Contrasting stable water isotope signals from convective and large-scale precipitation phases of a heavy precipitation event in southern Italy during HyMeX IOP 13: a modelling perspective

Lee, Keun-Ok; Aemisegger, Franziska; Pfahl, Stephan; Flamant, Cyrille; Lacour, Jean-Lionel; Chaboureau, Jean-Pierre

The dynamical context and moisture transport pathways embedded in large-scale flow and associated with a heavy precipitation event (HPE) in southern Italy (SI) are investigated with the help of stable water isotopes (SWIs) based on a purely numerical framework. The event occurred during the Intensive Observation Period (IOP) 13 of the field campaign of the Hydrological Cycle in the Mediterranean Experiment (HyMeX) on 15 and 16 October 2012, and SI experienced intense rainfall of 62.4 mm over 27 h with two precipitation phases during this event. The first one (P1) was induced by convective precipitation ahead of a cold front, while the second one (P2) was mainly associated with precipitation induced by large-scale uplift. The moisture transport and processes responsible for the HPE are analysed using a simulation with the isotope-enabled regional numerical model COSMOinline-formulaiso. The simulation at a horizontal grid spacing of about 7 km over a large domain (about 4300 km inline-formula×3500 km) allows the isotopes signal to be distinguished due to local processes or large-scale advection. Backward trajectory analyses based on this simulation show that the air parcels arriving in SI during P1 originate from the North Atlantic and descend within an upper-level trough over the north-western Mediterranean. The descending air parcels reach elevations below 1 km over the sea and bring dry and isotopically depleted air (median inline-formulaδ18Oinline-formula M4inlinescrollmathml - normal 25 32pt11ptsvg-formulamathimgdb1890df7e308eaa6b2e59d6a2629782 acp-19-7487-2019-ie00001.svg32pt11ptacp-19-7487-2019-ie00001.png  ‰, water vapour mixing ratio inline-formulaq≤2 g kginline-formula−1) close to the surface, which induces strong surface evaporation. These air parcels are rapidly enriched in SWIs (inline-formulaδ18Oinline-formula M8inlinescrollmathml - normal 14 32pt11ptsvg-formulamathimg4b6d96d3ccc3863313b0e41123beb273 acp-19-7487-2019-ie00002.svg32pt11ptacp-19-7487-2019-ie00002.png  ‰) and moistened (inline-formulaq≥8 g kginline-formula−1) over the Tyrrhenian Sea by taking up moisture from surface evaporation and potentially from evaporation of frontal precipitation. Thereafter, the SWI-enriched low-level air masses arriving upstream of SI are convectively pumped to higher altitudes, and the SWI-depleted moisture from higher levels is transported towards the surface within the downdrafts ahead of the cold front over SI, producing a large amount of convective precipitation in SI. Most of the moisture processes (i.e. evaporation, convective mixing) related to the HPE take place during the 18 h before P1 over SI. A period of 4 h later, during the second precipitation phase P2, the air parcels arriving over SI mainly originate from north Africa. The strong cyclonic flow around the eastward-moving upper-level trough induces the advection of a SWI-enriched African moisture plume towards SI and leads to large-scale uplift of the warm air mass along the cold front. This lifts moist and SWI-enriched air (median inline-formulaδ18Oinline-formula M12inlinescrollmathml - normal 16 32pt11ptsvg-formulamathimgeaadcf7f321f4f510b8414cd66eb757d acp-19-7487-2019-ie00003.svg32pt11ptacp-19-7487-2019-ie00003.png  ‰, median inline-formulaq≥6 g kginline-formula−1) and leads to gradual rain out of the air parcels over Italy. Large-scale ascent in the warm sector ahead of the cold front takes place during the 72 h preceding P2 in SI. This work demonstrates how stable water isotopes can yield additional insights into the variety of thermodynamic mechanisms occurring at the mesoscale and synoptic scale during the formation of a HPE.

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Lee, Keun-Ok / Aemisegger, Franziska / Pfahl, Stephan / et al: Contrasting stable water isotope signals from convective and large-scale precipitation phases of a heavy precipitation event in southern Italy during HyMeX IOP 13: a modelling perspective. 2019. Copernicus Publications.

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