共查询到20条相似文献，搜索用时 703 毫秒
1.
Over the past 15 years atmospheric surface-layer experiments over heterogeneous canopies have shown that the vertical transfer of sensible heat and water vapour exhibit a strong dissimilarity. In particular, the sensible-heat-to-water-vapour transport efficiencies generally exceed unity. One of the main consequences is that evaporation (latent heat flux) computed by the flux-variance method is overestimated, as persistently demonstrated by comparisons with evaporation obtained with the eddy-correlation method. Various authors proposed to take into account the temperature–humidity dissimilarity to extend the applicability of the flux-variance method in order to compute evaporation from non-uniform surfaces. They attempted to connect the sensible-heat-to-water-vapour transport efficiency (λ) to the correlation coefficient between temperature and humidity turbulent fluctuations (R
Tq
). This approach was found to be successful over ‘wet’ surfaces for which λ can be approximated by R
Tq
and ‘dry’ surfaces for which λ can be approximated by 1/R
Tq
. However, no solution has been proposed until now for intermediate hydrological conditions. We investigated this question using eddy-correlation measurements above and inside a pine forest canopy. For both levels, our data present a strong likeness with previously published results over heterogeneous surfaces. In particular, they confirm that λ is R
Tq
in wet conditions and 1/R
Tq
in dry conditions. Moreover, we defined the range of the Bowen ratio (Bo) values for which those two approximations are valid (below 0.1 and greater than 1, respectively) and established a relationship between λ, R
Tq
and Bo for the intermediate range of Bo. We are confident that this new parameterization will enlarge the applicability of the flux-variance method to all kinds of heterogeneous surfaces in various hydrological conditions 相似文献
2.
3.
Estimation of Turbulent Fluxes Using the Flux-Variance Method over an Alpine Meadow Surface in the Eastern Tibetan Plateau 总被引：1，自引：0，他引：1
The flux-variance similarity relation and the vertical transfer of scalars exhibit dissimilarity over different types of surfaces,resulting in different parameterization approaches of relative transport efficiency among scalars to estimate turbulent fluxes using the flux-variance method.We investigated these issues using eddycovariance measurements over an open,homogeneous and flat grassland in the eastern Tibetan Plateau in summer under intermediate hydrological conditions during rainy season.In unstable conditions,the temperature,water vapor,and CO2 followed the flux-variance similarity relation,but did not show in precisely the same way due to different roles(active or passive) of these scalars.Similarity constants of temperature,water vapor and CO2 were found to be 1.12,1.19 and 1.17,respectively.Heat transportation was more efficient than water vapor and CO2.Based on the estimated sensible heat flux,five parameterization methods of relative transport efficiency of heat to water vapor and CO2 were examined to estimate latent heat and CO2 fluxes.The strategy of local determination of flux-variance similarity relation is recommended for the estimation of latent heat and CO2 fluxes.This approach is better for representing the averaged relative transport efficiency,and technically easier to apply,compared to other more complex ones. 相似文献
4.
When density fluctuations of scalars such as CO2 are measured with open-path gas analyzers, the measured vertical turbulent flux must be adjusted to take into account fluctuations
induced by ‘external effects’ such as temperature and water vapour. These adjustments are needed to separate the effects of
surface fluxes responsible for ‘natural’ fluctuations in CO2 concentration from these external effects. Analogous to vertical fluxes, simplified expressions for separating the ‘external
effects’ from higher-order scalar density turbulence statistics are derived. The level of complexity in terms of input to
these expressions are analogous to that of the Webb–Pearman–Leuning (WPL), and are shown to be consistent with the conservation
of dry air. It is demonstrated that both higher-order turbulent moments such as the scalar variances, the mixed velocity-scalar
covariances, and the two-scalar covariance require significant adjustments due to ‘external effects’. The impact of these
adjustments on the turbulent CO2 spectra, probability density function, and dimensionless similarity functions derived from flux-variance relationships are
also discussed. 相似文献
5.
Dissimilarity of Scalar Transport in the Convective Boundary Layer in Inhomogeneous Landscapes 总被引：1，自引：1，他引：0
A land-surface model (LSM) is coupled with a large-eddy simulation (LES) model to investigate the vegetation-atmosphere exchange
of heat, water vapour, and carbon dioxide (CO2) in heterogeneous landscapes. The dissimilarity of scalar transport in the lower convective boundary layer is quantified
in several ways: eddy diffusivity, spatial structure of the scalar fields, and spatial and temporal variations in the surface
fluxes of these scalars. The results show that eddy diffusivities differ among the three scalars, by up to 10–12%, in the
surface layer; the difference is partly attributed to the influence of top-down diffusion. The turbulence-organized structures
of CO2 bear more resemblance to those of water vapour than those of the potential temperature. The surface fluxes when coupled with
the flow aloft show large spatial variations even with perfectly homogeneous surface conditions and constant solar radiation
forcing across the horizontal simulation domain. In general, the surface sensible heat flux shows the greatest spatial and
temporal variations, and the CO2 flux the least. Furthermore, our results show that the one-dimensional land-surface model scheme underestimates the surface
heat flux by 3–8% and overestimates the water vapour and CO2 fluxes by 2–8% and 1–9%, respectively, as compared to the flux simulated with the coupled LES-LSM. 相似文献
6.
Dennis Baldocchi 《Boundary-Layer Meteorology》1992,61(1-2):113-144
An integrated canopy micrometeorological model is described for calculating CO2, water vapor and sensible heat exchange rates and scalar concentration profiles over and within a crop canopy. The integrated model employs a Lagrangian random walk algorithm to calculate turbulent diffusion. The integrated model extends previous Lagrangian modelling efforts by employing biochemical, physiological and micrometeorological principles to evaluate vegetative sources and sinks. Model simulations of water vapor, CO2 and sensible heat flux densities are tested against measurements made over a soybean canopy, while calculations of scalar profiles are tested against measurements made above and within the canopy. The model simulates energy and mass fluxes and scalar profiles above the canopy successfully. On the other hand, model calculations of scalar profiles inside the canopy do not match measurements.The tested Lagrangian model is also used to evaluate simpler modelling schemes, as needed for regional and global applications. Simple, half-order closure modelling schemes (which assume a constant scalar profile in the canopy) do not yield large errors in the computation of latent heat (LE) and CO2 (F
c
) flux densities. Small errors occur because the source-sink formulation of LE andF
c are relatively insensitive to changes in scalar concentrations and the scalar gradients are small. On the other hand, complicated modelling frames may be needed to calculate sensible heat flux densities; the source-sink formulation of sensible heat is closely coupled to the within-canopy air temperature profile. 相似文献
7.
Temperature and humidity flux-variance relations determined by one-dimensional eddy correlation 总被引：3，自引：1，他引：3
Harold L. Weaver 《Boundary-Layer Meteorology》1990,53(1-2):77-91
It may be possible to estimate surface fluxes of scalar quantities from measurement of their variance and mean wind speed. The flux-variance relation for temperature and humidity was investigated over prairie and desert-shrub plant communities. Fluxes were measured by one-dimensional eddy correlation, humidity by fast-response wet-bulb psychrometers and Krypton open-path hygrometers, temperature by fine-wire thermocouples, and mean windspeed by a cup anemometer. The quality of the flux-variance relation proved to be good enough for application to flux measurement. Regressions of flux estimated by the variance technique versus measured flux usually had r
2 values greater than 0.97 for sensible heat flux and greater than 0.88 for water vapor flux. More uniform surfaces tended to yield the same flux-variance relations except when fluxes were small. This exception supported the hypothesis that sparse sources of flux may increase variance downwind. Nonuniform surfaces yielded flux-variance relations that were less predictable, although reasonably accurate once determined. The flux-variance relation for humidity was quite variable over dry surfaces with senescent vegetation. 相似文献
8.
Heping Liu James T. Randerson Jamie Lindfors William J. Massman Thomas Foken 《Boundary-Layer Meteorology》2006,120(1):65-85
We present an approach for assessing the impact of systematic biases in measured energy fluxes on CO2 flux estimates obtained from open-path eddy-covariance systems. In our analysis, we present equations to analyse the propagation of errors through the Webb, Pearman, and Leuning (WPL) algorithm [Quart. J. Roy. Meteorol. Soc. 106, 85–100, 1980] that is widely used to account for density fluctuations on CO2 flux measurements. Our results suggest that incomplete energy balance closure does not necessarily lead to an underestimation of CO2 fluxes despite the existence of surface energy imbalance; either an overestimation or underestimation of CO2 fluxes is possible depending on local atmospheric conditions and measurement errors in the sensible heat, latent heat, and CO2 fluxes. We use open-path eddy-covariance fluxes measured over a black spruce forest in interior Alaska to explore several energy imbalance scenarios and their consequences for CO2 fluxes. 相似文献
9.
Latent and sensible heat flux predictions from a uniform pine forest using surface renewal and flux variance methods 总被引：1，自引：0，他引：1
Gabriel Katul Cheng-I Hsieh Ram Oren David Ellsworth Nathan Phillips 《Boundary-Layer Meteorology》1996,80(3):249-282
A surface renewal model that links organized eddy motion to the latent and sensible heat fluxes is tested with eddy correlation measurements carried out in a 13m tall uniform Loblolly pine plantation in Duke Forest, Durham, North Carolina. The surface renewal model is based on the occurance of ramp-like patterns in the scalar concentration measurements. To extract such ramp-like patterns from Eulerian scalar concentration measurements, a newly proposed time-frequency filtering scheme is developed and tested. The time-domain filtering is carried out using compactly-supported orthonormal wavelets in conjunction with the Universal Wavelet Thresholding approach of Donoho and Johnstone, while the frequency filtering is carried out by a band-pass sine filter centered around the ramp-occurrence frequency as proposed by other studies. The method was separately tested for heat and water vapour with good agreement between eddy correlation flux measurements and model predictions. The usefulness of the flux-variance method to predict sensible and latent heat fluxes is also considered. Our measurements suggest that the simple flux-variance method reproduces the measured heat and momentum fluxes despite the fact that the variances were measured within the roughness sublayer and not in the surface layer. Central to the predictions of water vapour fluxes using the flux-variance approach is the similarity between heat and water vapour transport by the turbulent air flow. This assumption is also investigated for this uniform forest terrain. 相似文献
10.
Atmospheric Stability Effects on Wind Fields and Scalar Mixing Within and Just Above a Subalpine Forest in Sloping Terrain 总被引：1，自引：1，他引：0
Sean P. Burns Jielun Sun Donald H. Lenschow Steven P. Oncley Britton B. Stephens Chuixiang Yi Dean E. Anderson Jia Hu Russell K. Monson 《Boundary-Layer Meteorology》2011,138(2):231-262
Air temperature T
a
, specific humidity q, CO2 mole fraction χ
c
, and three-dimensional winds were measured in mountainous terrain from five tall towers within a 1 km region encompassing
a wide range of canopy densities. The measurements were sorted by a bulk Richardson number Ri
b
. For stable conditions, we found vertical scalar differences developed over a “transition” region between 0.05 < Ri
b
< 0.5. For strongly stable conditions (Ri
b
> 1), the vertical scalar differences reached a maximum and remained fairly constant with increasing stability. The relationships
q and χ
c
have with Ri
b
are explained by considering their sources and sinks. For winds, the strong momentum absorption in the upper canopy allows
the canopy sublayer to be influenced by pressure gradient forces and terrain effects that lead to complex subcanopy flow patterns.
At the dense-canopy sites, soil respiration coupled with wind-sheltering resulted in CO2 near the ground being 5–7 μmol mol−1 larger than aloft, even with strong above-canopy winds (near-neutral conditions). We found Ri
b
-binning to be a useful tool for evaluating vertical scalar mixing; however, additional information (e.g., pressure gradients,
detailed vegetation/topography, etc.) is needed to fully explain the subcanopy wind patterns. Implications of our results
for CO2 advection over heterogenous, complex terrain are discussed. 相似文献
11.
Application of an infrared carbon dioxide and humidity instrument to studies of turbulent transport 总被引：4，自引：0，他引：4
Eiji Ohtaki 《Boundary-Layer Meteorology》1984,29(1):85-107
A calibration equation and some results of the field performance of an infrared instrument, which is designed to measure simultaneous fluctuations of atmospheric carbon dioxide and water vapor, are described. Field observations show that the instrument is suitable for simultaneous measurement of turbulent fluxes of carbon dioxide and water vapor in conjunction with a sonic anemometer. Measured values of carbon dioxide and water vapor fluxes show diurnal variations characterized by crop activity with respect to assimilation, respiration and evapotranspiration. Carbon dioxide is transferred downward during the daytime and upward at night, while latent heat and sensible heat are transferred in the opposite sense. The non-dimensional gradient of carbon dioxide is expressed in the following form under weak unstable conditions:
c
= (1 – 16
v
)-1/2. Here,
v
is the Monin-Obukhov stability parameter including the humidity effect. This relation was originally proposed for temperature and humidity. Thus, the results indicate that the turbulent mechanisms of carbon dioxide fluctuations are similar to those of other scalar entities. This is strongly supported by the high correlation coefficient found between fluctuations of carbon dioxide and temperature or humidity in the air layer over crop fields. 相似文献
12.
Penélope Serrano-Ortiz Andrew S. Kowalski Francisco Domingo Borja Ruiz Lucas Alados-Arboledas 《Boundary-Layer Meteorology》2008,126(2):209-218
Errors in the estimation of CO2 surface exchange by open-path eddy covariance, introduced during the removal of density terms [Webb et al. Quart J Roy Meteorol
Soc 106:85–100, (1980) - WPL], can happen both because of errors in energy fluxes [Liu et al. Boundary-Layer Meteorol 120:65–85,
(2006)] but also because of inaccuracies in other terms included in the density corrections, most notably due to measurements
of absolute CO2 density (ρ
c
). Equations are derived to examine the propagation of all errors through the WPL algorithm. For an open-path eddy covariance
system operating in the Sierra de Gádor in south-east Spain, examples are presented of the inability of an unattended, open-path infrared gas analyzer (IRGA) to
reliably report ρ
c
and the need for additional instrumentation to determine calibration corrections. A sensitivity analysis shows that relatively
large and systematic errors in net ecosystem exchange (NEE) can result from uncertainties in ρ
c
in a semi-arid climate with large sensible heat fluxes (H
s
) and (wet) mineral deposition. When ρc is underestimated by 5% due to lens contamination, this implies a 13% overestimation of monthly CO2 uptake. 相似文献
13.
Joe Kidston Christian Brümmer T. Andrew Black Kai Morgenstern Zoran Nesic J. Harry McCaughey Alan G. Barr 《Boundary-Layer Meteorology》2010,136(2):193-218
Components of the surface energy balance of a mature boreal jack pine forest and a jack pine clearcut were analysed to determine
the causes of the imbalance that is commonly observed in micrometeorological measurements. At the clearcut site (HJP02), a
significant portion of the imbalance was caused by: (i) the overestimation of net radiation (R
n
) due to the inclusion of the tower in the field of view of the downward facing radiometers, and (ii) the underestimation
of the latent heat flux (λE) due to the damping of high frequency fluctuations in the water vapour mixing ratio by the sample tube of the closed-path
infrared gas analyzer. Loss of low-frequency covariance induced by insufficient averaging time as well as systematic advection
of fluxes away from the eddy-covariance (EC) tower were discounted as significant issues. Spatial and temporal distributions
of the total surface-layer heat flux (T), i.e. the sum of sensible heat flux (H) and λE, were well behaved and differences between the relative magnitudes of the turbulent fluxes for several investigated energy
balance closure (C) classes were observed. Therefore, it can be assumed that micrometeorological processes that affected all turbulent fluxes
similarly did not cause the variation in C. Turbulent fluxes measured at the clearcut site should not be forced to close the energy balance. However, at the mature
forest site (OJP), loss of low-frequency covariance contributed significantly to the systematic imbalance when a 30-min averaging
time was used, but the application of averaging times that were long enough to capture all of the low-frequency covariance
was inadequate to resolve all of the high-frequency covariance. Although we found qualitative similarity between T and the net ecosystem exchange (NEE) of carbon dioxide (CO2), forcing T to closure while retaining the Bowen ratio and applying the same factor to CO2 fluxes (F
C
) cannot be generally recommended since it remains uncertain to what extent long wavelength contributions affect the relationship
between T, F
C
and C. 相似文献
14.
Denny P. Alappattu D. Bala Subrahamanyam P. K. Kunhikrishnan Radhika Ramachandran K. M. Somayaji R. Venkatesh G. S. Bhat A. Bhagavath Singh 《Boundary-Layer Meteorology》2008,126(2):297-309
We report the spatio-temporal variability of surface-layer turbulent fluxes of heat, moisture and momentum over the Bay of
Bengal (BoB) and the Arabian Sea (AS) during the Integrated Campaign for Aerosols, gases Radiation Budget (ICARB) field experiment.
The meteorological component of ICARB conducted during March – May 2006 onboard the oceanic research vessel Sagar Kanya forms
the database for the present study. The bulk transfer coefficients and the surface-layer fluxes are estimated using a modified
bulk aerodynamic method, and then the spatio-temporal variability of these air-sea interface fluxes is discussed in detail.
It is observed that the sensible and latent heat fluxes over the AS are marginally higher than those over the BoB, which we
attribute to differences in the prevailing meteorological conditions over the two oceanic regions. The values of the wind
stress, sensible and latent heat fluxes are compared with those obtained for the Indian Ocean Experiment (INDOEX) period.
The variation of drag coefficient (C
D
), exchange coefficients of sensible heat and moisture (C
H
= C
E
) and neutral drag coefficient (C
DN
) with wind speed is also discussed.
相似文献
15.
The paper reveals that the variations in parameters like u*, the scaling velocity and θ*. The scaling tempera-ture during the various phases of monsoon might be linked with subsynoptic features. The rise in u* is mainly connected with the presence of lower tropospheric cyclonic vorticity over a subsynoptic scale of the site. However the variations in θ* is mainly linked with the various phases of monsoon and θ* shows a sharp rise in presence of low level convective cloud.Besides the correlation studies of u and u*, θv and θv* , θv-θv0 and θv* are undertaken. The correlation be?tween θv and θv* is poor. In other two cases correlations are good. Besides u/u* , has shown good coefficient of variation values within the ζ range. 相似文献
16.
A Simple Method of Estimating Scalar Fluxes Over Forests 总被引：1，自引：0，他引：1
Nelson Luis Dias Jinkyu Hong Monique Y. Leclerc T. Andrew Black Z. Nesic P. Krishnan 《Boundary-Layer Meteorology》2009,132(3):401-414
A simple aerodynamic-variance method is proposed to fill gaps in continuous CO2 flux measurements in rainy conditions, when open-path analysers do not function. The method requires turbulent conditions
(friction velocity greater than 0.1 ms–1), and uses measurements of mean wind speed, and standard deviations of temperature and CO2 concentration fluctuations to complement, and at times replace, eddy-covariance measurements of friction velocity, sensible
heat flux and CO2 flux. Friction velocity is estimated from the mean wind speed with a flux-gradient relationship modified for the roughness
sublayer. Since normalised standard deviations do not follow Monin-Obukhov similarity theory in the roughness sublayer, a
simple classification scheme according to the scalar turbulence scale was used. This scheme is shown to produce sensible heat
and CO2 flux estimates that are well correlated with the measured values. 相似文献
17.
J. D. Wilson 《Boundary-Layer Meteorology》2012,145(1):165-184
This paper explores the utility of specifying the eddy viscosity for the horizontally uniform boundary layer as the product of the variance of vertical velocity and an empirical time scale τ w , as opposed to the more usual formulation where k is the turbulent kinetic energy (TKE), λ k is a length scale and α is a dimensionless coefficient. Simulations were compared with the observations on Day 33 of the Wangara experiment, and with a plausible specification of τ w (or λ k ) each model simulated convective boundary-layer development reasonably well, although the closure produced a more realistic width for the entrainment layer. Under the light winds of Day 33, and with the onset of evening cooling, an excessively shallow and strongly-stratified nocturnal inversion developed, and limited its own further deepening. Boundary-layer models that neglect radiative heat transport and parametrize convective transport by eddy viscosity closure are prone to this runaway (unstable) feedback when forced by a negative (i.e. downward) surface flux of sensible heat. 相似文献
18.
Investigating a Hierarchy of Eulerian Closure Models for Scalar Transfer Inside Forested Canopies 总被引：3，自引：3，他引：0
Jehn-Yih Juang Gabriel G. Katul Mario B. Siqueira Paul C. Stoy Heather R. McCarthy 《Boundary-Layer Meteorology》2008,128(1):1-32
Modelling the transfer of heat, water vapour, and CO2 between the biosphere and the atmosphere is made difficult by the complex two-way interaction between leaves and their immediate microclimate. When simulating scalar sources and sinks inside canopies on seasonal, inter-annual, or forest development time scales, the so-called well-mixed assumption (WMA) of mean concentration (i.e. vertically constant inside the canopy but dynamically evolving in time) is often employed. The WMA eliminates the need to model how vegetation alters its immediate microclimate, which necessitates formulations that utilize turbulent transport theories. Here, two inter-related questions pertinent to the WMA for modelling scalar sources, sinks, and fluxes at seasonal to inter-annual time scales are explored: (1) if the WMA is to be replaced so as to resolve this two-way interaction, how detailed must the turbulent transport model be? And (2) what are the added predictive skills gained by resolving the two-way interaction vis-à-vis other uncertainties such as seasonal variations in physiological parameters. These two questions are addressed by simulating multi-year mean scalar concentration and eddy-covariance scalar flux measurements collected in a Loblolly pine (P. taeda L.) plantation near Durham, North Carolina, U.S.A. using turbulent transport models ranging from K-theory (or first-order closure) to third-order closure schemes. The multi-layer model calculations with these closure schemes were contrasted with model calculations employing the WMA. These comparisons suggested that (i) among the three scalars, sensible heat flux predictions are most biased with respect to eddy-covariance measurements when using the WMA, (ii) first-order closure schemes are sufficient to reproduce the seasonal to inter-annual variations in scalar fluxes provided the canonical length scale of turbulence is properly specified, (iii) second-order closure models best agree with measured mean scalar concentration (and temperature) profiles inside the canopy as well as scalar fluxes above the canopy, (iv) there are no clear gains in predictive skills when using third-order closure schemes over their second-order closure counterparts. At inter-annual time scales, biases in modelled scalar fluxes incurred by using the WMA exceed those incurred when correcting for the seasonal amplitude in the maximum carboxylation capacity (V cmax, 25) provided its mean value is unbiased. The role of local thermal stratification inside the canopy and possible computational simplifications in decoupling scalar transfer from the generation of the flow statistics are also discussed.
相似文献
“The tree, tilting its leaves to capture bullets of light; inhaling, exhaling; its many thousand stomata breathing, creating the air”. Ruth Stone, 2002, In the Next Galaxy
19.
Vertical turbulent fluxes of water vapour, carbon dioxide, and sensible heat were measured from 16 August to the 28 September
2006 near the city centre of Münster in north-west Germany. In comparison to results of measurements above homogeneous ecosystem
sites, the CO2 fluxes above the urban investigation area showed more peaks and higher variances during the course of a day, probably caused
by traffic and other varying, anthropogenic sources. The main goal of this study is the introduction and establishment of
a new gap filling procedure using radial basis function (RBF) neural networks, which is also applicable under complex environmental
conditions. We applied adapted RBF neural networks within a combined modular expert system of neural networks as an innovative
approach to fill data gaps in micrometeorological flux time series. We found that RBF networks are superior to multi-layer
perceptron (MLP) neural networks in the reproduction of the highly variable turbulent fluxes. In addition, we enhanced the
methodology in the field of quality assessment for eddy covariance data. An RBF neural network mapping system was used to
identify conditions of a turbulence regime that allows reliable quantification of turbulent fluxes through finding an acceptable
minimum of the friction velocity. For the data analysed in this study, the minimum acceptable friction velocity was found
to be 0.15 m s−1. The obtained CO2 fluxes, measured on a tower at 65 m a.g.l., reached average values of 12 μmol m−2 s−1 and fell to nighttime minimum values of 3 μmol m −2 s−1. Mean daily CO2 emissions of 21 g CO2 m−2d −1 were obtained during our 6-week experiment. Hence, the city centre of Münster appeared to be a significant source of CO2. The half-hourly average values of water vapour fluxes ranged between 0.062 and 0.989 mmol m−2 s−1and showed lower variances than the simultaneously measured fluxes of CO2. 相似文献
20.
Xiaofeng Guo Kun Yang Long Zhao Wei Yang Shenghai Li Meilin Zhu Tandong Yao Yingying Chen 《Boundary-Layer Meteorology》2011,139(2):307-332
We present a field investigation over a melting valley glacier on the Tibetan Plateau. In the ablation zone, aerodynamic roughness
lengths (z
0M
) vary on the order of 10−4–10−2 m, whose evolution corresponds to three melt phases with distinct surface cover and moisture exchange: snow (sublimation/evaporation),
bare ice (deposition/condensation), and ice hummocks (sublimation/evaporation). Bowen-ratio similarity is validated in the
stably stratified katabatic winds, which suggests a useful means for data quality check. A roughness sublayer is regarded
as irrelevant to the present ablation season, because selected characteristics of scalar turbulence over smooth snow are quite
similar to those over hummocky ice. We evaluate three parametrizations of the scalar roughness lengths (z
0T
for temperature and z
0q
for humidity), viz. key factors for the accurate estimation of sensible heat and latent heat fluxes using the bulk aerodynamic
method. The first approach is based on surface-renewal models and has been widely applied in glaciated areas; the second has
never received application over an ice/snow surface, despite its validity in (semi-)arid regions; the third, a derivative
of the first, is proposed for use specifically over rough ice defined as z
0M
> 10−3 m or so. This empirical z
0M
threshold value is deemed of general relevance to glaciated areas (e.g. ice sheet/cap and valley/outlet glaciers), above
which the first approach gives notably underestimated z
0T,q
. The first and the third approaches tend to underestimate and overestimate turbulent heat/moisture exchange, respectively,
frequently leading to relative errors higher than 30%. Comparatively, the second approach produces fairly low errors in energy
flux estimates both in individual melt phases and over the whole ablation season; it thus emerges as a practically useful
choice to parametrize z
0T,q
in glaciated areas. Moreover, we find all three candidate parametrizations unable to predict diurnal variations in the excess
resistances to humidity transfer, thus encouraging more efforts for improvement. 相似文献