Saharan dust, convective lofting, aerosol enhancement zones, and potential impacts on ice nucleation in the tropical upper troposphere. Issue 16 (31st August 2017)

• Record Type: Journal Article
• Authors: Twohy, C. H.; Anderson, B. E.; Ferrare, R. A.; Sauter, K. E.; L'Ecuyer, T. S.; van den Heever, S. C.; Heymsfield, A. J.; Ismail, S.; Diskin, G. S.
• Is Part Of: Journal of geophysical research. Volume 122:Issue 16(2017)
• Page Start: 8833
• Page End: 8851
• Publication Date: 2017-08-31
• Subjects: mineral dust -- convective clouds -- tropical convection -- ice nuclei -- aerosol particles -- indirect effect; Atmospheric physics -- Periodicals; Geophysics -- Periodicals; 551.5
• DOI: 10.1002/2017JD026933
• Languages: English
• ISSNs: 2169-897X

Abstract

Abstract: Dry aerosol size distributions and scattering coefficients were measured on 10 flights in 32 clear‐air regions adjacent to tropical storm anvils over the eastern Atlantic Ocean. Aerosol properties in these regions were compared with those from background air in the upper troposphere at least 40 km from clouds. Median values for aerosol scattering coefficient and particle number concentration >0.3 μm diameter were higher at the anvil edges than in background air, showing that convective clouds loft particles from the lower troposphere to the upper troposphere. These differences are statistically significant. The aerosol enhancement zones extended ~10–15 km horizontally and ~0.25 km vertically below anvil cloud edges but were not due to hygroscopic growth since particles were measured under dry conditions. Number concentrations of particles >0.3 μm diameter were enhanced more for the cases where Saharan dust layers were identified below the clouds with airborne lidar. Median number concentrations in this size range increased from ~100 l −1 in background air to ~400 l −1 adjacent to cloud edges with dust below, with larger enhancements for stronger storm systems. Integration with satellite cloud frequency data indicates that this transfer of large particles from low to high altitudes by convection has little impact on dust concentrations within the Saharan Air Layer itself. However, it can lead to substantial enhancement in large dust particles and, therefore, heterogeneous ice nuclei in the upper troposphere over the Atlantic. This may induce a cloud/aerosol feedback effect that could impact cloud properties in the region and downwind. Key Points: Relative to background upper tropospheric air, an aerosol enhancement zone (AEZ) exists at the bottom edge of tropical storm anvils Storms affected by the SAL have more large particles, likely mineral dust, in the AEZ and these contribute to the background concentration Convective lofting of dust by convective systems is predicted to enhance ice nucleating particle concentrations in the upper troposphere