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1.
B. Kruijt Y. Malhi J. Lloyd A. D. Norbre A. C. Miranda M. G. P. Pereira A. Culf J. Grace 《Boundary-Layer Meteorology》2000,94(2):297-331
The turbulence structure in two Amazon rain forestswas characterised for a range of above-canopystability conditions, and the results compared withprevious studies in other forest canopies and recenttheory for the generation of turbulent eddies justabove forest canopies. Three-dimensional wind speedand temperature fluctuation data were collectedsimultaneously at up to five levels inside and abovetwo canopies of 30–40 m tall forests, during threeseparate periods. We analysed hourly statistics, jointprobability distributions, length scales, spatialcorrelations and coherence, as well as power spectraof vertical and horizontal wind speed.The daytime results show a sharp attenuation ofturbulence in the top third of the canopies, resultingin very little movement, and almost Gaussianprobability distributions of wind speeds, in the lowercanopy. This contrasts with strongly skewed andkurtotic distributions in the upper canopy. At night,attenuation was even stronger and skewness vanishedeven in the upper canopy. Power spectral peaks in thelower canopy are shifted to lower frequencies relativeto the upper canopy, and spatial correlations andcoherences were low throughout the canopy. Integrallength scales of vertical wind speed at the top of thecanopy were small, about 0.15 h compared to avalue of 0.28 h expected from the shear lengthscale at the canopy top, based on the hypothesis that theupper canopy air behaves as a plane mixing layer. Allthis suggests that, although exchange is not totallyinhibited, tropical rain forest canopies differ from other forests in that rapid, coherentdownward sweeps do not penetrate into the lowercanopy, and that length scales are suppressed. This isassociated with a persistent inversion of stability inthat region compared to above-canopy conditions. Theinversion is likely to be maintained by strong heatabsorption in the leaves concentrated near thecanopy top, with the generally weak turbulence beingunable to destroy the temperature gradients over thelarge canopy depth. 相似文献
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P. P. Harris C. Huntingford J. H. C. Gash M. G. Hodnett P. M. Cox Y. Malhi A. C. Araújo 《Theoretical and Applied Climatology》2004,78(1-3):27-45
Summary A land-surface model (MOSES) was tested against observed fluxes of heat, water vapour and carbon dioxide for two primary forest sites near Manaus, Brazil. Flux data from one site (called C14) were used to calibrate the model, and data from the other site (called K34) were used to validate the calibrated model. Long-term fluxes of water vapour at C14 and K34 simulated by the uncalibrated model were good, whereas modelled net ecosystem exchange (NEE) was poor. The uncalibrated model persistently underpredicted canopy conductance (g
c
) from mid-morning to mid-afternoon due to saturation of the response to solar radiation at low light levels. This in turn caused a poor simulation of the diurnal cycles of water vapour and carbon fluxes. Calibration of the stomatal conductance/photosynthesis sub-model of MOSES improved the simulated diurnal cycle of g
c
and increased the diurnal maximum NEE, but at the expense of degrading long-term water vapour fluxes. Seasonality in observed canopy conductance due to soil moisture change was not captured by the model. Introducing realistic depth-dependent soil parameters decreased the amount of moisture available for transpiration at each depth and led to the model experiencing soil moisture limitation on canopy conductance during the dry season. However, this limitation had only a limited effect on the seasonality in modelled NEE. 相似文献
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The overall long-term behaviour of the total visual coma brightness of P/Halley during October 1982 – June 1986 is presented. The observations used in this study consist of the raw total magnitude data collected from the International Astronomical Union Circulars. We have compared the observed light curve with the predicted one. 相似文献
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P. S. Goraya B. B. Sanwal B. S. Rautela H. K. Duggal J. S. Malhi 《Earth, Moon, and Planets》1989,44(3):243-249
Spectral scans of the head of periodic Comet Halley (1982i) have been presented in the optical region (3200–7000 Å) for six nights. Emission features due to NH, CN, CH, C3, C2 and NH2 molecules have been identified. It is found that the comet exhibits night-to-night variation of emission bands. The abundances and production rates of CN and C2 species have been derived. 相似文献
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Juan C. Jiménez-Muñoz Cristian Mattar José A. Sobrino Yadvinder Malhi 《International Journal of Digital Earth》2016,9(5):477-498
Remote sensing and climate digital products have become increasingly available in recent years. Access to these products has favored a variety of Digital Earth studies, such as the analysis of the impact of global warming over different biomes. The study of the Amazon forest response to drought has recently received particular attention from the scientific community due to the occurrence of extreme droughts and anomalous warming over the last decade. This paper focuses on the differences observed between surface thermal anomalies obtained from remote sensing moderate resolution imaging spectroradiometer (MODIS) and climatic (ERA-Interim) monthly products over the Amazon forest. With a few exceptions, results show that the spatial pattern of standardized anomalies is similar for both products. In terms of absolute anomalies, the differences between the two products show a bias near to zero with a standard deviation of around 0.2?K, although the differences can be up to 1?K over particular regions and months. Despite this general agreement, the proper filtering of MODIS daily values in order to construct a new monthly product showed a dramatic reduction in the number of reliable pixels during the rainy season, while for the dry season this reduction is only seen in Northern Amazonia. 相似文献
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A. L. Ruhoff A. R. Paz L. E. O. C. Aragao Q. Mu Y. Malhi W. Collischonn 《水文科学杂志》2013,58(8):1658-1676
AbstractRemote sensing is considered the most effective tool for estimating evapotranspiration (ET) over large spatial scales. Global terrestrial ET estimates over vegetated land surfaces are now operationally produced at 1-km spatial resolution using data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the MOD16 algorithm. To evaluate the accuracy of this product, ground-based measurements of energy fluxes obtained from eddy covariance sites installed in tropical biomes and from a hydrological model (MGB-IPH) were used to validate MOD16 products at local and regional scales. We examined the accuracy of the MOD16 algorithm at two sites in the Rio Grande basin, Brazil, one characterized by a sugar-cane plantation (USE), the other covered by natural savannah vegetation (PDG) for the year 2001. Inter-comparison between 8-day average MOD16 ET estimates and flux tower measurements yielded correlations of 0.78 to 0.81, with root mean square errors (RMSE) of 0.78 and 0.46 mm d-1, at PDG and USE, respectively. At the PDG site, the annual ET estimate derived by the MOD16 algorithm was 19% higher than the measured amount. For the average annual ET at the basin-wide scale (over an area of 145 000 km2), MOD16 estimates were 21% lower than those from the hydrological model MGB-IPH. Misclassification of land use and land cover was identified as the largest contributor to the error from the MOD16 algorithm. These estimates improve significantly when results are integrated into monthly or annual time intervals, suggesting that the algorithm has a potential for spatial and temporal monitoring of the ET process, continuously and systematically, through the use of remote sensing data.
Editor D. Koutsoyiannis; Associate editor T. WagenerCitation Ruhoff, A.L., Paz, A.R., Aragao, L.E.O.C., Mu, Q., Malhi, Y., Collischonn, W., Rocha, H.R., and Running, S.W., 2013. Assessment of the MODIS global evapotranspiration algorithm using eddy covariance measurements and hydrological modelling in the Rio Grande basin. Hydrological Sciences Journal, 58 (8), 1658–1676. 相似文献
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A Re-Evaluation of Long-Term Flux Measurement Techniques Part I: Averaging and Coordinate Rotation 总被引：11，自引：16，他引：11
J. J. Finnigan R. Clement Y. Malhi R. Leuning H.A. Cleugh 《Boundary-Layer Meteorology》2003,107(1):1-48
Experience of long term flux measurements over tall canopiesduring the last two decades has revealed that the eddy flux of sensible plus latentheat is typically 30% smaller than the available radiant energy flux. This failureto close the energy balance is less common close to the surface over short roughnessbut is still sometimes seen, especially in complex topography. These observationscast doubt on the results obtained from long term flux studies where daily and annualnet ecosystem exchange is usually the small difference between large positive andnegative fluxes over 24 h. In this paper we investigate this problem by examiningsome fundamental assumptions entailed in analysis of surface exchange by the eddyflux method.In particular, we clarify the form and use of the scalar conservation equation thatunderlies this analysis and we examine the links between averaging period androtation of coordinates in the situation where coordinates are aligned with thewind vector. We show that rotating coordinates so that the x axis is alignedwith the mean wind vector has the effect of high pass filtering the scalar covariance,¯wc, such that contributions to the aerodynamic flux from atmosphericmotions with periods longer than the averaging period are lost while those of shorterperiod are distorted.We compare the effect of computing surface exchange by averaging many shortperiods, in each of which the coordinates are rotated so that the mean verticalvelocity is zero (the method currently adopted in most long-term flux studies),with analysis in long-term coordinates and show a systematic underestimationof surface exchange in the former case. This is illustrated with data from threelong-term forest field sites where underestimations of sensible and latent heatfluxes of 10–15% averaged over many days are seen.Crucial factors determining the loss of flux are the averaging period T, themeasurement height and the content of the scalar cospectrum at periods longerthan T. The properties of this cospectrum over tall canopies in both homogeneousand complex terrain are illustrated by measurements at our three sites and we see thatover tall canopies on flat ground in convectiveconditions, or on hilly sites in near neutralflow, the scalar cospectra have much more low frequency contentthan classical surface-layerspectral forms would predict. We believe that the filtering of this low frequencycovariance by the averaging-rotation operations in common use is a large contributoryfactor to the failure to close the energy balance over tall canopies. 相似文献
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P. S. Goraya B. S. Rautela B. B. Sanwal S. K. Gupta H. K. Duggal J. S. Malhi 《Earth, Moon, and Planets》1989,45(1):17-27
Spectral scans of the head of periodic Comet Halley (1982i) have been presented and analysed in detail in the optical region (3200–7000 Å); for ten nights during pre-perihelion period. Emission features due to NH, CN, CH, C3, and C2 molecules have been identified. The behaviour of the variation of different emission lines strength as a function of heliocentric distance has been investigated. It is found that the comet exhibits night-to-night variation of brightness. The abundances and production rates of CN and C2 species have also been derived. 相似文献