P2.22 Idealized simulations of the impact of dry Saharan Air Layer air on Atlantic hurricanes

Wednesday, 19 August 2009
Arches/Deer Valley (Sheraton Salt Lake City Hotel)
Scott Braun, NASA/GSFC, Greenbelt, MD; and J. Sippel and D. S. Nolan

The existence of the Saharan air layer (SAL), a layer of warm, dry, dusty air frequently present over the tropical Atlantic Ocean, has long been appreciated. The nature of its impact on hurricanes remains unclear, with some researchers arguing that the SAL amplifies hurricane development and with others arguing that it inhibits it. The potential negative impacts of the SAL include 1) low-level vertical wind shear associated with the African easterly jet; 2) warm air

aloft, which increases thermodynamic stability; and 3) dry air, which produces cold downdrafts. Some investigators have assumed the validity of these proposed negative influences and have frequently used them to explain the failure of individual storms to intensify or to explain the relative inactivity of recent hurricane seasons. Dry air appears to be a key mechanism for SAL influence. Idealized high-resolution simulations are used to evaluate the role of dry air. The simulations are initialized with a Rankine vortex with maximum winds of 15 m/s at the surface and an environment derived from a non-SAL mean sounding. To represent the SAL, dry air with relative humidity of 25% between 850-600 hPa is placed about 270 km north of the vortex. The vortex readily wraps the dry around around it, but early results suggest that the dry air has little impact on storm evolution or intensity.

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