Mesoscale heat waves induced by orography

This work is devoted to the analysis of an unusual and sudden thermal fluctuation that interestedportions of Friuli Venezia Giulia (Italy) during the night of 27 July 1983. The whole 1983 summer was ex-tremely warm in Europe and in particular on the Italian peninsula, from the Alps down to Sicily. Nevertheless,the day of 27 July 1983 in Friuli Venezia Giulia deserves special attention because the observed maximum tem-peratures did not occur during day-time but during night-time (from 23:00 up to 24:00 LT, 21:00–22:00 UTC).Peaks of 34.8◦C and values of relative humidity of the order of 28% were registered by the official network ofweather stations. This event interested mainly the central-eastern part of the plain of Friuli Venezia Giulia, afew kilometers far from the Slovenian border and relieves. The thermal anomalies lasted up to an hour, thentemperatures decreased toward values more usual for the climate of the month. The study of this event is car-ried out with the aid of the AR-WRF numerical atmospheric model, initialized through the ECMWF analysis.The numerical simulations highlight the important role played by orography, jointly with the peculiar thermalstructure of the atmosphere, for the enhancing of the internal wave pattern over that area. According to the sen-sitivity studies realized, the amplification of the internal wave pattern might represent a possible explanationfor that meteorological enigma.