## 3 Results  
### Events Description by ERA5 Reanalysis and MODIS Retrievals  
This event was chosen by making a preliminary cross-checking in which we have considered the following criteria: (i) plume intrusion in Mediterranean basin with the highest AOD values; and, (ii) availability of Xmed-dry data. Considering that XMed-dry campaign started in 2017 with a series of Intensive Observation Periods, the period (14-16 April 2018) we have investigated was the only one satisfying the two criteria above.  
The synoptic analysis of the dust event is described using the ERA5 reanalysis of the geopotential at 500 hPa. The geopotential is obtained downloading data from the Copernicus Climate data store [52]. In particular, the selected dataset concerns the ERA5 hourly data on pressure levels (1979 to present) from which the geopotential at 500 hPa may be downloaded in GRIdded Binary (GRIB) format. ERA5 represents the fifth generation of European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis of the global climate. It combines model data with observations from across the world into a globally complete and consistent dataset [53]. ERA5 [54] is produced using 4D-Var data assimilation in the ECMWF's Integrated Forecast System (IFS), with 137 sigma/pressure levels in the vertical and 31 km horizontal resolution.  
Figure 2a-f shows the comparison between the geopotential height at an atmospheric pressure of 500 hPa predicted by the WRF-Chem (Figure 2b,d,f) and GRIB data downloaded from ECMWF/ERA5-reanalysis (Figure 2a,c,e). Data are displayed for 14 April at 06:00 UTC (upper panel), 15 April 14 06:00 UTC (central panel) and 16 April at 18:00 UTC (bottom panel). On 14 April, ERA5-reanalysis (Figure 2a) shows a pressure minimum  
localized in the internal regions of Algeria fostering a southern circulation in the central Mediterranean basin. The following day (15 April), ERA5-reanalysis depicts that the pressure minimum is dislocated toward Tunisia reinforcing the circulation toward Italy. On 16 April at 18:00 UTC, the weakening of the pressure minimum is evident with the consequent zonal circulation (west-east) on the central and eastern Mediterranean basin.  
It is evident the close reproduction of the ERA5 (Figure 2a,c,e) geopotential at 500 hPa by WRF-Chem model (Figure 2b,d,f). This is quite important as the long-range transport of dust plumes is governed by these high-tropospheric winds. These synoptic conditions are extremely favorable for the generation and transport of Saharan dust toward the Mediterranean basin for two reasons: (i) the presence of a low-pressure system and relative high winds speed on the Northern Sahara Desert; and the consequent (ii) southern wind circulation on the Mediterranean basin.  
The transport of Saharan dust may be further verified by analyzing Figure 3 that represents the time averaged (14-16 April) map of MODIS/AOD at 550 nm. It is calculated  
Figure 2: Geopotential height (meters) from ERA5 reanalysis (**a,c,e**) and WRF-Chem runs (**b,d,f**), for 14 April at 06:00 UTC (upper panel), 15 April at 06:00 UTC (central panel), and 16 April at 18:00 UTC (bottom panel).  
combining the dark target (DT; [15]) and deep blue (DB; [55]) algorithms for land and ocean. The final product depicted at Figure 3 is obtained by merging MODIS-Terra (MOD08_D3 v6) and MODIS-Aqua (MYD08_D3 v6) intermediate maps. The extent of the dust plume represented by the time average map of Figure 3 reports the strong intrusion of Saharan dust (AOD peak ~2.0), from the emission regions encompassing the Sahara Desert portion of Algeria and Tunisia [4] toward southern Italy (15 April) and the eastern Mediterranean regions (16 April).