A Multi-Scale Dynamical Analysis of the Saharan Dust Outbreak Towards
the Cape Verde in Early November 2017
Abstract
On 13 November 2017, a strong continental-scale Saharan dust outbreak
was observed in satellite imagery over Mindelo, Cape Verde, located
about 650 km off the coast of Senegal in West Africa. Horizontal
visibility was reduced to 1100 m leading to major disruptions of the
local air traffic. Dust mobilization was already observed over the
foothills of the Saharan Atlas Mountains at 0600 UTC on 10 November 2019
but did not appear clearly in SEVIRI pink dust images in the subsequent
days. In this study, we examined the multi-scale dynamical processes
associated with this particular dust storm using ECMWF ERA-Interim
reanalysis data, newly performed very-high resolution WRF-CHEM
simulations with horizontal grids of 18 km and less, NAAPS aerosol
forecast output, ship-based observation dataset from the North Atlantic
Expedition MSM 68/2, as well as surface observations, and upper-air
soundings from weather stations in North Africa. Our analyses of this
storm highlights the following meteorological processes: (1) the event
was associated with a typical Harmattan surge, i.e., the post-frontal
strengthening of the northerly winds behind an eastward moving cyclone,
(2) a series of earlier Rossby Wave Breaking events (RWBs) made the
environment favorable for the Harmattan surge, (3) the dust storm was
composed of two distinct dust surges, (4) the dust aerosol from the
first surge was later mixed with the dust from the second surge while
simultaneously propagating south-westward and later westward, (5) the
PBL became adiabatic along the leading edge of the leftover cold front
between the southern/southeastern flank of the Atlas Mountains and
western/northwestern flank of the Hoggar Mountains, and (6) vertical
dust mixing then occurred due to very strong surface heating associated
with the development of a deep daytime PBL in the region behind the cold
front. The results of this research demonstrate that very-high spatial
resolution WRF-CHEM model can resolve the dynamical processes and
realistically simulate large-scale North African dust storm.