Preface

Antje Weisheimer^1,2 Florian Pappenberger¹ and Ian Shipsey²

¹ ECMWF
² University of Oxford

In 1963, Ed Lorenz showed that due to the chaotic nature of the atmosphere, even small initial errors would eventually prevent the weather being predicted in detail. However, the timescale for such unpredictable effects to occur would vary from one initial state to another, and sometimes small initial errors could affect the forecasts. Such flow-dependent error growth and predictability can be estimated from the spread of forecasts made from slightly different initial conditions. This led to the development of the operational ensemble prediction systems at ECMWF, starting in the second half of the 1980s, and ensembles are now at the heart of ECMWF’s global forecasts at all time ranges from the ensemble of data analysis to the medium-, extended and longest seasonal forecast ranges. Ensemble-based probabilistic forecasts have become immensely successful for both the scientific developments of weather, climate, and environmental predictions as well as for a wide spectrum of sectorial applications.

To celebrate the 30th anniversary of this significant milestone for numerical weather predictions, and to pay tribute to Tim Palmer’s manifold contributions to science, ECMWF and Oxford University hosted a symposium on the 5th and 6th of December 2022. The days of talks and reflections also celebrated Tim's career as one of ECMWF’s most influential protagonists. He was instrumental in the development of ensemble techniques and their widespread adoption by meteorologists around the world. Tim Palmer’s professional life has been profoundly linked to ECMWF’s ensemble prediction system leading the probabilistic forecasting and diagnostics activities at the centre. He was at the forefront of the development of the scientific basis for probabilistic forecasting and the implementation of the first operational ensemble predictions in the early 1990s. In 2011, Tim left ECMWF to take up a Royal Society Research Professorship at the University of Oxford’s Physics Department. He continues to work very closely with ECMWF, in the development of the use of low numerical precision and the use of AI in weather forecast models (amongst many other activities).

He also authored many highly cited scientific articles on the subject, co-edited the books Predictability of Weather and Climate and Stochastic Physics and Climate Modelling, as well as his own recently published book The Primacy of Doubt: From climate change to quantum physics, how the science of uncertainty can help predict and understand our chaotic world, in which he links climate change to quantum physics as well as economic modelling and conflict prediction with the aim to bring uncertainty in a thought-provoking way to scientists and the wider public. In addition to his work on ensemble forecasting for weather prediction, Tim has also made significant contributions to the field of climate science contributing to the IPCC as a lead author. He has written extensively on the subject and has been a major influence in the development of climate models.

The presentations of the first day of the symposium reviewed the history that led to the development of operational ensemble forecasts, the initial perturbation and model uncertainty strategies as key characteristics of an ensemble configuration, the use of ensembles across time scales from weather to climate, and their value for humanitarian disaster risk management and commercial applications in the world of energy markets. While we celebrate Tim Palmer’s career, ECMWF’s upgrade of the operational modelling cycle to a unified high-resolution 9-km global ensemble as the centre piece of our medium-range prediction is actively being tested to be implemented in 2023, a truly marvellous prospect for the future of ensemble forecasting.

We are proud to present this Festschrift in honour of Tim Palmer and his remarkable career. His contributions to meteorology and climate science have been immense and have had a profound impact on the way that scientists understand and predict the weather and climate.

A second day of talks marking Tim’s wider career was held the following day at the Physics Department of the University of Oxford. We are a very wide-ranging department, studying the universe from the smallest scales to the largest scales, and everything in between. From the birth of time to the present, we make predictions about the future, from how our universe will evolve, to how our climate is changing. We are helping to usher in quantum revolution 2.0, developing some of the world’s most efficient solar cells and bringing them to market next year, and advancing the national programme in nuclear fusion. Whether we are astrophysicists, or biophysicists, or study quantum materials or quantum computing, or climate, we are united by being physicists.

I was impressed when I joined Oxford eight years ago, that the department of physics had a sub-department of atmospheric, oceanographic and planetary physics, because it is something of a rarity. But it should not be so. It is right and fitting because climate and climate change are physics. The award of the 2021 Nobel prize in physics, for the physical warming of earth’s climate, quantifying variability, and reliably predicting global warming, highlights that climate is physics. That it is so is profound because it renders the question “do you believe in global warming?” meaningless. Whether the globe warms in response to greenhouse gases is determined by the physics of energy balance; it is not subject to a belief system.

Tim is a great advocate that new ideas do not somehow emerge out of the aether. But rather they arise when someone takes an idea from one field and applies it to another. And so people should be encouraged to move between different areas of physics and bring ideas with them - Tim has written eloquently and with energy and passion on this subject. I think he can write that way because that’s how he’s lived. He was awarded a prize at the Royal Society for work parameterizing the effect of breaking gravity waves which are produced by flow over mountains in weather forecast models. The parametrisation improved the accuracy of jet stream forecasts and saved the airlines a great deal of money because they didn’t need to carry so much spare fuel; that prize (ESSO Energy Award in 1986) was for improvements in energy efficiency. During the presentation given by Tim on the prize, someone in the audience asked Tim about his PhD work. Tim’s thesis at Oxford was entitled “Covariant conservation equations and their relation to the energy momentum concept in general relativity” and his doctoral advisors included the eminent cosmologist Dennis Sciama. This person observed that and said that Tim had made a truly big switch in fields. But Tim replied “Not really. I used to work on gravitational waves, and now I work on gravity waves”.

I remember having dinner with Tim a day or two after LIGO announced the observation of gravitational waves at Jesus College, and we discussed the immense significance of that amazing technological feat. We then had a really interesting and wide-ranging discussion on general relativity and quantum mechanics. Discussing physics with Tim is a delight, but it is also an education. I am very proud indeed that our department is home to Tim, and all my other colleagues in Atmospheric, Oceanographic and Planetary Physics. Tim is a scientific titan, who has made truly immense and sustained contributions to climate physics.

Thank you very much Tim!