Abstract. The Special Observation Period (SOP1

During the first HyMeX Special Observation Period (SOP1) field campaign, the target site of north‐eastern Italy (NEI) experienced a large amount of precipitation, locally exceeding the climatological values and distributed among several heavy‐rainfall episodes. In particular, two events that occurred during the last period of the campaign drew our attention. These events had common large‐scale patterns and a similar mesoscale setting, characterised by southerly low‐level flow interacting with the Alpine orography, but the precipitation distribution was very different. During Intensive Observing Period IOP18 (31 October–1 November 2012), convective systems were responsible for intense rainfall mainly located over a flat area of the eastern Po Valley, well upstream of the orography. Conversely, during IOP19 (4/5 November 2012), heavy precipitation affected only the Alpine area. In addition to IOP18 and IOP19, the present study analyses other heavy‐precipitation episodes that display similar characteristics and which occurred over NEI during the autumn of recent years. A high‐resolution (2 km grid spacing) non‐hydrostatic NWP model and available observations are used for this purpose. The two different observed precipitation patterns are explained in terms of interaction between the impinging flow and the Alps. Depending on the thermodynamic profile, convection can be triggered when the impinging flow is forced to rise over a pre‐existing cold‐air layer at the base of the orography. In this situation a persistent blocked‐flow condition and upstream convergence are responsible for heavy rain localized over the plain. Conversely, if convection does not develop, flow‐over conditions are established and heavy rain affects the Alps. Numerical parameters proposed in the literature are used to support the analysis. Finally, the role of evaporative cooling beneath the convective systems is evaluated. It turns out that the stationarity of the systems upstream of the Alps is mainly attributable to persistent blocked‐flow conditions, while convective outflow slightly modifies the location of precipitation.

[1] From late January to mid-February 2012 the Gulf of Trieste (North Adriatic Sea) was affected by a severe winter weather event characterized by cold air and strong northeasterly wind (Bora). The atmospheric forcing caused large surface heat fluxes which produced remarkable effects on the gulf, particularly the production of a very cold and dense water mass. Temperatures as low as 4 C were observed in the deepest part of the gulf, similar to that which was observed in winter 1929, which was probably the most severe winter in the region over more than a century. The density anomaly attained values up to 30.58 kg m À3 , even greater than in 1929. Surface heat fluxes were estimated using bulk formulas and the meteorological and marine observations available at three stations. Mean daily heat losses exceeded 1000 W m À2 . A comparison of this event with similar past events was made using proxy heat fluxes, available since 1978, to account for the air-sea interactions and using temperature and salinity observations, performed since 1996, to account for the effect of heat fluxes on ocean properties. The 2012 Bora episode turned out to be the most severe event of this kind in the Gulf of Trieste for at least the last 35 years and is comparable to that which occurred in 1929. A significant linear correlation was also found between the total surface heat loss and the density increase of the waters in the part of the gulf deeper than 20 m.Citation: Raicich, F

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