Rainfall variability and climatology of orographic banded convection in a Mediterranean area

Statistical studies of rainfall climatology on the mountainous Cévennes Vivarais region, representative of a typical Mediterranean area, have brought to the fore the differential role of the relief given the intense rainfall duration (Bois et al., 1997; Molinié et al., 2008). For a 24h time scale, it is observed a significant signature of the relief on the cumulated rainfall, whereas for a smaller time step (i.e. hourly time step) no specific signature is observed. Moreover, banded orographic rainfall events associated with shallow convection have been observed in the previous field campaign Cévennes 86-88. Such events are characterized by their stationarity and their privileged location on the relief (Miniscloux et al., 2001), which suggest that they contribute to the signature of 24h cumulated rainfall. Banded orographic rainfall events are thus meteorologically significant. Their impact on the rain climate in mountainous areas is probably strong despite their low intensity (less than 10mm.h-1). Indeed, orographic rainbands are able to bring locally substantial accumulations of more than 100mm.day-1 contributing to locally saturate the soil and therefore, to be a factor for the genesis of high hydrological risk. The understanding of the role of the relief in the rain triggering is one of the objectives of the future field experiment HyMeX designed to be run in the Mediterranean region in 2010 – 2020. Therefore, a climatologic study based on weather types of these situations could lead to quantify their contribution on the rain climate in this region. Numerical simulations of identified weather types allow to assess the rainfall response of this classification and to evaluate the contribution of orographic banded events in rainfall regime for the Cévennes Vivarais region in the global warming context. Since only two events of banded convection have already been observed, the first step was to elaborate a database of such events. Thanks to soundings and hourly rainfall data, 121 soundings were identified to be representative of these events (Godart et al., 2008). The analysis of the statistical characteristics of orographic banded rainfall has confirmed their stationarity and highlights favourable atmospheric conditions which seem to be necessary: 1) a southerly wind with a low directional wind shear; 2) a high wind velocity (about 15m.s-1 in low level layer), 3) an intense low-level relative humidity which decreases rapidly above 1500m, 5) a stable layer around 3000m. Thus, a generic sounding has been proposed. Meteorological simulation with the MesoNH model was done using this generic sounding as the meteorological forcing. Results showed banded rainfall patterns, which validated our selection. Our contribution to 13th Conference of Mountain Meteorology and 17th Conference on Applied Climatology aims at presenting the results of sensitivity studies based on the simulations of the rainfall variability associated to different atmospheric conditions representative of shallow banded convective events. Geostatistical analysis on the simulated rainfall fields allow to highlight the relationship between the relief and rainfall patterns and to characterize more precisely the banded organisation. The link between the variability of atmospherics conditions and its resulting rainfall allows the study of the climatology of these events. This climatology is approached thanks to a weather type classification. The 40 years of ERA-40 data are used and 8 classes are identified as representative of the local rainfall climatology. Our selected banded convection events are located within this classification in order to identify their associated synoptic organisation.

References:

Bois, P., C.Obled, M-F Saintignon et H.Mailloux, 1997 : Atlas expérimental des risques de pluies intenses (2ème édition). Pôle grenoblois d'études et de recherche pour la prévention des risques naturels.

Godart, A, S.Anquetin, E.Leblois, 2008 : Rainfall regime associated with banded convection in mountainous area, submitted to Meteorology and Atmospheric physics

Miniscloux F, JD.Creutin, S.Anquetin, 2001: Geostatistical analysis of orographic rainbands, J.Appl.Meteor, 40:1835-1854

Molinié, G., E. Yates, P. Bois, B. Boudevillain, S. Anquetin and J.-D. Creutin, 2008: A statistical Analysis of extreme rainfalls in a Mountainous Mediterranean region, submitted to J. of Hydrometeorology