Overview

Project Highlights:

  • Novelty: For the first time, the Free-Air CO2 Enrichment (FACE) Experiment facility is used to assess wind and turbulence inside woodland and to evaluate the first large-eddy simulation (LES) of turbulent flows inside a small woodland.
  • Feasibility: The existing LES modelling capability at Birmingham and the dataset (wind, turbulence and CO2) collected from the BIFoR site are well suited for the project.
  • Impact: The project will reveal the key mechanisms of CO2 exchange between patches of small-size heterogeneous woodland (SHW) and the atmosphere and provide an estimate of the enhanced CO2 exchange per unit area relative to that of large-size homogeneous forest (LHF).

Assessment of carbon exchange between large-size homogeneous forests (LHF) and the atmosphere largely relies on the FLUXNET approach[1] in which long-term eddy-covariance measurements from flux towers above forest canopy are used. This approach assumes spatial homogeneity with a large fetch and ignores many other factors (e.g. horizontal heterogeneity below the sensor level)[3]. Therefore, this approach cannot be applied to small-size heterogeneous woodlands (SHW) because previous studies showed that, depending on tree/leaf density, the length scale (from the edge) for a scalar to reach equilibrium can be tens of tree heights, much longer than the length scale for momentum[6]. SHW cover a significant proportion of land in the UK, Europe and and other parts of the world. The statistics, especially the intermittency, of wind and turbulence inside such woodlands play a crucial role of transferring CO2[2]; in addition, enhanced turbulence near the edge raise the exchange of CO2 there (Figure 1)[4]. In order to improve the assessment of CO2 exchange associated with SHW, we must enhance our understanding of the key player, turbulence, and its impact on CO2 transport for woodlands.  Scientific questions are: (i) what are the 3D characteristics of wind and turbulence inside and outside the tree canopy? (ii) Can we model in-canopy mixing processes in order to improve the knowledge that is currently lacking? (iii) Compared with LHFs of the same tree species, can we express the additional carbon uptake in terms of some measure of the SHW shape (e.g., edge length, area)? (iv) What impact does accounting for SHW make to estimates of the global terrestrial carbon sink?

This project aims to address the scientific questions listed above and to shed light on the impact of SHW on the CO2 exchange with the atmosphere aloft. Particular focuses will be on the spatial varibility of CO2 exchange across a SHW and the enhancement of the exchange per unit area with reference to LHF. The project has the following innovative elements: (1) the first LES for turbulent flows inside a SHW; (2) the first use of a FACE facility to assesss the mixing capability of woodland.

Enhanced exchange of a passive scalar near the edge of a forest simulated by LES (Kanani-Suhring and Raasch, 2015)

Methodology

LES will be adopted to reveal detailed in-canopy transport processes[4,5] that affect CO2 fluxes. To evaluate the model, turbulence data from 5 sonic anemometers inside and above the canopy of the BIFoR woodland and other data (met and CO2 concentrations) will be used. The BIFoR facility will also be used as a unique “field dispersion laboratory” where the CO2 amount required to maintain a concentration 150ppmv above ambient over several patches can be used to infer the mixing capability of the woodland.

Once evaluated, the model will be run for CO2 exchange scenarios (respectively for SHWs and LHF) by imposing various CO2 concentrations above the canopy, thus enabling an assessment of the CO2 exchange for SHWs and LHF. We seek functions to describe the difference between SHWs and LHF based on geometric and/or meteorologial parameters, and then assess the scaled-up estimates using land-cover maps and the new parameterisation for SHWs.

Training and Skills

CENTA students are required to complete 45 days training throughout their PhD including a 10 day placement. In the first year, students will be trained as a single cohort on environmental science, research methods and core skills. Throughout the PhD, training will progress from core skills sets to master classes specific to the student's projects and themes. 

Student will take a training course scheduled in 2018 for the WRF (Weather Research & Forecasting) model and its LES configuration. Training for in-house codes (inlet LES condition & postprocessing) will be conducted by a research fellow, Dr Zhong.  Computing related training courses will be provided by IT Services of University of Birmingham, e.g. UNIX and parallel computation. Student will also receive training on meteorological modelling by supervisors and research fellows of the modelling group. Furthermore, visits to the BIFoR site will also be arranged.

Timeline

Year 1:

  • Conduct literature review
  • Post-graduate subject training according to needs (eg carbon cycle, forest structure and function);
  • CENTA generic skills training
  • Specialist modelling training (WRF-LES & other codes)
  • Define and set-up model configurations;
  • Design the strategy of model evaluation;
  • Conduct initial analysis of BIFoR FACE field data.

Year 2:

  • Publish literature review
  • Publish initial data analysis with default model runs
  • Advanced CENTA training
  • Complete model evaluation
  • Design strategy of scenario simulations
  • Conduct simulations
  • Analyse model output

Year 3+:

  • Continue simulations and analysis
  • Write further journal paper(s)
  • Compile thesis

Partners and collaboration (including CASE)

CASE: Currently none. We will seek CASE partners from the BIFoR network if the proposal goes forward to advert. Potential CASE partners include The Arboricultural Association (contact: Stewart Wardrop, the CEO) and Commonwealth Forestry Association (BIFoR’s partner).

Other Parners: Professor Natascha Kljun, Geography, Swansea University, who has the expertise in footprint modelling, vegetation-atmosphere carbon exchange and upscaling of greenhouse gas flux measurements, is likely a collaborator of the project.

Further Details

Any further details of the project can be obtained from:

Dr Xiaoming Cai, School of Geography, Earth and Env. Sci., University of Birmingham, x.cai@bham.ac.uk

Prof Rob MacKenzie, School of Geography, Earth and Env. Sci., University of Birmingham, a.r.mackenzie@bham.ac.uk

http://www.birmingham.ac.uk/research/activity/bifor