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
Authors: Lee Keun-Ok , Aemisegger Franziska , Pfahl Stephan , Flamant Cyrille , Lacour Jean-Lionel , Chaboureau Jean-Pierre 
Published: June 2019

Abstract. 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
COSMOiso. The simulation at a horizontal grid spacing of about 7 km
over a large domain (about 4300 km ×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 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 SWIs (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 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 δ18O ≥-16 ‰,
median q≥6 g kg−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.