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
COSMO
The Mediterranean basin is frequently affected by deep convection, resulting
in heavy precipitation and potentially leading to devastating flash floods.
Deep convection generally results from complex multi-scale interactions
between large-scale, mesoscale, and microphysical processes. In the
north-western Mediterranean, the large-scale patterns associated with heavy
precipitation events (HPEs) have been shown to be connected to upper-level
troughs, responsible for generating low-level northward flow of marine air
masses characterized by high values of equivalent potential temperature and
precipitable water (Lin et al., 2001; Martius et al., 2006; Nuissier et al.,
2008, 2011; Ricard et al., 2012; Barthlott and Davolio, 2015). In this
favourable large-scale situation, organized deep convection can occur and
often produces high-impact events, with rainfall amounts larger than 100 mm
in less than 6 h. The origin of the moisture feeding the convective
systems is an important research topic that has been addressed using
different techniques and tools, such as trajectory and numerical tracer
analyses (e.g. Turato et al. 2004; Winschall et al., 2012; Duffourg and Ducrocq,
2013; Winschall et al., 2014; Röhner et al., 2016; Duffourg et al.,
2018; Lee et al., 2018). These studies found substantial contributions of
subtropical and tropical moisture coming from various sources such as Africa
(latitude
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)
In the past, some of the most prominent applications of SWIs have been in a paleoclimate context to infer past temperatures and moisture sources from natural archives, for groundwater studies, and in studies investigating the water vapour budget in the stratosphere (Sherwood and Dessler, 2000; Vimeux et al., 2001; Dessler and Sherwood, 2003; Jouzel et al., 2005). The process-based insight provided by the isotope composition of atmospheric water have more recently been extended to synoptic and sub-diurnal timescales and to the lower troposphere, where most atmospheric water vapour resides. Thanks to a tremendous expansion in the number of datasets of water vapour isotopic composition and a substantially improved set of theories and models for interpreting them, the related studies have been expanded during the past several years (e.g. Pfahl et al., 2008; Steen-Larsen et al., 2014; Bonne et al. 2014; Aemisegger et al., 2015; Dütsch et al., 2018; Lacour et al., 2017; Christner et al., 2018).
Recent studies have shown unique information about meteorological processes recorded in SWI data. For instance, 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. Aemisegger and Papritz (2018) and Aemisegger and Sjolte (2018) showed that the important moisture uptake by cold and dry airstreams during events of strong large-scale ocean evaporation carries a distinct SWI signature in water vapour. Recent studies (Schneider et al., 2016; Lacour et al., 2017) analysed the influence of the Saharan heat low on the isotopic budget of offshore west Africa on various temporal and spatial scales, highlighting the importance of the Saharan heat low dynamics on the moistening and the SWI enrichment of air parcels in the free troposphere over the North Atlantic. In addition, Risi et al. (2008) used stable isotopic signals to better understand convective precipitation processes. These previous studies evidenced the usefulness of water vapour isotope data to better understand meteorological processes and moisture transport. Nevertheless, there are still very few studies (Risi et al., 2008, 2010; Tremoy et al., 2014) focusing on the application of water vapour isotopes to investigate moisture processes associated with HPEs at the mesoscale, particularly in the extratropics.
SWI measurements are mainly obtained from space-borne retrievals (e.g. Schneider et al., 2016; Lacour et al., 2017) and ground-based in situ laser spectroscopy (e.g. Aemisegger et al., 2015). The space-borne measurements provide continuous datasets in space at the global scale with coarse vertical resolution and limited precision. On the other hand, ground-based measurements with high temporal resolution are only available from a few locations and from dedicated field campaigns. In particular, the data availability for the Mediterranean region is very limited. A notable exception is the airborne dataset acquired around Corsica (Sodemann et al., 2017) during the first Special Observing Period of the Hydrological cycle in the Mediterranean Experiment (HyMeX SOP-1, Ducrocq et al., 2014). However, it does not include SWI observations for the days under scrutiny in this paper. Due to these limitations we use a model to demonstrate the usefulness of SWI data for understanding moisture processes associated with a Mediterranean HPE.
Accumulated precipitation during IOP 13 from 00:00 UTC on
15 October 2012 to 03:00 UTC on 16 October 2012 obtained from
Our study focuses on the transport of moisture associated with a HPE that occurred over southern Italy (SI) on 15–16 October 2012 and produced precipitation over land exceeding 60 mm in 27 h (Fig. 1a). The HPE consists of two precipitation peaks, the first peak in the late afternoon of 15 October and the second peak around midnight on that day. The target HPE occurred during the Intensive Observation Period (IOP) 13 of the HyMeX SOP-1. Using a combination of ground-based, airborne, and space-borne observations and numerical simulations of this HPE, Lee et al. (2016) investigated the detailed dynamic and thermodynamic environments of the two precipitation phases of the HPE. During Phase 1 (P1), rainfall was connected to convection triggered by local low-level convergence ahead of a cold front and was favoured by moist conditions in the lower troposphere over the Tyrrhenian Sea. Heavy precipitation during Phase 2 (P2) first occurred over Algeria and was favoured by the southerly flow ahead of the upper-level trough and high low-level moisture content and high sea surface temperatures in the Strait of Sicily. The penetration of the mistral over the Mediterranean and SI at the end of 15 October terminated the convective activity. Thanks to the unprecedented data acquired offshore and inland during IOP 13, the detailed moisture structure upstream of the HPE was investigated by Lee et al. (2016). However, the origin and transport pathways of moisture have not been studied to date.
Here 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. More importantly, it allows the questions we are interested in to be addressed, namely the following: which isotope signals are due to local processes, and which are due to large-scale advection? A detailed description of the data and methodology is presented in Sect. 2. Section 3 provides an overview of the meteorological conditions during the two precipitation peaks related to the HPE during IOP 13. Section 4 discusses the isotope signals and relates them to the moisture transport history. A summary and a discussion of the findings of the present study are given in Sect. 5.
The COSMO model (Steppeler et al., 2003) is a non-hydrostatic, limited-area
numerical weather and climate prediction model and is operationally used by
several European weather services. The isotope implementation (COSMOiso;
Pfahl et al., 2012) is similar to other Eulerian isotope models (e.g.
Jaussaume et al., 1984; Sturm et al., 2005; Blossey et al., 2010). COSMOiso
has already shown its capability to simulate the variations of stable water
isotopes at the event timescale (Pfahl et al., 2012; Aemisegger et al., 2015)
as well as in a climatological context (Christner et al., 2018; Dütsch et
al., 2018). It includes two additional parallel water cycles for each of the
heavy isotopes (
In this study, a horizontal grid spacing of 0.0625
Air parcel backward trajectories (Wernli and Davies, 1997; Sprenger and
Wernli, 2015) are calculated using the 3-D wind fields from
the COSMOiso simulation. In total 1440 trajectories per hourly time step are
started from 60 grid points within a box over SI (bounded by 15.2
COSMOiso-produced domain-averaged total precipitation (bar), synoptic precipitation (black solid line), and convective precipitation (dashed line) in domain of southern Italy (SI) over the land during IOP 13. Temporal evolution of observed maximum rainfall within the SI domain is shown by a line with dot. The location of domain SI is depicted by the box in Fig. 1.
As variations in
Mixing and distillation of water vapour with various origins can take place
over a wide range of combinations and generate
From 00:00 UTC on 15 October to 03:00 UTC on 16 October 2012, the SI area (box
marked by “SI” in Fig. 1) was affected by a HPE, with two phases of
precipitation. The large amount of maximum precipitation (in total 62.4 mm
over 27 h) recorded by the rain gauge network (Fig. 1a) is realistically
reproduced by the COSMOiso simulation (maximum precipitation of 59 mm; Fig. 1b),
both in terms of amplitude and spatial distribution. The temporal evolution
of the COSMOiso domain-averaged total precipitation within the SI area (bars
in Fig. 2) shows precipitation in excess of 10 mm within the SI region
between 19:00 UTC on 15 October and 01:00 UTC on 16 October. The period has two
distinct precipitation phases: (1) a convective precipitation phase
(P1) in the late afternoon (19:00–21:00 UTC) on 15 October (dashed line
in Fig. 2) and (2) a large-scale precipitation phase (P2) just
before midnight (22:00–00:00 UTC) on that day (solid line). The precipitation
associated with 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 (grey line with dot in Fig. 2), while the precipitation
during P2 is closely reproduced by the simulation with a good timing
(
Horizontal distributions of sea-level pressure (shades) and
geopotential height at 500 hPa (contour)
The moisture structure upstream of the HPE studied by Lee et al. (2016) has
been further analysed. Three features are highlighted below: (1) the presence
of an African moisture plume favouring the efficiency of the convection to
produce more precipitation, (2) the significance of the southerly flow from
the warmer Mediterranean Sea to the south of Sicily in strengthening the
convergence ahead of the cold front, and (3) the role of the extended
upper-level trough over southern France and the western Mediterranean in
enhancing convection at the leading edge of the surface front. At 16:00 UTC on
15 October 2012, an upper-level trough, located over south-eastern France,
extends to northern Algeria. Sea-level pressure values lower than 1006 hPa
can be observed over south-eastern France extending to northern Italy (Fig. 3a),
with the associated cyclonic flow seen at 850 hPa. Strong northerly
mistral and tramontane winds associated with cold and dry air with
Horizontal distributions of water vapour mixing ratio at 850 hPa
The hourly evolution of the moist and SWI-enriched air mass over the TY
during the period 16:00–20:00 UTC can also be seen in Fig. 5, which shows the
average
The averaged values of potential temperature (
At 20:00 UTC (Fig. 3c, d), southerly winds (10–15 m s
At 00:00 UTC, when the trough is located in the southern Tyrrhenian Sea with
the low-level mistral air mass (
During the two precipitation phases at 20:00 and 00:00 UTC, both
The temporal evolution of the domain-averaged
Domain-averaged
This section aims to investigate the history of the air masses involved in
the convective precipitation phase P1. Figure 7 displays the history of the air
parcel arriving at SI in the layer 800–700 hPa at 20:00 UTC on 15 October 2012.
The 3 d backward trajectories in Fig. 7 indicate that the air
parcels arriving at SI in the 800–700 hPa layer originated over the North
Atlantic. These air parcels remain dry (
History of air parcel arriving at SI in the 800–700 hPa layer
at 20:00 UTC on 15 October 2012.
Figure 8 displays the
Scatter diagram of
Between 6 and 3 h before their arrival in SI, the upper- to low-level
trajectories (green to purple dots in Fig. 9a, b) follow a mixing line
(dashed line) during their descent, while the lowermost trajectories (black
and grey dots) are distributed over a wider domain and do not follow a
Rayleigh distillation line exactly (solid line). This shows that the descending dry
air parcels mix with the warm and moist air parcels from lower altitudes,
which also increases surface evaporation. During P1 (Fig. 9c), the
Scatter diagram of
At 20:00 UTC, the precipitation over SI is associated with a convective line, which extends from SI to the Strait of Sicily (area closed
by dashed line in Fig. 10a) and is located ahead of the surface cold front.
Westerly and north-westerly winds prevail at 542 m a.s.l. (Fig. 10b), while
south-westerly wind is dominant at 2455 m a.s.l. (Fig. 10d). Within the
precipitation area, lower
Horizontal distributions of
At the same time, the African moisture plume is associated with SWI-enriched
vapour with
The Lagrangian analysis indicates that most processes inducing precipitation
during P1 take place during the last 18 h over the Tyrrhenian Sea and
the Strait of Sicily. The descending air parcels from the middle troposphere
reach altitudes below 1 km a.s.l. along the cold front and take up large
amounts of evaporated moisture near the warm sea surface of the Tyrrhenian
Sea. Additional moisture is then taken up at altitudes below 2 km a.s.l. from
mixing with the African moisture plume that extends from the African
continent to the Strait of Sicily. During the period from 18 to 6 h
before the precipitation peak P1,
The 3 d backward trajectories in Fig. 11 evidence that the air parcels
arriving at SI in the layer between 800 and 700 hPa at 00:00 UTC on
16 October come from north Africa and partly from the southern Iberian Plateau. The air
parcels are consistently moist along the tracks (Fig. 11a), with average
Same as Fig. 7 but for the air parcel arriving at SI in the 800–700 hPa layer at 00:00 UTC on 16 October 2012.
These moist and SWI-enriched air parcels are also evident from the scatter
diagram of
Scatter diagram of
At 00:00 UTC on 16 October during P2, stronger precipitation than that of P1 is
produced, and the precipitation system is located mainly over SI (marked
area closed by dashed line in Fig. 13a). In the vicinity of the
precipitating region, strong cyclonic south-westerly flow
Same as Fig. 10 but for 00:00 UTC on 16 October 2012.
Schematics summarizing the main features of water vapour
isotopologues and processes for deep convection upstream of SI and leading to
Phase 1
The Lagrangian analysis indicates that the moisture which feeds the
convection during P2 is related to large-scale ascent from north Africa, and
the air parcels take up additional moisture (2–3 g kg
On 15 to 16 October 2012, SI experiences a HPE (total precipitation of 62.4 mm)
with two phases of precipitation. The first one (P1) is induced by moist
convection, while the second one (P2) is mainly associated with large-scale
uplift along a front. The moisture transport and processes responsible for
the HPEs that occurred over the SI area during IOP 13 have been analysed
here using SWI data obtained from a numerical simulation with COSMO
The 3 d backward trajectory analysis shows that the air parcels arriving in SI during P1 originate from the North Atlantic and descend within the upper-level trough over the north-western Mediterranean Sea. The SWI-depleted air mass within the descending air parcels rapidly takes up a large amount of water vapour from ocean evaporation (green encapsulated area in Fig. 14a) over the Tyrrhenian Sea and also from evaporated moisture from falling precipitation. Additional moisture is taken up over the Strait of Sicily from mixing with the enriched African moisture plume. The SWI-enriched low-level air masses arriving upstream of SI are convectively pumped to higher altitudes, producing precipitation, and the SWI-depleted moisture is transported towards the surface within the downdrafts ahead of the cold front (red and blue arrows, Fig. 14a).
During P2 (Fig. 14b), just a few hours after P1, the origin of the air
parcels arriving at SI is distinct, i.e. mostly from north Africa. The air
parcels are moist and associated with large
Using the hourly 3-D water vapour isotope data, we highlight the large
variety of moisture sources and transport pathways that induced the two
phases of the HPE in southern Italy during IOP13 and the isotopic
characteristics of various air masses associated with the upper-level
trough, cold front, mistral, and African moisture plume that were involved
in convection development. We also highlight the role of the upper-level
trough over the southern Tyrrhenian Sea in driving the advection of the
SWI-enriched plume from north Africa into the region of the deep convective
system, resulting in heavy precipitation over SI. Moreover, we demonstrate
the importance of various moisture processes such as mixing, condensation, and
re-evaporation along the pathway based on the
Our study is the first study to investigate the potential benefit of SWIs in the context of a HPE in the Mediterranean. As such, our study provides a proof of concept of the usefulness of SWI data to understand the variety of origins and moisture processes associated with air masses feeding the convection over SI. This will be further investigated in future research using SWI measurements obtained from various platforms, e.g. ground-based, near-surface, airborne (Sodemann et al., 2017), and space-borne. Our modelling study will also allow forthcoming tailored field campaigns in the Mediterranean region to be designed. To further study the details of the fractionation processes in and around deep convective systems, complementary investigations will be conducted using higher-resolution convection-permitting simulation with a 2 km grid to shed light on cloud microphysical processes inside deep convection.
COSMOiso output data are available from the authors upon request (keun-ok.lee@aero.obs-mip.fr).
KOL, FA, SP, and CF planned the paper and analyses. SP and KOL designed the numerical simulation, and SP performed it. JLL and JPC contributed to the discussion of the results. KOL prepared the paper with contributions from all co-authors.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Hydrological cycle in the Mediterranean (ACP/AMT/GMD/HESS/NHESS/OS inter-journal SI)”. It is not associated with a conference.
We thank Jean-Pierre Pinty (Laboratoire d'Aerologie) for insightful suggestions and three anonymous reviewers for their interesting comments and suggestions. This work was supported by the French Agence Nationale de la Recherche (ANR) via the IODA-MED grant ANR-11-BS56-0005, the MUSIC grant ANR-14-CE01-014, and the MISTRALS/HyMeX programme.
This research has been supported by the French Agence Nationale de la Recherche (ANR) (the IODA-MED grant no. ANR-11-BS56-0005 and the MUSIC grant no. ANR-14-CE01-014) and the MISTRALS/HyMeX programme.
This paper was edited by Christian Barthlott and reviewed by three anonymous referees.