Profiling of Saharan dust from the Caribbean to western Africa a?? Part??1: Layering structures and optical properties from shipborne??polarization/Raman lidar observations

摘要

pstrongAbstract./strong 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??iMeteor/i during the 1-month transatlantic cruise from Guadeloupe to Cabo Verde over 4500span class="thinspace"/spankm from 61.5 to 20?°span class="thinspace"/spanW at 14a??15?°span class="thinspace"/spanN in Aprila??May 2013. First results of the shipborne lidar measurements, conducted in the framework of SALTRACE (Saharan Aerosol Long-range Transport and Aerosola??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 532span class="thinspace"/spannm) of the extinction-to-backscatter ratio (lidar ratio) of 17?±5span class="thinspace"/spansr (MAL) and 43?±8span class="thinspace"/spansr (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?±45span class="thinspace"/spanMmsupa??1/sup (MAL) and 68?±37span class="thinspace"/spanMmsupa??1/sup (SAL). The 532span class="thinspace"/spannm 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 50a??60span class="thinspace"/spansr and 0.3, respectively) in combination with backward trajectories indicate a smoke contribution to light extinction of the order of 20span class="thinspace"/span% during May 2013, at the end of the burning season in central-western Africa./p.