Dynamique des épisodes pluvieux intenses sur le Sud-Est du Massif Central - Aspects météorologiques et applications hydrologiques

The dynamics of intense rainy spells over the South-East of the Massif Central - Meteorological aspects and hydrological applications

Groupe Hydrologie, Institut de Mécanique de Grenoble B.P. 53X - 38041 Grenoble Cedex

Abstract

The whole south-eastern side of the Massif Central is regularly subject, in particular in Autumn, to periods of very intense rainfall (682 mm of rain in 48 h at the Mont-Aigoual on one occasion, on October 31st 1963 ; 85 mm in exactly an hour in three different stations on September 12th, 1976 ; etc.), which can cause violent and sometimes catastrophic floods in some basins. For a better understanding of the phenomenon and to obtain a closer view of the spatial characteristics of rainfall fields in the region, data on all intense rainfall periods from 1971 to 1979 were compiled and analysed, in hourly steps, using a network of 58 rainfall recording stations, i.e. a total of approximately thirty different episodes. Systematic plotting of the most interesting episodes revealed that during phases of high activity, rainfall had the very specific shape of a narrow rainy strip (a few kilometers wide), parallel to the topographical crest, but located distinctly in front of it at a distance of approximately twenty kilometers. The strip contains very intense nuclei within which rain frequently exceeds 50 mm an hour. These nuclei can cover a few dozen to a few hundred square kilometers. Joint analysis of hourly rainfall graphs and the concomitant meteorological fields has allowed for better understanding of the phenomenon, as well as the relationships which can exist between the main synoptic factors (intensity of flow, position of the front, etc.), and the organisation of the rainfall field. It was thus ascertained that the rainfall drip developing over the mountainside did not (except for some special cases or at the end of a rainy episode, following a sudden advance of the front) move systematically in a single direction, but that the rainfall system, once organised, remained quite stable, without any general movement. Sharp variations in local intensity can nevertheless be observed, associated with the magnitude of convective activity, making the phenomenon one of rather intermittent character. It should also be said that the study has not been completed, and is being continued with a larger volume of information : strengthening the network better to understand the situations in some zones, and taking the greater number of episodes into consideration.