Extreme gust measurements - are Dines or
cup anemometers the answer?
www.cawcr.gov.au
Bob Cechet (Geoscience Australia)
John Ginger (James Cook University)
John Holmes (JDH Consulting)
Jeff Kepert (CAWCR)
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
What is a Dines Anemometer?
• The “head” is a large-diameter pitot
tube, mounted on a vane.
Dines anemometer head, Townsville Airport.
Photo JCU/CTS.
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
How does it record the wind?
Inside the tank.
The float.
•Tubing carries the pressure signal from
the head to the interior of an openbottomed float (the manometer).
• Increased pressure expels water from
the float and causes it to rise.
Dines float chamber and
chart recorder.
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Why is the Dines important?
• How does the float respond to gusts?
• Are there resonant frequencies?
• Can the float bob up and down?
• Australian record wind gusts measured by Dines
anemometers:
• Cyclone Tracy, Dec 24 1974, 217 km/hr.
• Cyclone Trixie, Feb 19 1975, 246 km/hr.
• Cyclone Vance, March 23 1999, 267 km/hr.
• The Vance measurement had a co-located cup
anemometer that measured ~35 km/hr lower. Can we trust
the Dines?
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
What are the project aims?
• Modelling of the transient response of the float
chamber when forced by gusty winds (me).
• Measurements of a float chamber forced by varying
winds (John Ginger, JCU CTS)
• Comparison of Dines and cup anemometer
climatologies (Bob Cechet, GA)
• Transfer functions (John Holmes)
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Modelling the Transient Response of the
Dines Anemometer
www.cawcr.gov.au
Jeffrey D. Kepert
Head, High Impact Weather Research
Weather and Environmental Prediction Program
Southern Hemisphere Extreme Winds Workshop, Aug 4, 2010
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Modelling: Simplify the geometry
Area A
Piston
x1
Trapped air c(t)
-xe
xe
x2
0
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Modelling: The equations.
Water
Float
Air
F = ma for the float and water (with linear damping), plus Boyle’s
law for the trapped air.
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Linear solutions
Equilibrium solution:
Equilibrium water position
Equilibrium float position
Seeking coupled, linearised solutions of the form:
Low frequency, float and
water in phase
yields:
High frequency, float and
water out of phase
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Numerical solutions (no friction)
Float position
Actual mean float
position
Expected mean
float position
Mean and
instantaneous
water position
• In-phase and out-of-phase oscillations present.
• Positive bias in mean wind speed (over-speeding)
• Trapped air acts as a nonlinear spring.
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
5. Power Spectrum
• Power spectrum (float, water)
• Linear frequencies
dominate
• Numerous harmonics and
interharmonics (nonlinear, but
possibly not too much)
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Forced, damped response – resonances!
Amplitude
Positive bias in gusts at
resonant frequencies
Phase difference
Resonances and overspeeding
occur near linear frequencies.
Negative bias in gusts
at other frequencies
Frequency
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Observations!
Lab. measurements (Borges 1968)
Phase
Amplitude
Amplitude
Amplitude and phase of float
Phase
Frequency
Frequency
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Real geometry
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Standard Dines anemometer resonances
• Power spectral density (log scale) of float motion as a function of mean
wind speed, standard Dines anemometer.
• Oscillation at ~0.5 Hz is “in-phase”, slight wind-speed dependence
• Oscillation at 1 – 3 Hz is “out-of-phase”, marked wind-speed dependence
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
… and more observations!
Spectral power (kPa2/Hz)
CTS Lab. measurements (Henderson et al., 2010)
White noise forcing
Ratio forcing : response
Anemometer response
Frequency (Hz)
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
High-speed Dines resonances
• Power spectral density (log scale) of float motion as a function of mean
wind speed, high-speed Dines anemometer.
• Oscillation at ~0.3 Hz is “in-phase”
• Oscillation at 1.5 – 4 Hz is “out-of-phase”
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology
Conclusions
• Dines anemometer has two resonances
• low frequency (~0.5 Hz), water and float in phase
• high frequency, (1 – 3 Hz) water and float out of phase
• System is nonlinear but not strongly so
• Excellent agreement between model and observations
• Acknowledgements: Dept of Climate Change funding, JCU Cyclone Testing
Station, Jeff Callaghan, Bob Cechet, Dave Edwards, John Ginger, Bruce Harper,
David Henderson, John Holmes, Paul Leigh, Craig Miller, and Ian Muirhead.
The Centre for Australian Weather and Climate Research
A partnership between CSIRO and the Bureau of Meteorology