WRF-Chem model simulations of a dust outbreak over the Central Mediterranean and comparison with multi-sensor desert dust observations
Authors: Rizza Umberto , Barnaba Francesca , Miglietta Mario Marcello , Gobbi Gian Paolo , Mangia Cristina , Di Liberto Luca , Dionisi Davide , Costabile Francesca , Grasso Fabio 
Published: July 2016

Abstract. In this study, the Weather Research and Forecasting (WRF) Model with online coupled chemistry (WRF-Chem) is applied to simulate an intense Saharan dust outbreak event that took place over the Mediterranean in May 2014. The dust outbreak was generated in correspondence with an omega-like pressure configuration associated with a cyclogenesis in the Atlantic coasts of Spain. This pattern has been recognized as one of the three major cyclogenesis situations responsible for the transport of Saharan dust towards the Central and Western Mediterranean. In fact, in the case investigated here, a cyclone near the Atlantic coasts of Spain is responsible for strong westerly Atlantic winds (about 20 m s−1) reaching the northern Sahara and leading to the lifting of mineral dust. The northward transport is made possible by a ridge over the central Mediterranean associated with the omega-like pressure configuration. WRF-Chem simulations are able to reproduce the synoptic meteorological conditions and the transport outline of the dust outbreak that was in fact characterized by multiple, superimposed dust impulses. The model performances in reproducing the atmospheric desert dust load were evaluated using a multi-platform observational dataset of aerosol and desert dust properties, including optical properties from satellite and ground-based sun-photometers and lidars, plus in situ PM10 data. This comparison allowed us to investigate the model ability in reproducing both the horizontal and the vertical displacement of the dust plume, and its evolution in time. Results show a good agreement between the model and the AERONET-AOD in six sites in the Mediterranean. Comparison with the MODIS-AOD retrieval shows that WRF-Chem satisfactorily resolves the arrival, the time evolution and the horizontal pattern of the dust storm over Central Mediterranean. Comparison with lidar data confirms the desert dust advection to occur in several, superimposed ‘pulses’, as simulated by the model. In most cases the desert dust is shown to arrive above the PBL and then to descend and mix with the local aerosols within it. The vertical displacement of the dust was in good agreement with the lidar soundings with a mean discrepancy along the aerosol extinction of about 40–60 %. The model-measurements comparison for the PM10 and PM2.5 shows a good temporal matching, although there is a clear overestimation of PM10 and PM2.5, of the order of 70 % during the dust peak. This tendency is reduced or even inverted in weak-dust or no-dust conditions, in which model and measured PM10 and PM2.5 are within 30 % and 10–60 %, respectively. For the PM10 metrics it was also possible to investigate the accordance between the model-based and the measurements-based dust-PM10. This comparison confirmed the PM10 model overestimation to be related to over-predicted dust mass by a factor of 140 %.