Parameterization of shortwave radiation fluxes for nonuniform vegetation canopies in land surface models

Yang, Rongqian; Friedl, M. A.; Ni, Wenge

doi:10.1029/2001jd900180

Cited by 47 publications

(45 citation statements)

References 22 publications

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“…Thus, the two types of canopy structure differ more in the distribution of radiation than the total radiation absorbed by the stand. Yang et al [2001] reached similar conclusions with a 3D vegetation canopy in a soil-vegetationatmosphere-transfer model in the old jack pine stand of the southern study site of the Boreas project [Sellers et al, 1997].…”

Section: Net Radiationsupporting

confidence: 63%

“…Attempts to represent the canopy over large spatial domains and long-temporal integrations often adopt a simpler approach, and assume that the canopy can be represented by a uniform isotropic turbid medium, as employed by a number of models: FOREST-BGC [Running and Coughlan, 1988], Hybrid [Friend et al, 1997], 3-PG [Landsberg and Waring, 1997], SDGVM [Woodward et al, 1998] and RHESSys [Tague and Band, 2004]. However, even uniform crop canopies rarely meet the isotropic turbid medium assumption [Suits, 1983], and leaves in forest canopies are usually clumped at multiple spatial scales, severely violating the homogeneity assumption [Chen and Leblanc, 1997;Ni et al, 1997;Kucharik et al, 1999;Yang et al, 2001]. Understanding these compromises in terms of model skill that are accrued by making such assumptions is a logical first step for developing process-based ecosystem models that are robust in long-term space-time integration.…”

Section: Introductionmentioning

confidence: 99%

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Energy, water, and carbon fluxes in a loblolly pine stand: Results from uniform and gappy canopy models with comparisons to eddy flux data

et al. 2009

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[1] This study investigates the impacts of canopy structure specification on modeling net radiation (R n ), latent heat flux (LE) and net photosynthesis (A n ) by coupling two contrasting radiation transfer models with a two-leaf photosynthesis model for a maturing loblolly pine stand near Durham, North Carolina, USA. The first radiation transfer model is based on a uniform canopy representation (UCR) that assumes leaves are randomly distributed within the canopy, and the second radiation transfer model is based on a gappy canopy representation (GCR) in which leaves are clumped into individual crowns, thereby forming gaps between the crowns. To isolate the effects of canopy structure on model results, we used identical model parameters taken from the literature for both models. Canopy structure has great impact on energy distribution between the canopy and the forest floor. Comparing the model results, UCR produced lower R n , higher LE and higher A n than GCR. UCR intercepted more shortwave radiation inside the canopy, thus producing less radiation absorption on the forest floor and in turn lower R n . There is a higher degree of nonlinearity between A n estimated by UCR and by GCR than for LE. Most of the difference for LE and A n between UCR and GCR occurred around noon, when gaps between crowns can be seen from the direction of the incident sunbeam. Comparing with eddy-covariance measurements in the same loblolly pine stand from May to September 2001, based on several measures GCR provided more accurate estimates for R n , LE and A n than UCR. The improvements when using GCR were much clearer when comparing the daytime trend of LE and A n for the growing season. Sensitivity analysis showed that UCR produces higher LE and A n estimates than GCR for canopy cover ranging from 0.2 to 0.8. There is a high degree of nonlinearity in the relationship between UCR estimates for A n and those of GCR, particularly when canopy cover is low, and suggests that simple scaling of UCR parameters cannot compensate for differences between the two models. LE from UCR and GCR is also nonlinearly related when canopy cover is low, but the nonlinearity quickly disappears as canopy cover increases, such that LE from UCR and GCR are linearly related and the relationship becomes stronger as canopy cover increases. These results suggest the uniform canopy assumption can lead to underestimation of R n , and overestimation of LE and A n . Given the potential in mapping regional scale forest canopy structure with high spatial resolution optical and Lidar remote sensing plotforms, it is possible to use GCR for up-scaling ecosystem processes from flux tower measurements to heterogeneous landscapes, provided the heterogeneity is not too extreme to modify the flow dynamics.

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Section: Net Radiationsupporting

confidence: 63%

Section: Introductionmentioning

confidence: 99%

Energy, water, and carbon fluxes in a loblolly pine stand: Results from uniform and gappy canopy models with comparisons to eddy flux data

et al. 2009

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“…Pinty et al (2006) have shown that the 3-D canopy radiative transfer problem can be approximated by a 1-D model using an effective leaf area index as well as effective parameters for the vegetation optical properties. Different approaches have been proposed to relate a leaf area index to an effective leaf area index by defining a structural parameter that accounts for vegetation clumping (Yang et al, 2001;Pinty, 2004;Ni-Meister et al, 2010;Haverd et al, 2012). A new globally available data set of effective RT model parameters as derived from satellite observations ) could facilitate RT model parametrizations of DGVMs for different plant functional types.…”

Section: A Way Forward?mentioning

confidence: 99%

Do we (need to) care about canopy radiation schemes in DGVMs? Caveats and potential impacts

et al. 2014

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Abstract. Dynamic global vegetation models (DGVMs) are an essential part of current state-of-the-art Earth system models. In recent years, the complexity of DGVMs has increased by incorporating new important processes like, e.g., nutrient cycling and land cover dynamics, while biogeophysical processes like surface radiation have not been developed much further. Canopy radiation models are however very important for the estimation of absorption and reflected fluxes and are essential for a proper estimation of surface carbon, energy and water fluxes.The present study provides an overview of current implementations of canopy radiation schemes in a couple of stateof-the-art DGVMs and assesses their accuracy in simulating canopy absorption and reflection for a variety of different surface conditions. Systematic deviations in surface albedo and fractions of absorbed photosynthetic active radiation (faPAR) are identified and potential impacts are assessed.The results show clear deviations for both, absorbed and reflected, surface solar radiation fluxes. FaPAR is typically underestimated, which results in an underestimation of gross primary productivity (GPP) for the investigated cases. The deviation can be as large as 25 % in extreme cases. Deviations in surface albedo range between −0.15 ≤ α ≤ 0.36, with a slight positive bias on the order of α ≈ 0.04. Potential radiative forcing caused by albedo deviations is estimated at −1.25 ≤ RF ≤ −0.8 (W m −2 ), caused by neglect of the diurnal cycle of surface albedo.The present study is the first one that provides an assessment of canopy RT schemes in different currently used DGVMs together with an assessment of the potential impact of the identified deviations. The paper illustrates that there is a general need to improve the canopy radiation schemes in DGVMs and provides different perspectives for their improvement.

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“…However, this approach involves a number of simplifications such as the big-leaf paradigm and horizontally uniform canopy. Yang et al (2001) modified the two-stream method to include both the between-and within-crown gap probability in the discontinuous canopy. Niu and Yang (2004) used the scheme of Yang et al (2001) to study the effects of vegetation canopy on snow surface energy and mass balances.…”

Section: Introductionmentioning

confidence: 99%

“…Yang et al (2001) modified the two-stream method to include both the between-and within-crown gap probability in the discontinuous canopy. Niu and Yang (2004) used the scheme of Yang et al (2001) to study the effects of vegetation canopy on snow surface energy and mass balances. These modifications have demonstrated improvement in surface albedo simulations, particularly in late spring, when the solar zenith angle (SZA) is getting smaller and the ground surface under the canopy is still covered by snow.…”

Section: Introductionmentioning

confidence: 99%

Simulation of canopy radiation transfer and surface albedo in the EALCO model

2007

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A new canopy radiation transfer and surface albedo scheme is developed as part of the land surface model EALCO (Ecological Assimilation of Land and Climate Observations). The model uses a gap probabilitybased successive orders of scattering approach that explicitly includes the heterogeneities of stands and crown elements and the radiation multiple scattering. The model uses the optical parameters of ecosystem elements and physically represents ecosystem processes in surface albedo dynamics. Model tests using measurements from a boreal deciduous forest ecosystem show that the model well reproduced the observed diurnal and seasonal albedo dynamics under different weather and ecosystem conditions. The annual mean absolute errors between modeled and measured daily albedo and reflected radiation are 0.01 and 1.33 W m -2 , respectively. The model results provide a quantitative assessment of the impacts of plant shading and sky conditions on surface albedo observed in high-latitude ecosystems. The contribution of ground snow to surface albedo in winter was found to be less than 0.1 even though the canopy is leafless during this time. The interception of snow by the leafless canopy can increase the surface albedo by 0.1-0.15. The model results show that the spectral properties of albedo have large seasonal variations. In summer, the near infrared component is substantially larger than visible, and surface albedo is less sensitive to sky conditions. In winter, the visible band component is markedly increased and can exceed the near infrared proportion under cloudy conditions or when snow exists on the canopy. The spectral properties of albedo are also found to have large diurnal variations under the clear-sky conditions in winter.

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Parameterization of shortwave radiation fluxes for nonuniform vegetation canopies in land surface models

Cited by 47 publications

References 22 publications

Energy, water, and carbon fluxes in a loblolly pine stand: Results from uniform and gappy canopy models with comparisons to eddy flux data

Energy, water, and carbon fluxes in a loblolly pine stand: Results from uniform and gappy canopy models with comparisons to eddy flux data

Do we (need to) care about canopy radiation schemes in DGVMs? Caveats and potential impacts

Simulation of canopy radiation transfer and surface albedo in the EALCO model

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