C
d
was independent of σ when λ
p
= 7%. In the case of horizontal random arrays, the plot of the estimated C
d
against λ
f
showed a peak. The effect of both vertical and horizontal randomness of the layout on aerodynamic parameters can be explained
by the structure of the vortices around the blocks; the aspect ratio of the block is an appropriate index for the estimation
of such features. 相似文献
16.
This study investigates the organised motion near the canopy-atmosphere interface of a moderately dense spruce forest in heterogeneous,
complex terrain. Wind direction is used to assess differences in topography and surface properties. Observations were obtained
at several heights above and within the canopy using sonic anemometers and fast-response gas analysers over the course of
several weeks. Analysed variables include the three-dimensional wind vector, the sonic temperature, and the concentration
of carbon dioxide. Wavelet analysis was used to extract the organised motion from time series and to derive its temporal scales.
Spectral Fourier analysis was deployed to compute power spectra and phase spectra. Profiles of temporal scales of ramp-like
coherent structures in the vertical and longitudinal wind components showed a reversed variation with height and were of similar
size within the canopy. Temporal scales of scalar fields were comparable to those of the longitudinal wind component suggesting
that the lateral scalar transport dominates. The existence of a – 1 power law in the longitudinal power spectra was confirmed
for a few cases only, with a majority showing a clear 5/3 decay. The variation of effective scales of organised motion in
the longitudinal velocity and temperature were found to vary with atmospheric stability, suggesting that both Kelvin-Helmholtz
instabilities and attached eddies dominate the flow with increasing convectional forcing. The canopy mixing-layer analogy
was observed to be applicable for ramp-like coherent structures in the vertical wind component for selected wind directions
only. Departures from the prediction of m = Λ
w
L
s
−1 = 8–10 (where Λ
w
is the streamwise spacing of coherent structures in the vertical wind w and L
s
is a canopy shear length scale) were caused by smaller shear length scales associated with large-scale changes in the terrain
as well as the vertical structure of the canopy. The occurrence of linear gravity waves was related to a rise in local topography
and can therefore be referred to as mountain-type gravity waves. Temporal scales of wave motion and ramp-like coherent structures
were observed to be comparable. 相似文献
17.
Summary This paper assesses recent changes in extremes of seasonal rainfall in Ethiopia based on daily rainfall data for 11 key stations
over the period 1965–2002. The seasons considered are Kiremt (‘main rains’, June–September) and Belg (‘small rains’, February/March–May). The Mann-Kendall and linear regression trend tests show decreasing trends in the Kiremt and the Belg extreme intensity and maximum consecutive 5-day rains over eastern, southwestern and southern parts of Ethiopia whereas no
trends are found in the remaining part of Ethiopia. In general, no trends are found in the yearly maximum length of Kiremt and Belg dry spells (days with rainfall below 1 mm) over Ethiopia. 相似文献
18.
Wind and temperature profile prediction for diabatic surface layers including strong inversion cases
Heinz H. Lettau 《Boundary-Layer Meteorology》1979,17(4):443-464
The micrometeorological research program in Antarctica has provided extensive data on wind and temperature profile structure under strong to extreme inversion conditions (Dalrymple et al., 1966; Lettau et al., 1977). The basic similarity hypotheses and limiting conditions for prediction of diabatic surface layer profiles are summarized. The model by Businger et al. (1971) for dimensionless shear and temperature gradients is revised to conform with the new results for strong stability. A novel similarity hypothesis is introduced to complete the step from shear and gradient prediction to prediction of absolute wind speed, wind energy, and temperature on the basis of prescribed external factors of surface layer structure. The physics of interactions between predicted profile tilting and curving are discussed and used to explain several micrometeorological paradoxes, including that of the elevated minimum of air temperature observed occasionally near the active surface when the energy budget is of the nocturnal type. 相似文献
19.
Numerical Investigations of Mean Winds Within Canopies of Regularly Arrayed Cubical Buildings Under Neutral Stability Conditions 总被引:2,自引:2,他引:0
Recently, several attempts have been made to model the wind velocity in an urban canopy in order to accurately predict the
mixing and transport of momentum, heat, and pollutants within and above the canopy on an urban scale. For this purpose, unverified
assumptions made by Macdonald (Boundary-Layer Meteorol 97:25–45, 2000) to develop a model for the profile of the mean wind
velocity within an urban canopy have been used. In the present study, in order to provide foundations for improving the urban
canopy models, the properties of the spatially-averaged mean quantities used to make these assumptions have been investigated
by performing large-eddy simulations (LES) of the airflow around square and staggered arrays of cubical blocks with the following
plan area densities: λ
p
= 0.05, 0.11, 0.16, 0.20, 0.25, and 0.33. The LES results confirm that the discrepancy between the spatial average of wind
velocity and Macdonald’s five-point average of wind velocity can be large in both types of arrays for large λ
p
. It is also confirmed that Prandtl’s mixing length varies significantly with height within the canopy, contrary to Macdonald’s
assumption for both types of arrays and for both small and large λ
p
. On the other hand, in accordance with Macdonald’s assumption, the sectional drag coefficient is found to be almost constant
with height except in the case of staggered arrays with high λ
p
. 相似文献
20.
On the Formation of an Elevated Nocturnal Inversion Layer in the Presence of a Low-Level Jet: A Case Study 总被引:1,自引:1,他引:0
We report on observed nocturnal profiles, in which an inversion layer is located at the core of a low-level jet, bounded between two well-mixed layers. High-resolution vertical profiles were collected during a field campaign in a small plain in the Israeli desert (Negev), distant 100 km from the eastern shore of the Mediterranean Sea. During the evening hours, the synoptic flow, superposed on the late sea breeze, forms a low-level jet characterized by a maximum wind speed of 12 m s −1 at an altitude of 150 m above the ground. The strong wind shear at the jet maximum generates downward heat fluxes that act against the nocturnal ground cooling. As a result, the typical ground-based nocturnal inversion is “elevated” towards the jet centre, hence a typical early morning thermal profile is observed a few hours after sunset. Since the jet is advected into the region, its formation does not depend on the presence of a surface nocturnal inversion layer to decouple the jet from surface friction. On the contrary, here the advected low-level jet acts to hinder the formation of such an inversion. These unusual temperature and wind profiles are expected to affect near-ground dispersion processes. 相似文献