Explicit simulation of electrified clouds: From idealized to real case studies

Original results obtained with the revised version of the explicit cloud electrification module developed in the French mesoscale model MesoNH are presented. The module includes three ingredients: (i) a prognostic budget equation of the electric charges carried by each type of hydrometeor and by the positive and negative ions, (ii) a computation of the electric field by inverting the Gauss equation and (iii) a statistical algorithm to produce intra-cloud and cloud-to-ground discharges. Electric charges are generated by non-inductive (NI) processes involving ice particles that collide in the presence of supercooled water. The geometry of the lightning flashes obeys a fractal law that characterizes the densely branched flash structures with a marked horizontal extent in electrically charged regions of the clouds.For the first time, the electrical module of MesoNH is used to simulate the electrical aspects of a heavy precipitation event over the Cevennes area in the South of France. MesoNH is initialized with "AROME" analyses of Meteo-France for the 6-8 September 2010 event that produced locally 250 to 300 mm of precipitation over the Cevennes ridge. A 54 hour simulation starting 6 September 2010 at 00 UTC and without electricity, is performed at 1 km resolution over a 384 × 384 km2 area in order to check the ability of the model to reproduce the very fine scale structure of the rainfall. MesoNH is then restarted on 7 September 2010 at noon for a 6 hour simulation involving the full electrical module. Several aspects of the electrical state of the clouds could be simulated at this scale: the cloud charge structure, the electric field and the lightning flash characteristics. Finally a map of flash density is compared to observations taken from two conventional detection networks (LInet and ATDnet). Both networks suggest that the model produces ten times too many flashes but arguments are put forward to explain this discrepancy, e.g., filtering the less energetic flashes improves the comparison. In conclusion, this study calls for a reference evaluation of the electrical module of MesoNH against statistics of flash data at high resolution in a next planned field experiment in 2012.

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