CLM4.5 includes BOTH the old CLM4.0 physics AND the new CLM4.5 physics and you can toggle between two. The "standard" practice for CLM4.0 is to run with CN on, and with Qian atmospheric forcing. While the "standard" practice for CLM4.5 is to run with BGC on, and CRUNCEP atmospheric forcing. "BGC" is the new CLM4.5 biogeochemistry and include CENTURY-like pools, vertical resolved carbon, as well as Nitrification and de-Nitrification (see the Section called Some Acronym's and Terms We'll be Using in Other resources to get help from).
When running with CLMCN (either CLM4.0 or CLM4.5 physics) or CLM4.5-BGC, it is critical to begin with initial conditions that are provided with the release or to spin the model up following the CN spinup procedure before conducting scientific runs (see the Section called Spinning up the CLM4.5 biogeochemistry (CLMBGC spinup) in Chapter 4 for CLM4.5 or the Section called Spinning up the CLM4.0 biogeochemistry Carbon-Nitrogen Model (CN spinup) in Chapter 4 for CLM4.0). Simulations without a proper spinup will effectively be starting from an unvegetated world. See the Section called Setting Your Initial Conditions File in Chapter 1 for information on how to provide initial conditions for your simulation.
Initial condition files are provided for CLM4.0-CN as before, for fully coupled BCN and offline ICN cases for 1850 and 2000 at finite volume grids: 1deg (0.9x1.25), 2deg (1.9x2.5), and T31 resolutions. We also have interpolated initial conditions for BCN for 1850 and 2000 for two finite volume grids: 10x15, 4x5 and two HOMME grids (ne30np4 and ne120np4). There's also an initial condition file for ICN with the prognostic crop model for 2000 at 2deg resolution, and one with CLMSP for 2000 at 2deg resolution. We also have initial conditions for offline CNDV for 1850. The 1850 initial condition files are in 'reasonable' equilibrium. The 2000 initial condition files represent the model state for the year 2000, and have been taken from transient simulations. Therefore, by design the year 2000 initial condition files do not represent an equilibrium state. Note also that spinning the 2000 initial conditions out to equilibrium will not reflect the best estimate of the real carbon/nitrogen state for the year 2000.
Initial condition files are also provided for CLM4.5 for several configurations and resolutions. For CLM4.5-SP and CLM4.5-BGC with CRUNCEP forcing we have initial conditions at 1deg resolution for both 1850 and 2000. The CLM4.5-BGC initial conditions for 1850 (again with CRUNCEP forcing) were also interpolated to 2deg, CRUNCEP half degree (360x720cru), and ne30np4 resolutions. Also the CLM4.5-BGC initial conditions for 1850 (with CRUNCEP forcing) were interpolated to 1deg CLM4.5-BGC-DV and 2deg CLM4.5-BGC-Crop.
Users can generate initial condition files at different resolutions by using the CLM tool interpinic to interpolate from one of the provided resolutions to the resolution of interest. Interpolated initial condition files may no longer be in 'reasonable' equilibrium.
In CLM4.5 for both CLM4.5-CN and CLM4.5-BGC the new fire model requires lightning frequency data, and human population density (both are read inside of CLM). By default we have provided a climatology dataset for lightning frequency and a dataset with coverage from 1850 to 2010 for population density. Both of these datasets are interpolated from the native resolution of the datasets to the resolution you are running the model on. If you are running with an atmosphere model or forcing that is significantly different than present day -- the lightning frequency may NOT appropriately correspond to your atmosphere forcing and fire initiation would be inappropriate.
Aerosol deposition is a required field to both CLM4.0 and CLM4.5 physics, sent from the atmosphere model. Simulations without aerosol deposition will exhibit unreasonably high snow albedos. The model sends aerosol deposition from the atmospheric model (either CAM or DATM). When running with prescribed aerosol the atmosphere model will interpolate the aerosols from 2-degree resolution to the resolution the atmosphere model is running at.