Land Model

[Catherine Nicolo]

Itis a collaborative project between scientists in the Terrestrial Sciences Section (TSS) and the Climate and Global Dynamics Division (CGD) at the National Center for Atmospheric Research (NCAR) and the CESM Land Model and Biogeochemistry Working Groups. Other principal working groups that contribute to the CLM are the Chemistry-Climate, Paleoclimate, Climate Change, and Land Ice Working Groups.

The model formalizes and quantifies concepts of ecological climatology. Ecological climatology is an interdisciplinary framework to understand how natural and human changes in vegetation affect climate. It examines the physical, chemical, and biological processes by which terrestrial ecosystems affect and are affected by climate across a variety of spatial and temporal scales. The central theme is that terrestrial ecosystems, through their cycling of energy, water, chemical elements, and trace gases, are important determinants of climate. The land surface is a critical interface through which climate change impacts humans and ecosystems and through which humans and ecosystems can effect global environmental change (iLEAPS Newsletter article on CLM).

The model represents several aspects of the land surface including surface heterogeneity and consists of components or submodels related to land biogeophysics, the hydrologic cycle, biogeochemistry, human dimensions, and ecosystem dynamics. Specific processes that are represented include:

The Common Land Model (CLM) was developed for community use by a grassroots collaboration of scientists who have an interest in making a general land model available for public use and further development. The major model characteristics include enough unevenly spaced layers to adequately represent soil temperature and soil moisture, and a multilayer parameterization of snow processes; an explicit treatment of the mass of liquid water and ice water and their phase change within the snow and soil system; a runoff parameterization following the TOPMODEL concept; a canopy photo synthesis-conductance model that describes the simultaneous transfer of CO2 and water vapor into and out of vegetation; and a tiled treatment of the subgrid fraction of energy and water balance. CLM has been extensively evaluated in offline mode and coupling runs with the NCAR Community Climate Model (CCM3). The results of two offline runs, presented as examples, are compared with observations and with the simulation of three other land models [the Biosphere-Atmosphere Transfer Scheme (BATS), Bonan's Land Surface Model (LSM), and the 1994 version of the Chinese Academy of Sciences Institute of Atmospheric Physics LSM (IAP94)].

Abstract. We implement and analyze 13 different metrics (4 moist thermodynamic quantities and 9 heat stress metrics) in the Community Land Model (CLM4.5), the land surface component of the Community Earth System Model (CESM). We call these routines the HumanIndexMod. We limit the algorithms of the HumanIndexMod to meteorological inputs of temperature, moisture, and pressure for their calculation. All metrics assume no direct sunlight exposure. The goal of this project is to implement a common framework for calculating operationally used heat stress metrics, in climate models, offline output, and locally sourced weather data sets, with the intent that the HumanIndexMod may be used with the broadest of applications. The thermodynamic quantities use the latest, most accurate and efficient algorithms available, which in turn are used as inputs to the heat stress metrics. There are three advantages of adding these metrics to CLM4.5: (1) improved moist thermodynamic quantities; (2) quantifying heat stress in every available environment within CLM4.5; and (3) these metrics may be used with human, animal, and industrial applications.

We demonstrate the capabilities of the HumanIndexMod in a default configuration simulation using CLM4.5. We output 4 daily temporal resolution globally. We show that the advantage of implementing these routines into CLM4.5 is capturing the nonlinearity of the covariation of temperature and moisture conditions. For example, we show that there are systematic biases of up to 1.5 C between monthly and 0.5 C between 4 daily offline calculations and the online instantaneous calculation, respectively. Additionally, we show that the differences between an inaccurate wet bulb calculation and the improved wet bulb calculation are 1.5 C. These differences are important due to human responses to heat stress being nonlinear. Furthermore, we show heat stress has unique regional characteristics. Some metrics have a strong dependency on regionally extreme moisture, while others have a strong dependency on regionally extreme temperature.

This new integrated land model, LM3, has improved capabilities to represent snow and rain interception on vegetation, as well as water phase change in the soil and snow pack. LM3 includes water storage and flow through the global river network. Carbon transport through the soil column, and carbon flux in the river network, are under development.

To address the v1 BGC science questions regarding the effects of nutrient limitations on carbon-climate feedbacks and their sensitivity to model structural uncertainty, we developed two biogeochemistry approaches in ELMv1 (see figure). In one approach, we integrated in a new theory, Equilibrium Chemical Approximation (ECA), for representing nutrient competition between microbes, plants, and abiotic processes (Tang, 2015; Tang and Riley, 2017; Zhu et al., 2016; Zhu et al., 2017), as well as for leaf-level photosynthesis controls (Ghimire et al., 2016). These processes have been evaluated against site-level observations and implications for global-scale carbon and nitrogen cycle responses have been investigated (Riley et al., 2018; Tang and Riley, 2018; Zhu and Riley, 2015; Zhu et al., 2018). The second approach is the Converging Trophic Cascade (CTC) biogeochemistry framework, which includes phosphorus dynamics (Yang et al., 2016) and plant storage pools as new capabilities. The representation of nutrient limitation and competition among plants and microbes for available nutrients in the CTC framework was investigated in several model-data integration studies, including the use of stable isotope observations (Mao et al., 2016; Raczka et al., 2016; Duarte et al., 2017). A sophisticated and efficient uncertainty quantification capability has been integrated with ELM v1 and is being used to perform global sensitivity analyses (Sargsyan et al. 2014, Safta et al., 2015, Ricciuto et al., 2018) and parameter optimization.

Since many processes controlling ecosystem carbon and nutrient cycling result from vertically-resolved soil processes, a sophisticated multi-phase, multi-species, and multi-transport mechanism reactive transport solver (BeTR) has been implemented in ELMv1 and applied to examine global-scale implications of several nutrient constraint hypotheses (Tang and Riley, 2018). Another new reactive transport capability based on the community model PFLOTRAN has also been integrated (Tang et al., 2016a and 2016b). In preparation for the analysis of results from the v1 coupled biogeochemistry experiments, new detection and attribution methods have been evaluated (Mao et al. 2016).

To support the water cycle experiments, we developed a new soil hydrology model and a new river transport model (MOSART; Li et al., 2013; 2015a) to improve simulations of surface hydrology and river flow, along with their response to anthropogenic forcings. The new soil hydrology solver, the Variably Saturation Flow Model (VSFM), improves on the physics implementation in our predecessor model without degrading computational efficiency (Bisht et al., 2018a).

Besides the above features that have been included in E3SM v1, Phase I activities also included development of model features to enhance capabilities in E3SM v2. Building on the framework of MOSART, we developed a floodplain inundation parameterization (MOSART-inundation) and evaluated it over the Amazon basin (Luo et al., 2017). We also added one-way coupling between MOSART with ELM and a water management model (WM) to simulate the effects of reservoir operations on streamflow (Voisin et al., 2017). Two new modules have also been added to MOSART to represent stream temperature (MOSART-heat) (Li et al., 2015b) and sediment transport (MOSART-sediment). A demographic and dynamic vegetation model, Functionally Assembled Terrestrial Ecosystem Simulator (FATES), has been coupled to ELM and used to assess differences in ecosystem responses to CO2 and climate (Holm et al., 2018), forest recovery from disturbance, and predicted global plant function trait (PFT) distributions without climate envelopes. Lastly, we also explored the implications of human-Earth system interactions using the integrated Earth System Model (iESM; Collins et al., 2015). Thornton et al. (2017) demonstrated the effects of changes in terrestrial productivity on energy, land use, and terrestrial carbon storage in an RCP4.5 scenario using the iESM. A follow-on paper (Calvin et al., in review) exploring these effects in both medium (RCP4.5) and high (RCP8.5) forcing worlds is also in review. Effort is underway to port the iESM code to E3SM, which should be completed by the end of Phase I and ready for testing in v2 as a key feature for the v2 biogeochemistry experiments.

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