On the role of aerosols in the production of orographically-induced extreme rainfall in near-maritime environments

The present study aims at disentangling the role of marine aerosols in extreme precipitation events of orographic nature over near-maritime locations. Being the hazardous character of these events mainly associated to runoffs and river floodings, their destructive potential makes them subjects of special interest. In pursuit of a deep understanding on the development mechanisms of such events, this study reproduces the conditions and development of an event ranging from 20 to 27 of December 2009 that took place in Southern Spain. During this event, a series of Atlantic fronts swept the southernmost part of the Iberian Peninsula which, along with water vapor entrainment in the form of an atmospheric river, produced rainfall accumulations up to 900 mm in some specific mountainous locations. With this purpose, a number of simulations are conducted to study the microphysical processes and aerosol-cloud interactions governing the rain production. The WRF model is utilized with two different configurations of the Morrison double-moment microphysics scheme: (1) with prescribed type and concentrations of aerosols (PA); and (2) a setup in which a chemistry module is coupled to the meteorological core (Interactive Aerosols, IA), allowing for the consideration of prognostic aerosol concentrations and other relevant aspects such as the hygroscopicity and dry size of the different chemical species. According to the validation performed with a database constructed from measurements of rain gauges, results show that using prognostic aerosols concentrations for natural aerosols, namely sea salt and dust, improves the model's skill for reproducing extreme rainfall with respect to the setup using the default aerosols distribution prescribed in the WRF-alone simulations. Additionally, IA simulations increase windward precipitation and reduce leeward rainfall, thus largely amplifying the spatial variability of precipitation along the direction of the moisture flux. Besides, it is observed that this effect is independent of the model resolution. Consequently, it is concluded that a realistic aerosols concentration, along with a detailed treatment of both the size and hygroscopic properties of aerosols, have a crucial impact on the microphysical processes involved in this type of orographically-induced extreme precipitation events. Specifically, under a regime of low concentration of cloud condensation nuclei originating from natural aerosols, the formed cloud droplets are more prone to grow to precipitating size, thus enhancing the auto-conversion rate.