Influence of the Saharan Air Layer on Hurricane Nadine (2012). Part I: Observations from the Hurricane and Severe Storm Sentinel (HS3) Investigation and Modeling Results

Abstract

AbstractThis study uses a model with aerosol-cloud-radiation coupling to examine the impact of Saharan dust and other aerosols on Hurricane Nadine (2012). In order to study aerosol direct (radiation) and indirect (cloud microphysics) effects from individual, as well as all aerosol species, eight different NU-WRF simulations were conducted. In several simulations, aerosols led to storm strengthening, followed by weakening relative to the Ctrl simulation. This variability of the aerosol impact may be related to whether aerosols are ingested into clouds within the outer rainbands or the eyewall. Upper tropospheric aerosol concentrations indicate vertical transport of all aerosol types in the outer bands but only vertical transport of sea salt in the inner core. The results suggest that aerosols, particularly sea salt, may have contributed to a stronger initial intensification, but that aerosols ingestion into the outer bands at later times may have weakened the storm in the longer term. In most aerosol experiments, aerosols led to a reduction in cloud and precipitation hydrometeors, the exception being the dust-only case that produced periods of enhanced hydrometeor growth. The Saharan Air Layer (SAL) also impacted Nadine by causing a region of strong easterlies impinging on the eastern side of the storm. At the leading edge of these easterlies, cool and dry air near the top of the SAL was being ingested into the outer-band convection. This midlevel low equivalent-potential-temperature air gradually lowered toward the surface and eventually contributed to significant cold pool activity in the eastern rain band and in the northeast quadrant of the storm. Such enhanced downdraft activity could have led to weakening of the storm, but it is not presently possible to quantify this impact.