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

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.

The role of air–sea fluxes for the water vapour isotope signals in the cold and warm sectors of extratropical cyclones over the Southern Ocean

“…range. The effective reflectivity along with the Doppler velocity and the spectral range is derived from the raw Doppler spectra using the algorithm of Maahn and Kollias (2012). In order to estimate the snowfall rate, we use the reflectivity-snowfallrate (Z-S) relationship derived by Grazioli et al (2017) based on the measurements at Dumont d'Urville station.…”

“…Lagrangian studies based on COSMO iso simulations have shown that δ 2 H and d in near-surface water vapour are strongly influenced by ocean evaporation and, over land, by evapotranspiration and mixing with moist air, while liquid-and mixedphase cloud formation contributes to the δ 2 H and d variability (Aemisegger et al, 2015;Dütsch et al, 2018). The importance of different air mass origins and pathways of airstreams for the SWI evolution during frontal passages has been illustrated using a COSMO iso simulation in the Mediterranean Sea (Lee et al, 2019). Idealised COSMO iso simulations of an extratropical cyclone showed that the meridional advection of air in the cold and warm sector strongly shapes the characteristic high δ values in the warm and low δ values in the cold sector (Dütsch et al, 2016, see also Fig.…”

How Rossby wave breaking modulates the water cycle in the North Atlantic trade wind region

“…A dynamical phenomenon with a particularly strong impact on the vertical extent of shallow cumulus clouds are so-called dry air tongues (Mapes and Zuidema, 1996) or dry intrusions (Browning, 1993;Raveh-Rubin, 2017). These dry intrusions have been shown to originate from the midlatitudes (Yoneyama and Parsons, 1999) using soundings and global atmospheric analysis datasets from the Coupled Ocean-Atmosphere Response Experiment of the Tropical Ocean and Global Atmosphere programme (TOGA COARE; Webster and Lukas, 1992).…”

The role of air–sea fluxes for the water vapour isotope signals in the cold and warm sectors of extratropical cyclones over the Southern Ocean

“…These additional water cycles are affected by the same physical processes as the light water molecule, except for isotopic fractionation during phase change processes. COSMO iso has been previously used to study various aspects of the regional atmospheric water cycle over Europe and the USA (Pfahl et al, 2012;Aemisegger et al, 2015;Dütsch et al, 2018;Christner et al, 2018;Lee et al, 2019). Here, nine COSMO iso simulations were conducted in the Southern Ocean covering the ACE measurement time period.…”

“…Lagrangian studies based on COSMO iso simulations have shown that δ 2 H and d in near-surface water vapour are strongly influenced by ocean evaporation and, over land, by evapotranspiration and mixing with moist air, while liquid and mixed phase cloud formation contributes to the δ 2 H-and d-variability (Aemisegger et al, 2015;Dütsch et al, 2018). The importance of different air mass origins and pathways of airstreams for the SWI evolution during frontal passages has been illustrated using a COSMO iso simulation in the Mediterranean Sea (Lee et al, 2019). Idealised COSMO iso simulations of an extratropical cyclone showed that the meridional advection of air in the cold and warm sector strongly shape the characteristic high δ values in the warm and low δ values in the cold sector (Dütsch et al, 2016, see also Fig.…”