Dropsonde for Weather PHOTO COURTESYPHOTO of NOAA, USA

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Marit Finne 2000 NWS Isaac Cline Meteorology Award Editor-in-Chief Vaisala News Vaisala Helsinki Finland Dropsonde for Weather PHOTO COURTESYPHOTO OF NOAA, USA.

Vaisala’s RD93 GPS dropsonde is used in hurricane and weather research.

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r Reconnaissance

James L. Franklin, a hurricane specialist with the U.S. National Weather Service’s (NWS) National Hurricane Center (NHC) in Miami, Florida, has won the 2000 NWS Isaac Cline Meteorology Award for his study of the hurricane eyewall based on data obtained with the Vaisala GPS dropwindsonde.

he 2000 NWS Isaac debated National Hurricane Cline Meteorology Center’s (NHC) practices and Award is given an- the NHC received criticism T nually to individuals from some quarters for its use or teams that have made signi- of relatively high ratios. Mr. ficant contributions in support Franklin’s work should end the of U.S. National Weather debate. Service’s (NWS) strategic and Then a hurricane researcher operational plans, and recog- for NOAA’s Hurricane Re- nizes operational excellence search Division, Mr. Franklin within key facets of the core was part of the development NWS mission. Mr. Franklin’s team at the National Center for study was cited for resulting in Atmospheric Research (NCAR) “excellent improvements to the that first developed the RD93 accuracy of National Weather GPS dropwindsonde (Hock Service hurricane analyses, and Franklin, 1999). The drop- through innovative interpreta- sonde, now manufactured by tion and application of Global Vaisala under license from Positioning System (GPS) NCAR, is routinely deployed dropsonde data”. by reconnaissance aircraft in the hurricane eyewall, the most Meteorological debate intense and turbulent part of the hurricane circulation. The “Meteorologists have struggled dropsondes provide, for the for decades to accurately and first time, detailed, accurate objectively assess a hurricane’s profiles of the inner core of a maximum surface wind speeds, hurricane from flight level due to the lack of in-situ sur- down to the surface. face observations in the storm’s core. Even with aircraft recon- 500 dropsonde profiles naissance data, forecasters could in the eyewall only estimate the surface wind from observations typically ob- Mr. Franklin recently com- tained from a flight level of 700 pleted an analysis of more than Mb (10,000 ft),” Mr. Franklin 500 dropsonde profiles in the explains. eyewall and outer vortex re- In recent years forecasters gions obtained during the usually estimated the surface 1997–99 hurricane seasons, de- wind speed as 80–90 % of the termining the mean wind speed flight-level wind. Limited com- profile over the lowest 10,000 ft parisons of flight-level and in various portions of the hur- buoy data, however, suggested ricane circulation. His study a different range, 63–73 %. In (Franklin et al. 2000) produced view of the buoy studies, the the following results and opera- scientific community openly tional recommendations.

NOAA’s P-3 ‘Orion’ Hurricane Research Aircraft are among the most advanced airborne environmental research platforms for the study of severe storms and global climate change.

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• Operational Hurricane Analysis. The average reduction factor (R) between the surface and flight-level (700 Mb) wind in the hurricane eyewall is 0.89. This con- firms NHC’s operational practice of using higher factors than implied from buoy intercomparisons. Mr. Franklin also determined reduction factors for other typical flight levels. That is, for the 850 Mb level (com- monly flown in tropical James L. Franklin has won the 2000 NWS Isaac Cline Meteorology Award for his study storms), R = 0.80. For inves- of the hurricane eyewall based on data obtained with the Vaisala GPS dropwindsonde. tigative flights at 1,000 ft, R = 0.85. In the past, forecasters did not generally consid- er the altitude of the recon- Over 80 hurricane naissance aircraft in their es- eyewall penetrations timations. Mr. Franklin (43) relates: “I re- • Representativeness. Storm ceived my B.Sc. and M.Sc. de- to storm variability in the grees in Earth, Atmospheric, wind reduction factors was and Planetary Science from the documented, and a strong Massachusetts Institute of Tech- dependence upon the char- nology in Cambridge, Mas- acter of convective activity sachusetts. In 1982 I joined the and on cyclone strength was National Oceanic and found. A method was devel- Atmospheric Administration’s oped to evaluate eyewall (NOAA) Hurricane Research dropsonde profiles to ensure Division (HRD) as a research that the assigned wind meteorologist, where I studied speeds accurately represent- synoptic-scale influences on ed the 1-min sustained wind hurricane track. At HRD, I was standard. the principle investigator of an experiment designed to im- • Public Safety. The drop- prove hurricane forecasts by re- sonde data revealed that leasing Omega-based drop- winds at the top of a 30- windsondes in the hurricane story building average about environment, an experiment 20 kt (one Saffir-Simpson which led to NOAA’s purchase Hurricane Scale category) of a Gulfstream IV jet aircraft higher than at the surface. and the development of the Franklin’s average eyewall GPS dropwindsonde in the profile required reassessing mid-1990’s. “ the vulnerability of high-rise During his time at HRD, he This figure shows the mean eyewall profile buildings and elevated ter- made over 80 hurricane eyewall determined from the dropsonde data. rain. The study suggests penetrations in NOAA’s WP- people seeking shelter in 3D research aircraft, including high-rises should stay at the several into Hurricane Mitch, a lowest floors necessary to category five hurricane on the avoid the storm surge. Saffir-Simpson scale. He joined

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the National Hurricane Center as a hurricane specialist in 1999. In this position, he is one of a group of six NHC forecasters re- sponsible for all tropical and subtropical cyclone advisories, forecasts, and warnings for the Atlantic Ocean, Gulf of Mexico, Caribbean Sea, and the eastern North Pacific Ocean.

National Hurricane COURTESYPHOTO OF NOAA, USA. Center

“The National Hurricane Center’s mission is to save lives and protect property by issuing watches, warnings, forecasts, and analyses of hazardous weather conditions in the trop- ics,” Mr. Franklin says. “To do this, NHC maintains a contin- uous watch on tropical cyclones over these waters annu- Hurricane Floyd on September 13, 1999. ally from 15 May through November 30. The Center pre- pares and distributes hurricane Towards improved hurricane forecasts watches and warnings for the Hurricanes are products of the tropical ocean and atmosphere. Powered general public, and also pre- by heat from the sea, they are steered by the easterly trade winds and pares and distributes marine the temperate westerlies, as well as by their own ferocious energy. Around and military advisories for their core, winds grow to a great velocity, generating violent seas. Moving other users.” ashore, they sweep the ocean inward while spawning tornadoes and During the ‘off-season’, fore- producing torrential rains and floods. casters provide training for Satellite images of hurricanes show a unique and characteristic cloud U.S. emergency managers and formation, one that signals an intense tropical weather system. The power- representatives from many ful storms spawned in these tightly coiled systems produce heavy rains other countries that are affect- and maximum sustained winds of 33 m/s (74 mph / 64 knots) or higher. ed by tropical cyclones. NHC Each year an average of ten tropical storms (of which six become hurri- also conducts applied research canes), develop over the Atlantic Ocean, Caribbean Sea, or Gulf of to evaluate and improve hurri- Mexico. Every three years, about five hurricanes strike the United States cane forecasting techniques, coastline. Two of the five will be major hurricanes (category 3 or greater, and is involved in public aware- on the Saffir-Simpson Hurricane Scale. The scale from minimum to maxi- ness programs. mum is 1 to 5). Designed for weather reconnaissance, hurricane and weather research, the Airborne Vertical Atmospheric Profiling System (AVAPS) incorporates a Vaisala RD93 GPS dropsonde. This advanced research system was developed by the U.S. National Center for Atmospheric Research (NCAR) in cooperation with the German Aerospace Research Establishment (DLR) and the National Oceanic and Atmospheric Administration (NOAA). NCAR has licensed the production of the new GPS dropsonde.

References

Franklin, J. L., M. L. Black and K. Valde, 2000: Eyewall wind profiles in hurricanes determined by GPS dropwindsondes. Preprints, 24th Conf. Hurr. Trop. Meteor., Ft. Lauderdale, Amer. Meteor. Soc., 446-447.

Hock, T. F., and J. L. Franklin, 1999: The NCAR GPS dropwindsonde. Bull. Amer. Meteor. Soc., 80, 407-420.

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