Searching for resemblance between large-scale sea level pressure patterns leading to “intense” precipitation events over Italy.

How strong is the resemblance between two states of the atmosphere leading to the same outcome? In the following pages this question was addressed by studying the resemblance of the surface pressure fields, obtained by NCEP reanalyses, corresponding to ‘‘intense’’ precipitation events over 5 Italian climatic areas. Such ‘‘intense’’ precipitation events were selected by means of 45 daily precipitation records covering the period from 1951 to 2000. Two pressure patterns that resemble one another are termed soft-analogues in this paper. The mathematical measures used to assess the difference between two pressure patterns were: the root-mean-square difference, the correlation, and the S1 score. The comparison outcomes were categorized into classes according to the previous measures. Once a couple of the pressure patterns were found to be good soft-analogues (that was a rare occurrence), these pressure patterns were compared with all pressure patterns excluded by the initial selection being not associated with intense precipitation events. The few soft-analogues associated with ‘‘intense’’ precipitation events were found to be also softanalogue to several, and sometimes a great deal of, pressure patterns not associated with the selected ‘‘intense’’ precipitation events.