the centre for australian weather and climate research a partnership between csiro and the bureau of...

The Centre for Australian Weather and Climate ResearchA partnership between CSIRO and the Bureau of Meteorology

Extreme gust measurements - are Dines or cup anemometers the answer?

Bob Cechet (Geoscience Australia)

John Ginger (James Cook University)

John Holmes (JDH Consulting)

Jeff Kepert (CAWCR)

www.cawcr.gov.au

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?

•Tubing carries the pressure signal from the head to the interior of an open-bottomed float (the manometer).• Increased pressure expels water from the float and causes it to rise.

Dines float chamber and chart recorder.

The float.

Inside the tank.

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 ResearchA partnership between CSIRO and the Bureau of Meteorology

Modelling the Transient Response of the Dines Anemometer

Jeffrey D. KepertHead, High Impact Weather Research

Weather and Environmental Prediction Program Southern Hemisphere Extreme Winds Workshop, Aug 4, 2010

www.cawcr.gov.au

The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology

Modelling: Simplify the geometry

0

x1

Piston

-xeTrapped air c(t)

x2

xe

Area A

The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology

Modelling: The equations.

F = ma for the float and water (with linear damping), plus Boyle’s law for the trapped air.

Wa

ter

Flo

at

Air

The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology

Linear solutions

Equilibrium solution:

Seeking coupled, linearised solutions of the form:

yields:

Equilibrium water position

Equilibrium float position

Low frequency, float and water in phase

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)

• In-phase and out-of-phase oscillations present.

• Positive bias in mean wind speed (over-speeding) • Trapped air acts as a nonlinear spring.

Mean and instantaneous water position

Expected mean float position

Actual mean float position

Float position

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!

Resonances and overspeeding occur near linear frequencies.

Positive bias in gusts at resonant frequencies

Negative bias in gusts at other frequencies

Frequency

Ph

ase

diff

ere

nce

Am

plit

ud

e

The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology

Observations!

Amplitude and phase of float Lab. measurements (Borges 1968)

Am

plitu

de

Frequency

Pha

se

Am

plitu

de

Frequency

Pha

se

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

• 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

Standard Dines anemometer resonances

The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology

… and more observations!

CTS Lab. measurements (Henderson et al., 2010)

White noise forcing

Ratio forcing : response

Anemometer response

Frequency (Hz)

Spe

ctra

l pow

er (

kPa2 /

Hz)

The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology

• 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”

High-speed Dines resonances

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.