Wednesday, 5 June 2002
Extra large particle images at 40,000 ft. in a hurricane eyewall: Evidence of partially frozen raindrops?
The conventional wisdom about hurricanes indicates that supercooled water is scarce in the eyewall, and almost non-existent at temperatures colder than about –5°C (Black and Hallett 1986). However, there is evidence that some hurricanes are different. Black et al (1992) reported one such storm, 1987 Hurricane Emily. This storm featured updrafts at the melting level of ~ 20 m s-1, elevated radar reflectivity maxima at 8 – 10 km (much higher than usual), and lightning. Questions about the existence of high-altitude supercooled water cannot be answered with only the instruments aboard the typical hurricane hunter aircraft, the Lockheed WP-3D Orion. However, during the summer of 1998, the NASA DC-8 aircraft made repeated penetrations of the eyewall of Hurricane Bonnie, collecting the first truly high-altitude (~40,000 ft MSL) PMS 2-D imagery of the precipitation particles in a hurricane. While the vast majority of the images appeared to be of irregular shape and less than 1 mm in diameter, a few were rounded and > 2 mm in diameter. Some of these appeared to be at least partially liquid. The occasional presence of such deeply supercooled raindrops at these altitudes may greatly impede efforts to accurately quantify condensate mass remotely from radiometric data such as that provided by the TRMM satellite.