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© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 20 Nov 2018

Research article | 20 Nov 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Atmospheric Chemistry and Physics (ACP).

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

Keun-Ok Lee1, Franziska Aemisegger2, Stephan Pfahl2,3, Cyrille Flamant4, Jean-Lionel Lacour5, and Jean-Pierre Chaboureau1 Keun-Ok Lee et al.
  • 1Laboratoire d'Aérologie, Université de Toulouse, CNRS, UPS, Toulouse, France
  • 2Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland
  • 3Institute of Meteorology, Freie Universität Berlin, Berlin, Germany
  • 4LATMOS/IPSL, CNRS, Sorbonne Université and Université Paris-Saclay, Paris, France
  • 5Institute of Earth Sciences, University of Iceland, Reykjavik, Iceland

Abstract. The dynamical context and moisture transport pathways associated with a heavy precipitation event (HPE) in Southern Italy (SI) are investigated with the help of stable water isotopes (SWIs). 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. SI experienced intense rainfall of 62.4 mm over 27 hr with two precipitation phases during this event. The first one (P1) was induced by convective precipitation linked to a frontal feature, 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 COSMOiso. 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 δ18O ≤ −25 ‰, water vapour mixing ratio q ≤ 2 g kg−1) close to the surface, which induces strong surface evaporation. These air parcels are rapidly enriched in SWI (δ18O ≥ −14 ‰) and moistened (q ≥ 8 g kg−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 precipitation of convective nature in SI. Most of the moist processes (i.e. evaporation, convective mixing) related to the HPE take place during the 18 hours preceding the occurrence of P1 over SI. Four hours 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 moist and SWI-enriched African plume towards SI, and leads to large-scale uplift of the warm African air mass along the cold front. This brings moist and SWI-enriched air masses (median δ18O ≥ −18 ‰, median q ≥ 6 g kg−1) to higher altitudes 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 hours preceding P2 in SI. This work sheds light on the variety of thermodynamic mechanisms occurring at the meso- and synoptic scales and leading to two distinct precipitation phases of a HPE over the densely populated SI region.

Keun-Ok Lee et al.
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