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. The present study examines the impact of the environmental moisture structurein the lower troposphere (below 2 km above sea level, a.s.l.) on theprecipitation development, observed in southern France during IntensiveObservation Period (IOP) 13 of the first Special Observation Period of theHydrological cycle in the Mediterranean Experiment (HyMeX SOP-1), through aseries of sensitivity experiments using the non-hydrostatic mesoscaleatmospheric numerical model (Meso-NH). The control simulation (CNTL) and allthe other 12 sensitivity experiments examined in this study succeed inreproducing a heavy precipitation event (HPE) in the coastal mountainousregion of Var in south-eastern France as observed. The sensitivityexperiments are designed to investigate the response of the HPE to thevariability of the water vapour content upstream in the moist marineatmospheric boundary layer (MABL) and the drier air above. The comparisonsbetween CNTL and the 12 sensitivity experiments show how the life cycle ofprecipitation associated with the HPE, but also the upstream flow (over thesea), is modified, even for moisture content changes of only 1 g kg −1 below 2 km a.s.l. Within the low-level wind convergence between southerliesand south-westerlies, a small increase of moisture content in the MABLprolongs moderate precipitation ( ≥5  mm in 15 min) and enlarges thearea of weak precipitation ( ≥1  mm in 15 min). The moistening in the1–2 km a.s.l. layer, just above the MABL, prolongs the duration ofmoderate precipitation, for a similar total precipitation amount as in CNTL.The drier MABL and 1–2 km a.s.l. layer shorten the lifetime ofprecipitation and reduce the total precipitation amount with respect to CNTL.We also found that the moisture in the MABL has a stronger impact onproducing enhanced precipitation (both in terms of amount and intensity) thanthe moisture just above (1–2 km a.s.l.). Also, it is worth noting thatadding moisture in the MABL does not necessarily lead to enhancedprecipitation amount. In moistening the MABL, the duration of moderateprecipitation increases with increasing moisture as does the area covered byweak precipitation, while the area covered by the intense precipitation ( ≥30  mm) decreases. Despite a simplified moisture-profile modificationapproach, this study suggests that moisture structure in the lowertroposphere is key for accurate prediction at short-term range of the timingand location of precipitation in the coastal mountainous region in southernFrance.

Impact of upstream moisture structure on a back-building convective precipitation system in south-eastern France during HyMeX IOP13