Land models are classically used as tools to integrate terrestrial contributions and responses to weather, climate variability, and climate change. In addition, modern land models are increasingly expected to provide insight into weather and climate impacts of societally relevant quantities such as water availability, crop and timber yields, wildfire risk, human heat stress, and other ecosystem services. The Community Land Model (CLM), which is the land component of the Community Earth System Model (CESM), has been developed and expanded over the last decade to provide an increasingly comprehensive platform that researchers can use to address these types of questions. More explicitly, CLM has been developed in accordance with two central themes: 1) terrestrial ecosystems, through their cycling of energy, water, momentum, carbon, nitrogen, and other trace gases, are important determinants of weather and climate, and 2) the land is a critical interface through which climate variability and climate change influence humans and ecosystems and through which humans and ecosystems can affect global environmental change.
Here, we will introduce recent model developments that have been included in CLM version 5 (CLM5), which is the default land component for CESM2. We assess an ensemble of simulations, including prescribed and prognostic vegetation state, multiple forcing datasets, and CLM4, CLM4.5, and CLM5, against a range of metrics including from the International Land Model Benchmarking (ILAMBv2) package. CLM5 includes new and updated processes and parameterizations: (1) dynamic land units, (2) updated parameterizations and structure for hydrology and snow (spatially explicit soil depth, dry surface layer, revised groundwater scheme, revised canopy interception and canopy snow processes, updated fresh snow density, simple firn model, and Model for Scale Adaptive River Transport), (3) plant hydraulics and hydraulic redistribution, (4) revised nitrogen cycling (flexible leaf stoichiometry, leaf N optimization for photosynthesis, carbon costs for plant nitrogen uptake), (5) global crop model with six crop types and time-evolving irrigated areas and fertilization rates, (6) updated urban building energy, (7) carbon isotopes, and (8) updated stomatal physiology. New optional features include demographically structured dynamic vegetation model (Functionally Assembled Terrestrial Ecosystem Simulator, FATES), ozone damage to plants, fire trace gas emissions coupling to the atmosphere, and representative hillslopes. Conclusive establishment of improvement or degradation of individual variables or metrics is challenged by forcing uncertainty, parametric uncertainty, and model structural complexity, but the multi-variate metrics presented here suggest a general broad improvement from CLM4 to CLM5. CLM5 is increasingly suited for research into a broad range of societally-relevant scientific questions related to the terrestrial system. We will conclude with an outlook of future model development needs and plans.
Dr. David Lawrence is a senior scientist and the terrestrial sciences section head in NCAR’s Climate and Global Dynamics Laboratory. Dr. Lawrence received his Ph.D. from the University of Colorado in Atmospheric and Ocean Sciences in 1999 and then became a postdoc at the University of Reading in the UK. Dr. Lawrence joined NCAR in 2003 and has coordinated and contributed to developing several recent versions of the Community Land Model, including CLM3.5, CLM4, and CLM4.5. His research interests mainly focused on land surface processes and climate change, emphasizing investigations into Arctic terrestrial climate system feedback, including the impact of permafrost degradation on carbon, water, and energy cycles into land-use change impacts on climate. He is also interested in land-atmosphere interactions and how land-atmosphere interactions contribute to climate variability and change. Dr. Lawrence has been co-chair of the Community Earth System Model (CESM) Land Model Working Group and is currently serving as a CESM Scientific Steering Committee member. He is also a co-lead of the Community Terrestrial Systems Model (CTSM), an effort to unify land modeling across NCAR and the broader research community. Dr. Lawrence’s excellent research has received wide recognition in the earth system modeling field. He is a topical editor of the journal Geoscientific Model Development, a co-chair of the Vulnerability of Permafrost Carbon RCN Model Integration group, and a co-lead of the Water and Energy group of the International Land Model Benchmarking project. Because of his outstanding accomplishments, he received a CCSM distinguished achievement award in 2008, a CGD special recognition award in 2012, a NERSC achievement award in 2016, and a CESM distinguished achievement award in 2020.