On the origin of hurricane spiral bands

Abstract

Vertical radar cross-sections of echo intensity in Hurricane Esther are used to build a three dimensional model of a spiral band. This model and PPI time-lapse movies show that the large persistent bands are essentially stratiform and have convective type clouds only at their upwind ends. The convective clouds are observed to develop upband outward relative to the eye, while spewing out precipitation particles which stream down-wind in plume· like fashion. The net effect is echo motion along the band in both direction; the plume extends downwind while the source cloud propagates upwind. The outward radial component of propagation of the source cloud gives the band its outward velocity. The major growth is concentrated in the stratiform region of the band at middle and low levels below 18,000 ft (0° C level at 14,000 ft). The presence of the bright band in the melting layer precludes that the moisture supply for growth occurs in the form of strong convective currents. A mechanism is proposed in which cooling by evaporation and melting snow con tributes to the establishment of a spiral line of convergence near the advancing edge of the band and gives rise to a cumuliform roll cloud. Cooling by melting snow within the core of the hand establishes a 0° C isothermal and causes instability which results in a finer scale handedness of stratocumulus below the melting level. The mechanism is consistent with direct meteorological observations; namely, that a few per cent of the precipitation area is comprised of "hot" convective towers, that the intermediate spiral bands are usually cold core, and that relatively modest turbulence is experienced within them.