International audienceThe Mediterranean region is frequently affected by heavy precipitation events associated with flash floods, landslides, and mudslides that cause hundreds of millions of euros in damages per year and often, casualties. A major field campaign was devoted to heavy precipitation and flash floods from 5 September to 6 November 2012 within the framework of the 10-year international HyMeX (Hydrological cycle in the Mediterranean Experiment) dedicated to the hydrological cycle and related high-impact events. The 2- month field campaign took place over the Northwestern Mediterranean Sea and its surrounding coastal regions in France, Italy, and Spain. The observation strategy of the field experiment was devised to improve our knowledge on the following key components leading to heavy precipitation and flash flooding in the region: i) the marine atmospheric flows that transport moist and conditionally unstable air towards the coasts; ii) the Mediterranean Sea acting as a moisture and energy source; iii) the dynamics and microphysics of the convective systems producing heavy precipitation; iv) the hydrological processes during flash floods. This article provides the rationale for developing this first HyMeX field experiment and an overview of its design and execution. Highlights of some Intense Observation Periods illustrate the potential of the unique datasets collected for process understanding, model improvement and data assimilation

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[1] NO x transport and production by lightning for the 10 July 1996 StratosphereTroposphere Experiment-Radiation, Aerosols, and Ozone convective storm is examined using radar, in situ observations and cloud model simulations. Observations and model simulations indicate that most of the NO x produced by the storm was transported out into the anvil. The analyzed NO x flux into the anvil, combined with results from a cloud model simulation, indicate that approximately 60 percent of the NO x transported into the anvil during the observational period is produced by lightning. Lightning flash rate and channel length measurements, obtained using the Office National d'Etudes et de Recherche Arospatiales lightning interferometer, are combined with the NO x budget to give estimates of average lightning NO x production per interferometer flash and per unit flash channel length. The analysis yields production rates of approximately 43.2 moles (2.6 Â 10 25 molecules) NO x per interferometer flash and 1.7 Â 10 À3 moles (1.0 Â 10 21 molecules) NO x per meter of flash channel. These production rates fall within the bounds of rates derived in previous studies using completely different approaches.

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Concurrent measurements from the CSU-CHILL multiparameter Doppler radar, the Office National d'Etudes et de Recherches Aérospatiales VHF lightning interferometer, and the National Lightning Detection Network, obtained during phase A of the Stratosphere-Troposphere Experiments: Radiation, Aerosols, Ozone (STERAO-A) field project, provided a unique dataset with which to study the relationships between convective storm microphysics and associated lightning. Two storms have been examined in detail in this study: 10 and 12 July 1996. Both storms were long lived, existing in some form for over 4 h apiece, and produced very low cloud-to-ground (CG) lightning flash rates, in particular negative CG flash rates (generally Ͻ1 min Ϫ1 and often no CG flashes for periods ranging from 10 to almost 30 min), during all or a portion of their lifetimes while simultaneously producing relatively high intracloud (IC) flash rates (Ͼ30 min Ϫ1 at peak). For both storms, radar reflectivity intensity and the production of hail were anticorrelated with the production of significant negative cloud-toground lightning. These observations are shown to be consistent with an elevated charge hypothesis and suggest a possible way of correlating updraft speed, hail, and storm severity to CG and IC flash rates.