Abstract
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.
Highlights
The Mediterranean basin is frequently affected by deep convection, resulting in heavy precipitation and potentially leading to devastating flash floods
The precipitation associated with Phase 1 (P1) is delayed by 4 h in the COSMOiso simulation compared to the precipitation recorded by the rain gauge network, which shows a peak at 16:00–18:00 UTC, while the precipitation during Phase 2 (P2) is closely reproduced by the simulation with a good timing (∼ 1 h early, with the measured peak occurring at 23:00–01:00 UTC)
The first one (P1) is induced by moist convection, while the second one (P2) is mainly associated with large-scale uplift along a front
Summary
The Mediterranean basin is frequently affected by deep convection, resulting in heavy precipitation and potentially leading to devastating flash floods. To improve our understanding of the water vapour transport upstream of HPEs and the moisture cycling during such events, humidity observations based on measurements of the most abundant stable water isotope (SWI) H126O alone can be limited. Using ground-based SWI measurements and numerical simulations, Pfahl et al (2012) and Aemisegger et al (2015) investigated the mixing processes of different air masses, as well as isotope fractionation and equilibration related to precipitation evaporation during the passage of cold fronts. We investigate these moisture transport processes using trajectory calculations and SWI data obtained from a COSMOiso numerical simulation with 7 km horizontal resolution with parameterized convection This setup results from a trade-off between having high enough resolution for including detailed dynamics of the mesoscale systems and being able to run efficiently over a large domain that includes the moisture transport from Africa.
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