Finding Tiny Particles In Hurricanes May Help With Predictions

During a flight into Hurricane Humberto in 2001, researchers identified a large number of ice particles up to 7 millimeters in diameter in the upper reaches of the storm, around 39,000 feet high. The research was part of CAMEX-4 (The Convection And Moisture EXperiment), a joint effort between NASA, the National Oceanic and Atmospheric Administration (NOAA) and a number of universities.

"We've found that a high concentration of small particles at the top portion of a hurricane are very reflective, and they send more energy back to space than previously thought," said Andrew J. Heymsfield, researcher from the National Center for Atmospheric Research (NCAR), Boulder, Colo., and a lead author on a paper on the experiment.

Large concentrations of small ice crystals can reflect more sunlight and can lead to strong cooling near the tops of hurricanes, creating more of a temperature contrast between the bottom and top of the storm. That contrast of air temperatures could possibly contribute to its strength, just as a cold front moving into warm, moist air triggers thunderstorms.

"Updrafts of wind in hurricanes are generally pretty small, so some researchers did not expect large (up to 7 millimeters) particles, which generally form at warm temperatures, to exist at higher altitudes," Heymsfield said. Because large particles weigh more, they fall faster than small particles, and small particles can evaporate.

Aaron R. Bansemer, co-author on the paper and on the experiment noted that the data from Humberto showed some of the larger particles were transported to at least 35,000 feet, or may have grown to large sizes through aggregation of smaller particles, a process that is usually thought to occur at warmer temperatures, usually around 18,000 feet.

Scientists are curious about how ice particles and condensed water is distributed in a hurricane because it tells them where most of the mass or ìweightî is in a hurricane. Knowing this distribution of mass will help researchers determine if a hurricane is strengthening. If most of the hurricaneís mass (condensed water and ice particles) is high in the storm, some may escape into other parts of the upper troposphere where it evaporates, weakening it by reducing the overall precipitation efficiency of the hurricane.

This discovery and observations of particles higher up in hurricanes are being entered into computer models that simulate hurricanes, and will give researchers a better idea of the workings of hurricanes. "The bottom line here is that by identifying where larger ice particles are in a hurricane, we are adding to the data we put in computer models simulating the storms, and that will improve predictions on whether a hurricane is strengthening or weakening," Heymsfield said.

This paper, "In Situ Measurements of Particle Size Distributions in Hurricane Humberto," will be presented on Monday, April 29, 2002, at the American Meteorological Society Hurricane and Tropical Meteorology Conference, San Diego, Calif.

This research was entirely funded by NASA through the CAMEX-4 Mission.