Airplanes open the spatial dimension of the atmosphere to observation in a highly versatile, three-dimensional way. Particularly fruitful is the study of air-surface exchange through the atmosphere's boundary layer. This lowest several hundred meters of air is characterized by turbulence, a chaotic, three-dimensional flow that transports mass, momentum, and energy, both upward and downward. Thus heat, water vapor, CO2, and other quantities are exchanged between the atmosphere and the surface below, whether land or sea. Such exchanges drive both the weather and the climate. Improvement in understanding them promises better forecasts, both short-term and long-term. Airborne turbulence measurements are valuable away from the boundary layer as well. Clear-air turbulence in the upper troposphere is important to aviation and military operations. Atmospheric thermals and the three-dimensional wind field around them are important to air-pollution dispersion and to mesoscale weather forecasting.
The picture shows NOAA Long-EZ airplane on which the physical probe was primarily developed. Tim Crawford, whose vision brought the BAT probe into being, died of a massive stroke in September 2002. Since he was flying the Long-EZ at the time, we lost both him and the airplane. Nevertheless, a hallmark of his leadership was to develop the talents of those he was leading. He has left behind a group of capable people and a mature system. A hallmark of the BAT probe is its low cost and adaptability to nearly any airplane. It is now commercially available and in use in multiple places around the globe. NOAA's Air Resources Laboratory (ARL) is committed to seeking replacement of the capability within the calendar year 2003. We look forward to continuing the excellence in airborne turbulence measurement that was Tim Crawford's vision. The photograph came by courtesy of the NOAA ARL Field Research Division in Idaho Falls.
Wind is determined from an airplane by taking the airflow relative to the airplane and adding the airplane's (wind-flow sensor's) motion relative to earth. Since the airplane flies at least ten times as fast as the wind, these two vectors nearly cancel. The wind vector is a small residual. Obtaining Research-grade measurements of atmospheric turbulence from an airplane is thus a demanding task. Over the last fifty years, multiple instrument configurations have been used. In the most successful, relative wind was derived from the pressure distribution on the airplane's nose cone, while the airplane's motion (not the nose cone's) was determined by an Inertial Navigation System (INS). The nose cone's motion had to be inferred from the INS information. These systems involved complex, heavy, and expensive components, usually built into the structure of a dedicated airplane. Ease of replication on general aircraft was not a design criterion. Furthmore, such systems were unsuitable for small inexpensive aircraft. Efforts with mechanical gyro systems and flux-gate-magnetometer compasses in small airplanes gave marginal performance. Since about 1990, however, technological developments in all aspects of airborne turbulence measurements, have completely changed the situation.
The Best Aircraft Turbulence Probe (BAT) probe is a joint effort by NOAA and Airborne Research Australia (ARA). The acronym derives from the shape of the probe, which resembles a baseball bat. The purpose of this design is to extend the sensor head forward from the airplane into a region of minimal flow disturbance. The full name reflects a commitment to continuing excellence in turbulence measurement. The system was conceived by Tim Crawford in the late 1980's and initially developed by his team at ATDD. It has been in operational use since 1989 over which time its design has evolved to improve accuracy, modularity, and duplicability. The current version is in use worldwide.
The BAT probe incorporates sensors of the latest design in a self-contained package requiring only connections for power and for serial data transmission. The probe's internal temperature is controlled to maintain calibration stability over a wide range of ambient temperatures. The probe has been used from tropical to polar regions, over land and sea, and from 5 m to 15,000 m above sea level. The sensors measure both the wind relative to the probe and the probe's motion relative to the earth, all within 0.2 m of each other. The probe's motion is derived from the Global Positioning System (GPS). The primary GPS antenna can be seen in the diagram, directly behind the sensor head. It feeds signals to high-quality multi-channel, dual frequency receivers. High-precision velocity measurements are obtained from the Doppler shift in the carrier frequency of the GPS signal. Furthermore, the position of the sensor head is found within a few tens of millimeters by differential correction using the phase of the carrier wave. The relative wind is found from the pressure distribution over the hemispherical head of the probe. The system provides fifty measurements each second of the pressure, temperature, and three components of wind. Its mass is about 3 kg. It requires 10 W of (10 to 30) VDC power. Installation, though not yet trivial, is far easier than for most airborne flux systems. Any airplane from an ultra-light to a B-52 can achieve high fidelity turbulence measurements with this probe.