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
(\u201cImpacto de las intrusiones de masas de aire con polvo
aFRicano y de masas de aire EStratosf\ue9rico en la
Pen\uednsula Ib\ue9rica. Influencia de El Atlas\u201d) 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