Small tropical cyclones: challenges of estimating the intensity analysis and prediction

CIRA Director's Conference Room
Wednesday, September 11, 2013 - 17:30
Bureau of Meteorology Perth, Australia
Joe Courtney
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It is widely acknowledged that there has been only modest improvement in tropical cyclone intensity forecast skill despite ongoing efforts to focus research in that area.

Very small, 'compact' or 'midget' tropical cyclones having a radius of gales of just 60nm or less occur more frequently in the Australian region than in other TC basins and account for about 30 per cent of all cyclones in parts of northwest Australia. Such cyclones present an even greater forecasting challenge because:
Traditional intensity analysis tools under-estimate the true maximum winds. This includes Dvorak technique, microwave (AMSU), scatterometry and surface observation interpretation.
They fluctuate in intensity much faster than is typical including undergoing rapid intensification at an earlier stage.
They respond quickly to changes in environmental influences such as wind shear and will even fluctuation diurnally.
NWP typically under-perform as the intensity is greatly influenced by small scale convective processes that may not be well-resolved by the global models.

Forecasting strategies to improve intensity prediction include:
Early recognition of the small-scale nature of the cyclone at or before the time of formation.
More appropriate interpretation of intensity analysis tools particularly the use of 85-91GHz microwave imagery.
The importance of recognizing the recent intensity trend as a key forecast tool.
The likely requirement for increased uncertainty range on intensity forecasts. Although ranges of intensity are not part of the formal product suite, it is possible to add descriptive remarks to products and on advisory information to emergency managers.