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 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 replicability. 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.
Last updated in 2003 November
by Ronald Dobosy. Questions should be directed to:
Jeff French(Jeff.French@noaa.gov) at NOAA/FRD J�rg M. Hacker (Jorg.Hacker@AirborneResearch.com.au)
at ARA
The first prototype BAT probe, combining
both NOAA and Flinders technology, was completed in February 1997.
However, the NOAA housing with sensors has been in use on the
LongEZ since 1989. Similarly, the A/D system has been used by
Flinders since 1995. Since the first proof-of-concept flight in
1987, many technological improvements have been added. Probes
are in service, being installed, or planned on the aircraft pictured
below. Clearly evident in these pictures is our attention to aerodynamically
clean installation.
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The Long-EZ N3R, now lost, first
carried the prototype BAT probe in 1989. Over the years,
the Long-EZ served both as a scientific instrument and a
test vehicle for further development. During 1991, differential
GPS technology was added, giving the first satisfactory
wind measurements. Aerodynamically, the LongEZ installation
was exceptional because the probe extended FIVE wing cord
lengths in front of the canard and main wing. As a result,
upwash and flow distortion were minimal. ARL is committed
to seeking replacement of the small-airplane capability
represented by the Long-EZ during calendar year 2003.
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Single-engine tractor aircraft are
generally poor platforms from which to make high-quality
wind measurements because of flow distortion. To mitigate
this problem on the Egrett high altitude research aircraft,
three BAT probes were installed. One probe was installed
high on the tail with an additional probe placed six meters
outboard on each wing. This symmetrical installation cancels
side-wash distortion while increasing measurement redundancy
and improving resolution. Additional information on the
unusual Egrett high altitude research aircraft is available
from the Egrett's home page.
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The new Sky Arrow is manufactured
by Iniziative Industriali Italiane. It features
a strong, all carbon-fiber airframe, is powered by a 100HP
pusher engine, and is equipped with a fixed pitch propeller,
a rectangular wing supported by two struts and a fixed landing
gear. Its max speed is 105 kts, with a low stall speed of
35 kts. The aircraft is certified (FAR23/AC2311) in the
normal category but restricted to VFR. Complete Sky Arrow
aircraft specifications
are available. The 650 ERA model designation stands for
Environmental Research Aircraft. This model has necessary
hard points, portholes and power distribution to mount the
BAT probe and Mobile Flux Platform system. The aircraft
is especially suited for high-fidelity measurements at 60
kts (30 ms-1) flight speed and altitudes up to
12,000 ft MSL.
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The first production Sky Arrow 650
ERA is owned and operated by San Diego State University.
It is tasked to perform CO2 flux measurements
on the North Slope of Alaska. In the picture, Ed Dumas and
Joe Verfaillie are making the aircraft and instrument systems
ready for a research flight from its Prudhoe Bay, Alaska
operations base. To learn more about the SDSU Sky Arrow
visit the Global Change
Research Group's web page.
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RECAB (REgional assessment and modelling
of the CArbon Balance within Europe ) operates a Sky Arrow
in Europe under funding from the European Commission. Regional
flux measurements are performed in two- to three-week campaigns
in several European countries, both in summer and in winter.
To learn more about the RECAB Sky Arrow, visit RECAB web page.
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In support of ONR's Coupled Boundary
Layer Air-Sea Transfer (CBLAST) Hurricane
research program, we are in the process of installing
BAT probes and support instrumentation on the NOAA P3's.
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- Dobosy, R. J. and T. L. Crawford, 1991:
Developments in turbulence measurement by pressure sphere. In
Proc. of the Conference on Meteorological Observations and
Instrumentation,New Orleans, LA, American Meteorological
Society, Boston, MA, 151-155.
- Crawford, TL and RJ Dobosy: 1992, A
sensitive fast-response probe to measure turbulence and heat
flux from any airplane. Boundary-Layer Meteorology,
59, 257-278.
- Dobosy, R. J., T. L. Crawford, and R.
T. McMillen, 1992: Simpler wind measurements from moving vehicles
using the global positioning system (GPS). Presented at the
72nd American Meteorological Society Annual Meeting, Third Symposium
on Global Change Studies, Atlanta GA.
- Crawford, T. L., R. T. McMillen, R.
J. Dobosy, and I. MacPherson, 1993: Correcting airborne flux
measurements for aircraft speed variation. Boundary-Layer
Meteorology 66: 237-245.
- Crawford,
T. L., R. J. Dobosy, and E. Dumas, 1995: Aircraft wind measurement
considering lift-induced upwash. Boundary-Layer Meteorology
80: 79-94
- Schlichting,
H.: 1968, Boundary-Layer Theory, McGraw-Hill, Yew York,
747pp
- Crawford,
T. L. and R. J. Dobosy, 1997: Pieces
to a puzzle: Air-surface exchange and climate,GPSWORLD,
8(11), 32 - 39.
- Hacker,
J. M. and T. L. Crawford, 1999 The BAT-probe:
The ultimate tool to measure turbulence from any kind of aircraft
(or sailplane), J of Technical Soaring, XXIII:2,
43-46
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