Dual-Aircraft Investigation of the Inner Core of Hurricane Nobert Dual-Aircraft Investigation of the Inner Core of Hurricane Nobert. Part II: Mesoscale Distribution of Ice Particles Houze, R. A., Jr., Frank D. Marks, Jr., and Robert A. B., 1992: J. Atmos. Sci., 49, 943-962.

Introduction The precipitation in the inner-core region of a hurricane is characterized by a ring of the heavy eyewall rain and a surrounding region of lighter stratiform precipitation. The convective updrafts in the eyewall region contain graupel along with some supercooled drops. The stratiform areas are characterized by aggregated snowflakes. Ice particles generated in and detrained from the top of convection at a particular location along eyewall form a slowly descending plume that spirals gradually outward from the eyewall while winding around the entire storm. In this study, authors examine the data collected by the upper-level microphysics aircraft in Hurricane Nobert.

Data collection and analysis a. Flight pattern (Hurricane Nobert 1984) 0215 ~ 0402 UTC 0018 ~ 0215 UTC The flight tracks of the upper-level (6 km) 0402 ~ 0518 UTC

b. Radar reflectivity data (WP-3D aircraft) http://www.aoc.noaa.gov/aircraft_lockheed.htm

further subdivided according to particle shape c. Classification of particle images The remaining good particle images were classified by size according to the equivalent circle diameters of the images on each probe. small medium large 2DC (2-D cloud) 0.05 ~ 0.5 mm 0.55 ~ 1.05 mm > 1.05 mm 2DP (2-D precipitation) 0.2 ~ 2 mm 2 ~ 4 mm > 4 mm PMS (Particle Measuring System) further subdivided according to particle shape very few particles were observed In this study, particle shape was assumed to be indicated by the eccentricity of an ellipse fitted to the actual image. eccentricity was < 0.4 (columns) eccentricity was > 0.9 (nearly round) 0.9≧eccentricity was ≧0.4 (aggregate of ice crystals)

d. Mapping of ice-particle data Number of data samples assigned to 5 km × 5 km.

3D radar reflectivity structure a. Eyewall echo pattern at 1-, 3-, 6-, and 9-km levels 0018 ~ 0215 UTC

b. Vertical cross sections through the eyewall

convective-scale fluctuations c. Vertical cross sections through the stratiform region outside the eyewall 240o convective-scale fluctuations 150o bright band

Mapping of ice-particle data obtained at the 6-km level a. Mass concentration The total mass of water per unit volume of air sampled by the PMS 2DC and 2DP probes. Most of the sampled hydrometeor mass was contained in particles > 0.2 mm in dimension (smallest size that cloud be resolved by the 2DP).

b. Median volume diameter The median volume diameter, D*, is defined by D is particle size, N(D) is the observed particle size distribution (m-4), V is the volume of a spherical particle of diameter D. D* for the size distributions measured by the PMS 2DC and 2DP probes. The medium volume diameters are generally larger for the 2DP.

c. Number concentration The total number of particles per unit volume of air sampled by the PMS 2DC and 2DP probes (can be regarded as small particles).

d. Particle shape eccentricity was < 0.4 eccentricity was > 0.9

e. Schematic pattern of ice-particle characteristics at the 6-km level Pattern of ice particle characteristics at 6 km.

Relationship of vertical and horizontal air motion at flight level to precipitation and microphysical fields

Trajectory analysis The region of high particle concentration between the two zones of large particles was located radially just outside strong eyewall updrafts, and be consistent with large concentrations of small particles. The eyewall-echo region was associated with strong convective-like vertical motions coinciding with the radius of maximum wind and was characterized by large particles. The precipitation band southwest of the storm center was stratiform, was characterized by weak vertical air motions, contained large particles that appear to have been produced by aggregation.

b a

Conclusions Graupel fall speeds are too great for the graupel to have been advected a significant distance in the radial direction by the storm’s secondary circulation. High concentrations of ice particles ( ~ 100-300 per liter) tended to occur on the outside edges of eyewall convective-scale updrafts. Radially outside the zone of high particle concentrations, a second band of large particles occurred. This band was located within the zone of maximum stratiform precipitation southwest of the storm. The trajectories of the snow particles from the convective cells in the eyewall to their ultimate fallout in the region of maximum stratiform precipitation. The faster falling graupel particles that remained in the radial zone of maximum wind while they fell out.