Direct radiative effects during intense Mediterranean desert dust outbreaks
Authors: Gkikas Antonis , Obiso Vincenzo , Pérez García-Pando Carlos , Jorba Oriol , Hatzianastassiou Nikos , Vendrell Lluis , Basart Sara , Solomos Stavros , Gassó Santiago , Baldasano José Maria 
Published: June 2018

Abstract. The direct radiative effect (DRE) during 20 intense and widespread dust
outbreaks, which affected the broader Mediterranean basin over the period
March 2000–February 2013, has been calculated with the NMMB-MONARCH model at
regional (Sahara and European continent) and short-term temporal (84 h)
scales. According to model simulations, the maximum dust aerosol optical
depths (AODs) range from ∼ 2.5 to ∼ 5.5 among the identified
cases. At midday, dust outbreaks locally induce a NET (shortwave plus
longwave) strong atmospheric warming (DREATM values up to
285 W m−2; Niger–Chad; dust AODs up to ∼ 5.5) and a strong
surface cooling (DRENETSURF values down to −337 W m−2),
whereas they strongly reduce the downward radiation at the ground level
(DRESURF values down to −589 W m−2 over the Eastern
Mediterranean, for extremely high dust AODs, 4.5–5). During night-time,
reverse effects of smaller magnitude are found. At the top of the atmosphere
(TOA), positive (planetary warming) DREs up to 85 W m−2 are found over
highly reflective surfaces (Niger–Chad; dust AODs up to ∼ 5.5) while
negative (planetary cooling) DREs down to −184 W m−2 (Eastern
Mediterranean; dust AODs 4.5–5) are computed over dark surfaces at noon.
Dust outbreaks significantly affect the mean regional radiation budget, with
NET DREs ranging from −8.5 to 0.5 W m−2, from −31.6 to
2.1 W m−2, from −22.2 to 2.2 W m−2 and from −1.7 to
20.4 W m−2 for TOA, SURF, NETSURF and ATM, respectively. Although the
shortwave DREs are larger than the longwave ones, the latter are comparable
or even larger at TOA, particularly over the Sahara at midday. As a response
to the strong surface day-time cooling, dust outbreaks cause a reduction in
the regional sensible and latent heat fluxes by up to 45 and 4 W m−2,
respectively, averaged over land areas of the simulation domain. Dust
outbreaks reduce the temperature at 2 m by up to 4 K during day-time,
whereas a reverse tendency of similar magnitude is found during night-time.
Depending on the vertical distribution of dust loads and time, mineral
particles heat (cool) the atmosphere by up to 0.9 K (0.8 K) during day-time
(night-time) within atmospheric dust layers. Beneath and above the dust
clouds, mineral particles cool (warm) the atmosphere by up to 1.3 K (1.2 K)
at noon (night-time). On a regional mean basis, negative feedbacks on the
total emitted dust (reduced by 19.5 %) and dust AOD (reduced by
6.9 %) are found when dust interacts with the radiation. Through the
consideration of dust radiative effects in numerical simulations, the model
positive and negative biases for the downward surface SW or LW radiation,
respectively, with respect to Baseline Surface Radiation Network (BSRN)
measurements, are reduced. In addition, they also reduce the model
near-surface (at 2 m) nocturnal cold biases by up to 0.5 K (regional
averages), as well as the model warm biases at 950 and 700 hPa, where the
dust concentration is maximized, by up to 0.4 K. However, improvements are
relatively small and do not happen in all episodes because other model
first-order errors may dominate over the expected improvements, and the
misrepresentation of the dust plumes' spatiotemporal features and optical
properties may even produce a double penalty effect. The enhancement of dust
forecasts via data assimilation techniques may significantly improve the
results.