THE PROBABILITY DENSITY AND AUTOCORRELATION OF SHORT-TERM GLOBAL AND BEAM IRRADIANCE

For individual hours, a characteristic bimodal pattern of short-term global and beam irradiance is frequently observed, with modes at high and low irradiances and with low probabilities near the hourly averages. For such hours, averaging over the hour will imply smoothing of quite significant variations within the hour. Models for the probability density distributions of short-term (5 min or less) irradiances are presented in this paper. These distributions are not unique functions of the hourly averages, but depend heavily also on the irradiance variability within the hour. This intrahour variability is found to depend on the averaging time and also on the interhour variability among three hourly averages. namely, the hour in question, the preceding and the deceding hour. The distribution differences between 5 min averages and instantaneous values are, however, negligible for most practical purposes. The lag one autocorrelation is evaluated as a function of averaging time. and a first order autoregressive model is presented. With hourly averages as the only input the probability density and autoregressive model in combination produce time series of short-term intrahour averages having realistic distributions and autocorrelation structure.