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

Abstract

Abstract. Vegetation optical properties have a direct impact on canopy absorption and\nscattering and are thus needed for modeling surface fluxes. Although plant functional type (PFT) classification varies between different land surface\nmodels (LSMs), their optical properties must be specified. The aim of this\nstudy is to revisit the “time-invariant optical properties table” of the\nSimple Biosphere (SiB) model (later referred to as the “SiB table”) presented\n30 years ago by Dorman and Sellers\xa0(1989), which has since been adopted by\nmany LSMs. This revisit was needed as many of the data underlying the\nSiB table were not formally reviewed or published or were based on older\npapers or on personal communications (i.e.,\xa0the validity of the optical\nproperty source data cannot be inspected due to missing data sources,\noutdated citation practices, and varied estimation methods). As many of\ntoday s LSMs (e.g.,\xa0the Community Land Model (CLM), the Jena Scheme of Atmosphere\nBiosphere Coupling in Hamburg (JSBACH), and the Joint UK Land Environment\nSimulator (JULES)) either rely on the optical properties of the SiB table or\nlack references altogether for those they do employ, there is a clear need\nto assess (and confirm or correct) the appropriateness of those being used\nin today s LSMs. Here, we use various spectral databases to synthesize and\nharmonize the key optical property information of PFT classification shared\nby many leading LSMs. For forests, such classifications typically\ndifferentiate PFTs by broad geo-climatic zones (i.e.,\xa0tropical, boreal,\ntemperate) and phenology (i.e.,\xa0deciduous vs.\xa0evergreen). For short-statured\nvegetation, such classifications typically differentiate between crops, grasses, and photosynthetic pathway. Using the PFT classification of the\nCLM (version 5) as an example, we found the optical properties of the\nvisible band (VIS; 400–700\u2009nm) to fall within the range of measured values.\nHowever, in the near-infrared and shortwave infrared bands (NIR and SWIR; e.g.,\xa0701–2500\u2009nm, referred to as “NIR”) notable differences between CLM default and\nmeasured values were observed, thus suggesting that NIR optical properties\nare in need of an update. For example, for conifer PFTs, the measured mean\nneedle single scattering albedo (SSA, i.e.,\xa0the sum of reflectance and\ntransmittance) estimates in NIR were 62\u2009% and 78\u2009% larger than the CLM\ndefault parameters, and for PFTs with flat leaves, the measured mean leaf\nSSA values in NIR were 20\u2009%, 14\u2009%, and 19\u2009% larger than the CLM defaults.\nWe also found that while the CLM5 PFT-dependent leaf angle values were\nsufficient for forested PFTs and grasses, for crop PFTs the default\nparameterization appeared too vertically oriented, thus warranting an\nupdate. In addition, we propose using separate bark reflectance values for\nconifer and deciduous PFTs and demonstrate how shoot-level clumping\ncorrection can be incorporated into LSMs to mitigate violations of turbid\nmedia assumption and Beer s law caused by the nonrandomness of finite-sized\nfoliage elements.