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PhD Projects

High-dimensional probabilistic dynamic models for global vegetation change and weather forecasting

Primary Supervisor: Victor Elvira (School of Mathematics, University of Edinburgh, victor.elvira@ed.ac.uk)

Co-Supervisors: Dr Encarni Medina-Lopez (School of Informatics, University of Edinburgh), Professor John Marsham (School of Earth and Environment, University of Leeds, and Met Office)

CASE Partner: Space Intelligence Ltd

 

A map of Leaf Area Index (LAI) produced by remote sensing. Leaf area varies greatly with seasons and between years, and differentiating long-term trends from short-term fluctuations and sensor calibration issues is a difficult task

The Earth’s vegetation is changing as a result of both human activity and climate change. Large scale shifts in vegetation will fundamentally alter terrestrial ecosystems, with a range of potential consequences – from impacts on biodiversity to altered carbon and hydrological cycling. In northern high latitudes plants are growing more as the climate warms, resulting in a “greening” of the land surface. Within the next 50 years the tundra biome is expected to become climatically suitable for trees, the boreal treeline is already shifting northwards and woody shrub abundance in tundra is increasing. These changes will have a profound impact on ecosystem function and climate feedbacks; while CO2 uptake from the atmosphere through photosynthesis is likely to increase, taller denser plant canopies will decrease the reflectivity of the land surface, resulting in greater warming. To understand the implications of changing vegetation distributions, it is vital we can model important biophysical parameters from space over time. Moreover, interactions between land and atmosphere also affect the weather. Thus, understanding and modeling these interactions through statistical models is of utmost importance for weather forecasting.

These spatio-temporal problems can be described through statistical models that relate the sequential observed data to a dynamical hidden process through some unobserved static model parameters. These parameters are often interpretable and are essential for prediction purposes, also providing domain experts with further understanding. In the Bayesian framework, the probabilistic estimation of the unknowns is represented by the posterior distribution of these parameters. Learning those distributions is crucial not only for prediction/forecasting purposes but also for uncertainty quantification. Unfortunately, in most realistic models for earth observation problems, the posteriors of both static and dynamic parameters are intractable and must be approximated. Importance Sampling (IS)-based algorithms are Monte Carlo methods that have shown a satisfactory performance in many problems of Bayesian inference, both for inferring static parameters and the hidden states [1]. Particle filters (also called sequential Monte Carlo methods) are the de facto IS-based computational tools in this context. See [2] and [3] for two tutorials in adaptive IS and particle filtering, respectively.

In this project, we focus in developing inferential tools for probabilistic spatio-temporal models with applications in earth observation problems. We consider the challenging problem of estimating biophysical parameters and atmospheric entrainment parameters from remote sensing (satellite) observations acquired across time. Just as an example, let us focus in the aforementioned problem where the estimation of the evolving Leaf Area Index (LAI) is key for forecasting the change of Earth’s vegetation. It is important to track evolution of LAI through time in every spatial position on Earth because LAI plays an important role in vegetation processes such as photosynthesis and transpiration, and is connected to meteorological/climate and ecological land processes [4, 5]. We will propose novel computational methods in order to overcome current limitations of more traditional IS-based techniques in such a challenging context, including adaptive IS methods for learning static parameters in high dimensional spaces [6] and extensions of [7] to observational spaces with big amount of data. Many applications in earth observation can be benefited from the development of these methodologies. See [5] and [8] for the application of recent IS methodological advances in remote sensing problems.

 

References

[1] C. P. Robert and G. Casella, Monte Carlo Statistical Methods. Springer, 2004.
[2] M. F. Bugallo, V. Elvira, L. Martino, D. Luengo, J. Miguez, and P. M. Djuric, “Adaptive importance sampling: the past, the present, and the future,” IEEE Signal Processing Magazine, vol. 34, no. 4, pp. 60 79, 2017.
[3] A. Doucet and A. M. Johansen, “A tutorial on particle filtering and smoothing: Fifteen years later,” Handbook of nonlinear filtering, vol. 12, no. 656-704, p. 3, 2009.
[4] J. M. Chen and T. A. Black, “Defining leaf area index for non-flat leaves,” Plant, Cell & Environment, vol. 15, no. 4, pp. 421–429, 1992.
[5] L. Martino, V. Elvira, and G. Camps-Valls, “Group importance sampling for particle filtering and mcmc,” Digital Signal Processing, vol. 82, pp. 133–151, 2018.
[6] L. Martino, V. Elvira, D. Luengo, and J. Corander, “Layered adaptive importance sampling,” Stat. Comput., vol. 27,
no. 3, pp. 599–623, May 2017.
[7] V. Elvira, J. Miguez, and P. M. Djuric, “Adapting the Number of Particles in Sequential Monte Carlo Methods Through an Online Scheme for Convergence Assessment,” IEEE Trans. Sig. Proc., vol. 65, no. 7, pp. 1781–1794, 2017.
[8] L. Martino, V. Elvira, and G. Camps-Valls, “The recycling gibbs sampler for efficient learning,” Digital Signal Processing, vol. 74, pp. 1–13, 2018.