Characterizing Large-scale Circulation Triggerring Heavy Precipitation in the Northern French Alps

Abstract:

Large-scale circulation (LSC) explains a significant part of Alpine precipitation. Characterizing heavy precipitation LSCs is usually done using classification, which leads to an ineluctable loss of information. In this work, a continuous characterization of heavy precipitation LSCs is implemented, based on analogy in atmospheric descriptors describing both the dynamics of LSC and their relative position in the atmospheric space. This work is applied to two neighboring catchments in the Northern French Alps for the 1950-2011 period, considering 3-day annual maxima. The Isère River catchment is influenced by Oceanic circulations, whereas the Drac River catchment is influenced by both Oceanic and Mediterranean circulations. The 500 hPa geopotential height covering part of Western Europe is used to represent LSC. Our results reveal that heavy 3-day precipitation are triggered by quasi-stationary upper-level wind direction over 3 days, whatever the atmospheric influence. Oceanic circulations triggering heavy precipitation sequences feature among the most pronounced and reproducible centers of action, and show among the highest degree of clustering in the atmospheric space. Such combinations of LSC characteristics are rare, and correspond to among the most pronounced westerlies for Western Europe circulation. Heavy precipitation sequences triggered by Oceanic circulations mostly occur in winter when atmospheric humidity is low, highlighting the leading role of LSCs in triggering heavy 3-day precipitation. Mediterranean circulations triggering heavy precipitation sequences in the Drac River catchment also feature pronounced and reproducible centers of action and a high degree of clustering in the atmospheric space, but to a lesser extent than for Oceanic circulations. However, such combinations of LSC characteristics are rare for Mediterranean circulations. Heavy precipitation sequences triggered by Mediterranean circulations feature weaker upper-level winds than Oceanic circulations and all occur in autumn when atmospheric humidity remains high, suggesting a more balanced contribution of LSC and large-scale convection in triggering heavy 3-day precipitation.