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Emulation of Community Land Model Version 5 (CLM5) to quantify sensitivity of soil moisture to uncertain parameters

Abstract: Land surface models (LSMs) are limited in their ability to reproduce observed soil moisture partially due to uncertainties in model parameters. Here we conduct a variance-based sensitivity analysis to quantify the relative contribution of different model parameters and their interactions to the uncertainty in the surface and root-zone soil moisture in the Community Land Model 5.0 (CLM5). We focus on soil texture-related parameters (porosity, saturated matric potential, saturated hydraulic conductivity, shape parameter of soil-water retention model) and organic matter fraction. A Gaussian process emulator is constructed based on CLM5 simulations and used to estimate soil moisture across the five-dimensional parameter space for sensitivity analysis. The procedure is demonstrated for four seasons across various U.S. sites of distinct soil and vegetation types.

We find that the emulator captures well the CLM5 behavior across the parameter space for different soil textures and seasons. The uncertainties of surface and rootzone soil moisture are dominated by the uncertainties in porosity and shape parameter with negligible parametric interactions. However, relative importance of porosity versus shape parameter varies with soil textures (sites), depths (surface versus root-zone), and seasons. At most of the sites, surface soil moisture uncertainty is attributed largely to shape parameter uncertainty, while porosity uncertainty is more important for the root-zone soil moisture uncertainty. All individual parameter and interaction effects demonstrate less variability across different soil textures and seasons for root-zone than for surface soil moisture. These results provide scientific guidance to prioritize reducing the uncertainty of sensitive parameters for improving soil moisture modeling with CLM.

Aiming to avoid the worst effects of climate change, from severe droughts to extreme coastal flooding, the nearly 200 nations that signed the Paris Agreement set a long-term goal of keeping global warming well below two degrees Celsius. Achieving that goal will require dramatic reductions in greenhouse gas emissions, primarily through a global transition to low-carbon energy technologies. In the power sector, these include solar, wind, biomass, nuclear and carbon capture and storage (CCS).

Powering through the coming energy transition

We live at a time of increasing physical risk—exposure to detrimental climate change and/or weather extremes—as well as transition risk—particularly the financial impacts of fossil fuel assets losing their value in the needed rapid transition to a low-carbon economy aimed at stabilizing the climate. A better understanding of these risks could empower decision-makers in the public and private sectors to chart a more sustainable, equitable and prosperous future.

Global changes: Physical and transition risks
Integrated climate-change assessment scenarios and carbon dioxide removal

Abstract: To halt climate change this century, we must reduce carbon dioxide (CO2) emissions from human activities to net zero. Any emission sources, such as in the energy or land-use sectors, must be balanced by natural or technological carbon sinks that facilitate CO2 removal (CDR) from the atmosphere. Projections of demand for large-scale CDR are based on an integrated scenario framework for emission scenarios composed of emission profiles as well as alternative socio-economic development trends and social values consistent with them. The framework, however, was developed years before systematic reviews of CDR entered the literature. This primer provides an overview of the purposes of scenarios in climate-change research and how they are used. It also introduces the integrated scenario framework and why it came about. CDR studies using the scenario framework, as well as its limitations, are discussed. Possible future developments for the scenario framework are highlighted, especially in relation to CDR.

Extratropical Storm Tracks and the Mean State of the Atmosphere

The exact impacts of changes in the mean state of the atmosphere on the highfrequency phenomena that form the extratropical atmospheric circulation are uncertain. The extratropical storm tracks, regions of frequent extratropical cyclones, dominate weather in the extratropics, affecting the lives and livelihoods of billions of people. The results presented in this thesis connect changes in the mean state of the atmosphere to changes in the extratropical storm tracks.

The Northern Hemisphere summer extratropical storm track has weakened in observations over the satellite era, while evidence indicates convective precipitation in the extratropics has concurrently increased. Using the concept of mean available potential energy (MAPE) partitioned into nonconvective and convective components, the second chapter of this thesis demonstrates that the changes in storm track strength and convection are consistent with changes in the temperature and humidity structure of the atmosphere. Further, experiments with idealized atmospheres indicate how characteristic changes in surface temperatures over this period lead to diverging changes in the energy available to extratropical cyclones and their associated convection.

In the third chapter of this thesis, the storm track strength is examined in solar geoengineering scenarios using results from climate models. The Northern Hemisphere extratropical storm track weakens in response to increased CO2 by similar magnitudes regardless of whether solar geoengineering is used. In the Southern Hemisphere, the storm track strengthens in global warming scenarios, but weakens with solar geoengineering. Storm track intensity changes are shown to be consistent with changes in the structure of temperature and humidity using MAPE.

In the fourth chapter of this thesis, a new method to calculate MAPE is introduced and used to perform the first exact MAPE calculations in a  three-dimensional domain. Further, an eddy-size restriction on the MAPE calculation is developed and introduced, which provides a measure of available energy that could be accessed locally by an extratropical cyclone. This approach is also used to identify the thermodynamic potential for ascent on the eddy lengthscale, which is shown to relate strongly to the frequency of warm conveyor belts (WCBs), dynamic components of extratropical cyclones with large impacts on weather.

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