### 2.2.3.2. Initialization[¶](#initialization "Permalink to this headline")

Initialization of the land model (i.e., providing the model with initial temperature and moisture states) depends on the type of run (startup or restart) (see the CLM4.5 User’s Guide). A startup run starts the model from either initial conditions that are set internally in the Fortran code (referred to as arbitrary initial conditions) or from an initial conditions dataset that enables the model to start from a spun up state (i.e., where the land is in equilibrium with the simulated climate). In restart runs, the model is continued from a previous simulation and initialized from a restart file that ensures that the output is bit-for-bit the same as if the previous simulation had not stopped. The fields that are required from the restart or initial conditions files can be obtained by examining the code. Arbitrary initial conditions are specified as follows.

Soil points are initialized with surface ground temperature \\(T\_{g}\\) and soil layer temperature \\(T\_{i}\\), for \\(i=1,\\ldots,N\_{levgrnd}\\), of 274 K, vegetation temperature \\(T\_{v}\\) of 283 K, no snow or canopy water (\\(W\_{sno} =0\\), \\(W\_{can} =0\\)), and volumetric soil water content \\(\\theta \_{i} =0.15\\) mm3 mm\-3 for layers \\(i=1,\\ldots,N\_{levsoi}\\) and \\(\\theta \_{i} =0.0\\) mm3 mm\-3 for layers \\(i=N\_{levsoi} +1,\\ldots,N\_{levgrnd}\\). placeLake temperatures (\\(T\_{g}\\) and \\(T\_{i}\\) ) are initialized at 277 K and \\(W\_{sno} =0\\).

Glacier temperatures (\\(T\_{g} =T\_{snl+1}\\) and \\(T\_{i}\\) for \\(i=snl+1,\\ldots,N\_{levgrnd}\\) where \\(snl\\) is the negative of the number of snow layers, i.e., \\(snl\\) ranges from –5 to 0) are initialized to 250 K with a snow water equivalent \\(W\_{sno} =1000\\) mm, snow depth \\(z\_{sno} =\\frac{W\_{sno} }{\\rho \_{sno} }\\) (m) where \\(\\rho \_{sno} =250\\) kg m\-3 is an initial estimate for the bulk density of snow, and \\(\\theta \_{i}\\) =1.0 for \\(i=1,\\ldots,N\_{levgrnd}\\). The snow layer structure (e.g., number of snow layers \\(snl\\) and layer thickness) is initialized based on the snow depth (section 6.1). The snow liquid water and ice contents (kg m\-2) are initialized as \\(w\_{liq,\\, i} =0\\) and \\(w\_{ice,\\, i} =\\Delta z\_{i} \\rho \_{sno}\\), respectively, where \\(i=snl+1,\\ldots,0\\) are the snow layers, and \\(\\Delta z\_{i}\\) is the thickness of snow layer \\(i\\) (m). The soil liquid water and ice contents are initialized as \\(w\_{liq,\\, i} =0\\) and \\(w\_{ice,\\, i} =\\Delta z\_{i} \\rho \_{ice} \\theta \_{i}\\) for \\(T\_{i} \\le T\_{f}\\), and \\(w\_{liq,\\, i} =\\Delta z\_{i} \\rho \_{liq} \\theta \_{i}\\) and \\(w\_{ice,\\, i} =0\\) for \\(T\_{i} >T\_{f}\\), where \\(\\rho \_{ice}\\) and \\(\\rho \_{liq}\\) are the densities of ice and liquid water (kg m\-3) ([Table 2.2.7](#table-physical-constants)), and \\(T\_{f}\\) is the freezing temperature of water (K) ([Table 2.2.7](#table-physical-constants)). All vegetated and glacier land units are initialized with water stored in the unconfined aquifer and unsaturated soil \\(W\_{a} =4000\\) mm and water table depth \\(z\_{\\nabla }\\) at five meters below the soil column.

