Comment on acp-2021-219

Abstract

In this paper, a long-term large-scale Sahara dust transport event occurred during 14 June and 27 June 2020 is tracked with the spaceborne lidars ALADIN and CALIOP observations and the models ECMWF and HYSPLIT analysis. We evaluate the performance of the ALADIN and CALIOP on the observations of dust optical properties and wind fields and explore the capability in tracking the dust events and in calculating the dust horizontal mass fluxes with the combination of measurement data from ALADIN and CALIOP coupled with the products from ECMWF and HYSPLIT. Compared with the traditional assessments based on the data from CALIOP and models, the complement of Aeolus-produced aerosol optical properties and wind data will significantly improve the accuracy of dust horizontal flux estimations. The dust plumes are identified with AIRS/Aqua Dust Score Index and with the Vertical Feature Mask products from CALIPSO. The emission, dispersion, transport and deposition of the dust event are monitored using the data from HYSPLIT, CALIPSO and AIRS/Aqua. With the quasi-synchronization observations by ALADIN and CALIOP, combining the wind vectors and relative humidity, the dust horizontal fluxes are calculated. From this study, it is found that the dust event generated on 14 and 15 June 2020 from Sahara Desert in North Africa, and then dispersed and transported westward over the Atlantic Ocean, and finally deposited in the Atlantic Ocean, the Americas and the Caribbean Sea. During the transport and deposition processes, the dust plumes are trapped in the Northeasterly Trade-wind zone between the latitudes of 5° N and 30° N and altitudes of 0 km and 6 km (in this paper we name this space area as “Saharan dust eastward transport tunnel”). From the measurement results on 19 June 2020, influenced by the hygroscopic effect and mixing with other types aerosols, the backscatter coefficients of dust plumes are increasing along the transport routes, with 3.88 × 10−6 ± 2.59 × 10−6 m−1 sr−1 in “dust portion during emission phase”, 7.09 × 10−6 ± 3.34 × 10−6 m−1 sr−1 in “dust portion during development phase” and 7.76 × 10−6 ± 3.74 × 10−6 m−1 sr−1 in “dust portion during deposition phase”. Finally, the horizontal fluxes at different dust parts and heights on 19 June and on entire transport routine during transportation are computed. On 19 June, the dust horizontal fluxes are about 2.17 ± 1.83 mg m−2 s−1 in dust portion during emission phase, 2.72 ± 1.89 mg m−2 s−1 in dust portion during development phase and 3.01 ± 2.77 mg m−2 s−1 in dust portion during deposition phase. In the whole life-time of the dust event, the dust horizontal fluxes are about 1.30 ± 1.07 mg m−2 s−1 on 15 June 2020, 2.62 ± 1.88 mg m−2 s−1 on 16 June 2020, 2.72 ± 1.89 mg m−2 s−1 on 19 June 2020, 1.98 ± 1.41 mg m−2 s−1 on 24 June 2020 and 2.11 ± 1.74 mg m−2 s−1 on 27 June 2020. From this study, it is found that the minimum of the fluxes appears when the dust event is initially generated on 15 June. During the dust development stage, the horizontal fluxes gradually increase and reach to the maximum value on 19 June with the enhancement of the dust event. Then, the horizontal fluxes gradually decrease since most of the dust deposited in the Atlantic Ocean, the Americas and the Caribbean Sea. Combining the Chlorophyll concentrations data provided by MODIS-Aqua, the Saharan Dust is found transported across the oligotrophic regions Atlantic Ocean towards the Americas and Caribbean Sea, which are also oligotrophic regions. The mineral dust delivers micronutrients including soluble Fe and P to the deposition zones and has the potential to fertilizing the ocean and increase the primary productivity in the Atlantic Ocean and Caribbean Sea.