BAMOS Vol 32 No.2 June 2019 | Page 12

12 BAMOS Jun 2019 Figure 5: Scatter plot of wind speed increases by month. Table 1: Number of “straight-line” abrupt wind speed increases according to the quadrant of the initial wind direction. The abrupt wind speed increases, particularly the stronger events, are concentrated between September and April (Figure 5). The strongest winter event was 23 July 2003, when wind speed increased by 23 knots around 7am. In this case the direction change was only 5 degrees. There is not a strong relationship between time of day and number, or strength, of abrupt wind speed increases, although two thirds of cases occurred after 1200 Local Time. The three strongest cases occurred in the afternoon or early evening. Wind speed changes with large direction changes are more typical, but are normally well forecast and the associated hazardous conditions expected. The behaviour of the “straight- line” events is less familiar and the following discussion focuses on the one third of events with direction changes of 30° or less. “Straight line” wind speed increases The 86 events (which include the June 2018 event) were divided into quadrants based on initial wind direction. Numbers in each category are shown in Table 1. About 40 per cent of the cases are for winds in the North quadrant. East Quadrant There were only two cases in the east quadrant at Fawkner Beacon, both being abrupt speed increases in pre-existing south-easterly winds. South and West Quadrants Around half of the west and south quadrant speed increases showed a two stage wind change associated with the passage of a front. The initial wind change from northerly winds (often strong) to the south brings a drop in wind speed, a lull, before the onset of strong south-southwesterly winds, sometimes as a squall. One stark example is 22 January 2006 (Figure 6a). Around 7pm the 20 knot northerly winds backed to southerly and decreased to below 15 knots. An hour later the wind speed increased suddenly reaching a 10‐minute average of 40 knots before abating two hours later. The “Black Saturday” case shown in Figure 1 falls into this category. The other pattern was of short fluctuations within an extended period of generally strong winds. An example is the time series for 15 October 2011 (Figure 6b). The synoptic pattern showed a deep low moving south of Victoria. Figure 6: Wind speed and direction at Fawkner Beacon for (a) two stage wind change (example from 22 January 2006) and (b) short fluctuations (example from 15 October 2011).