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).