Formation and Maintenance of Tropical Cyclone Spiral Bands in Idealized Numerical Simulations

Spiral bands are one of the most prominent features of tropical cyclones (TCs). Since the advent of radar technology, numerous ideas have been proposed to explain the existence of TC spiral bands. Previous hypotheses include the manifestation of atmospheric waves emanating from the TC inner core, boundary layer instabilities, and the interaction between surface cold pools and low-level vertical wind shear. Despite much effort, no consensus has yet been reached on the underlying physical mechanism responsible for TC bands. We approach this problem by examining the formation of TC spiral bands in a set of idealized three-dimensional simulations using the System for Atmospheric Modeling. The simulations are run with doubly-periodic horizontal boundaries, constant sea surface temperature, interactive surface fluxes, and a constant rotation rate corresponding to 15N latitude. No background mean flow is imposed on the TC runs.

We find that, in simulations with full moist physics and interactive radiative fluxes, spiral bands are consistently collocated with surface cold pools and aligned normal to the low-level wind shear, similar to tropical squall-lines. However, convection still organizes into spiral bands in simulations in which surface cold pools are suppressed. Non-rotating experiments with imposed background wind shear taken from a TC simulation suggest that, in the absence of surface cold pools, vortex dynamics are necessary for convection to align into spiral bands. Finally, we examine numerical simulations of TC-like vortices that develop over a completely dry surface. We find that these dry TCs exhibit numerous spiral bands in the wind and temperature fields extending far away from the inner core. Initially, these perturbations are nearly stationary and exhibit overturning circulations consistent with boundary layer instabilities. Barotropic-baroclinic instability dominates the TC structure later in the simulation, reducing the outer region to just a few spiral bands.