Wind speed time averaging conversions for tropical cyclone conditions

A progress report will be presented on a WMO-supported project that aims to provide recommendations for converting between wind speeds having different time averaging periods under tropical cyclone conditions. The review was commissioned in response to a request arising from the Fourth Tropical Cyclone RSMC's Technical Coordination Meeting in Nadi (Fiji), November 2002. A draft set of recommendations was completed in 2004 and the final version is currently undergoing client and peer review.

The review considers past and contemporary theory and data relevant to the issue of wind averaging periods and conversions under tropical cyclone conditions both over the open ocean and in coastal situations. While there are still significant gaps in our understanding of atmospheric turbulence characteristics under strong wind conditions, a simplified approach has been recommended that should nevertheless lead to an increase in consistency of quoted and forecast winds. An existing mathematical model of wind over-land in extra-tropical conditions has been adapted for this purpose and nominally calibrated against a wide range of assembled tropical cyclone data. The recommended procedure is seen as a practical interim solution until such time as increased data collection and analysis provides more definitive advice.

Differences between the newly recommended conversion factors and those previously specified in the WMO (1993) Global Guide, are reasonably significant in a number of ways and will be discussed. For example, the ratio between the most commonly used 10-min average and the associated 1-min peak gust in marine exposure conditions reduces from previously used factors. The review has specifically highlighted the need to distinguish clearly between randomly sampled estimates of the mean wind speed based on any chosen averaging period and the peak gust wind speed of a given duration within a particular observation period. It is particularly noted that mean wind speed estimates cannot be converted between different averaging periods – only gust wind speeds.

One of the review outcomes is the identified need for WMO regional associations and panels to consider revising and standardising their wind terminology, definitions and associated use of averaging periods in the various operational plans. This will assist in ensuring that the historical record contains more consistent measurements and/or estimates that can be reliably transformed or converted for assisting in further development of the science.