LSM2 (NCAR Land Surface Model version 2.0)

LSM2 is NCAR Land Surface Model version 2.0

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Quoted from: https://doi.org/10.1175/1520-0442(2002)015%3C3123:TLSCOT%3E2.0.CO;2 

In LSM1, the geography of PFTs and the structure of vegetation (the height, roughness length, displacement height, and leaf and stem area of each PFT) are based on biomes. The type of biome determines the composition of the vegetation (i.e., the PFTs and their abundance). The PFT determines vegetation structure. This is because the high-spatial-resolution datasets needed to derive PFT composition and structure were not available during the development of LSM1. Direct specification of land cover in terms of PFTs is preferred over the use of biomes because of its more accurate depiction of spatial heterogeneity and the ability to separately specify vegetation composition and structure.

In LSM2, PFTs are inferred from 1-km satellite data. Oleson and Bonan (2000) describe this methodology for a region of the boreal forest. Bonan et al. (2002) describe the global implementation. The PFT determines plant physiology while vegetation structure is direct input to each grid cell for each PFT. This also allows the model to interface with models of ecosystem processes and vegetation dynamics such as the Lund–Potsdam–Jena (LPJ) dynamic global vegetation model (Sitch 2000Cramer et al. 2001McGuire et al. 2001). LPJ also uses PFTs to simulate the carbon cycle and vegetation dynamics, changing over time the structure and composition of patches of PFTs within a grid cell in response to disturbance (e.g., fire) and climate change. LSM2 is a restructuring of LSM1 to meet these objectives.

In LSM2, a grid cell is divided into five primary land cover types: glacier, lake, wetland, urban, and vegetation (Fig. 1). An urban land cover is included so that future versions of the model can study urbanization, but currently the urban cover is zero. The vegetated portion of a grid cell is further divided into patches of up to 4 of 16 PFTs, each with its own leaf area index, stem area index, and canopy top and bottom heights. Not all grid cells contain four PFTs. Homogenous vegetation may have fewer PFTs (e.g., one) than mixed vegetation (e.g., four). Bare ground is represented not as a primary land cover type, but rather as an unvegetated patch occurring among the PFTs.

As described by Bonan et al. (2002), 0.5° maps of the abundance of seven primary PFTs (needleleaf evergreen or deciduous tree, broadleaf evergreen or deciduous tree, shrub, grass, crop) were derived from the 1-km International Geosphere–Biosphere Program Data and Information System (IGBP DISCover) dataset (Loveland et al. 2000) and the 1-km University of Maryland tree cover dataset (DeFries et al. 19992000a,b). Temperature and precipitation were used to distinguish arctic, boreal, temperate, and tropical plants, C3 and C4 grasses, and evergreen and deciduous shrubs. Monthly leaf area index for each PFT in each 0.5° grid cell was obtained from 1-km Advanced Very High Resolution Radiometer (AVHRR) red and near-infrared reflectances for April 1992–March 1993 (Bonan et al. 2002). Stem area index, canopy top height, and canopy bottom height were based on the LSM1 values prescribed for each PFT (Bonan et al. 2002). Physiological parameters for the 16 PFTs were obtained from the 12 LSM1 PFTs (Bonan 1996) so that although the list of PFTs expanded, no new physiologies were introduced.

Coupling with the LPJ dynamic global vegetation model (Sitch 2000Cramer et al. 2001McGuire et al. 2001) necessitated three changes in plant physiology from LSM1. First, roughness length and displacement height were changed to proportions of canopy top height because plant height changes during vegetation dynamics. These ratios were obtained from LSM1 values prescribed for each PFT and are similar to the values of 0.1 and 0.7 often cited for roughness length and displacement height, respectively (Bonan 2002). Second, coupling with LPJ revealed an inappropriate scaling of leaf stomatal conductance to the canopy. In LSM1, leaf physiology is scaled to the canopy using sunlit and shaded leaves, which vary in photosynthesis and stomatal conductance. The LSM1 formulation of these processes for shaded leaves was found to be unrealistic, allowing for net carbon gain at high leaf area index. In LSM2, the canopy scaling is replaced by an assumption similar to that of the Simple Biosphere model version 2 (SiB2) whereby only sunlit leaves photosynthesize (Sellers et al. 19921996). Third, values of maximum carboxylation at 25°C (Vmax25), a key determinant of leaf photosynthesis and stomatal conductance, were increased from LSM1 values to maintain realistic canopy photosynthesis. These values of Vmax25, roughness length, and displacement height are also used in CLM2 and are listed with other CLM2 parameter values (section 2c).

An additional feature of LSM2 is that soil texture (percent sand and clay) varies with depth according to the IGBP soil dataset (Global Soil Data Task 2000). This was motivated by a desire to include dust emissions as a component of the land model. Preliminary simulations with a dust emission parameterization found better entrainment of dust into the atmosphere in the Sahara Desert, a high dust source region, with the sandier top soil layers of the IGBP dataset rather than a uniform soil profile as in LSM1.

The surface dataset for LSM2 includes: the glacier, lake, wetland, and urban portions of the grid cell (vegetation occupies the remainder); the fractional cover in the vegetated portion of the grid cell of the four most abundant PFTs; monthly leaf and stem area index and canopy top and bottom heights for each PFT; soil color; and soil texture. These fields are aggregated to the CCM3 T42 grid from high-resolution surface datasets (Table 1). In contrast to LSM1, there is no irrigation of crops. This is because LSM1 recognizes irrigated crops as a biome, but LSM2 only recognizes a crop PFT.

Table 2 summarizes the differences between LSM1 and LSM2. The primary difference is related to surface datasets: the representation of subgrid land cover, vegetation structure, and soil texture. Biogeophysical parameterizations are the same except for canopy scaling and leaf physiology.

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NCAR LSM2 team (2021). LSM2 (NCAR Land Surface Model version 2.0), Model Item, OpenGMS, https://geomodeling.njnu.edu.cn/modelItem/df8f9331-732b-43e0-8517-fb5b8ddacdc6
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