RAINFALL PATTERNS OVER INDIA - CLASSIFICATION WITH FUZZY C-MEANS METHOD

Seasonal (June through September) percentage departure from normal rai nfall patterns over India for the period 1871-1994 have been classifie d using Fuzzy c-means method (FCM) to identify the dominant modes of s patiotemporal variability in the Indian monsoon rainfall. Unlike the h ard clustering methods, for example the Map-to-Map (MM) correlation me thod and the k-means (KM) clustering method, this method does not forc e a pattern to get classified into only one cluster but assigns varyin g membership to every cluster. Thus marginal patterns get classified i nto all clusters with different memberships. Patterns for the 124-year period are represented by the four dominant clusters. The spatial pat terns associated with the extreme (deficient/excess) Indian monsoon ra infall (IMR) get high membership in one of the clusters only, while th e patterns associated with the normal LMR get almost equal membership to all clusters. Even the spatial patterns during the El Nino/La Nina episodes show high preference to a particular cluster. Time variation of each cluster shows that there are epochs where a particular cluster dominates. Possible dynamic causes leading to the clusters are examin ed. Merits and demerits of the FCM method are also discussed.