Wind Speed Time Averaging Conversions for Tropical Cyclone Conditions

1. Wind Speed Time Averaging Conversions for Tropical Cyclone Conditions Bruce Harper Andrew Burton Systems Engineering Australia Pty Ltd

2. Acknowledgements Authors Bruce Harper Systems Engineering Australia Jeffrey Kepert Bureau of Meteorology Centre for Australian Weather and Climate Research, Melbourne. John Ginger James Cook University Cyclone Testing Station, Townsville. • WMO Client Representative • Jim Davidson, Bureau of Meteorology, Brisbane. • Internal reviews • James Franklin, Mark Powell, Peter Black, Craig Miller IBTrACS Workshop, Asheville, NC, USA, 5-7 May 2009.

3. Questions/Issues What do we mean by “mean wind” and “gust”? How do you compare wind speeds of different averaging periods? What does Vmax represent? (What have we recorded in our Best Tracks?) Dvorak Database Goals • Homogeneity • Transparency IBTrACS Workshop, Asheville, NC, USA, 5-7 May 2009.

4. What do you mean by the “mean” wind? Highest 3-sec mean in 10-min Highest 1-min mean in 10-min 10-min mean Thin line is the 1-sec averaged wind from a 10Hz sampling of 10 minutes of wind. Circles are the 3-sec averages. Thick horizontal bars are the 1-min averages. Thin horizontal line is the 10-min average. You cannot convert between any of these estimates of the mean wind but you can apply a spectral sampling theory to estimate the probability distribution of the variations about the true mean wind and obtain the most likely gust wind speeds of a given duration within a specific period of observation. IBTrACS Workshop, Asheville, NC, USA, 5-7 May 2009.

5. Gust factors A gust factor is a theoretical conversion between an estimate of the mean wind speed and the expected highest or “gust” wind speed of a given duration within a stated observation period. applicable only in a statistical sense isolated comparisons cannot be expected to match theoretical values Gust factors depend on exposure To be representative, certain conditions must be met, many of which may not be exactly satisfied during a specific weather event or at a specific location. IBTrACS Workshop, Asheville, NC, USA, 5-7 May 2009.

6. Conflict between the Spectral Gap and the Need for Stationarity Schematic energy spectrum of near-ground wind speed after Van der Hoven (1957) IBTrACS Workshop, Asheville, NC, USA, 5-7 May 2009.

7. Converting Between Agency Estimates of Vmax Table E-1 Comparison of recommended and traditional conversion factor approaches. IBTrACS Workshop, Asheville, NC, USA, 5-7 May 2009.

8. Dvorak issues • Most TC intensity estimates are based on Dvorak • Dvorak is silent on wind averaging issues • Dvorak calibration issues • Generally held that Dvorak Vmax is 1-min average • Atkinson and Holliday implies V60,300 IBTrACS Workshop, Asheville, NC, USA, 5-7 May 2009.

9. Dvorak & Transparency • In most cases CI is the primary metric of intensity. • For transparency CI must be in BT databases • What do we do with historical BTs? (Where CI is not included) IBTrACS Workshop, Asheville, NC, USA, 5-7 May 2009.

10. Reverse Engineering CI IBTrACS Workshop, Asheville, NC, USA, 5-7 May 2009.

11. Transparency • Remove ambiguities, standardise wind definitions and nomenclature • Rigorously record all metrics and metadata IBTrACS Workshop, Asheville, NC, USA, 5-7 May 2009.

12. Summary IBTrACS Workshop, Asheville, NC, USA, 5-7 May 2009. • Transparency is critical for future improvement • Improving homogeneity requires transparency • Harper et al study provides: • a simplified yet consistent relationship for wind averaging conversions • improved consistency (homogeneity) and recoverability (transparency) • a strong basis for future work as better data increasingly becomes available

13. Thank You (and Bruce and Jeff) IBTrACS Workshop, Asheville, NC, USA, 5-7 May 2009.