The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Extreme gust measurements - are Dines

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The Centre for Australian Weather and Climate Research A 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 Slide 2 The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Slide 3 Slide 4 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. Slide 5 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. Slide 6 The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Slide 7 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? Slide 8 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) Slide 9 The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Slide 10 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 Jeffrey D. Kepert Head, High Impact Weather Research Weather and Environmental Prediction Program Southern Hemisphere Extreme Winds Workshop, Aug 4, 2010 www.cawcr.gov.au Slide 11 The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Modelling: Simplify the geometry 0 x1x1 Piston -x e Trapped air c(t) x2x2 xexe Area A Slide 12 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 Boyles law for the trapped air. Water Float Air Slide 13 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 Slide 14 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 Slide 15 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) Slide 16 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 Phase difference Amplitude Slide 17 The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Observations! Amplitude and phase of floatLab. measurements (Borges 1968) Amplitude Frequency Phase Amplitude Frequency Phase Slide 18 The Centre for Australian Weather and Climate Research A partnership between CSIRO and the Bureau of Meteorology Real geometry Slide 19 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 Slide 20 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) Spectral power (kPa 2 /Hz) Slide 21 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 Slide 22 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.