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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.
Summary A K-type diffusion model coupled with a massconsistent wind model is applied for one of the rural biological waste disposal
sites in Austrian Alps. The site is situated in the P?ls valley in the eastern Alps, 250 km south-west of Vienna in Austria
Aim of the study is to demonstrate dispersion of H2S from the site to near by village. Model simulations are carried out each for an evening and a morning transition case characterized
by flow reversals. The role of locally generated wind in changing the pollutant distribution over nearby residential area
is investigated. Surface observations at two stations toward the open boundaries of the main valley are used to derive the
turbulence parameters and then to obtain initial inputs of wind profiles. The turbulence parameters behave analogous to that
over a plane terrain after the establishment of the valley wind. The model simulations are done for eight hours during the
evening transition and eighteen hours for the morning transition by incorporating the wind field from a mass consistent wind
model. The results are compared with SF6 tracer experiments conducted during those periods. The model outputs and the observations at various points inside the valley
are in good correlation except for NW part of the valley after the reversal of valley wind. The results also reveal the potential
of a simple approach with minimized inputs.
Received August 15, 1997 Revised August 15, 1998 相似文献
3.
1998年夏季HUBEX/GAME期间热量和水汽收支(英) 总被引：4，自引：0，他引：4
By using the high-resolution GAME reanalysis data, the heat and moisture budgets during the period of HUBEX/GAME in the summer of 1998 are calculated for exploring the thermodynamic features of Meiyu over the Changjiang-Huaihe (CH) valley. During the CH Meiyu period, an intensive vertically-integrated heat source and moisture sink are predominant over the heavy rainfall area of the CH valley, accompanied by strong upward motion at 500 hPa. The heat and moisture budgets show that the main diabatic heating component is condensation latent heat released by rainfall. As residual terms, the evaporation and sensible heating are relatively small. Based on the vertical distribution of the heat source and moisture sink, the nature of the rainfall is mixed, in which the convective rainfall is dominant with a considerable percentage of continuous stratiform rainfall. There are similar time evolutions of the main physical parameters(〈Q1〉,〈Q2〉,and vertical motion ω at 500 hPa).The time variations of〈Q1〉and〈Q2〉are in phase with those of -ω500, and have their main peaks within the CH Meiyu period. This shows the influence of the heat source on the dynamic structure of the atmosphere. The wavelet analyses of those time series display similar multiple timescale characteristics. During the CH Meiyu period, both the synoptic scale(～6 days) and mesoscale (～2 days and ～12 hours) increase obviously and cause heavy rainfall as well as the appearances of the maxima of the main physical parameters. Among them, the mesoscale systems are the main factors. 相似文献
4.
Summary ?Above orographically structured terrain considerable differences of the regional wind field may be identified during large-scale
extreme wind events. So far, these regional differences could not be resolved by climate models. To determine the relationships
between large-scale atmospheric conditions, the influence of orography, and the regional wind field, data measured in the
upper Rhine valley within the framework of the REKLIP Regional Climate Project were analyzed and calculations were made using
the KAMM mesoscale model. In the area of the upper Rhine valley, ratios of the wind velocity in the Rhine valley at 10 m above
ground level, νval, and the large-scale flow velocity, νlar, are between νval/νlar ≈ 0.1 and νval/νlar ≈ 1. The νval/νlar ratio exhibits a strong dependence on thermal stratification, δ, and decreases from νval/νlar ≈ 1 at δ = 0 K m−1 to νval/νlar ≈ 0.2 at δ = 0.0075 K m−1. In areas, where the lateral mountainous border of the Rhine valley is interrupted, the νval/νlar ratio increases again with increasing stability or decreasing Froude number. This is obviously due to flow around the Black
Forest under stable stratification. It is demonstrated by model calculations that a complex wind field develops in the Rhine
valley at small Froude numbers (Fr < 1) irrespective of the direction of large-scale flow. The νval/νlar ratio is characterized by small values in the direct lee side (νval/νlar ≈ 0.2) and high values on the windward side of the lateral mountainous border of the Rhine valley (νval/νlar ≈ 0.8).
Received October 22, 2001; revised June 18, 2002; accepted June 23, 2002 相似文献
5.
The characteristics of dynamics and thermodynamics of the atmospheric boundary layer in a part of the Colorado River Valley, centered around Lake Mohave, have been investigated by analysis of measurements conducted during a field program in late spring and early summer of 1986 and a series of numerical simulations by a three-dimensional second-moment turbulence-closure model. The model was validated against measurements described in a companion article (Engeret al., 1993). According to airsonde measurements performed on eight nights, the depth of the surface inversion was around 200 m with an average temperature gradient of about 30 K km–1. Analysis of acoustic sounder data collected during one month revealed significant diurnal variations ofU andV wind-speed components related to slope and valley flows, respectively. Some of the dynamics properties have been explained by the simulation results. It has been shown that the appearance of supergeostrophic southerly valley flow is associated with the westerly component of the geostrophic flow. Since a westerly component of the geostrophic wind is quite common for this area in summer, this effect also explains the frequently observed southerly valley flow in summer. Elevated minima of the measured wind speed around valley ridges appear to be related to the interaction of conservation of momentum in theX andY directions. The critical direction of the geostrophic wind relevant for reversal of up-valley flow to down-valley flow has also been studied. The critical direction is about 300° for one of the measurement sites and, depending on the angle between valley axis and south-north direction, the critical direction is expected to vary by about 15–20°. The scale analysis of the simulated equations of motion and turbulence kinetic energy emphasizes the strong impact of meandering of the flow due to actual topographic complexity. 相似文献
6.
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
7.
Results from large-eddy simulations and field measurements have previously shown that the velocity field is influenced by
the boundary layer height, z
i
, during close to neutral, slightly unstable, atmospheric stratification. During such conditions the non-dimensional wind
profile, φ
m
, has been found to be a function of both z/L and z
i
/L. At constant z/L, φ
m
decreases with decreasing boundary layer height. Since φ
m
is directly related to the parameterizations of the air–sea surface fluxes, these results will have an influence when calculating
the surface fluxes in weather and climate models. The global impact of this was estimated using re-analysis data from 1979
to 2001 and bulk parameterizations. The results show that the sum of the global latent and sensible mean heat fluxes increase
by 0.77 W m−2 or about 1% and the mean surface stress increase by 1.4 mN m−2 or 1.8% when including the effects of the boundary layer height in the parameterizations. However, some regions show a larger
response. The greatest impact is found over the tropical oceans between 30°S and 30°N. In this region the boundary layer height
influences the non-dimensional wind profile during extended periods of time. In the mid Indian Ocean this results in an increase
of the mean annual heat fluxes by 2.0 W m−2 and an increase of the mean annual surface stress by 2.6 mN m−2. 相似文献
8.
Zbigniew Sorbjan 《Boundary-Layer Meteorology》2007,123(3):365-383
We examine daily (morning–afternoon) transitions in the atmospheric boundary layer based on large-eddy simulations. Under
consideration are the effects of the stratification at the top of the mixed layer and of the wind shear. The results describe
the transitory behaviour of temperature and wind velocity, their second moments, the boundary-layer height Z
m
(defined by the maximum of the potential temperature gradient) and its standard deviation σ
m
, the mixed-layer height z
i
(defined by the minimum of the potential temperature flux), entrainment velocity W
e, and the entrainment flux H
i
. The entrainment flux and the entrainment velocity are found to lag slightly in time with respect to the surface temperature
flux. The simulations imply that the atmospheric values of velocity variances, measured at various instants during the daytime,
and normalized in terms of the actual convective scale w*, are not expected to collapse to a single curve, but to produce a significant scatter of observational points. The measured
values of the temperature variance, normalized in terms of the actual convective scale Θ*, are expected to form a single curve in the mixed layer, and to exhibit a considerable scatter in the interfacial layer. 相似文献
9.
利用2010年1-2月深圳LAP3000型风廓线雷达资料, 对湍流耗散率进行了估算, 针对典型晴天条件下的湍流耗散率ε、折射率结构常数C2n、水平风速和风切变, 分析了其时空变化特征。得出如下结论: (1) 深圳地区低空大气ε的量级在10-7~10-1 m-2·s-3之间, 与理论模拟值基本一致; (2) 时间分布特征为, 2 km以下ε有很明显的日变化特征, 夜晚和上午ε较大, 下午及傍晚减少;(3) 空间分布特征表现为, ε随高度大致呈递减分布;ε量级达10-2.5 m2·s-3所在高度可作为深圳地区2010年1月14-15日边界层顶高度的判断依据。 相似文献
10.
Wan-Li Cheng 《Meteorology and Atmospheric Physics》2000,75(3-4):251-258
Summary In the central region of Taiwan, ozone episodes occur most often during autumn. Two field experiments were conducted during
the autumns of 1998 and 1999 to analyze the vertical profile of the boundary layer and determine its effects on ozone concentration
over the region. The vertical virtual potential temperature and wind profiles were derived from tethersonde data. The NOx, NMHC and O3 concentration vertical profiles were monitored up to a height of 500 meters using black-covered Teflon tedler sampling bags.
During the experimental periods, nighttime terrestrial long wave radiation could cause the inversion height to reach 500 meters
by the following morning. It was shown that these types of synoptic structures suppress the vertical diffusion of NOx, NMHC and O3. During the daytime, measurements indicate that pollutants were well mixed in the upper portion of the mixing layer. At night,
the ground level ozone concentration was on the decrease but increased with altitude to a height of 500 m. The NOx decreased with altitude whereas the NMHC showed no significant variations.
Received April 13, 2000 Revised July 24, 2000 相似文献
11.
Florence Bocquet Ben Balsley Michael Tjernström Gunilla Svensson 《Boundary-Layer Meteorology》2011,138(1):43-60
Tethered Lifting System (TLS) estimates of the dissipation rate of turbulent kinetic energy (e){(\varepsilon)} are reasonably well correlated with concurrent measurements of vertical velocity variance (sw2){(\sigma_{w}^{2})} obtained from sonic anemometers located on a nearby 60-m tower during the CASES-99 field experiment. Additional results in
the first 100 m of the nocturnal stable boundary layer confirm our earlier claim that the presence of weak but persistent
background turbulence exists even during the most stable atmospheric conditions, where e{\varepsilon} can exhibit values as low as 10−7 m2 s−3. We also present a set of empirical equations that incorporates TLS measurements of temperature, horizontal wind speed, and
e{\varepsilon} to provide a proxy measurement for sw2{\sigma_{w}^{2}} at altitudes higher than tower heights. 相似文献
12.
S. Khodayar N. Kalthoff M. Fiebig-Wittmaack M. Kohler 《Meteorology and Atmospheric Physics》2008,99(3-4):181-198
Summary The boundary-layer structure of the Elqui Valley is investigated, which is situated in the arid north of Chile and extends
from the Pacific Ocean in the west to the Andes in the east. The climate is dominated by the south-eastern Pacific subtropical
anticyclone and the cold Humboldt Current. This combination leads to considerable temperature and moisture gradients between
the coast and the valley and results in the evolution of sea and valley wind systems. The contribution of these mesoscale
wind systems to the heat and moisture budget of the valley atmosphere is estimated, based on radiosoundings performed near
the coast and in the valley.
Near the coast, a well-mixed cloud-topped boundary layer exists. Both, the temperature and the specific humidity do not change
considerably during the day. In the stratus layer the potential temperature increases, while the specific humidity decreases
slightly with height. The top of the thin stratus layer, about 300 m in depth, is marked by an inversion. Moderate sea breeze
winds of 3–4 m s−1 prevail in the sub-cloud and cloud layer during daytime, but no land breeze develops during the night.
The nocturnal valley atmosphere is characterized by a strong and 900 m deep stably stratified boundary layer. During the day,
no pronounced well-mixed layer with a capping inversion develops in the valley. Above a super-adiabatic surface layer of about
150 m depth, a stably stratified layer prevails throughout the day. However, heating can be observed within a layer above
the surface 800 m deep. Heat and moisture budget estimations show that sensible heat flux convergence exceeds cold air advection
in the morning, while both processes compensate each other around noon, such that the temperature evolution stagnates. In
the afternoon, cold air advection predominates and leads to net cooling of the boundary layer. Furthermore, the advection
of moist air results in the accumulation of moisture during the noon and afternoon period, while latent heat flux convergence
is of minor relevance to the moisture budget of the boundary layer.
Correspondence: Norbert Kalthoff, Institut für Meteorologie und Klimaforschung, Universit?t Karlsruhe/Forschungszentrum Karlsruhe,
Postfach 3640, 76021 Karlsruhe, Germany 相似文献
13.
The purpose of this study was to develop a diffusion model for a continuous point source which takes into account the increase
of wind speed with height, and to compare this model with short-range diffusion experiments. The main problem was to find
a good expression for the vertical diffusion coefficient. It turned out that good agreement between theory and experiment
could only be obtained by introducing a settling speed W for the tracer combined with a conventional expression for the vertical diffusion (K(Z) = K
0Z1−p). An empirical relation was found between K
0 and τ
vU and between W and bar
σ
vU2. 相似文献
14.
A model is developed to simulate the potential temperature and the height of the mixed layer under advection conditions. It includes analytic expressions for the effects of mixed-layer conditions upwind of the interface between two different surfaces on the development of the mixed layer downwind from the interface. Model performance is evaluated against tethersonde data obtained on two summer days during sea breeze flow in Vancouver, Canada. It is found that the mixed-layer height and temperature over the ocean has a small but noticeable effect on the development of the mixed layer observed 10 km inland from the coast. For these two clear days, the subsidence velocity at the inversion base capping the mixed layer is estimated to be about 30 mm s–1 from late morning to late afternoon. When the effects of subsidence are included in the model, the mixed-layer height is considerably underpredicted, while the prediction for the mean potential temperature in the mixed layer is considerably improved. Good predictions for both height and temperature can be obtained when values for the heat entrainment ratio,c, 0.44 and 0.68 for these two days respectively for the period from 1000 to 1300 LAT, were used. These values are estimated using an equation including the additional effects on heat entrainment due to the mechanical mixing caused by wind shear at the top of the mixed layer and surface friction. The contribution of wind shear to entrainment was equal to, or greater than, that from buoyant convection resulting from the surface heat flux. Strong wind shear occurred near the top of the mixed layer between the lower level inland flow and the return flow aloft in the sea breeze circulation.Symbols
c
entrainment parameter for sensible heat
-
c
p
specific heat of air at constant pressure, 1010 J kg–1 K–1
-
d
1
the thickness of velocity shear at the mixed-layer top, m
-
Q
H
surface sensible heat flux, W m–2
-
u
m
mean mixed-layer wind speed, m s–1
-
u
*
friction velocity at the surface, m s–1
-
w
subsidence velocity, m s–1
-
W
subsidence warming,oC s–1
-
w
e
entrainment velocity, m s–1
-
w
*
convection velocity in the mixed layer, m s–1
-
x
downwind horizontal distance from the water-land interface, m
-
y
dummy variable forx, m
-
Z
height above the surface, m
-
Z
i
height of capping inversion, m
-
Z
m
mixed-layer depth, i.e.,Z
i–Zs, m
-
Z
s
height of the surface layer, m
-
lapse rate of potential temperature aboveZ
i, K m–1
-
potential temperature step atZ
i, K
- u
h
velocity step change at the mixed-layer top
-
m
mean mixed-layer potential temperature, K 相似文献
15.
The characteristics of the boundary layer over complex terrain (Lannemezan - lat.: 43.7° N and, long.: 0.7 ° E) are analyzed for various scales, using measurements obtained during the COCAGNE Experiment. In this first part, the dynamic characteristics of the flow are studied with respect to atmospheric stability and the relief at small (~20 km) and medium scales (~100 km). These relief scales depend on the topographical profile of the Lannemezan Plateau along the dominant axis of the wind (E-W) and the Pyrénées Mountains located at the south of the experimental site. The terrain heterogeneities have a standard deviation of ~48 m and a wavelength of ~2 km.The averaged vertical profiles of wind speed and direction over the heterogeneous terrain are analyzed. The decrease of wind speed within the boundary layer is greater than over flat terrain (WANGARA Experiment). However, a comparison between ETTEX (complex terrain) and COCAGNE vertical wind speed profiles shows good agreement during unstable conditions. In contrast, during neutral conditions a more rapid increase with normalized height is found with COCAGNE than with ETTEX and WANGARA data. The vertical profiles of wind direction reveal an influence of the Pyrénées Mountains on the wind flow. The wind rotation in the BL is determined by the geostrophic wind direction-Pyrénées axis angle (negative deviation) as the geostrophic wind is connected with the Mountain axis.When the geostrophic wind does not interact with the Pyrénées axis, the mean and turbulent wind flow characteristics (drag coefficient C
D, friction velocity u
*) depend on the topography of the plateau. When the wind speed is strong (>6 m s -1), an internal boundary layer is generated from the leading edge of the Plateau. 相似文献
16.
Hironori Iwai Yasuhiro Murayama Shoken Ishii Kohei Mizutani Yuichi Ohno Taichiro Hashiguchi 《Boundary-Layer Meteorology》2011,141(1):117-142
To study the wind field within the atmospheric boundary layer over the Tokyo metropolitan area, Doppler lidar observations
were made 45 km north of Sagami Bay and 30 km west of Tokyo Bay, from 14 May to 15 June 2008. Doppler lidar on 27 May 2008
observed the vertical and horizontal wind structure of a well-developed sea-breeze front (SBF) penetrating from Sagami Bay.
At the SBF, a strong updraft (maximum w approximately equal to 5 m s−1) was formed with a horizontal scale of about 500 m and vertical scale of 2 km. The spatial relationship between the strong
updraft over the nose of the SBF and prefrontal thermal suggests that the strong updraft was triggered by interaction between
the SBF and the thermal. After the updraft commenced, a collocated ceilometer observed an intense aerosol backscatter up to
2 km above ground level. The observational results suggest that the near-surface denser aerosols trapped in the head region
of the SBF escaped from the nose of the SBF and were then vertically transported up to the mixing height by the strong updraft
at the SBF. This implies that these phenomena occurred not continuously but intermittently. The interaction situations between
the SBF and prefrontal thermal can affect the wind structure at the SBF and the regional air quality. 相似文献
17.
Relation of the Second Type Thermal Helicity to Precipitation of Landfalling Typhoons: A Case Study of Typhoon Talim 
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下载免费PDF全文 This study utilized the MM5 mesoscale model to simulate the landfalling process of Typhoon Talim. The simulated typhoon track,
weather patterns, and rainfall process are consistent with the observation. Using the simulation results, the relation of
the second type thermal helicity (H
2) to rainfall caused by the landfalling typhoon Talim was analyzed. The results show that H
2 could well indicate the heavy inland rainfall but it did not perform as well as the helicity in predicting rainfall during
the beginning stage of the typhoon landfall. In particular, H
2 was highly correlated with rainfall of Talim at 1-h lead time. For 1–5-h lead time, it also had a higher correlation with
rainfall than the helicity did, and thus showing a better potential in forecasting rainfall intensification. Further analyses
have shown that when Talim was in the beginning stage of landfall, 1) the 850–200-hPa vertical wind shear around the Talim
center was quite small (about 5 m s−1); 2) the highest rainfall was to the right of the Talim track and in the area with a 300-km radius around the Talim center,
exhibiting no obvious relation to low-level temperature advection, low-level air convergence, and upper-level divergence;
3) the low-level relative vorticity reflected the rainfall change quite well, which was the main reason why helicity had a
better performance than H
2 in this period. However, after Talim moved inland further, 1) it weakened gradually and was increasingly affected by the
northern trough; 2) the vertical wind shear was enhanced as well; 3) the left side of the down vertical wind shear lay in
the Lushan and Dabieshan mountain area, which could have contributed to triggering a secondary vertical circulation, helping
to produce the heavy rainfall over there; hence, H
2 showed a better capacity to reflect the rainfall change during this stage. 相似文献
18.
Ph. Drobinski S. Bastin J. Dusek G. Zängl P. H. Flamant 《Meteorology and Atmospheric Physics》2006,92(3-4):285-306
Summary This paper investigates the characteristics of channelled airflow in the vicinity of a junction of three idealized valleys
(one valley carrying the incoming flow and two tributaries carrying the outflow), using a two-dimensional single-layer shallow
water model. Particular attention is given to the flow splitting occurring at the junction. Nondimensionalized, the model
depends on the valley geometry, the Reynolds number, which is related to the eddy viscosity, and on the difference of the
hydrostatic pressure imposed at the exit of the tributaries. At the spatial scale considered in this study, the Rossby number
relating the inertial and Coriolis forces is always larger than 1, implying that the effect of earth rotation can be neglected
to a first approximation.
The analysis of the flow structure within the three valleys as well as the calculation of the split ratio (fraction of the
air flow diverted into one of the two downstream valleys with respect to the total mass flux in the upstream valley) show
that (i) the flow pattern depends strongly on the Reynolds number while the split ratio is comparatively insensitive; (ii)
the valley geometry and the difference between the upstream and downstream hydrostatic pressures affect the flow pattern,
the location of the split point and the split ratio; (iii) the relative contribution of flow deflection by the sidewalls and
the blocking/splitting mechanism differs between the settings of a “Y-shape” valley and a “T-shape” valley.
Quantitative comparison of the present results with numerical simulations of realistic cases and with observations collected
in the region of the Rhine and Seez valleys (Switzerland) (“Y-shape” valley) and in the region of the Inn and Wipp valleys
(Austria) (“T-shape” valley) during the Mesoscale Alpine Programme (MAP) field experiment shows good agreement provided that
the normalized valley depth NΔH/Uu significantly exceeds 1, i.e., when “flow around” is expected. A structural disagreement between the idealized simulations
and the observed wind field is found only when NΔH/Uu ≃ 1, that is, in the “flow over” regime. This shows that the dimensionless valley depth
is indeed a good indicator for flow splitting, implying that the stratification is a key player in reality. 相似文献
19.
We present results of a technique for examining the scale-dependence of the gradient Richardson number, Ri, in the nighttime residual layer. The technique makes use of a series of high-resolution, in situ, vertical profiles of wind
speed and potential temperature obtained during CASES-99 in south-eastern Kansas, U.S.A. in October 1999. These profiles extended
from the surface, through the nighttime stable boundary layer, and well into the residual layer. Analyses of the vertical
gradients of both wind speed, potential temperature and turbulence profiles over a wide range of vertical scale sizes are
used to estimate profiles of the local Ri and turbulence structure as a function of scale size. The utility of the technique lies both with the extensive height range
of the residual layer as well as with the fact that the sub-metre resolution of the raw profiles enables a metre-by-metre
‘sliding’ average of the scale-dependent Richardson number values over hundreds of metres vertically. The results presented
here show that small-scale turbulence is a ubiquitous and omnipresent feature of the residual layer, and that the region is
dynamic and highly variable, exhibiting persistent turbulent structure on vertical scales of a few tens of metres or less.
Furthermore, these scales are comparable to the scales over which the Ri is less than or equal to the critical value of Ri
c of 0.25, although turbulence is also shown to exist in regions with significantly larger Ri values, an observation at least consistent with the concept of hysteresis in turbulence generation and maintenance. Insofar
as the important scale sizes are comparable to or smaller than the resolution of current models, it follows that, in order
to resolve the observed details of small Ri values and the concomitant turbulence generation, future models need to be capable of significantly higher resolutions. 相似文献
20.
Flux Footprints in the Convective Boundary Layer: Large-Eddy Simulation and Lagrangian Stochastic Modelling 总被引：1，自引：1，他引：0
We investigated the flux footprints of receptors at different heights in the convective boundary layer (CBL). The footprints
were derived using a forward Lagrangian stochastic (LS) method coupled with the turbulent fields from a large-eddy simulation
model. Crosswind-integrated flux footprints shown as a function of upstream distances and sensor heights in the CBL were derived
and compared using two LS particle simulation methods: an instantaneous area release and a crosswind linear continuous release.
We found that for almost all sensor heights in the CBL, a major positive flux footprint zone was located close to the sensor
upstream, while a weak negative footprint zone was located further upstream, with the transition band in non-dimensional upwind
distances −X between approximately 1.5 and 2.0. Two-dimensional (2D) flux footprints for a point sensor were also simulated. For a sensor
height of 0.158 z
i, where z
i is the CBL depth, we found that a major positive flux footprint zone followed a weak negative zone in the upstream direction.
Two even weaker positive zones were also present on either side of the footprint axis, where the latter was rotated slightly
from the geostrophic wind direction. Using CBL scaling, the 2D footprint result was normalized to show the source areas and
was applied to real parameters obtained using aircraft-based measurements. With a mean wind speed in the CBL of U = 5.1 m s−1, convective velocity of w
* = 1.37 m s−1, CBL depth of z
i = 1,000 m, and flight track height of 159 m above the surface, the total flux footprint contribution zone was estimated to
range from about 0.1 to 4.5 km upstream, in the case where the wind was perpendicular to the flight track. When the wind was
parallel to the flight track, the total footprint contribution zone covered approximately 0.5 km on one side and 0.8 km on
the other side of the flight track. 相似文献