Studies of energy balance that rely on eddy covariance（EC） are always challenged by energy balance closure, which is mainly caused by the underestimations of latent heat flux（LE） and sensible heat flux（Hs）. The ...Studies of energy balance that rely on eddy covariance（EC） are always challenged by energy balance closure, which is mainly caused by the underestimations of latent heat flux（LE） and sensible heat flux（Hs）. The Bowen ratio（BR） and energy balance residual（ER） approaches are two widely-used methods to correct the LE. A comprehensive comparison of those two approaches in different land-use types is essential to accurately correcting the LE and thus improving the EC experiments. In this study, two energy balance approaches（i.e., BR and ER） were compared to correct the LE measured at six EC sites（i.e., three vegetated, one mixed and two non-vegetated sites） in an oasis-desert ecotone of the Heihe River Basin, China. The influences of meteorological factors on those two approaches were also quantitatively assessed. Our results demonstrated that the average energy closure ratio（（LE＋Hs）/（Rn–Gs）; where Rn is the surface net radiation and Gs is the surface soil heat flux） was approximately close to 1.0 at wetland, maize and village sites, but far from 1.0 at orchard, Gobi and desert sites, indicating a significant energy imbalance at those three latter sites. After the corrections of BR and ER approaches that took into account of soil heat storage, the corrected LE was considerably larger than the EC-measured LE at five of six EC sites with an exception at Gobi site. The BR and ER approaches yielded approximately similar corrected LE at vegetated and mixed sites, but they generated dissimilar results at non-vegetated sites, especially at non-vegetated sites with low relative humidity, strong wind, and large surface-air temperature difference. Our findings provide insight into the applicability of BR and ER approaches to correcting EC-based LE measurements in different land-use types. We recommend that the BR-corrected and ER-corrected LE could be seriously reconsidered as validation references in dry and windy areas.展开更多
Hydrometeorological models are often evaluated and optimized on the basis of micrometeorological measurements. However, it has been known for more than three decades that surface measurements of sensible and latent he...Hydrometeorological models are often evaluated and optimized on the basis of micrometeorological measurements. However, it has been known for more than three decades that surface measurements of sensible and latent heat energy (LE) are systematically underestimated. We studied this problem using six years of eddy-correlation measurements for four fields (corn, soybean, and prairie) in central Iowa, USA. We recorded major components of the energy equation (i.e. net radiation, sensible heat flux, LE, and soil heat flux, photosynthesis), and indirectly estimated most of the minor components of energy balance (namely storage in the soil, canopy and air). Storage in the canopy was related to leaf area index (LAI) acquired from Moderate Resolution Imaging Spectrometer (MODIS). In this paper, a diagnostic approach is investigated where systematic error is identified first. Three dimensional (3D) plots of the residual of energy equation vs. potential variables indicated the imbalance was largest mainly during the cold non-growing season when the soil was dry. Correlations between energy balance residual (EBR) and energy components showed that soil storage was not precisely estimated. Finally, an a-posteriori analysis (constrained linear multiple regression (CMLR)) was conducted to quantify the contribution of major/minor components of the energy equation towards EBR. The result highlights that the contribution of pertinent components of energy to EBR is mainly controlled by prevailing monthly hydrometeorological conditions;however, precise quantification of causes of imbalance is site-specific. A comparison between the a-posteriori analysis technique and the Bowen-ratio method demonstrates that the Bowen-ratio basically presumes a higher level of underestimation in LE. The results obtained in this study suggest that a-posteriori analysis may offer a superior methodology to correct measured eddy-correlation measurements. Furthermore, the overall trends in the correction of LE measurements suggest that there is a potential for rough monthly corrections of LE, irrespective of the type of crop.展开更多
In order to compare and evaluate the performances of the Laplacian viscosity closure, the biharmonic viscosity closure, and the Leith closure momentum schemes in the MPAS-Ocean model, a variety of physical quantities,...In order to compare and evaluate the performances of the Laplacian viscosity closure, the biharmonic viscosity closure, and the Leith closure momentum schemes in the MPAS-Ocean model, a variety of physical quantities, such as the relative reference potential energy(RPE) change, the RPE time change rate(RPETCR), the grid Reynolds number, the root mean square(RMS) of kinetic energy, and the spectra of kinetic energy and enstrophy, are calculated on the basis of results of a 3D baroclinic periodic channel. Results indicate that: 1) The RPETCR demonstrates a saturation phenomenon in baroclinic eddy tests. The critical grid Reynolds number corresponding to RPETCR saturation differs between the three closures: the largest value is in the biharmonic viscosity closure, followed by that in the Laplacian viscosity closure, and that in the Leith closure is the smallest. 2) All three closures can effectively suppress spurious dianeutral mixing by reducing the grid Reynolds number under sub-saturation conditions of the RPETCR, but they can also damage certain physical processes. Generally, the damage to the rotation process is greater than that to the advection process. 3) The dissipation in the biharmonic viscosity closure is strongly dependent on scales. Most dissipation concentrates on small scales, and the energy of small-scale eddies is often transferred to large-scale kinetic energy. The viscous dissipation in the Laplacian viscosity closure is the strongest on various scales, followed by that in the Leith closure. Note that part of the small-scale kinetic energy is also transferred to large-scale kinetic energy in the Leith closure. 4) The characteristic length scale L and the dimensionless parameter Г in the Leith closure are inherently coupled. The RPETCR is inversely proportional to the product of Г and L. When the product of Г and L is constant, both the simulated RPETCR and the inhibition of spurious dianeutral mixing are the same in all tests using the Leith closure. The dissipative scale in the Leith closure depends on the parameter L, and the dissipative intensity depends on the parameter Г. 5) Although optimal results may not be achieved by using the optimal parameters obtained from the 2D barotropic model in the 3D baroclinic simulation, the total energies are dissipative in all three closures. Dissipation is the strongest in the biharmonic viscosity closure, followed by that in the Leith closure, and that in the Laplacian viscosity closure is the weakest. Mesoscale eddies develop the fastest in the biharmonic viscosity closure after the baroclinic adjustment process finishes, and the kinetic energy reaches its maximum, which is attributed to the smallest dissipation of enstrophy in the biharmonic viscosity closure. Mesoscale eddies develop the slowest, and the kinetic energy peak value is the smallest in the Laplacian viscosity closure. Results in the Leith closure are between that in the biharmonic viscosity closure and the Laplacian viscosity closure.展开更多
Radiation is a major driver to the carbon,water, and energy exchanges of an ecosystem. For local radiation balance measurements, one essential question is whether the measurement systems should be installed horizontal...Radiation is a major driver to the carbon,water, and energy exchanges of an ecosystem. For local radiation balance measurements, one essential question is whether the measurement systems should be installed horizontally or parallel to inclined slope surface. With a case study over a temperate deciduous forest on a moderate inclined(9°) northwest-facing slope, we quantified the slope effect on net radiation(Rn) and its components and the energy balance closure measured by an eddy covariance(EC) system.Compared with the slope-parallel radiometer, the horizontal sensor overestimated the incident solar radiation(SR) by 7%, the incoming photosynthetically active radiation(PAR) by 1.5%, and the incoming near-infrared radiation(NIR) by 10%;while underestimated the reflected shortwave radiation(SR)by 4% and NIR by 5%. The influence of radiometerorientation on incoming longwave radiation(LR) was about 3%, while that on outgoing LR was negligible.Summing all these components, horizontal sensor overestimated the Rn by 9%. Converting the horizontally-measured incident radiation to slopesurface reduced a half of the biases on incoming SR and Rn. Measuring the Rn with slope-parallel radiometer and correcting the slope-effect on horizontally-measured incident SR improved the energy balance ratio(EBR) by 8% and 5%,respectively. A mini-review indicated that, the horizontal sensor underestimated(overestimated) the EBR on north-facing(south-facing) slopes in temperate zone in the Northern Hemisphere, with an inclination angular sensitivity of EBR as high as 1.17%per degree of inclination angle. We recommend measuring radiations on inclined terrains with slopeparallel radiometers, or correcting at least for the incident SR in energy balance studies.展开更多
We find an asymptotic expression for the characteristic timescales of decorrelation processes in weakly compressible and isothermal turbulence. This result is used in the Eddy-Damped Quasi-Normal Markovian equation to...We find an asymptotic expression for the characteristic timescales of decorrelation processes in weakly compressible and isothermal turbulence. This result is used in the Eddy-Damped Quasi-Normal Markovian equation to derive the scalings of compressible energy spectra: (1) if the acoustic waves are dominant, the compressible energy spectra exhibit \(-7/3\) scaling; (2) if local eddy straining is dominant, the compressible energy spectra are scaled as \(-3\). Meanwhile, the energy spectra of incompressible components display the same scaling of \(-5/3\) as those in incompressible turbulence. The direct numerical simulations of weakly compressible turbulence are used to examine the scaling.展开更多
Introduction:Low energy balance closure(EBC)at a particular eddy-covariance flux site has increased the uncertainties of carbon,water,and energy measurements and has thus hampered the urgent research of scaling up and...Introduction:Low energy balance closure(EBC)at a particular eddy-covariance flux site has increased the uncertainties of carbon,water,and energy measurements and has thus hampered the urgent research of scaling up and modeling analyses through site combinations in regional or global flux networks.Methods:A series of manipulative experiments were conducted in this study to explore the role of net radiation(Rn)in the EBC in relation to spatial variability of vegetation characteristics,source area,and sensor type in three sites of the Inner Mongolian grassland of northern China.Results:At all three sites,the residual fluxes of EBC peaked consistently at 110 W m^(-2).The spatial variability in net radiation was 19 W m^(-2)(5%of R_(n))during the day and 7 W m^(-2)(16%)at night,with an average of 13 W m^(-2)(11%)from eight plot measurements across the three sites.Large area measurements of Rn significantly increased by 9 W m^(-2)during the day and decreased by 4 W m^(-2)at night in the unclipped treatments.Net radiation decreased by 25 W m^(-2)(6%of Rn)at midday and 81 MJ m^(-2)(6%)during a growing season with heavier regular clipping than that in unclipped treatments.The Rn was lower by 11–21 W m^(-2)(~20–40%of Rn)measured by CNR1 than by Q7.1 at night,while there was only 6 W m^(-2)(~1–2%of Rn)difference during the daytime between these two types of commonly used net radiometers.Conclusions:Overall,the inclusion of the uncertainty in available energy accounted for 65%of the~110 W m^(-2)shortfalls in the lack of closure.Clearly,the unclosed energy balance at these three grassland sites remains significant,with unexplored mechanisms for future research.展开更多
基金supported by the National Natural Science Foundation of China (41430855, 91125004)
文摘Studies of energy balance that rely on eddy covariance（EC） are always challenged by energy balance closure, which is mainly caused by the underestimations of latent heat flux（LE） and sensible heat flux（Hs）. The Bowen ratio（BR） and energy balance residual（ER） approaches are two widely-used methods to correct the LE. A comprehensive comparison of those two approaches in different land-use types is essential to accurately correcting the LE and thus improving the EC experiments. In this study, two energy balance approaches（i.e., BR and ER） were compared to correct the LE measured at six EC sites（i.e., three vegetated, one mixed and two non-vegetated sites） in an oasis-desert ecotone of the Heihe River Basin, China. The influences of meteorological factors on those two approaches were also quantitatively assessed. Our results demonstrated that the average energy closure ratio（（LE＋Hs）/（Rn–Gs）; where Rn is the surface net radiation and Gs is the surface soil heat flux） was approximately close to 1.0 at wetland, maize and village sites, but far from 1.0 at orchard, Gobi and desert sites, indicating a significant energy imbalance at those three latter sites. After the corrections of BR and ER approaches that took into account of soil heat storage, the corrected LE was considerably larger than the EC-measured LE at five of six EC sites with an exception at Gobi site. The BR and ER approaches yielded approximately similar corrected LE at vegetated and mixed sites, but they generated dissimilar results at non-vegetated sites, especially at non-vegetated sites with low relative humidity, strong wind, and large surface-air temperature difference. Our findings provide insight into the applicability of BR and ER approaches to correcting EC-based LE measurements in different land-use types. We recommend that the BR-corrected and ER-corrected LE could be seriously reconsidered as validation references in dry and windy areas.
文摘Hydrometeorological models are often evaluated and optimized on the basis of micrometeorological measurements. However, it has been known for more than three decades that surface measurements of sensible and latent heat energy (LE) are systematically underestimated. We studied this problem using six years of eddy-correlation measurements for four fields (corn, soybean, and prairie) in central Iowa, USA. We recorded major components of the energy equation (i.e. net radiation, sensible heat flux, LE, and soil heat flux, photosynthesis), and indirectly estimated most of the minor components of energy balance (namely storage in the soil, canopy and air). Storage in the canopy was related to leaf area index (LAI) acquired from Moderate Resolution Imaging Spectrometer (MODIS). In this paper, a diagnostic approach is investigated where systematic error is identified first. Three dimensional (3D) plots of the residual of energy equation vs. potential variables indicated the imbalance was largest mainly during the cold non-growing season when the soil was dry. Correlations between energy balance residual (EBR) and energy components showed that soil storage was not precisely estimated. Finally, an a-posteriori analysis (constrained linear multiple regression (CMLR)) was conducted to quantify the contribution of major/minor components of the energy equation towards EBR. The result highlights that the contribution of pertinent components of energy to EBR is mainly controlled by prevailing monthly hydrometeorological conditions;however, precise quantification of causes of imbalance is site-specific. A comparison between the a-posteriori analysis technique and the Bowen-ratio method demonstrates that the Bowen-ratio basically presumes a higher level of underestimation in LE. The results obtained in this study suggest that a-posteriori analysis may offer a superior methodology to correct measured eddy-correlation measurements. Furthermore, the overall trends in the correction of LE measurements suggest that there is a potential for rough monthly corrections of LE, irrespective of the type of crop.
基金sponsored by the National Natural Science Foundation of China Program (No.41175089)
文摘In order to compare and evaluate the performances of the Laplacian viscosity closure, the biharmonic viscosity closure, and the Leith closure momentum schemes in the MPAS-Ocean model, a variety of physical quantities, such as the relative reference potential energy(RPE) change, the RPE time change rate(RPETCR), the grid Reynolds number, the root mean square(RMS) of kinetic energy, and the spectra of kinetic energy and enstrophy, are calculated on the basis of results of a 3D baroclinic periodic channel. Results indicate that: 1) The RPETCR demonstrates a saturation phenomenon in baroclinic eddy tests. The critical grid Reynolds number corresponding to RPETCR saturation differs between the three closures: the largest value is in the biharmonic viscosity closure, followed by that in the Laplacian viscosity closure, and that in the Leith closure is the smallest. 2) All three closures can effectively suppress spurious dianeutral mixing by reducing the grid Reynolds number under sub-saturation conditions of the RPETCR, but they can also damage certain physical processes. Generally, the damage to the rotation process is greater than that to the advection process. 3) The dissipation in the biharmonic viscosity closure is strongly dependent on scales. Most dissipation concentrates on small scales, and the energy of small-scale eddies is often transferred to large-scale kinetic energy. The viscous dissipation in the Laplacian viscosity closure is the strongest on various scales, followed by that in the Leith closure. Note that part of the small-scale kinetic energy is also transferred to large-scale kinetic energy in the Leith closure. 4) The characteristic length scale L and the dimensionless parameter Г in the Leith closure are inherently coupled. The RPETCR is inversely proportional to the product of Г and L. When the product of Г and L is constant, both the simulated RPETCR and the inhibition of spurious dianeutral mixing are the same in all tests using the Leith closure. The dissipative scale in the Leith closure depends on the parameter L, and the dissipative intensity depends on the parameter Г. 5) Although optimal results may not be achieved by using the optimal parameters obtained from the 2D barotropic model in the 3D baroclinic simulation, the total energies are dissipative in all three closures. Dissipation is the strongest in the biharmonic viscosity closure, followed by that in the Leith closure, and that in the Laplacian viscosity closure is the weakest. Mesoscale eddies develop the fastest in the biharmonic viscosity closure after the baroclinic adjustment process finishes, and the kinetic energy reaches its maximum, which is attributed to the smallest dissipation of enstrophy in the biharmonic viscosity closure. Mesoscale eddies develop the slowest, and the kinetic energy peak value is the smallest in the Laplacian viscosity closure. Results in the Leith closure are between that in the biharmonic viscosity closure and the Laplacian viscosity closure.
基金supported by the National Natural Science Foundation of China (32171765 and 41503071)the Program for Changjiang Scholars and Innovative Research Team in University (IRT_15R09)。
文摘Radiation is a major driver to the carbon,water, and energy exchanges of an ecosystem. For local radiation balance measurements, one essential question is whether the measurement systems should be installed horizontally or parallel to inclined slope surface. With a case study over a temperate deciduous forest on a moderate inclined(9°) northwest-facing slope, we quantified the slope effect on net radiation(Rn) and its components and the energy balance closure measured by an eddy covariance(EC) system.Compared with the slope-parallel radiometer, the horizontal sensor overestimated the incident solar radiation(SR) by 7%, the incoming photosynthetically active radiation(PAR) by 1.5%, and the incoming near-infrared radiation(NIR) by 10%;while underestimated the reflected shortwave radiation(SR)by 4% and NIR by 5%. The influence of radiometerorientation on incoming longwave radiation(LR) was about 3%, while that on outgoing LR was negligible.Summing all these components, horizontal sensor overestimated the Rn by 9%. Converting the horizontally-measured incident radiation to slopesurface reduced a half of the biases on incoming SR and Rn. Measuring the Rn with slope-parallel radiometer and correcting the slope-effect on horizontally-measured incident SR improved the energy balance ratio(EBR) by 8% and 5%,respectively. A mini-review indicated that, the horizontal sensor underestimated(overestimated) the EBR on north-facing(south-facing) slopes in temperate zone in the Northern Hemisphere, with an inclination angular sensitivity of EBR as high as 1.17%per degree of inclination angle. We recommend measuring radiations on inclined terrains with slopeparallel radiometers, or correcting at least for the incident SR in energy balance studies.
基金supported by the National Natural Science Foundation of China (Grants 11302238, 11232011, 11572331, and 11490551)the support from the Strategic Priority Research Program (Grant XDB22040104)the Key Research Program of Frontier Sciences of Chinese Academy of Sciences and the National Basic Research Program of China (973 Program) (Grant 2013CB834100: Nonlinear science)
文摘We find an asymptotic expression for the characteristic timescales of decorrelation processes in weakly compressible and isothermal turbulence. This result is used in the Eddy-Damped Quasi-Normal Markovian equation to derive the scalings of compressible energy spectra: (1) if the acoustic waves are dominant, the compressible energy spectra exhibit \(-7/3\) scaling; (2) if local eddy straining is dominant, the compressible energy spectra are scaled as \(-3\). Meanwhile, the energy spectra of incompressible components display the same scaling of \(-5/3\) as those in incompressible turbulence. The direct numerical simulations of weakly compressible turbulence are used to examine the scaling.
基金This study was supported by the Natural Science Foundation of China(31170454,31229001,31130202)the State Key Basic Research Development Program of China(2013CB956600)the NASA-NEWS Program(NN-H-04-Z-YS-005-N),and the USCCC。
文摘Introduction:Low energy balance closure(EBC)at a particular eddy-covariance flux site has increased the uncertainties of carbon,water,and energy measurements and has thus hampered the urgent research of scaling up and modeling analyses through site combinations in regional or global flux networks.Methods:A series of manipulative experiments were conducted in this study to explore the role of net radiation(Rn)in the EBC in relation to spatial variability of vegetation characteristics,source area,and sensor type in three sites of the Inner Mongolian grassland of northern China.Results:At all three sites,the residual fluxes of EBC peaked consistently at 110 W m^(-2).The spatial variability in net radiation was 19 W m^(-2)(5%of R_(n))during the day and 7 W m^(-2)(16%)at night,with an average of 13 W m^(-2)(11%)from eight plot measurements across the three sites.Large area measurements of Rn significantly increased by 9 W m^(-2)during the day and decreased by 4 W m^(-2)at night in the unclipped treatments.Net radiation decreased by 25 W m^(-2)(6%of Rn)at midday and 81 MJ m^(-2)(6%)during a growing season with heavier regular clipping than that in unclipped treatments.The Rn was lower by 11–21 W m^(-2)(~20–40%of Rn)measured by CNR1 than by Q7.1 at night,while there was only 6 W m^(-2)(~1–2%of Rn)difference during the daytime between these two types of commonly used net radiometers.Conclusions:Overall,the inclusion of the uncertainty in available energy accounted for 65%of the~110 W m^(-2)shortfalls in the lack of closure.Clearly,the unclosed energy balance at these three grassland sites remains significant,with unexplored mechanisms for future research.
