Numerical simulation of orographic rainbands

Abstract

This study, based on a statistical analysis of simulated warm rain event and radar data, aims at highlighting the main physical mechanisms that lead to organize shallow convection on the relief. The region of investigation, the Cévennes‐Vivarais, is located in the southeast part of France. Radar images from the Cévennes experiment (fall 1986–1988) reveal a characteristic and repetitive structure of the rain distribution organized in narrow bands or plumes, oriented south‐north in the case of stationary southerly Mediterranean flow. The event of 14 November 1986 has been selected and constitutes the data set of this numerical study. This work is closely associated with the previous work by Miniscloux et al. [2001] which presents in detail the results of a geostatistical analysis of the radar data set extracted from the Cévennes experiment data base. The main results highlight the physical characteristics and the dynamics of the rain patterns. Following the recent work of Cosma et al. [2002], high‐resolution (Δ = 1 km) simulations have been continued with the nonhydrostatic three‐dimensional (3D) atmospheric model MesoNH, in order to reproduce the observed rainbands over the Cévennes region. The numerical model correctly reproduces the structure and the dynamics of the rainbands. The geostatistical analysis has been applied for the simulated rain fields. The model slightly overestimates the northward advection velocity of the rain cells within the bands (75 km h−1 against 60 km h−1 for the observation), and the simulated rainbands are narrower and more organized around the N180° direction than the observed rain field. The comparison allows the qualification and validation of the choice of the numerical methodology and realism of the physical parameterizations. The analysis of the 3D simulated fields confirms the physical mechanisms responsible for the rain organization demonstrated by Cosma et al. [2002] through idealized simulations. The statistical analysis highlights the presence of mean topographic features under low‐level convergence zones composed of a succession of ridges and penetrating valleys orientated east‐west. The rainbands are generated upstream of these topographic features and enhanced on the leeside due to the convergence created by the flow deflection around the obstacle and its penetration into the valleys. The simulated triggering takes place further to the south than the observed one, and the triggering is active as soon as the relief is suitably described in the model.