Observations of the Distribution of Ice in Hurricanes

Abstract

Observations of the type and distribution 0°C isotherm in three Atlantic hurricanes are presented. Supercooled drops, graupel, columns and aggregated snowflakes were observed. The supercooled drops were found only in convective updrafts stronger than 5 m s⁻¹, but not all updrafts > 5 m s⁻¹ contained appreciable liquid. Graupel was found in all updrafts at temperatures < −2°C, and small columns were sometimes found in downdrafts. Nonconvective rainbands contained 15–30 L⁻¹ of snow composed of columns and what appeared to be large aggregates. Other stratiform regions contained 1–15 L⁻¹ of medium and large aggregates; columns were occasionally found there also but only within about 15 km of convection. Hurricane convection is almost completely glaciated at the −5°C level. It is suggested that the ice particles observed at 6.0 km inside the convection result primarily from downward mixing on both sides of the eyewall updraft of ice formed in the convective areas at higher, colder levels. The ice in the stratiform areas is believed to have fallen from the high-level (6.km and higher) eyewall outflow.