The footprints of Saharan air layer and lightning on the formation of tropical depressions over the eastern Atlantic Ocean
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The roles of the Saharan Air Layer (SAL) and lightning during genesis of Tropical Depression (TD) 8 (2006) and TD 12 (2010) were investigated in relation to the interaction of the dust outbreaks with each system and their surrounding environment. This study applied data collected from the 2006 NASA African Monsoon Multidisciplinary Analysis and 2010 Genesis and Rapid Intensification Processes projects. Satellite observations from METEOSAT and Moderate Resolution Imaging Spectroradiometer (MODIS)—Aerosol Optical Depth (AOD) were also employed for the study of the dust content. Lightning activity data from the Met Office Arrival Time Difference (ATD) system were used as another parameter to correlate moist convective overturning and a sign of cyclone formation. The AOD and lightning analysis for TD 8 demonstrated the time-lag connection through their positive contribution to TC-genesis. TD 12 developed without strong dust outbreak, but with lower wind shear (2 m s−1) and an organized Mesoscale Convective System (MCS). Overall, the results from the combination of various data analyses in this study support the fact that both systems developed under either strong or weak dust conditions. From these two cases, the location (i.e., the target area) of strong versus weak dust outbreaks, in association with lightning, were essential interactions that impacted TC-genesis. While our dust footprints hypothesis applied under strong dust conditions (i.e., TD 8), other factors (e.g., vertical wind shear, pre-existing vortex and trough location, thermodynamics) need to be evaluated as well. The results from this study suggest that the SAL is not a determining factor that affects the formation of tropical cyclones (i.e., TD 8 and TD 12).
KeywordsTropical Cyclone Dust Particle Aerosol Optical Depth Vertical Wind Shear Mesoscale Convective System
The critical reviews of anonymous reviewers and the editor were very helpful. We acknowledge the suppliers of datasets utilized in this research. Computations were performed at the National Center for Atmospheric Research. We wish to express our appreciation to J. Dunion and G. Jenkins for the suggestions for the development of this research. Comments and suggestions on the revised manuscript from Duane Stevens and Craig Clements were much appreciated. This research was supported by the National Science Foundation Grant AGS-1209296.
- CIMSS Tropical Cyclone Team (2013) Saharan air layer (SAL)—product description. http://tropic.ssec.wisc.edu/misc/sal/info.sal.m8split.html
- Gaffard C, Nah J, Atkinson N, Bennett A, Callaghan G, Hibbett E, Taylor P, Turp M, Schulz W (2008) Observing lightning around the globe from the surface. In: 20th international lightning detection conferenceGoogle Scholar
- Rhome JR, Sisko CA, Knabb RD (2006) On the calculation of vertical shear: an operational perspective. In: Preprints, 27th conference on hurricanes and tropical meteorology, Monterey, CA, Amer Meteor Soc, 14A.4. http://ams.confex.com/ams/pdfpapers/108724.pdf