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1.
The model devised by Lhommeet al. (1988) allows one to calculate the sensible heat flux over a homogeneous crop canopy from radiometric surface temperature by adding a so-called canopy aerodynamic resistance to the classical aerodynamic resistance calculated above the canopy. This model is reformulated in order to simplify the mathematical procedure needed to calculate this additional resistance. Analytical expressions of micrometeorological profiles within the canopy are introduced. Assuming a constant leaf area density, an analytical expression of canopy aerodynamic resistance is inferred, which is a function of wind velocity, inclination angle of the radiometer and crop characteristics such as crop height, leaf area index, inclination index of the foliage and leaf width. Sensitivity of this resistance to the different parameters is investigated. The most significant are wind velocity and LAI. Finally, the predictions of the model are tested against two sets of measurements obtained for two different crops, potato and maize. 相似文献
2.
An urban canopy model is developed for use in mesoscale meteorological and environmental modelling. The urban geometry is
composed of simple homogeneous buildings characterized by the canyon aspect ratio (h/w) as well as the canyon vegetation characterized by the leaf aspect ratio (σ
l
) and leaf area density profile. Five energy exchanging surfaces (roof, wall, road, leaf, soil) are considered in the model,
and energy conservation relations are applied to each component. In addition, the temperature and specific humidity of canopy
air are predicted without the assumption of thermal equilibrium. For radiative transfer within the canyon, multiple reflections
for shortwave radiation and one reflection for longwave radiation are considered, while the shadowing and absorption of radiation
due to the canyon vegetation are computed by using the transmissivity and the leaf area density profile function. The model
is evaluated using field measurements in Vancouver, British Columbia and Marseille, France. Results show that the model quite
well simulates the observations of surface temperatures, canopy air temperature and specific humidity, momentum flux, net
radiation, and energy partitioning into turbulent fluxes and storage heat flux. Sensitivity tests show that the canyon vegetation
has a large influence not only on surface temperatures but also on the partitioning of sensible and latent heat fluxes. In
addition, the surface energy balance can be affected by soil moisture content and leaf area index as well as the fraction
of vegetation. These results suggest that a proper parameterization of the canyon vegetation is prerequisite for urban modelling. 相似文献
3.
The seasonal trend of vertical temperature profiles within and above an urban canopy has been investigated. We measured air temperatures and wind velocities along a 29-m tower in a residential area of Tokyo, Japan continually for 14 months. The height of the daily maximum temperature ZTmax varied with the season; ZTmax was at the roof level in winter but near the ground in summer. The seasonal change of ZTmax is likely due to the change of height at which solar energy is absorbed. At the time of the maximum temperature, the atmosphere above the canopy is always unstable whereas the air within the canopy is unstable in summer but stable in winter. 相似文献
4.
开展森林小气候研究,对于协调生物与环境的关系、保护生物多样性以及实现森林的可持续高效管理,都是十分重要的。选择5月和10月典型的晴朗无风天气条件,以侧柏纯林为对照,研究了郁闭度为0.25、0.54和0.87的侧柏-山毛桃群落演替初期小气候特征。结果表明,侧柏-山毛桃混交林对春季和秋季林内空气温度的早间增温效应和午间降温效应,随着郁闭度增加而增大。春季的高温峰值出现在15时,晚于秋季的高温峰值13时。春季和秋季林内10 cm处地温日间呈持续增加趋势,春季以郁闭度为0.25的B点为最高,秋季侧柏纯林在12时前为最低,14时后为最高。春季林内空气相对湿度从07时到11时呈快速降低的趋势,然后逐渐缓慢回升;而秋季林内的相对湿度,随着郁闭度的增加而增大,但日间变化幅度比较小。无论春季还是秋季,侧柏和山毛桃的叶片温度日间变化均呈先升高后缓慢降低的趋势。春季郁闭度对侧柏和山毛桃的叶片温度影响不显著,但秋季11-15时,随着郁闭度的增加,叶片温度有降低的趋势。在太行山区,以山毛桃为保护树种,快速提高侧柏-山毛桃混交林的郁闭度,对有效地改善林内小气候,促进森林更新和管理都是十分重要的。 相似文献
5.
Daytime variations of ozone eddy fluxes to maize 总被引：1，自引：0，他引：1
The vertical fluxes of ozone, momentum and heat in the atmospheric surface layer have been measured by eddy correlation above
both mature and senescent maize canopies. Aerodynamic formulae are applied to find that the bulk canopy surface resistancer
c
to ozone uptake and destruction varies between 4.0 and 0.5 s cm−1 during the daytime. Apparently, surface properties tend to control the removal of ozone at the surface of the earth. For
a lush canopy, the stomatal diffusion resistance is the most important property, while changes in surface temperature have
little effect. Destruction at the soil and exterior plant surfaces appears to account for 20–50% of the total loss if leaf
mesophyll resistances are assumed to be very small. Free water at leaf surfaces may at times inhibit ozone removal by both
senescent and healthy plants. 相似文献
6.
William P. Kustas Martha C. Anderson John M. Norman Fuqin Li 《Boundary-Layer Meteorology》2007,122(1):167-187
In many land-surface models using bulk transfer (one-source) approaches, the application of radiometric surface temperature
observations in energy flux computations has given mixed results. This is due in part to the non-unique relationship between
the so-called aerodynamic temperature, which relates to the efficiency of heat exchange between the land surface and overlying
atmosphere, and a surface temperature measurement from a thermal-infrared radiometer, which largely corresponds to a weighted
soil and canopy temperature as a function of radiometer viewing angle. A number of studies over the past several years using
multi-source canopy models and/or experimental data have developed simplified methods to accommodate radiometric–aerodynamic
temperature differences in one-source approaches. A recent investigation related the variability in the radiometric–aerodynamic
relation to solar radiation using experimental data from a variety of landscapes, while another used a multi-source canopy
model combined with measurements over a wide range in vegetation density to derive a relationship based on leaf area index.
In this study, simulations by a detailed multi-source soil–plant–environment model, Cupid, which considers both radiative
and turbulent exchanges across the soil–canopy–air interface, are used to explore the radiometric–aerodynamic temperature
relations for a semi-arid shrubland ecosystem under a range of leaf area/canopy cover, soil moisture and meteorological conditions.
The simulated radiometric-aerodynamic temperatures indicate that, while solar radiation and leaf area both strongly affect
the magnitude of this temperature difference, the relationships are non-unique, having significant variability depending on
local conditions. These simulations also show that soil–canopy temperature differences are highly correlated with variations
in the radiometric–aerodynamic temperature differences, with the slope being primarily a function of leaf area. This result
suggests that two-source schemes with reliable estimates of component soil and canopy temperatures and associated resistances
may be better able to accommodate variability in the radiometric–aerodynamic relation for a wider range in vegetated canopy
cover conditions than is possible with one-source schemes. However, comparisons of sensible heat flux estimates with Cupid
using a simplified two-source model and a one-source model accommodating variability in the radiometric-aerodynamic relation
based on vegetation density gave similar scatter. On the other hand, with experimental data from the shrubland site, the two-source
model generally outperformed the one-source scheme. Clearly, vegetation density/leaf area has a major effect on the radiometric–aerodynamic
temperature relation and must be considered in either one-source or two-source formulations. Hence these adjusted one-source
models require similar inputs as in two-source approaches, but provide as output only bulk heat fluxes; this is not as useful
for monitoring vegetation conditions. 相似文献
7.
A new method to estimate tree biomass heat storage from thermal infrared (TIR) imaging of biomass surface temperature is presented.
TIR images of the canopy are classified into trunk, branches, and leaves. The one-dimensional heat equation in cylindrical
coordinates is forced with trunk and branch surface temperatures to simulate the temperature distribution and heat storage
in tree trunks and branches. Assuming uniform leaf temperatures, heat storage in leaves is computed from the surface temperature
of the leaves separately for the sunlit upper and shaded lower canopy. The sum of trunk, branches, leaf, and air heat storage
gives the canopy heat storage. Measurements in a walnut orchard near Davis, California, in early June 2007 showed that biomass
heat storage was of the same order as air heat storage and about 1% of daytime and 9% of nighttime net radiation. 相似文献
8.
Aerodynamic and canopy resistance of short-rotation forest in relation to leaf area index and climate 总被引：3，自引：0，他引：3
Anders Lindroth 《Boundary-Layer Meteorology》1993,66(3):265-279
The aerodynamic and canopy resistances of a willow short-rotation stand were estimated during the course of a growing season on the basis of micrometeorological measurements. The normalized roughness length (z
0/h) decreased from about 0.10 at a leaf area index of one to 0.05 at a leaf area index of seven. This implies that the aerodynamic resistance at peak leaf area index is more than twice the value at zero leaf area index, all other variables unchanged. The canopy resistance depended strongly on air water concentration deficit and on leaf area index. The Lohammar equation showed good agreement between estimated and measured canopy resistances over the whole course of leaf development. The stand was well-coupled to the atmosphere only for very small values of leaf area indices, less than one, and it was practically de-coupled for leaf area indices above two. From the point of view of factors controlling evaporation, this type of stand acts as a traditional forest at the beginning and end of the season and as an agricultural crop in the middle of the season. 相似文献
9.
Roni Avissar 《Boundary-Layer Meteorology》1993,64(1-2):127-148
Plant stomata play a key role in the redistribution of energy received on vegetated land into sensible and latent heat. As a result, they have a considerable impact on the atmospheric planetary boundary layer, the hydrologic cycle, the climate, and the weather. Current parameterizations of the stomatal mechanism in state-of-the-art atmospheric models are based on empirical relations that are established at the leaf scale between stomatal conductance and environmental conditions. In order to evaluate these parameterizations, an experiment was carried out on a potato field in New Jersey during the summer of 1989. Stomatal conductances were measured within a small homogeneous area in the middle of the potato field and under a relatively broad range of atmospheric conditions. A large variability of stomatal conductances was observed. This variability, which was associated with the variability of micro-environmental and physiological conditions that is found even in a homogeneous canopy, cannot be simulated explicitly on the scale of a single agricultural field and,a fortiori, on the scale of atmospheric models. Furthermore, this variability could not be related to the environmental conditions measured at a height of 2 m above the plant canopy simultaneously with the conductances, reinforcing the concept of scale decoupling suggested by Jarvis and McNaughton (1986) and McNaughton and Jarvis (1991). Thus, for atmospheric modeling purposes, a parameterization of stomatal conductance at the canopy scale using external environmental forcing conditions seems more appropriate than a parameterization based on leaf-scale stomatal conductance, as currently adopted in state-of-the-art atmospheric models. The measured variability was characterized by a lognormal probability density function (pdf) that remained relatively stable during the entire measuring period. These observations support conclusions by McNaughton and Jarvis (1991) that, unlike current parameterizations, a parameterization of plant stomata applied on a field (or larger) scale, might not require inclusion of the complex relations found at the leaf scale between stomata and their microenvironment. 相似文献
10.
H. Swaid 《Theoretical and Applied Climatology》1992,46(2-3):179-191
Summary Adaptive geometrical configurations are presented, which aim to create intelligent urban forms, and which include screening methods applicable to the linear- and grid-type building layouts. They are especially suitable for mid-latitude cities characterized by seasonally swinging climates which necessitate heating in winter and cooling in summer. The screens are envisaged as shading devices in the summer blocking the incoming solar radiation during day-time (that is, on-position), while being removed at night to enhance nocturnal radiative cooling (that is, off-position). In winter they are assumed to be in off-position during sunshine hours to promote the access of solar radiation and in on-position at night to obstruct the sky energy sink and reduce radiant heat losses. Their implications for the urban street canyon climate and the thermal performance of the built environment are simulated using the cluster thermal time constant (CTTC) model. The diurnal variation of both the ambient air temperature and net radiant flux within the urban canopy layer serve as criteria by which the climatetempering effectiveness of the screens is assessed.With 9 Figures 相似文献
11.
Robert Kurzeja 《Boundary-Layer Meteorology》2010,134(1):181-193
Experiments and calculations were conducted with a 0.13 mm fine wire thermocouple within a naturally-aspirated Gill radiation
shield to assess and improve the accuracy of air temperature measurements without the use of mechanical aspiration, wind speed
or radiation measurements. It was found that this thermocouple measured the air temperature with root-mean-square errors of
0.35 K within the Gill shield without correction. A linear temperature correction was evaluated based on the difference between
the interior plate and thermocouple temperatures. This correction was found to be relatively insensitive to shield design
and yielded an error of 0.16 K for combined day and night observations. The correction was reliable in the daytime when the
wind speed usually exceeds 1 m s−1 but occasionally performed poorly at night during very light winds. Inspection of the standard deviation in the thermocouple
wire temperature identified these periods but did not unambiguously locate the most serious events. However, estimates of
sensor accuracy during these periods is complicated by the much larger sampling volume of the mechanically-aspirated sensor
compared with the naturally-aspirated sensor and the presence of significant near-surface temperature gradients. The root-mean-square
errors therefore are upper limits to the aspiration error since they include intrinsic sensor differences and intermittent
volume sampling differences. 相似文献
12.
An intensive measurement campaign within and above a maize row canopy was carried out to investigate flow characteristics within this vegetation. Attention was given to finding adequate scaling parameters of the within-canopy windspeed and air temperature profiles under above-canopy stable stratification.During clear and calm nights the within-canopy condition differs considerably from the abovecanopy state. In contrast to the daytime, the windspeed and temperature profiles do not scale with the above-canopy friction velocity,u
*
, and the scaling temperature,T
*
, respectively. A free convection flow regime is generated, forced by the soil heat flux at the canopy floor and by cooling at the top of the canopy. However, the windspeed and temperature profiles appear to scale well with the free convective velocity scale,w
*
, and the free convective temperature scale,T
f
, respectively. The free convective state within the canopy agrees well with the free convection criterion Gr>16Re2(u
*
), where Gr is the Grashof number and Re(u
*
) the Reynolds number, a criterion often used in technical flow problems. Also it is shown that under within-canopy free convection, there is a unique relation between the Grashof number, Gr, and the Reynolds number if the latter is based on the free convective velocity scale.Under within-canopy free convective conditions, it appears that within the canopy the fluxes of heat and water vapour can be estimated well with the relatively simple variance technique. Under these conditions, the Grashof, or Rayleigh number, represents a measure for the kinetic energy of the turbulence within the canopy. 相似文献
13.
A model for inferring canopy and underlying soil temperatures from multi-directional measurements 总被引：1，自引：0，他引：1
Thermal emission is modeled from a canopy/soil surface, where the soil and the leaves are at different temperatures,T g andT c respectively. The temperatureT m corresponding to a radiometer reading is given by $$B_\lambda (T_m ) = \chi B_\lambda (T_g ) + (1 - \chi )B_\lambda (T_c ) ,$$ whereB λ denotes the Planck blackbody function at wavelength λ, χ specifies the fraction of the field of view occupied by the soil at a given view direction, and an emissivity of 1.0 is assumed for the plants and the soil. The dependence of the soil-fraction χ on the view direction and the structure is expressed by the viewing-geometry parameter, which allows for concise and simple formulation. We observe from our model that at large view zenith angles, only the plants are effectively seen (that is, χ tends to zero), and thereforeT c can be determined from observations at large zenith angles, to the extent that such observations are practical. Viewing from the zenith, χ = exp(-L hc), whereL hc is the projection of the canopy leaf-area (per unit surface area) on a horizontal plane. For off-zenith observations, the soil-fraction χ depends on the distribution in the azimuth of the projected areas of various leaf categories, in addition to the dependence on the sum total of these projections,L hc.L hc, rather than the leaf-area index, emerges as the parameter characterizing the optical thickness of the canopy. Inferring bothT c andT g from observations from the zenith and from large zenith angles is possible ifL hc is known from other measurements. Drooping of leaves under water-stress conditions affects the observed temperatureT m in a complicated way because a leaf-inclination change produces a change inL hc (for the same leaf area) and also a change in the dependence of χ on the view direction. Water stress can produce an increase of the soil-fraction χ and thus tends to produce an exaggerated increase in the observed temperature compared to the actual increase in canopy temperature. These effects are analyzed for a simulated soybean canopy. 相似文献
14.
The deposition velocity (V
d) of nitric acid vapor over a fully leafed deciduous forest was estimated using flux/gradient theory. HNO3 deposition velocities ranged from 2.2 to 6.0cm/s with a mean V
don the order of 4.0cms-1. Estimates of V
dfrom a detailed canopy turbulence model gave deposition velocities of similar magnitude. The model was used to investigate the sensitivity of V
dto the leaf boundary-layer resistance and leaf area index (LAI). Although modeled deposition velocities were found to be sensitive to the parameterization of the leaf boundary-layer resistance, they were less sensitive to the LAI. Modeled V
d's were found to peak at LAI = 7. 相似文献
15.
Coherent structures in turbulent flow above a midlatitude deciduous forest are identified using a wavelet analysis technique. Coupling between motions above the canopy (z/h=1.5, whereh is canopy height) and within the canopy (z/h=0.6) are studied using composite velocity and temperature fields constructed from 85 hours of data. Data are classified into winter and summer cases, for both convective and stable conditions. Vertical velocity fluctuations are in phase at both observation levels. Horizontal motions associated with the structures within the canopy lead those above the canopy, and linear analysis indicates that the horizontal motions deep in the canopy should lead the vertical motions by 90°. On average, coherent structures are responsible for only about 40% of overall turbulent heat and momentum fluxes, much less than previously reported. However, our large data set reveals that this flux fraction comes from a wide distribution that includes much higher fractions in its upper extremes. The separation distanceL
s between adjacent coherent structures, 6–10h, is comparable to that obtained in previous observations over short canopies and in the laboratory. Changes in separation between the summer and winter (leafless) conditions are consistent withL
s being determined by a local horizontal wind shear scale. 相似文献
16.
Abstract Dawn‐to‐dusk evolution of air turbulence, sensible heat and latent heat above a forest during cloud‐free or near‐cloud‐free summer conditions is modelled by way of a system of differential equations. Temperatures in and above the canopy, near canopy‐top wind velocities, early morning leaf moisture (dew) and afternoon canopy ventilation (i.e. heat released from the canopy and from below the canopy) are included in the mathematical treatment. Computed results are compared with field data for atmospheric temperature and wind speed profiles up to 1200 m, within‐canopy temperature, and canopy‐level radiation, turbulent fluxes and wind speeds. Data were collected at a central New Brunswick mixed‐wood forest site dominated by spruce (Picea spp. ) and shade‐tolerant hardwoods for four representative summer days. It was found that the effective canopy temperature was not only affected by insolation, but also by the extent of canopy ventilation and the amount of dew on the foliage. The growth of the mixing layer was affected by canopy ventilation and by above‐canopy wind speeds. Model calculations closely simulated the meteorological observations. 相似文献
17.
Water-flume experiments are conducted to study the structure of turbulent flow within and above a sparse model canopy consisting
of two rigid canopies of different heights. This difference in height specifies a two-dimensional step change from a rough
to a rougher surface, as opposed to a smooth-to-rough transition. Despite the fact that the flow is in transition from a rough
to a rougher surface, the thickness of the internal boundary layer scales as x
4/5, consistent with smooth-to-rough boundary layer adjustment studies, where x is the downstream distance from the step change. However, the analogy with smooth-to-rough transitions no longer holds when
the flow inside the canopy and near the canopy top is considered. Results show that the step change in surface roughness significantly
increases turbulence intensities and shear stress. In particular, there is an adjustment of the mean horizontal velocity and
shear stress as the flow passes over the rougher canopy, so that their vertical profiles adjust to give maximum values at
the top of this canopy. We also observe that the magnitude and shape of the inflection in the mean horizontal velocity profile
is significantly affected by the transition. The horizontal and vertical turbulence spectra compare well with Kolmogorov’s
theory, although a small deviation at high frequencies is observed in the horizontal spectrum within the canopy. Here, for
relatively low leaf area index, shear is found to be a more effective mechanism for momentum transfer through the canopy structure
than vortex shedding. 相似文献
18.
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
19.
Summary ?The long-term variations of upward terrestrial (E) and downward atmospheric (A) long-wave radiation fluxes above a pine stand
in the southern part of the upper Rhine valley plain are analysed based on monthly mean values from 27 years of monitoring.
Equivalent blackbody temperatures of the canopy and the atmosphere are calculated and compared to air temperatures at nearby
sites.
Based on 324 monthly values each, correlations between A and E as well as A and global radiation G are analysed. Only the
former are highly correlated. Monthly mean values of long-wave radiation A can be calculated from air temperature, water vapour
pressure and cloud cover.
The long-term yearly average of equivalent blackbody temperature of the canopy is 0.6 K lower than kinetic air temperature
at nearby sites.
Only small, insignificant increases of both blackbody temperatures and air temperature are found. Despite the strong forest
growth, it is surprising that the ratio of canopy emission temperature to air temperature did not change significantly.
Relationships between the changes of general atmospheric circulation and equivalent blackbody temperatures of the canopy point
to a strong dependence on shifts of general atmospheric flow.
Received February 24, 2000; revised April 18, 2002; accepted July 20, 2002 相似文献
20.
J. Otterman J. Susskind T. Brakke D. Kimes R. Pielke T. J. Lee 《Boundary-Layer Meteorology》1995,74(1-2):163-180
The thermal-infrared (longwave) emission from a vegetated terrain is generally anisotropic, i.e., the emission temperature varies with the view direction. If a directional measurement of temperature is considered to be equal to the effective temperature of the hemispheric emission, then the estimate of the latter can be significantly in error. The view-direction (zenith angle
eq
) at which the emission equivalence does hold is determined in our modeling study. In a two-temperature field-of-view (soil and plants),
eq
falls in a narrow range depending on plant density and canopy architecture.
eq
does not depend on soil and (uniform) plant temperatures nor on their ratio, even though the pattern of emission vs. the view direction depends crucially on this ratio. For a sparse canopy represented as thin, vertical cylindrical stalks (or vertical blades uniformly distributed in azimuth) with horizontal facets,
eq
ranges from 48 to 53° depending on the optical density of the vertical elements alone. When plant elements are modeled as small spheres,
eq
lies between 53 to 57° (for the same values of the canopy optical density). Only for horizontal leaves (a truly planophile canopy) is the temperature measured from any direction equal to the temperature of the hemispheric emission. When the emission temperature changes with optical depth within the canopy at a specified rate,
eq
depends to some extent on that rate. For practically any sparsely vegetated surface, a directional measurement at the zenith angle of 50° offers an appropriate evaluation of the hemispheric emission, since the error in the estimate will, at most, only slightly exceed 1% (around 4 W m–2). Estimates of the hemispheric emission through a nadir measurement, on the other hand, can be in error in some cases by about 10%, i.e., on the order of 40 W m–2....by a small sample we may judge the whole piece... Miguel de Cervantes, 1605, inDon Quixote de la Mancha, Pt. I, Ch. 4 相似文献