Abstract
AbstractThe Community Land Model (CLM) is the land component of the Community Earth System Model (CESM) and is used in several global and regional modeling systems. In this paper, we introduce model developments 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 data sets, 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, and 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), ozone damage to plants, and fire trace gas emissions coupling to the atmosphere. Conclusive establishment of improvement or degradation of individual variables or metrics is challenged by forcing uncertainty, parametric uncertainty, and model structural complexity, but the multivariate metrics presented here suggest a general broad improvement from CLM4 to CLM5.
Highlights
Land models are classically used as tools to integrate terrestrial contributions and responses to weather, climate variability, and climate change
We introduce model developments included in Community Land Model (CLM) version 5 (CLM5), the default land component for CESM2 which will be used for the Coupled Model Intercomparison Project (CMIP6)
We present a representative sample of analyses that are selected to emphasize strengths and weaknesses of CLM5, relative to CLM4 and CLM4.5, as well as to highlight new features of the model
Summary
Land models are classically used as tools to integrate terrestrial contributions and responses to weather, climate variability, and climate change. 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. Priorities for model development are set collectively by the CLM research and development community and are broadly focused on the enhancement of the capacity of the model to be applied to emerging questions that lie at the intersection of weather and climate with terrestrial processes. We include a high‐level assessment of the integrated impact of these developments on the overall performance of the model, utilizing the International Land Model Benchmarking package (ILAMB, Collier et al, 2018), ecosystem experiment data, and other metrics (section 4). A full technical description of the model is available online (http://www.cesm.ucar.edu/models/cesm2/land/CLM50_ Tech_Note.pdf)
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