Identification of flood producing atmospheric circulation patterns

Floods in mesoscale catchments (with a size of a few hundreds to a few thousand square kilometers) are often related to typical large scale weather phenomena which affect a large area in continental scale. The purpose of this paper is to develop a methodology for the identification of flood producing daily atmospheric circulation patterns (CPs) from large-scale pressure maps. The CPs are described by fuzzy rules which indicate the locations of the pressure centers-lows and highs. The flood producing circulation patterns are identified using the positive increments of the observed discharge series. The rules are assessed using a discrete optimization procedure based on simulated annealing. The goal of the optimization is to find CPs corresponding to high discharge increments. The methodology is applied in two catchments: the Ardèche in France and the Llobregat in Spain. In both cases typical flood producing CPs could be identified. The occurrence of these CPs does not necessarily lead to floods, but most important floods were related to them.