Abstract
We present final and quality-assured results of multiwavelength
polarization/Raman lidar observations of the Saharan air layer (SAL) over the
tropical Atlantic. Observations were performed aboard the German research
vessel R/V Meteor during the 1-month transatlantic cruise from
Guadeloupe to Cabo Verde over 4500 km from 61.5 to 20° W at
14–15° N in April–May 2013. First
results of the shipborne lidar measurements, conducted in the framework of
SALTRACE (Saharan Aerosol Long-range Transport and Aerosol–Cloud Interaction
Experiment), were reported by Kanitz et al.(2014). Here, we present four
observational cases representing key stages of the SAL evolution between
Africa and the Caribbean in detail in terms of layering structures and
optical properties of the mixture of predominantly dust and aged smoke in the
SAL. We discuss to what extent the lidar results confirm the validity of the
SAL conceptual model which describes the dust long-range transport and
removal processes over the tropical Atlantic. Our observations of a clean
marine aerosol layer (MAL, layer from the surface to the SAL base) confirm
the conceptual model and suggest that the removal of dust from the MAL, below
the SAL, is very efficient. However, the removal of dust from the SAL assumed
in the conceptual model to be caused by gravitational settling in combination
with large-scale subsidence is weaker than expected. To explain the observed
homogenous (height-independent) dust optical properties from the SAL base to
the SAL top, from the African coast to the Caribbean, we have to assume that
the particle sedimentation strength is reduced and dust vertical mixing and
upward transport mechanisms must be active in the SAL. Based on lidar
observations on 20 nights at different longitudes in May 2013, we found, on
average, MAL and SAL layer mean values (at 532 nm) of the
extinction-to-backscatter ratio (lidar ratio) of 17±5 sr (MAL)
and 43±8 sr (SAL), of the particle linear depolarization ratio
of 0.025±0.015 (MAL) and 0.19±0.09 (SAL), and of the particle
extinction coefficient of 67±45 Mm−1 (MAL) and 68±37 Mm−1 (SAL). The 532 nm optical depth of the lofted SAL was
found to be, on average, 0.15±0.13 during the ship cruise. The
comparably low values of the SAL mean lidar ratio and depolarization ratio
(compared to typical pure dust values of 50–60 sr and 0.3,
respectively) in combination with backward trajectories indicate a smoke
contribution to light extinction of the order of 20 % during May 2013, at
the end of the burning season in central-western Africa.
Citation
ID:
185641
Ref Key:
rittmeister2017atmosphericprofiling