Evaluation Of Soil Moisture Control On Surface Fluxes In Earth System Models (e-stress)

Lead Research Organisation: NERC Centre for Ecology and Hydrology
Department Name: Hydro-climate Risks

Abstract

Soil water plays a key role in a range of processes which are important for weather and climate. During extended periods without rain, the soil can dry out due to the vegetation transpiring and evaporation of water direct from bare soil surfaces. At some point in this drying cycle, evaporation itself becomes limited by the lack of soil water. Under such water-stressed conditions, there is a change in the way that incoming radiation from the sun is partitioned at the land surface; less energy is required for evapotranspiration so more energy goes into heating up the ground and overlying air. As well as raising air temperatures, this change can have important effects on atmospheric circulations, clouds and rain. In addition to these physical effects, the drying out of soils also has important biogeochemical impacts. The seasonal evolution of crops and natural vegetation is often sensitive to drought, in turn affecting crop yields and natural habitats. In the longer-term, drying soils can trigger changes in the regional composition of vegetation, for example favouring shrubs over trees. Such changes in vegetation are expected to play an important future role in the global climate system; vegetation offsets much of the carbon dioxide which is emitted from man's activities, and loss of trees weakens this carbon sink. Soil water also affects a number of other important trace gases, such as ozone and volatile organic compounds. During heatwaves, soil water deficits contribute to high concentrations of such trace gases, as well as high temperatures, with impacts on human health.

We rely on complex numerical codes run on powerful computers to make predictions of the atmosphere. For several decades, weather prediction models have incorporated simple descriptions of how soil water affects the atmosphere. Driven by a growing realisation of the importance of soil and vegetation processes for future climate, land surface models within so-called Earth System Models (ESMs) have become more complex, allowing us to simulate vegetation dynamics and trace gas responses to drought amongst other factors. These models rely on basic equations designed to capture the physical processes of e.g. evapotranspiration and soil drainage at a point in space. However, between locations there are huge and sometimes unknown differences in the nature of vegetation and soil which control these processes. All the same, the ESMs apply these equations over diverse areas of many thousands of square kilometres. Critically, there are no accurate in situ measurements at such large spatial scales which can be used to check how well the model simulates key land processes.

This project will exploit the availability of images collected by satellites over recent years. These can provide both spatial detail (down to 1km) and global coverage of key land properties. We will look at how the temperature of the land surface rises as the soil dries, how long a dry spell is required for these temperatures to rise, and how they influence the occurrence of heat waves. We will look at these relationships at the same coarse spatial scale as the ESMs and identify which regions and vegetation types are more prone to drought stress. We will produce several measures which for the first time, will allow us to test how well the key processes are represented by the ESMs across the globe. We will identify specific weaknesses within the UK ESM, and also evaluate a number of other models used for the latest Intergovernmental Panel on Climate Change to make projections of future climate. We will make our new observational datasets available to climate and weather modelling groups around the world. This will allow the next generation of ESMs to benefit from our research, and in turn contribute to improved prediction on time scales from hours to decades.

Planned Impact

As well as having considerable scientific impact (as outlined in the academic beneficiaries section), this project will strongly support impact through four major routes:
1. Effective engagement with government departments, such as DECC, Defra and DFID, concerned with policy and environment regulation .
2. The operational meteorological community who would also wish to implement model benchmarking and process improvement in land surface models for numerical weather prediction (NWP).
3. The space agencies and space industry which wish to understand the exploitation of EO data, the utility of satellite instruments for EO, and the future scenarios and instrument requirements for new missions both in the scientific and commercial sectors.
4. Outreach into schools to demonstrate the value of research, the power of and need for satellite observations and Earth System Models (ESM), and to encourage and learn from school children about their interests in STEM subjects.

Much of our impact activity in this project will be leveraged through investments and activities on-going in these areas but with a specific focus on EO exploitation and ESM model testing implications. Extreme temperature events will be a particular focus. Government departments will be engaged both through existing contacts and through our commitment to programme-wide initiatives in the NERC ESM projects and in LWEC. Operational organisations will be targetted through a specific workshop. The space sector will be informed both through national interactions, e.g. with the Space Catapult and the existing LWEC engagement in this area, and international impact agenda considerations, e.g. through the European Space Agency. Finally, outreach activities will be enhanced through specific topic development (project-related) in the national Space Academy and the peer-reviewed Blue Marble projects.
 
Description Computer models used for climate and weather prediction need to represent how the land dries out during dry spells in order to capture key features of our weather such as heat waves. This project has used satellite data of the temperature of the land surface to provide a unique new global picture of this process, distinguishing between different climatic regions and vegetation types. We have used these data to identify where different climate models accurately predict behaviour during dry spells, and where they are less successful, for example by raising temperatures unrealistically. We have created a tool for scientists around the world to use to improve their computer models, and thus contribute to better predictions.
Exploitation Route We have created a tool to evaluate climate model behaviour against observations across the world. This will feed in to future model development.
Sectors Environment