The Role of the Saharan Air Layer and Dust in Atlantic Tropical Cyclone Evolution As Diagnosed From MERRA-2 Reanalyses

The Saharan Air Layer (SAL) is thought to have potential impacts on the genesis and evolution of tropical cyclones in the Atlantic basin. Both the magnitude and the sign of this impact has been debated, with several studies suggesting that dust, dry air, higher stability, and increased vertical wind shear have detrimental impacts on storm development and intensification. Other studies have suggested very limited impact, positive impacts, or both positive and negative impacts on intensification. An earlier study by the author (from 2010) used NOAA Global Forecast System (GFS) reanalyses to find that the SAL had limited impacts on intensification in the days following genesis. The NOAA reanalyses had relatively coarse resolution and did not incorporate the impacts of Saharan dust on dynamic and thermodynamic fields. In the current study, higher resolution NASA Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA2) reanalyses, which use the NASA Goddard Earth Observing System (GEOS) model with coupled aerosol physics, are used to reassess the impacts of the SAL, including dust, on the evolution of developing tropical cyclones. Fifty-five Atlantic storms forming in the Main Development Region are tracked within the reanalysis fields extending from two days before to two days after genesis (the time of the first storm report in National Hurricane Center's Tropical Cyclone Reports). The presentation will discuss the extent to which SAL properties near the time of genesis are correlated with future intensity change. Meteorological and dust mass fields are correlated to intensity change in the period 2-4 days after genesis using a Spearman's rho rank correlation analysis. Overall, an increased influence of warm and dry air is seen relative to the 2010 analysis; however, the dry air influence is primarily found at middle levels (600-500 hPa) and extending above the low-level dry SAL air while the influence of warm SAL air is of relatively short duration. Only a very weak influence of dust is found, positive prior to genesis, negative afterwards. As with the 2010 study, the strongest relationship is seen in the zonal winds with stronger easterlies/weaker westerlies north of the storms associated with strengthening storms.