Impact of Aerosols on the Evolution of a Medicane in November 2011

A Medicane is a high impact weather system over the Mediterranean Sea. This tropospheric depression occurs approximately once a year mainly in the autumn and winter months [1], [2]. It has similarities to a hurricane, which is also in the coinage “Medicane” in combination with “Mediterranean Sea”. In such a system a circular, cloudless eye is surrounded by an eye wall with a roughly axisymmetric cloud pattern. The wind speed is not as high as within a hurricane and the spatial extent is smaller as well [3]. Nevertheless, the damage potential is high due to high wind speeds, heavy precipitation, and flooding when making landfall. Therefore, a good prediction of such a weather system is essential. We address the questions whether and to which extent aerosols affect the track, intensity and structure of the Medicane, the formation and lifetime of related clouds and the locations, duration and amount of the ensuing precipitation. We have already found locally a strong impact of sea salt particles on precipitation. In comparison with observation the accumulated precipitation is in better agreement if sea salt is taken into account. The relative importance of the influence of natural and anthropogenic aerosols on high impact weather is still an open question. Aerosol-Cloud-Precipitation feedback processes are very complex and not completely understood. In modeling studies the impact of aerosol on precipitation differs in sign and magnitude from case to case [4], [5]. Consequently, it is not known whether it is necessary to take into account this impact in numerical weather prediction models. Aerosols have an impact on cloud formation and therefore on precipitation. They alter the energy budget by absorbing and scattering radiation. Aerosols can influence the water cycle as well. This is done by acting as cloud condensation nuclei or ice nuclei. Thereby, they also alter the physical properties of the cloud. The air over the Mediterranean Sea consists of a special mixture of aerosol composition. There are natural aerosols like sea salt, which is emitted directly out of the sea and mineral dust, which is transported out of the Saharan Dessert over the Mediterranean Sea. Anthropogenic aerosols occur there, too. Sources are coastal cities, traffic, and emissions of ships. In our study we focus on the development of a Medicane that occurred in November 2011. Simulations of such systems have been done before [4], [5]. However, the impact of sea salt, dust and anthropogenic aerosols was not investigated yet. In order to quantify this impact we applied the comprehensive model system COSMO-ART. COSMO (Consortium of Small-scale MOdeling, [6]) is the non-hydrostatic numerical weather forecast model of the German Weather Service (DWD). The online-coupled extension ART (Aerosol and Reactive Trace gases, [7]) accounts for gases and aerosols. It treats gas phase and aerosol chemistry, aerosol dynamics, and cloud microphysical processes in detail. The aerosol distribution is described by twelve modes, which are shaped by overlapping lognormal functions. There is one coarse mode, three modes for each sea salt and mineral dust and one mode with pure soot. Two modes are internally mixed with sulfate, nitrate, ammonium, secondary organic matter and water. The last two modes represent aged particles, which consist of an internal mixture of the compounds mentioned in the two modes before and soot. Emissions of sea salt and mineral dust are calculated as functions of the meteorological conditions and soil data whereas anthropogenic emissions are provided by external data. The standard cloud scheme in COSMO for NWP applications is a one-moment bulk microphysics scheme. In contrast, this study uses a two-moment scheme [8], which adds to the prediction of mass densities also the number densities of the hydrometeors. This two-moment-scheme is extended by an activation parameterization based on Köhler Theory [9], [10] and the adsorption theory FHH [11]. The parameterization in [12] is used to describe the formation of ice nuclei. Applying COSMO-ART we simulate the effect of aerosols on the evolution of the Medicane, the track, and precipitation. We do a triple nesting where the highest horizontal grid mesh size is 2.8 km. The higher the wind speed the more sea salt is emitted. Thus the Medicane creates and feeds its own sea salt field. Then these particles can act as cloud condensation nuclei and alter the microphysical properties of the cloud. Fig. 1 shows the horizontal distribution of sea salt particles and the horizontal wind vectors. During the simulation episode, mineral dust is transported from the Saharan Dessert over the Mediterranean Sea. These dust particles can also act as cloud and also as ice condensation nuclei. In addition, mineral dust particles alter the radiation budget by efficient scattering and absorption. The results of our simulations are used to answer the questions of the beginning. [1] M. Tous and R. Romero, International Journal of Climatology 33.1 (2013) [2] L. Cavicchia and H. von Storch, Climate Dynamics 39.9-10 (2012) [3] K. Emanuel, Advances in Geosciences 2.2 (2005) [4] W. K. Tao et al., Reviews of Geophysics 50.2 (2012) [5] A. P. Khain, Environmental Research Letters 4.1 (2009) [6] M. Baldauf et al., Monthly Weather Review 139.12 (2011) [7] B. Vogel et al., Atmospheric Chemistry and Physics 9.22 (2009) [8] A. Seifert and K. D. Beheng, Meteorology and atmospheric physics 92.1-2 (2006) [9] C. Fountoukis and A. Nenes, Journal of Geophysical Research: Atmospheres (1984–2012) 110.D11 (2005) [10] M. Bangert et al., Atmospheric Chemistry and Physics 12.9 (2012) [11] P. Kumar et al., Atmospheric Chemistry and Physics 9.7 (2009) [12] D. Barahona and A. 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