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Cyclone processes leading to extreme rainfall in the Mediterranean region

Periodic Reporting for period 1 - ExMeCy (Cyclone processes leading to extreme rainfall in the Mediterranean region)

Reporting period: 2016-05-01 to 2018-04-30

"Understudying cyclone dynamics and related rainfall is of high interest to the socio-economic activity in the Mediterranean. Cyclones in the region constitute a major environmental risk as the associated heavy rainfall and thunderstorms affect more than 480 million people on three continents. In fact, several Mediterranean cyclones may attain wind intensities comparable to those of named Atlantic hurricanes. The recent storm ""Rolf"" (Fig.1)– a tropical-like cyclone that occurred in November 2011 to the south of France – caused fatalities and severe damage due to wind gusts of more than 40 m s-1, sea waves that exceeded 6 m, and heavy rainfall that locally reached exceptionally high 6-day accumulated values of about 1000 mm. As a result, it is essential to better understand the dynamics of cyclones to better understand the variability of the regional climate and its extremes. The overall objective of EXMECY is related to the delineation of the relative contribution of the multi-scale synergistic of several processes such as deep convection (DC), air-sea interactions and large-scale forcings, to the intensification of cyclones and the formation of extreme rainfall, as well as to identify the dynamical and water ingredients that turn cyclones into high impact weather events.

Our results complement previous studies on the role of DC and the warm conveyor belts (WCB; fast ascending air parcels within the cyclones warm sector) to produce heavy rainfall. Most importantly, we quantified in a climatological framework the relative contribution of these processes and showed their relationship with cyclone rainfall. Especially for tropical-like cyclones, we concluded in a scenario that describes their lifecycle and finally, we addressed the issue of Mediterranean cyclones water budget, sources and precipitation efficiency.
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The work performed in the framework of EXMECY has been organised in five work packages. Work activities comprise scientific research, the development of new diagnostic tools for the analysis of cyclones water budget and dynamics as well as the performance of open public outreach.

In terms of research, four scientific publications have been submitted (three already published or accepted for publication) where the MSCA support through EXMECY project has been adequately acknowledged. The main scientific results of this action concern cyclones relationship to rainfall. Especially concerning deep convection (DC), an innovative approach has been followed, where lightning observations have been used for the climatological analysis of DC cells. Furthermore, cyclones have been analysed with respect to their dynamics and relationship with heavy rainfall and finally with respect their water budget. Main results are summarized as following:

In terms of development of implemented diagnostic tools into atmospheric models, two diagnostic modules have been developed and were implemented to the Weather Research and Forecasting (WRF) model. The tools have been/will be used for the scientific production of EXMECY:
- Diagnostic tool on extracting the potential vorticity budget of cyclones.
- Diagnostic tool on extracting the water budget terms.

A rich outreach activity portfolio has been also achieved including participation to Researcher's night, publications publicly accessed on the host institution forecasting webpage (www.meteo.gr; ~350.000 daily visitors), four international press interviews and youtube animation.
"The state-of the art in the field of Mediterranean cyclones suffers from the lack of studies addressing the relationship of cyclones with extreme rainfall, but most important from the quantification of the different processes contributing to these extremes. New observation and modelling approaches have been used in EXMECY, providing unprecedented results on the quantification of the contribution of WCBs and DC to rainfall close to cyclones. Our results provide an insight into the processes where forecasters and model developers should give priority for improving weather or climate prediction. Furthermore, we focused on DC and its climatology through the detection of thunderstorms. A novel methodology has been applied to a wide dataset of lightning observations in order to isolate lightning clusters, eventually identified as thunderstorms. Results documented for the first time the thunderstorms climatology over the Mediterranean, providing an insight to their morphological characteristics (Fig. 2). Concerning cyclone dynamics, we focused on tropical-like cyclones providing a new point of view on the processes that govern the life-cycle of these systems. The application of dedicated tools to tropical-like Mediterranean cyclones permitted for the first time the quantification of the relative contribution of upper tropospheric precursors and DC to the intensification of these systems. Results revealed a rather close relationship between tropical-like cyclones in the Mediterranean and subtropical systems occurring over the Oceans. In this sense, a new cyclones category has been suggested to occur in the Mediterranean basin. This result bridges Mediterranean cyclones as a unique cyclones category with other cyclone systems occurring over Oceans and thus open perspectives to new collaborations and to transferring knowledge, tools and methods from the widely researched tropical/subtropical/mid-latitude systems to the relatively ""grey"" area of Mediterranean cyclones. Finally, a novel modelling diagnostic tool, developed in the framework of EXMECY, has been applied to Mediterranean cyclones in order to delineate the contribution of different processes to the ""water ingredients"" forming extreme rainfall in the Mediterranean due to cyclones (Fig. 3). This methodology provides a new insight on the location of water sources to Mediterranean cyclones, providing an insight to the role of synoptic scale meteorology not only in provoking Mediterranean cyclones but also in fuelling these systems with moist air originating from the Atlantic Ocean. These new results suggest that proper forecasting of weather extremes over the Mediterranean do not only reside on the credible reproduction of Mediterranean cyclones, but also to the realistic representation of evaporation over the Atlantic Ocean. This sets new priorities to the credible prediction of Mediterranean high-impact weather.
Overall, the research conducted within the framework of EXMECY is inline with the constant demand of forecasters and civil protection for credible prediction of rainfall extreme events at various timescales ranging from the short-term through seasonal and up to climate scale. In the long term, this project provides priorities for improving numerical models to simulate Mediterranean cyclones and their impacts on the environment. The new methodologies and tools will remain available for continuous exploitation from the Researcher but also from collaborators who may freely make use of them upon request. A solid four-publications record assures the visibility of EXMECY in subsequent years and certifies the quality of work achieved during this project.
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Fig.1: Met-9, 8 November 2011, 10:00 UTC Channel 12 (HRV) and Metop-A ASCAT winds
Fig.2: Density map of thunderstorms per season.
Fig. 3: Water budget terms for cyclones with high rain and differences from those with low rain.