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2.12.1. Vertical Discretization¶
Currently, there is one lake modeled in each grid cell (with prescribed or assumed depth d, extinction coefficient \(\eta\), and fetch f), although this could be modified with changes to the CLM subgrid decomposition algorithm in future model versions. As currently implemented, the lake consists of 0-5 snow layers; water and ice layers (10 for global simulations and 25 for site simulations) comprising the “lake body;” 10 “soil” layers; and 5 bedrock layers. Each lake body layer has a fixed water mass (set by the nominal layer thickness and the liquid density), with frozen mass-fraction I a state variable. Resolved snow layers are present if the snow thickness \(z_{sno} \ge s_{\min }\), where _s_min = 4 cm by default, and is adjusted for model timesteps other than 1800 s in order to maintain numerical stability (section 2.12.6.5). For global simulations with 10 body layers, the default (50 m lake) body layer thicknesses are given by: \(\Delta z_{i}\) of 0.1, 1, 2, 3, 4, 5, 7, 7, 10.45, and 10.45 m, with node depths \(z_{i}\) located at the center of each layer (i.e., 0.05, 0.6, 2.1, 4.6, 8.1, 12.6, 18.6, 25.6, 34.325, 44.775 m). For site simulations with 25 layers, the default thicknesses are (m): 0.1 for layer 1; 0.25 for layers 2-5; 0.5 for layers 6-9; 0.75 for layers 10-13; 2 for layers 14-15; 2.5 for layers 16-17; 3.5 for layers 18-21; and 5.225 for layers 22-25. For lakes with depth d \(\neq\) 50 m and d \(\ge\) 1 m, the top layer is kept at 10 cm and the other 9 layer thicknesses are adjusted to maintain fixed proportions. For lakes with d \(<\) 1 m, all layers have equal thickness. Thicknesses of snow, soil, and bedrock layers follow the scheme used over non-vegetated surfaces (Chapter 2.6), with modifications to the snow layer thickness rules to keep snow layers at least as thick as _s_min (section 2.12.6.5).