Stroboscopic analysis of quasi-periodic anomalies in the pluviometric series of Rome

Abstract

The hystorical series of meteorological data were often studied in order to detect climatological cycles, usually with some kind of harmonic analysis. Several cycles were detected, but the results were not satisfying, because the component waves show often abrupt variations in amplitude and phase. More recently, a different approach, was introduced considering the series of data as a stochastic sequence, in which quasi-periodic positive or negative anomalies are embedded. This paper deals with the hystorical series of yearly rainfall values at Collegio Romano from 1782 to 1981. First the randomness of the series was tested by comparing the Hurst exponents of the series and of an equally long series of random numbers. Then the series was analysed by a stroboscopic method. The «speeds» of the strobing window that minimize the second moments of the distribution yield an indication of anomalies periods. Being the time step of the original series 1 year, best results are obtained with a width of the strobing window of 3 years. In order to avoid truncation errors, the maximum period is 1/4 of the duration of the series, namely 50 y. Among the periods detected are the cycles of Wagner (15 y), luni-solar Toaldian (18 y), the double solar cycle of Hale (22 y), the cycle of Brukner (30 y) and a cycle of about 44 y detected by Vercelli. All those periods correspond to those calculated by Njau from orbital considerations.