Introduction

The danger and cost of hurricanes continue to be demonstrated with each hurricane season. Both the intensification and eventual decline of these storms are largely determined by turbulent exchanges of heat, water vapor, and momentum between the atmosphere and the underlying surface. Scientists are therefore keenly interested in obtaining direct turbulence measurements within hurricanes. However, standard turbulence sensors are not designed to withstand the extreme winds, rain, and spray associated with these storms. The Field Research Division is currently developing and deploying a new type of turbulence sensor that is designed for hurricane environments. This sensor, referred to as the Extreme Turbulence (ET) probe, can make accurate measurements in very high winds (and rain) without being damaged.

The ET Probe is an innovative stationary sensor based on the same technology used in aircraft gust probes, including the NOAA Air Resources Laboratory's Best Aircraft Turbulence (BAT) probe. Aircraft gust probes are routinely operated at airspeeds well exceeding 50 m s^-1, so this technology is fully capable of withstanding hurricane-force winds. Unlike aircraft probes, the ET probe must be omnidirectional so that it can respond to winds coming from any compass direction. In addition, aircraft with gust probes tend to avoid precipitation, whereas the ET probe is specially adapted to function in heavy precipitation.

A special strengthened tower has been designed for deploying the ET probe in hurricanes. The probes and towers are small enough to carry inside standard passenger vans, and the towers are easy to set up. This gives us the portability and flexibility necessary to quickly react to unexpected changes in a hurricane's path.

The ET probe is based on a 43 cm fiberglass sphere with three rows of pressure ports, one row along the sphere's "equator" and the other two 18º above and below the equator. Within each row the ports are spaced 36º apart, so there are 10 ports total in each row. The temperature sensors are in a "mushroom" housing on top of the sphere. Later in the development, support wires were added to the mushroom housing to provide additional strength in the highest winds.

The probe in the photo uses the passive technique to keep rain from fouling the ports. These are called big-hole probes, because they use ports that are 6.4 mm in diameter instead of the 1.6 mm ports used with the original ET prototype and still used in the active design. The large ports are connected to tubes that slope upward to the top of the sphere. The combination of large holes and gravity drainage is surprisingly effective in keeping the ports clear of water. This design also has the advantage that no additional power is required for the rain defense.

The active design (not shown) has ports in the same location as the photo, but they are only 1.6 mm in diameter and are connected to smaller tubing inside. Of course, running an air pump to backflush the ports requires additional power, which is a problem with a battery-operated system. Data processing is also more problematic with the active design, because rain drops striking the small ports produce spikes in the pressure data that must be removed in postprocessing. However, the active design might have some advantages in certain environments, such as a buoy deployment of an ET probe.

The 2004 hurricane season was very active, with four huricanes striking Florida within a short period. ARL deployed ET probes into two of these storms: Hurricanes Frances and Ivan. Frances struck the East Coast of Florida in early September 2004. For this storm, the ARL deployment team based out of Melbourne, FL, and ET probes were deployed along the coast in Sebastian and Vero Beach. The probes at both locations worked well.

Hurricane Ivan struck near the Florida-Alabama border in mid September. The deployment team was based in Tallahassee, FL, and deployed two ET probes (one passive and one active design) at an inactive Navy airfield near Pensacola, FL. We had planned to deploy a third probe at another location, but were delayed by the heavy, compacted soil at the airfield, which caused considerable difficulties in anchoring the towers. Ivan was stronger than Frances, but the probes still functioned well.

The figure below shows some example data collected during the Ivan deployment. It shows the turbulent kinetic energy (TKE) for a thirty-hour period starting at 1200 UTC on 15 September 2004. The TKE at the peak of the storm was about a factor of ten larger than the more normal values before and after the storm. In many hurricanes it is actually the turbulent gusts rather than the sustained winds that are responsible for much of the property damage, so data like this are of great importance.

Turbulent kinetic energy observed by big-hole probe during Hurricane Ivan.