Objective identification of wind change timing from single station observations Part 1: methodology and comparison with subjective wind change timings

摘要

Wind changes associated with the southeasternAustralian ‘cool change’ contribute to significant fire control issues, and so the forecasting of the passage of these cool changes is a vital component of the meteorological support for fire management operations. These ‘wind change’ forecasts have been subjectively verified for some years, and this report proposes an objective systemthat identifies wind change timing froma single station time series of observations. Thus the verification of wind change forecasts using this objective systemis reproducible, defensible, and also offers potential efficiencies in terms of personnel. The algorithms developed use fuzzy logic functions of rate of direction and rate of speed change, and gust speed to determine a ‘change time’, and these objectively determined change times are compared with the subjectively determined wind change timings for four fire seasons. It is shown that some 85 per cent of the objective timings are within 2.5 hours of the subjective timings. More importantly, the objective system identifies a start time and an end time for the change period, and more than 95 per cent of the subjective wind change timings occur within these time windows. A number of wind change events are discussed in some detail to demonstrate both the performance of the system and the complexity of wind change structures over southeastern Australia on days when the Victorian Regional Forecast Centre had issued Wind Change Forecast Charts.