UPPER-LEVEL DISTURBANCES AND THE IMPACT OF DUST OUTBREAKS IN SPAIN

The interaction between upper-level disturbances and major orographic features in north-western Africa is a key element for cyclogenesis and the development of convective storms, which promote instability at lower levels and lead to dust mobilization. It may also induce stratospheric to tropospheric exchange. The analysis of these processes and their subsequent impact over the Iberian Peninsula (IP) is the objective of the FRESA (“Impacto de las intrusiones de masas de aire con polvo aFRicano y de masas de aire EStratosférico en la Península Ibérica. Influencia de El Atlas”) research project. After a long term (2004-2016) analysis of jet streams in NW Africa and the analysis of a number of representative cases, in this work we present a description of the largescale meteorological processes leading to massive dust mobilization in NW Africa and transport to the IP after passing over the Saharan Atlas for three episodes (October 2008, September 2007, and February 2016). The three events had a strong impact either at the ground level or in the vertical. Contrarily to most of African events over the IP, none of these events occurred during the summertime, when African dust outbreaks are more common but the jet streams are by far less frequent over the study area. The interaction of upperlevel troughs and cut-off lows with the Atlas Mountains as well as the interaction of tropical plumes and/or the subtropical jet with the Hoggar Massif were the largescale relevant processes, but the precise location of the meteorological features is found to be crucial as indicated by the differences among the three episodes. In the case of October 2008, a cut-off low moving westwards over the Atlas was the cause of both dust mobilization (cold pool and density current formation, leading to a haboob that ran parallel to the SE slope of the Atlas) and transport to the IP. On September 2007, a low to the SW of St. Vincent Cape in combination with the North African high advected dust to the IP at mid-low tropospheric altitude after mobilization due to moist convection produced to the SW and N of the Hoggar. On February 2016, three main synoptic features at midupper levels were involved: a cut-off low to the SW of St. Vincent Cape, which was mostly responsible for the dust advection to the IP during the first part of the event; a tropical moist plume accompanied by an intense southwesterly subtropical jet streak that interacted with the Hoggar and triggered the instability at low levels; and the North African high (extended to the north when the trough was cut-off and the polar front jet retreated northwards) which advected the dust laden air masses to the east of the IP in the second part of the event. The analysis of back-trajectories calculated at multiple heightsfor the event of February 2016 in connection with aerodrome present-weather reports and satellite observations shows the orographic uplifting of the dust laden flows in the southern slope of the Saharan Atlas and their entrance into the IP at mid-low levels, in good agreement with the backscatter coefficient profiles of the CEAMA (Granada) ceilometer (Cazorla et al., submitted). This work was supported by the Spanish MINECO under grant CGL2015-70741-R (FRESA). Cazorla et al. (submitted to ACP), Near real time processing of ceilometer network data: characterizing and extraordinary dust outbreak over the Iberian Peninsula.