BAMOS
Jun 2019
Figure 7: Wind speed and direction at Fawkner Beacon for (a) Lull in wind speed (example from
17 July 2017), (b) abrupt onset of northerly winds (example from 23 July 2003) and (c) Squall or
spike in wind speed (example from 25 September 2002).
Again there is a preference for the warmer months with fourty
of the fifty cases for these quadrants occurring between
October and April. The winter/spring cases tended to result
from fluctuations in strong westerly flow, rather than the double
wind change category.
North Quadrant
About 40 per cent of the “in-line” abrupt wind speed increases
occurred for winds in the North quadrant. They fall into three
subjective categories: (a) after a lull in an extended period of
significant winds (12 of the 35 events), (b) an abrupt onset of
strong northerly winds from a period of light winds (15 of 35
events), and (c) squalls or spikes in wind speed typically lasting
less than 20 minutes (8 cases). An example of (b) occurred on
7 February 2009 (Figure 1), and it was this type of event that
is described in the case study (page 15). Another example for
each category is shown in Figure 7.
The “lull” cases occurred in synoptic situations with a series of
fronts or a major low with embedded fronts crossing Victoria.
The “abrupt onset” cases occurred in northerly flow ahead of a
front approaching from the west, but still located over South
Australia.
In the “abrupt onset” cases the 925 hPa wind (about 700 metre
altitude) was typically northerly at 30–40 knots several hours
before the onset of the strong winds at the surface. These events
are likely due to the mixing down of the low-level jet once the
surface inversion has been eroded, as is hypothesised for the
case of 11 June 2018 (case study, page 15). All except one of the
15 “abrupt onset” cases occurred before 1300 Local Time.
The onset of strong winds at Fawkner Beacon is generally more
abrupt than the simultaneous time series of winds at other
stations in the north of Port Phillip. For example, of the 15
“abrupt onset” cases at Fawkner Beacon only two would have
met the same onset criteria (an increase of 16 knots in half an
hour) at Point Wilson. Several of the very marked cases occurred
in the cooler months when the water over the bay is quite
cold and does not respond to the diurnal warming that would
take place over land. In some cases there are even hints of an
incipient sea breeze.
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