Reexamination and further development of two‐stream canopy radiative transfer models for global land modeling

Abstract

Four representative two‐stream canopy radiative transfer models were examined and intercompared using the same configuration. Based on the comparison results, two modifications were introduced to the widely used Dickinson‐Sellers model and then incorporated into the Community Land Model (CLM4.5). The modified model was tested against Monte‐Carlo simulations and produced significant improvements in the simulated canopy transmittance and albedo values. In direct comparison with MODIS albedo data, the modified model shows good performance over most snow/ice‐free vegetated areas, especially for regions that are covered by dense canopy. The modified model shows seasonally dependent behavior mainly in the near‐infrared band. Thus, the improvements are not present in all seasons. Large biases are still noticeable in sparsely vegetated areas, in particular for the snow/ice covered regions, that is possibly related to the model, the land surface input data, or even the observations themselves. Further studies focusing on the impact of the seasonal changes in leaf optical properties, the parameterizations for snow/ice covered regions and the case of sparsely vegetated areas, are recommended.

Evaluation of leaf-level optical properties employed in land surface models

“…Generally speaking, the need for improving the RT models employed in LSMs has been acknowledged, and progress has already been made (Yuan et al, 2017;McGrath et al, 2016). For example, a "domain-averaged structural factor" (i.e., effective LAI accounting for inhomogeneous horizontal distribution such as tree clumping and canopy gaps) and multilayer canopy vertical albedo profile were recently added by McGrath et al (2016) for the ORganising Carbon and Hydrology In Dynamic EcosystEms (ORCHIDEE, SVN r2566) model.…”

“…They modeled grasses and crops as homogenous blocks, without internal structure, and defined the tunable "correction factor" to account clumping effects. For the CLM, recent advancements were done by Yuan et al (2017), who compared four representative 1-D RT models under the same framework and implemented the appropriate modifications for the CLM4.5 (Oleson et al, 2013). They proposed changes for the employed LAD equation and two modifications following the paper by Pinty et al (2006) regarding the treatment of incident diffuse radiation and backward scattering coefficient for incident direct radiation.…”

“…As remotely sensed products are used as an input in LSMs, advances in RT modeling employed in remote sensing should ideally be reflected by LSM RT parameterizations. In addition, more effort in LSM RT modeling is needed for developing scaling routines to account for seasonal changes in optical properties (and SAI), and for improving parameterizations for snow and ice (Yuan et al, 2017).…”

“…Although there are many types of canopy RT models with varying complexities -from light extinction algorithms to those applying turbid medium and geometricoptical methods -they must specify the following: optical properties (i.e., reflectance "R" and transmittance "T ") of canopy elements such as foliage and bark, canopy foliage area (e.g., leaf area index (LAI, m 2 m −2 )), and vegetation spatial ordering (e.g., leaf inclination angle, LIA, i.e., the angle between the leaf surface normal and the zenith). At present, most LSMs are limited to one-dimensional (1-D) radiative exchange relying on solutions derived from twostream approximations based on plane-parallel turbid media assumptions (Loew et al, 2014;Yuan et al, 2017).…”

Evaluation of leaf-level optical properties employed in land surface models – example with CLM 5.0

“…Leaf Inclination Angle, LIA, angle between the leaf surface normal and the zenith). At present, most LSMs are limited to one-dimensional radiative exchange relying on solutions derived from twostream approximations based on plane-parallel turbid media assumptions (Loew et al, 2014, Yuan et al, 2017. 30 years ago, Dorman and Sellers (1989) presented a 'time-invariant optical properties table' for the Simple Biosphere (SiB) model (later referred as 'SiB-table' or 'SiB-classes') which was compiled using available data and field notes of the time.…”