Climak: a Stochastic Model For Weather Data Generation

BACKGROUND. Many human activities and ecological processes are affected by climatic conditions. Despite the difficulties in long term meteorological forecasting, a statistical description of climate is possible and used for planning purposes and strategic decisions. With this aim, stochastic models for the generation of daily weather data have been developed (Climak). The generated meteorological data can be used: a) to perform Monte Carlo simulations with deterministic models (e.g., crop growth and ecological models, models for climate risk assessment); b) to better classify the climates; c) to assess environmental scenarios for the effects of climatic changes by “what if” procedures; d) to spatially interpolate the climate parameters, so obtaining data for location not covered by meteorological stations. These data can be used as input for agro-ecological models, mainly when a probabilistic evaluation of climatic uncertainty and risk is of interest. Climak has been developed to directly take into account the among-years variability. METHODS. Climak generates, as first, the occurrence of rain and the rainfall amount for rainy days. After rainfall generation, minimum and maximum air temperatures are generated, with different parameters for rainy and dry days. Solar radiation is obtained from the astronomical photoperiod and from the daily thermal excursion. Evapotranspiration (reference or potential) is generated from solar radiation, if available; if not, it is obtained from photoperiod and daily maximum temperature. RESULTS. The model was evaluated using meteorological data coming from different locations of northern Italy and comparing its capability to reproduce the climatic properties with that of another weather generator (Wxgen). The behaviour of Climak was quite satisfactorily even if some minor problems have been highlighted. The minimum number of years of data to correctly estimate the climatic parameters was also determined. CONCLUSIONS. Climak was shown to be satisfactorily accurate in generating meteorological data representing the climate of a site, even when compared with a well known weather generator. At present the model still has some limitations and it has not been tested in climates other then the temperate ones. These drawbacks will be improved in the next versions of Climak.