Numerical modelling of heavy precipitation events over Eastern Pyrenees: Analysis of orographic effects

Abstract

This study examines the effect of the orography on two heavy precipitation events (HPE1 and HPE2) which exceeded 100mm in 24h and affected the Eastern Pyrenees (Andorra, NE Spain and SE France) in the NW Mediterranean. High resolution simulations (3km) performed with the Weather and Research Forecasting model (WRF) were compared with precipitation observations to assess the ability of the model to reproduce the spatial distribution of the rainfall field in this mountainous area and determine the prominent factors of intense precipitation occurrence. The analysis has been focused on the association of the heavy rainfall to the existing atmospheric disturbance and the presence of orographic induced vertical motions. For both episodes at 3km resolution, on exposed mountain sides, vertical moisture fluxes characterized with the orographic moisture flux index (OMF) yielded values above 7gkg−1ms−1, consistent with previous studies. The spatial distribution of the OMF maximum is in agreement with the observed rainfall maxima over the Eastern Pyrenees according to the combination of two factors, namely the normal orientation of mountain range with respect to the prevailing moisture flows and large wind speeds of the impinging upstream flows (~20m/s). Moreover, the analyses of a triggering term and moist Froude number also reflect the complexity of applying theoretical results obtained in idealized simulations with 2D mountain ridges to numerical simulations with dynamic evolving conditions over areas with complex orography as the Eastern Pyrenees.