Offtopic : A Simple Stochastic Climate Model: Introduction | ClimateSight
Andrew Lockley
http://climatesight.org/2013/06/18/a-simple-stochastic-climate-model-introduction/
This winter I took a course in computational physics, which has probably been my favourite undergraduate course to date. Essentially it was an advanced numerical methods course, but from a very practical point of view. We got a lot of practice using numerical techniques to solve realistic problems, rather than just analysing error estimates and proving conditions of convergence. As a math student I found this refreshing, and incredibly useful for my research career.
We all had to complete a term project of our choice, and I decided to build a small climate model. I was particularly interested in the stochastic techniques taught in the course, and given that modern GCMs and EMICs are almost entirely deterministic, it was possible that I could contribute something original to the field.
The basic premise of my model is this: All anthropogenic forcings are deterministic, and chosen by the user. Everything else is determined stochastically: parameters such as climate sensitivity are sampled from probability distributions, whereas natural forcings are randomly generated but follow the same general pattern that exists in observations. The idea is to run this model with the same anthropogenic input hundreds of times and build up a probability distribution of future temperature trajectories. The spread in possible scenarios is entirely due to uncertainty in the natural processes involved.
This approach mimics the real world, because the only part of the climate system we have full control over is our own actions. Other influences on climate are out of our control, sometimes poorly understood, and often unpredictable. It is just begging to be modelled as a stochastic system. (Not that it is actually stochastic, of course; in fact, I understand that nothing is truly stochastic, even random number generators - unless you can find a counterexample using quantum mechanics? But that's a discussion for another time.)
A word of caution: I built this model in about eight weeks. As such, it is highly simplified and leaves out a lot of processes. You should never ever use it for real climate projections. This project is purely an exercise in numerical methods, and an exploration of the possible role of stochastic techniques in climate modelling.
Over the coming weeks, I will write a series of posts that explains each component of my simple stochastic climate model in detail. I will show the results from some sample simulations, and discuss how one might apply these stochastic techniques to existing GCMs. I also plan to make the code available to anyone who's interested -it's written in Matlab, although I might translate it to a free language like Python, partly because I need an excuse to finally learn Python.
I am very excited to finally share this project with you all! Check back soon for the next installment.
Alan Robock
This is nothing new. I did a similar thing for my Ph.D., and published it in 1978:
Robock, Alan, 1978: Internally and externally caused climate change. J. Atmos. Sci., 35, 1111-1122. http://climate.envsci.rutgers.edu/pdf/RobockInternalExternalJAS1978.pdf
The Sellers energy balance model I used had a too high snow albedo feedback, something I addressed in subsequent work. So I overestimated the importance of stochastic forcing (natural variability) in this paper.
A major work on this topic published just before mine was:
Hasselmann, К., 1976: Stochastic climate models, Part L. Theory. Tellus, 28, 473-485.
This is an important topic and modeling technique. I recommend that you review the literature on what has been done on this in the past.
Alan Alan Robock, Distinguished Professor Editor, Reviews of Geophysics Director, Meteorology Undergraduate Program Associate Director, Center for Environmental Prediction Department of Environmental Sciences Phone: +1-848-932-5751 Rutgers University Fax: +1-732-932-8644 14 College Farm Road E-mail: rob...@envsci.rutgers.edu New Brunswick, NJ 08901-8551 USA http://envsci.rutgers.edu/~robock http://twitter.com/AlanRobock
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