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. Standard turbulence sensors, however, are not designed to withstand the extreme winds, rain, and spray associated with these storms. The Field Research Division has developed and deployed 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 sensor intended for deployment on a stationary tower or buoy, but it is 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 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.

During early testing in 2003 and 2004, the probes were deployed near the coast as hurricanes made landfall. A special strengthened tower was used for these deployments, which lasted just a few days. These deployments were primarily intended to be proof-of-concept demonstrations for the probes. Since 2009, the probes have been deployed for longer periods on fixed platforms over the sea. These recent deployments are designed to provide observations of air-sea turbulent exchange in high-wind conditions.

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. A fast-response temperature sensor is in a housing on top of the sphere. During longer deployments sea birds were found to cause problems by perching on the temperature housing. Bird spikes were therefore added at the top of the probes.

All the probes now use 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 early ET prototypes. 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.

In the latest generation of the probe design, all the analog temperature and pressure channels are digitized at 50 Hz by a small data acquisition board inside the sphere. Low-pass filters are used to eliminate aliasing before the channels are digitized. The digitized data are sent via a serial line to a single-board computer running the Linux operating system. This computer is located in a separate housing both for easy access and to improve the odds of data recovery if the probe itself is lost in a storm. Raw pressure and temperature data coming from the probe are used by the computer to generate the 50 Hz wind vector measurements.

In the 2010 deployments, the probes included cell-phone modems that allowed remote two-way communication. The bandwidth was sufficient to issue remote commands to the computer and to download time-average statistics, but not for transferring any 50 Hz data files. A satellite modem could also be used at more remote locations.

As noted above, the early ET probe deployments took place along the coast as hurricanes made landfall. In September 2004 two probes were deployed along the Gulf coast near the Florida-Alabama border for Hurricane Ivan. The hurricane's right eyewall passed near the probes. The figure to the right shows the turbulent kinetic energy (TKE) during Ivan's landfall 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.

Since 2009 the probes have been deployed on over-water platforms for months at a time during the hurricane season. The photo below shows a probe deployed on the Tennessee Reef navigation light in the Florida Keys. Platforms such as this are useful because they allow for over-water flux measurements but can still be accessed by relatively small boats. The long-term deployments have introduced a new set of challenges that were not encountered during the earlier coastal deployments. These include providing continuous power, including a means of communication, accounting for heat buildup in the electronics, and keeping birds from making a mess of the probes by perching on them. ARL has come up with solutions to these challenges. Both the 2009 and 2010 hurricane seasons resulted in few tropical cyclones near the U.S., so the long-term deployments have not yet resulted in a hurricane strike.

ET probe deployed on the Tennessee Reef navigation light in the Florida Keys. The white probe sphere is visible on the tower to the left of the light enclosure.