A fuzzy set, intuitively, is a set with uncertain boundaries and it can be mathematically represented through a function whose values range between 0 and 1 , interpreted as the membership grade of an element to the fuzzy set. A fuzzy set can be used as a model for linguistic variables such as for example low, medium or high similarity of the model output to observation. A fuzzy rule-based system is able to incorporate expert knowledge into predictions, by using the compositional rule of inference, that is able to evaluate fuzzy rules. An example of a very simple rule based system is as follows:
If model to observation similarity is High (resp. Medium, Low) then the model's weight in the aggregation is High (resp. Medium, Low)
In order to make the system meaningful, we will use model to observation fuzzy similarity measures based on climate models.