Dual-Aircraft Investigation of the Inner Core of Hurricane Norbert. Part II: Mesoscale Distribution of Ice Particles

Abstract

Horizontal fields of cloud microphysical parameters, vertical air motion, and horizontal wind at the 6-km level in Hurricane Norbert (1984) were obtained by mapping and interpolating data collected on board a WP-3D aircraft along numerous flight tracks executed within the central region of the storm. Although the storm was characterized by a strong vortex of winds reaching peak values > 50 m s⁻¹ all around the storm, the precipitation was concentrated on the southwest side of the storm. A sloping eyewall was located within 20 to 30 km of the eye. Stratiform precipitation dominated the region outside the eyewall. A 25-km-wide band of maximum stratiform precipitation was centered 60–70 km southwest of the storm center. The ice particles at flight level tended to be relatively large both in the eyewall and in the outer band of stratiform precipitation. Particles were smaller and more numerous (100–300 L⁻¹) in the zone between the eyewall and outer stratiform band. These particles occurred on the outside edges of the eyewall convective updrafts, indicating that they may have been produced by splintering in association with graupel formation in the updrafts. The large particles in the eyewall tended to be graupel. In the outer stratiform region, characterized by weak, average vertical air motion and an absence of strong convective drafts, the predominant particle type was aggregates. The region of large graupel particles in the eyewall coincided with the radius of maximum tangential wind and was apparently produced by the azimuthal advection of the graupel particles. Since graupel particles fall rapidly, they were not susceptible to advection out of the eyewall region by the weaker radial wind component. On the other hand, some of the more slowly falling, less dense aggregates produced in the eyewall region were evidently advected radially as well as azimuthally, thus accounting for the location of the outer region of maximum stratiform precipitation intensity.