APPLICATION OF WAVELET TRANSFORM TO METEOSAT-DERIVED COLD CLOUD INDEXDATA OVER SOUTH-AMERICA

Cold cloud index (CCI) data derived from Meteosat infrared imagery are used to detect periodicities in convective activity in South America. The generally used Fourier transform (FT) cannot provide time-localiz ed information but gives information on the average periodicity of osc illations over the entire time domain. As many events in the atmospher e are intermittent, wavelet transform (WT) is used to identify periodi c events in CCI data. First, the Morlet WT is applied to different com binations of time series data of known periodicities to demonstrate th e advantage of WT over FT. Later it is applied to CCI data over four 9 degrees square areas between the latitudes 4.5 degrees N and 31.5 deg rees S, and longitudes 54 degrees-45 degrees W. Near the equator perio dic convective activities are observed to be more prominent in the bor eal summer than in the austral summer. Between the latitudes 4.5 degre es and 22.5 degrees S, 1-: 2-3-, approximately 5-, and 8-10-day oscill ations are seen in the austral summer and seldom is any convective act ivity seen in the winter. In January semidiurnal variation of cloudine ss is also observed for a few days. Farther south in the extratropics, approximately 10- and approximately 20-day periodic events, which ref er to the baroclinic waves, are seen more prominently in the austral a utumn and winter, and 1- and approximately 5-day oscillations are seen in the summer, perhaps due to convective cloudiness.