Extra large particle images at 12 km in a hurricane eyewall: Evidence of high-altitude supercooled water?

The conventional wisdom about hurricanes suggests that updrafts are weak and supercooled water is scarce in the eyewall, and almost non-existent at temperatures colder than about -5degreesC [Black and Hallett, 1986]. However, there is evidence that some hurricanes are different. Questions about the existence of high-altitude supercooled cloud water cannot be answered with only the instruments aboard the typical propeller-driven aircraft. During the summer of 1998, the NASA DC-8 aircraft made penetrations of the intensifying eyewall of Hurricane Bonnie at 12 km MSL, collecting the first truly high-altitude 2-D particle imagery in a hurricane. The similarity of the splash images in Hurricane Bonnie to those from raindrops obtained at higher temperatures in other hurricanes suggests that the large images obtained by the DC-8 were soft, low density graupel, rather than hard, high-density graupel particles or frozen raindrops. This implies that these particles grew to several millimeters in diameter at altitude, rather than simply advecting from lower, warmer altitudes. This growth in turn requires the presence of deeply supercooled cloud droplets. Thermal emission from supercooled water aloft increases the microwave brightness temperatures, giving a misleading impression that there is much less ice aloft than actually exists. The extra attenuation from the occasional presence of large graupel at these altitudes reduces the ability of microwave sensors to see precipitation at lower altitudes. Both of these effects impede efforts to accurately quantify condensate mass remotely from radiometric data such as that provided by the TRMM satellite.