Abstract: Weather is of paramount importance among environmental elements for both natural and managed ecosystems. System analysis makes it increasingly imperative to simulate weather in research projects related to ecosystems. In this paper, a summary firstly introduces mathematical principles of an American weather generator (WGEN), which is constructed with Markov chain——gamma distribution as a model to produce daily precipitation data and weakly stationary process——harmonic analysis as tools to build another model generating daily maximum temperature, minimum temperature and solar radiation for a given location. Then, WGEN derives ALWGEN and BJWGEN for Alabama and Beijing, respectively, after the model parameters are estimated by procedures of conventional statistics or the grey target bleaching. Taking the paper size into account, however, the concrete procedures are not demonstrated here; and also, the model verification and validation by Monte Carlo bootstraping are mentioned only without detailed description.Finally, a brief discussion on applications of the weather simulators is made. Risk analysis in IPM programs, decision making in ecosystems management, ecologically planning of agricultural areas, are all need weather simulators really for the present or potentially for the future.

Key words: Ecosystem, Weather simulation, Markov chain, Gamma distribution.Weakly stationary process, Harmonic analysis