Dust impacting Cloud Microphysical Properties of a Mesoscale Convective System over West Africa

Mesoscale convective systems (MCSs) contribute to more than 50 % of the annual precipitation in southern West Africa (SWA). As indications have been found that dust changes the precipitation pattern in MCSs, a better understanding of the microphysical processes related to cloud particle formation in MCSs is of high interest particularly over semi-arid regions. However, in-situ cloud observations in West Africa are scarce and the high uncertainties in cloud properties limit the quality of weather forecast and climate models in this region.

Here, we present a case study of a MCS that was impacted by two dust layers at different altitudes. In situ cloud and aerosol properties were measured with a 2D-S optical array probe, a Cloud and Aerosol Spectrometer (CAS-DPOL) and a set of condensation particle counters and optical particle counters onboard the DLR Falcon research aircraft. During the DACCIWA (Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa) campaign, which took place in June and July 2016 and involved ground-based and aircraft-based measurements in five countries in SWA, the DLR Falcon conducted 12 research flights, two of which were targeting the outflow of MCSs at an altitude of 10 to 12 km. Hysplit back trajectories are calculated to study the transport pathway of the dust layers.

We find that the MCS that was studied on 13 July 2016 over SWA transported dust particles originating from the Bodélé depression in the Sahara to an altitude above 10 km. Here they acted as ice nucleating particles (INPs) and participated in the formation of ice crystals in the outflow of the MCS. Parameterized INP concentrations agree with or slightly exceed the concentration of ice crystals. This and the relatively pristine shapes of the ice crystals indicate primary ice formation as the main process of ice crystal formation at this high altitude. Furthermore, it shows that MCSs can lift their own INPs to altitudes above 10 km.

A second cloud layer was measured at 7-8 km altitudes. Apparently, these mid-level clouds also formed on dust particles advected from the Bodélé depression. However, at this lower altitude ice crystal number concentrations exceed parameterized INP concentrations by two orders of magnitude, suggesting that secondary ice production took place, in line with earlier observations of ice formation in MCSs.