Understanding rainfall extreme values in Tuscany (Italy)

The study area is the Tuscany Region, in Central Italy. The dataset is composed by the time series of annual rainfall maxima of different durations (i.e. 1 day, 1, 3, 6, 12, and 24 hours) of about 700 recording rain gauges, spatially distributed over an area of about 23.000 km2. The record period covers mainly the second half of 20th century. We use here a hierarchical modeling approach to investigate a collection of spatially referenced time series of rainfall extreme values. We assume that the observations follow a generalized extreme value (GEV) distribution whose locations are spatially and temporally dependent where the dependence is captured using a geoadditive model. Geoadditive models analyze the spatial distribution of the studied variable while accounting for the explicit consideration of linear and nonlinear relations with relevant explanatory variables, as well as the spatial correlation described by a standard spatial autocorrelation function. Under the additivity assumption they can handle the covariate effects by combining the ideas of additive models and kriging, both represented as linear mixed model. This approach, based on the generalized mixed model/splines paradigm, has achieved a valuable success during the last decade as useful tool with which to study the spatial distribution of climate variables as well as in other contexts. The preliminary results of the analysis are described and discussed here.