ON THE DISAGGREGATION OF CLIMATOLOGICAL MEANS AND ANOMALIES

The tool C2W (climate-to-weather disaggregator) is introduced which is aimed at disaggregating climatological means and anomalies into reali stic weather processes. For the core variables of minimum and maximum temperature and precipitation, a probit normalization is conducted whi ch transforms each quantity into one which is normally distributed wit h mean 0 and standard deviation 1. In this way, the total spatial and seasonal spectrum of climatological variability is filtered out, leavi ng a 3-dimensional process of 'normalized weather'. The climatology is contained in the set of parameters which define the probit function. In a second step, a first order autoregressive model is fitted to the normalized weather process which is then, by construction, globally ap plicable for any time of the year. The determination of probit paramet ers is achieved, roughly, by a parameterization of climate variability in terms of climate mean. By way of Monte-Carlo simulations, a univer sal map can be defined which transmits, in a 1-1 way, information betw een the climate mean and the probit function in such a way that the me ans of the simulated weather converge statistically, if simulated long enough, to the given climate mean. The simulated variability, however , is generally not preserved. Depending on the specific region and tim e of the year, C2W exhibits deviations from observed variability, with errors increasing in extreme climates; for most temperate climates, t he simulated variability is comparable to the observed. The disaggrega tion of climatological means is then extended to include Various aggre gations, such as monthly or seasonal means, by interpreting them as an omalies from the mean. Besides being a simple and handy weather genera tor, C2W is best applied as a postprocessing scheme for gridded data s ets, such as those from General Circulation Models (GCMs) or gridded c limate maps. In that way, C2W works as a simple link between GCMs and, for example, dynamic global vegetation models, C2W is available as a Fortran program module.