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

Abstract

Abstract. Vegetation optical properties have a direct impact on canopy absorption and scattering and are thus needed for modeling surface fluxes. Although plant functional type (PFT) classification varies between different land surface models (LSMs), their optical properties must be specified. The aim of this study is to revisit the “time-invariant optical properties table” of the Simple Biosphere (SiB) model (later referred to as the “SiB table”) presented 30 years ago by Dorman and Sellers (1989), which has since been adopted by many LSMs. This revisit was needed as many of the data underlying the SiB table were not formally reviewed or published or were based on older papers or on personal communications (i.e., the validity of the optical property source data cannot be inspected due to missing data sources, outdated citation practices, and varied estimation methods). As many of today's LSMs (e.g., the Community Land Model (CLM), the Jena Scheme of Atmosphere Biosphere Coupling in Hamburg (JSBACH), and the Joint UK Land Environment Simulator (JULES)) either rely on the optical properties of the SiB table or lack references altogether for those they do employ, there is a clear need to assess (and confirm or correct) the appropriateness of those being used in today's LSMs. Here, we use various spectral databases to synthesize and harmonize the key optical property information of PFT classification shared by many leading LSMs. For forests, such classifications typically differentiate PFTs by broad geo-climatic zones (i.e., tropical, boreal, temperate) and phenology (i.e., deciduous vs. evergreen). For short-statured vegetation, such classifications typically differentiate between crops, grasses, and photosynthetic pathway. Using the PFT classification of the CLM (version 5) as an example, we found the optical properties of the visible band (VIS; 400–700 nm) to fall within the range of measured values. However, in the near-infrared and shortwave infrared bands (NIR and SWIR; e.g., 701–2500 nm, referred to as “NIR”) notable differences between CLM default and measured values were observed, thus suggesting that NIR optical properties are in need of an update. For example, for conifer PFTs, the measured mean needle single scattering albedo (SSA, i.e., the sum of reflectance and transmittance) estimates in NIR were 62 % and 78 % larger than the CLM default parameters, and for PFTs with flat leaves, the measured mean leaf SSA values in NIR were 20 %, 14 %, and 19 % larger than the CLM defaults. We also found that while the CLM5 PFT-dependent leaf angle values were sufficient for forested PFTs and grasses, for crop PFTs the default parameterization appeared too vertically oriented, thus warranting an update. In addition, we propose using separate bark reflectance values for conifer and deciduous PFTs and demonstrate how shoot-level clumping correction can be incorporated into LSMs to mitigate violations of turbid media assumption and Beer's law caused by the nonrandomness of finite-sized foliage elements.

Effects of Incorporating Measured Leaf Optical Properties in Land Surface Models

“…These PFTs can be further classified as tropical (Tro), temperate (Tem) and boreal (Bor) by broad geoclimatic zones. Many current LSMs (e.g., the Community Land Model (CLM), the Jena Scheme of Atmosphere Biosphere Coupling in Hamburg (JSBACH), and the Joint United Kingdom Land Environment Simulator (JULES)) either rely on the "time-invariant optical properties look-up table" of the Simple Biosphere (SiB) model presented 30 years ago by Dorman and Sellers (1989) or lack references for the properties they do employ (Majasalmi and Bright, 2019). By examining the prescribed single-scattering albedo [SSA, i.e., the sum of reflectance (α) and transmittance (τ)] values of CLM (Lawrence et al, 2019), CoLM (Dai et al, 2003;Dai et al, 2004;Ji and Dai, 2010), Noah-Multi parameterization LSM (Noah-MP; Niu et al, 2011), JULES (Clark et al, 2011), SiB/SiB2 (Dorman and Sellers, 1989) and the Community Atmosphere Biosphere Land Exchange model (CABLE; Haverd et al, 2018), as shown in Figure 1, SSA values are very close to each other and almost identical to those in the SiB/SiB2 model.…”

“…The reported LOP values from early literature (e.g., Goudriaan, 1977;Dickinson, 1983) are different from those in the listed models (Figure 1). More recently, Majasalmi and Bright (2019) used various spectral databases to synthesize and harmonize the key optical property information of the PFT classification shared by many leading LSMs and found notable differences between the CLM default and measured LOP values in the NIR band. The LOP values for different PFTs provided by Majasalmi and Bright (2019) are highlighted in Figure 1 and referred to as "measured".…”

“…More recently, Majasalmi and Bright (2019) used various spectral databases to synthesize and harmonize the key optical property information of the PFT classification shared by many leading LSMs and found notable differences between the CLM default and measured LOP values in the NIR band. The LOP values for different PFTs provided by Majasalmi and Bright (2019) are highlighted in Figure 1 and referred to as "measured". Except for tropical broadleaf trees (i.e., BET-Tro and BDT-Tro), the measured SSA values are generally 0.1-0.29 higher than the prescribed values in the NIR band (Figure 1A).…”

“…In the VIS band, no difference greater than 0.04 is found (Figure 1B). Thus, Majasalmi and Bright (2019) suggested that NIR optical properties require an update.…”

“…The effects of such a large difference in LOP values between prescribed model and measured values in model simulations are still unclear. To this end, in this study, we incorporate the measured LOP values, which are provided by Majasalmi and Bright (2019), in CLM5 and CoLM and compare the difference in simulated albedo as a result of changed LOP values. Moreover, we analyze the changes in surface radiation partition, surface fluxes, net radiation, and land surface temperatures.…”