BAMOS
Jun 2019
Figure 3: Scatter diagram
showing direction and speed
changes for the cases of
abrupt wind speed increase.
The change in direction is simply taken as the difference
between the initial and final compass bearing over the half
hour. Positive (clockwise) changes are referred to as veering
while negative (anticlockwise) changes are backing. However,
the change in direction is limited to 180 degrees, leading to
ambiguity if the directional change exceeded 180 degrees.
Examination of the data shows that such cases are rare.
It should be emphasised that the discussion here concerns the
north of Port Phillip which is a large body with many variations
in wind across it. In particular, there may be large variations in
wind strength from north to south in strong easterly or westerly
flow.
Overall Results
There were 248 cases of abrupt increases in wind speed of 16
knots or more over the 21 years of data representing about
12 cases per year. There were 56 cases of abrupt wind speed
decreases.
Wind Speed Increases
Figure 3 shows a scatter plot of the speed and direction changes.
One third of the events (86 cases out of the 248) have direction
changes less than or equal to 30°, and in the discussion below
are termed “straight line” wind speed increases. For the cases
with a change in wind direction more than 30°, two-thirds
(108 out of 162) were cases of a backing wind and one third
associated with a veering wind.
Of note is that in four events the wind speed increased by
more than 30 knots within 30 minutes. All were associated with
significant backing in wind direction. Two examples are shown
in Figure 4. Around 4am on18 November 2001 winds increased
from 7 to 39 knots between successive 10‐minute data points.
Winds stayed around 30 knots for most of the remainder of
the day. An even sharper but shorter-lived event was 14 March
2009. Around 6:30pm the wind speed increased from 8 to 47
knots in a 20 minute period. The synoptic situation in each case
involved a low moving over Victoria.
In nine cases there were abrupt increases and decreases within
adjacent or overlapping 30 minute periods. A review of available
weather radar data showed that these events were typically
associated with a line of thunderstorms crossing Port Phillip.
Figure 4. Wind speed and direction at Fawkner Beacon for (a) 18 November 2001 and (b) 14 March 2009.
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