417 Formation of Tropical Cyclone Spiral Rainbands in Idealized Numerical Simulations

Tuesday, 24 January 2017
4E (Washington State Convention Center )
Diamilet Perez-Betancourt, MIT, Cambridge, MA; and K. A. Emanuel

Spiral rainbands are regions of clouds and precipitation that spiral inward toward the center of tropical cyclones (TCs). These features are often the source of various TC-related threats, such as inland flooding, mudslides, and tornadoes. Their characteristics are widely used to estimate TC intensity from satellite data, particularly in the absence of in situ measurements. Moreover, previous studies suggest that examining spiral rainbands may play a role in TC intensity changes.
The purpose of this research is to identify the underlying physical mechanisms responsible for the formation and organization of TC rainbands. Several previous studies characterize these bands as atmospheric waves emanating from the TC inner core. Others argue that rainbands are similar to tropical squall lines, which evolve through the interaction of spreading cold pools with the ambient vertical wind shear. Despite these and many other efforts, there is still a lack of consensus about the cause of TC rainbands. We approach this problem by examining TC spiral bands in idealized three-dimensional simulations from the System for Atmospheric Modeling. The simulations are run at fixed sea surface temperature with interactive radiation and surface fluxes. We find that, in a simulation with no background flow, spiral rainbands are consistently collocated with surface cold pools. These rainbands are oriented roughly perpendicular to the low-level vertical wind shear ahead of the cold pool, consistent with predictions from the theory of tropical squall lines. These initial results suggest that cold pool - shear interactions play an important role in the evolution of TC rainbands in this idealized setup. Preliminary insights into the role of atmospheric waves in TC rainband formation are also discussed.
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