10
BAMOS
Jun 2019
Article
Abrupt wind speed changes on
northern Port Phillip
Terry Hart and Graham Mills
Emails: [email protected]; [email protected]
People of Melbourne are familiar with the changeability in
its weather. Melburnians may head to work in solid rain but
return home in warm, bright sunshine. On hot summer days
many residents look forward to the “cool change” in which the
hot, northerly wind is swept away by a southerly wind and the
temperature drops by 15°C or more. However, through sad
experience Melburnians know the dangerous impact of these
sudden wind changes on bushfires. Sudden wind changes
are also a concern for mariners venturing on to Port Phillip. At
Mornington there is a memorial to 15 young men lost in May
1892 after a squall hit their boat as they returned from a football
match at Mordialloc.
One example of an abrupt change in wind speed and direction,
familiar through tragedy, is that on Black Saturday, 7 February
2009. The plot of the wind during the day at Fawkner Beacon
(Figure 1) shows several significant wind changes.
The “cool change” arrived around 5pm when the 30 knot
northerly backed to the south and gradually eased. A sudden
increase in wind speed occurred around 10pm EDT from near
calm to 28 knots within half an hour. Early in the day there was
another rapid increase when the northerly wind increased
in strength from 8 to 23 knots within half an hour with no
significant change in direction.
for small boats too. Such an event on 11 June 2018, as described
in the case study (page 15), led to the question “How common
are such cases on Port Phillip and what causes them?”. This
investigation aims, from the perspectives of would-be sailors
and meteorologists, to provide an indicative answer.
Data
This study concentrates on Fawkner Beacon which is located in
Port Phillip approximately six kilometres west of Sandringham
as representative of wind conditions in the northern area of
Port Phillip (see Figure 2). Automatic Weather Station (AWS)
data were acquired from the Bureau of Meteorology. The wind
observations contain 10-minute average wind speed and
direction every ten minutes, covering the period from 1998 to
January 2019. Gusts are also a significant safety consideration
but this study only considers changes in the 10-minute average
wind.
These abrupt wind changes in speed and/or direction are a
major concern for fire-fighters, but such increases in wind
speed could be at least unpleasant but potentially hazardous “Abrupt wind speed changes” were taken as half hour wind
speed changes exceeding 30 km/hr (16 knots). These were
identified by computing the wind speed change over half
an hour for every 10‐minute increment. In cases of several
successive ten minute starting times the case of the greatest
wind speed change was selected. A further test was added
so that cases more than an hour apart would be counted as a
separate event, meaning that there could be more than one
event (either increase or decrease) on a given day. Local time
is used.
Figure 1: Wind speed and direction at Fawkner Beacon for
7 February 2009. Figure 2: Location map of Bureau observation sites around
Melbourne. Source: Bureau of Meteorology website.