As part of the CBLAST-Hurricane program, the NOAA Air Resources Laboratory (ARL) is developing a surface-based, robust probe that is capable of measuring the mean wind, turbulence, and vertical fluxes in the high winds and rain associated with strong tropical cyclones. This probe is based on pressure-sphere anemometry, which has successfully been used on research aircraft for many years. On aircraft, systems of this type are routinely operated at air speeds exceeding 50 m/s, so a surface-based system should have no difficulty operating in hurricane-force winds. The system under development at ARL is called the Extreme Turbulence (ET) probe. It is based on a 43 cm diameter spherical shell made from fiberglass-epoxy composite. Pressure ports are positioned at regular intervals around the sphere. The data acquisition software uses the pressure data from these ports to compute a 50 Hz time series of the three-dimensional velocity vector. In addition, a temperature sensor is located in a small housing on top of the sphere. The system has no moving parts.

Several prototypes of the ET probe have been produced, and they have undergone a series of field tests. Since the probe does not begin to operate properly until the wind speed exceeds a minimum threshold, many of the field tests took place with an ET probe mounted on a vehicle side-by-side with standard wind instruments. These vehicles were driven at highway speeds to test the probes. Limited tests have also been performed with an ET probe statically deployed near a sonic anemometer. Such static tests are possible only on days with strong ambient winds. Most of the field tests so far have been in dry conditions. These indicate that the ET probe performs well once the wind speeds exceed a threshold of about 10 m/s. More extensive testing in wet conditions is planned for fiscal year 2004.

In September 2003, an ET probe was deployed on the North Carolina Coast as Hurricane Isabel made landfall. ARL designed special portable towers for the probes capable of withstanding the high winds in a hurricane. The battery-powered tower system operated for over 36 hours without failure during the Isabel landfall, collecting about 1.2 GBytes of 50 Hz data. The eye of the hurricane passed just north of the probe. A crew from Texas Tech University deployed a sonic anemometer near the ET probe, which will allow comparisons to be made between the sonic and the ET probe.

Initial analysis of the Isabel data indicates that water fouling of the pressure ports was a problem. This was known to be an issue since early in the project, but funding delays in fiscal year 2003 precluded the development of a fix to the problem before the Isabel deployment. Two different approaches are currently in development to avoid water fouling in future deployments. One is a passive approach that relies on gravity drainage. The second is an active approach that backflushes the pressure ports using either an air pump or compressed air. Testing is planned in fiscal year 2004 to determine the performance of these approaches.