Parameterization improvements and functional and structural advances in Version 4 of the Community Land Model

Abstract

The Community Land Model is the land component of the Community Climate System Model. Here, we describe a broad set of model improvements and additions that have been provided through the CLM development community to create CLM4. The model is extended with a carbon‐nitrogen (CN) biogeochemical model that is prognostic with respect to vegetation, litter, and soil carbon and nitrogen states and vegetation phenology. An urban canyon model is added and a transient land cover and land use change (LCLUC) capability, including wood harvest, is introduced, enabling study of historic and future LCLUC on energy, water, momentum, carbon, and nitrogen fluxes. The hydrology scheme is modified with a revised numerical solution of the Richards equation and a revised ground evaporation parameterization that accounts for litter and within‐canopy stability. The new snow model incorporates the SNow and Ice Aerosol Radiation model (SNICAR) ‐ which includes aerosol deposition, grain‐size dependent snow aging, and vertically‐resolved snowpack heating – as well as new snow cover and snow burial fraction parameterizations. The thermal and hydrologic properties of organic soil are accounted for and the ground column is extended to ∼50‐m depth. Several other minor modifications to the land surface types dataset, grass and crop optical properties, surface layer thickness, roughness length and displacement height, and the disposition of snow‐capped runoff are also incorporated. The new model exhibits higher snow cover, cooler soil temperatures in organic‐rich soils, greater global river discharge, and lower albedos over forests and grasslands, all of which are improvements compared to CLM3.5. When CLM4 is run with CN, the mean biogeophysical simulation is degraded because the vegetation structure is prognostic rather than prescribed, though running in this mode also allows more complex terrestrial interactions with climate and climate change.

Fate of water pumped from underground and contributions to sea-level rise

“…in the Community Land Model 4 (CLM4) 12 of the CESM (fully coupled). For the GWD estimates, we combined model estimates 7 and previously published results 7,8,22 (approximately 120 km 3 for the year 2000).…”

Scalability of grid- and subbasin-based land surface modeling approaches for hydrologic simulations

“…As another attempt on the use of subbasinbased representation in a land surface model, Tesfa et al [2014] introduced a new subbasin-based approach built upon version 4 of the Community Land Model (CLM4) [Oleson et al, 2010;Lawrence et al, 2011] with some important technical advances including meteorological and land surface inputs derived from highresolution data sets coupled with a new physically based river routing model [Li et al, 2013]. The Community Land Model version 4 (CLM4) [Lawrence et al, 2011] has a large user community, and its use of the TOPMODEL approach for parameterizing runoff may allow it to take more advantages of the subbasinbased land surface representation.…”

“…CLM4 is the latest version of the land component of the Community Earth System Model (CESM) [Collins et al, 2006;Gent et al, 2010;Lawrence et al, 2011], which has been designed and used for studies of interannual and interdecadal variability, paleoclimate regimes, and projections of future changes of the global earth system. Compared to previous versions, CLM4 represents significant improvements in its model parameterizations and structure, including runoff generation, soil hydrology thermodynamics, snow, and albedo parameters [Lawrence et al, 2011]. In CLM4, runoff is generated based on the simplified TOPMODEL-based runoff formulation, where both surface and subsurface runoff generations are parameterized as exponential functions of the water …”

Smoke consequences of new wildfire regimes driven by climate change

“…Vegetation feedbacks to the atmosphere are therefore a crucial component in modeling meteorology, climate, and smoke chemistry and transport. For example, the Community Land Model Version 4 (CLM4) couples dynamic vegetation with carbon and nitrogen dynamics from a terrestrial biogeochemistry model [Thornton et al, 2009;Bonan et al, 2011;Lawrence et al, 2011]. Land surface models such as included in CLM4 establish boundary conditions (from below) for the atmospheric-physics equations that are solved numerically in RCMs [Bonan, 2008], analogously, though at much finer scales, to boundary conditions (at the lateral boundaries and from above) provided by GCMs to RCMs.…”