Posts by old_user702252

1) Message boards : climateprediction.net Science : Regional attribution models - ensemble type? (Message 49022) Posted 2 May 2014 by old_user702252 Post: Dear all, I saw your discussion and as a project scientist, I think I can provide a reply. The UK floods and ANZ heatwave projects are initial condition ensembles (we slightly perturb the sea surface temperatures in each simulation, i.e. each of them is different). There are other projects where we perturb parameters, but not in these ones. Deterministic projections usually refer to only one projection, which is not what we do here, as we have thousands of simulations of almost the same, so it is probabilistic. I think how to perturb the parameters is relatively easy to understand. Every process that occurs on a scale smaller than the grid (50km x 50km for the European region at the moment) has to be parameterised because it cannot be calculate but maybe you already know this. Of course, we do not really know the exact value of these parameters, for example how fast snow falls. Snow fall speed depends on the shape, size and weight of each snow flake, what the air temperature is, how the wind blows, etc, and therefore each snow flake falls at a different speed, but we cannot go on that level of detail, so experts on snow fall velocity give their best guess of the average velocity of snow fall. Perturbing the parameters then "simply" involves changing slightly this best guess value to see how much it actually influence the climate. Often 2-3 alternative values are selected for each of the parameters (maybe a dozen of them, so this makes a big ensemble in the end, especially if initial conditions perturbation is performed on top of that). Perturbing some parameters hardly has any effect on the climate system, perturbing others however may have a big impact, and therefore, a lot of effort is made better know what the exact value of these parameter are (for example through lab experiments of snow falling in a known environment). I hope this made sense!
2) Message boards : climateprediction.net Science : Do climate models use bayesian probability? (Message 49021) Posted 2 May 2014 by old_user702252 Post: Hello! I am a project scientist and just saw that your question had no answer... The climate models you are running are based on physical principles, or in other words, on equations (for example the conservation of mass and momentum). These equations basically calculate the values of temperature, precipitation, wind, etc, for the next time step, and then for the next one, and so on. There is no bayesian or frequentist approaches at this stage. We are running physically-based climate models, which are not the same as statistical models used to project future global mean temperature for example. Both are quite different techniques, and used to answer different types of questions. Of course, with the outputs that you calculate, we perform statistical analyses to evaluate them. Depending again on the question we want to answer, we use frequentist or bayesian approaches. But often we simply start by calculating the mean and standard deviation of all ensemble members!