Poleward Transport of African Dust to the Iberian Peninsula Organized by
Multi-scale Terrain-Induced Circulations: Observations and
High-Resolution WRF-Chem Simulation Analyses
Abstract
This study presents the meso-β/γ scale dynamical features involved in an
extreme African dust outbreak, which occurred during 20-21 February 2016
over the Iberian Peninsula (IP), the southwest corner of Europe. During
this episode, nearly 90% of the air quality stations in Spain exceeded
the European Union’s PM10 daily limit. We used observations and
performed nested-grid 2km simulations with the Weather Research and
Forecasting model using coupled Chemistry (WRF-Chem) to understand the
development of this dust outbreak. The surface observations and the
false-color RGB dust product from the Spinning Enhanced Visible and
Infrared Imager (SEVIRI) revealed that the dust storm was initiated on
the southeastern flank of the Saharan Atlas Mountains at two distinct
phases of dust emissions. The first dust plume crossed the Saharan Atlas
during midday on the 20th, the second one followed in the afternoon of
the 21st. The first dust plume was advected towards the Western IP,
while the second one towards the Eastern IP. The WRF-Chem simulation
results indicated that the phase I dust emission was associated with
strong barrier jet (BJ) formation on the southeastern foothills of the
Saharan Atlas Mountains. The BJ strengthened just after sunrise on the
20th and emitted a massive amount of dust resulting in the first strong
dust storm. In phase II, a long-lived westward propagating mesoscale
gravity wave (MGW) was triggered near the northeastern edge of the
Tinrhert Plateau in eastern Algeria. When this westward propagating
long-lived MGW crossed the Tademaït Plateau, multiple hydraulic jumps
were formed on its lee side. The strong winds accompanying these
multiple hydraulic jumps emitted and mixed dust aerosols upwards which
enabled the second strong dust plume to reach the IP. The lifted dust
extended over 2-3 km in altitude in the growing daytime planetary
boundary layer (PBL) and was advected poleward by the
southerly/southeasterly wind at 700hPa. Our results underline the
importance of resolving meso-scale processes to understand dust storm
dynamics in detail, which are difficult to represent in
coarse-resolution (aerosol-) climate models.