BAMOS
Jun 2019
June 2018 event—Case study
I (TH) was helping out on the start boat for the Australian Women's
Keelboat Regatta held off St Kilda on 11 June 2018. The forecast
had been for moderate, northerly winds strengthening during
the day. The MSLP analysis showed a front well to the west, but
it was a sparkling sunny day after a chilly morning with no cloud
in the sky. We had one race in northerly winds, starting about 7
knots and dropping to 3 knots. Looking across to Williamstown
the water appeared glassy smooth and we feared that the wind
might drop out altogether. The second race started in about
6 knots but, looking out to Williamstown again, we saw a line
of white caps approaching and soon after, the wind suddenly
increased to a 10‐minute average of 15 knots with gusts over 25
knots. Needless to say, this caused a bit of mayhem for a while
with spinnakers flying from the tops of masts or dragging in
the water, and over-powered boats leaning at dramatic angles.
The increase in wind speed was even more abrupt and intense
further down the bay at Fawkner Beacon, for which the time
series shows a very impressive jump in 10‐minute average wind
speed from 4 knots to 24 knots in successive data points (i.e
within 10‐minutes and probably close to instantaneously if my
experience further up the bay is representative).
By contrast the corresponding graphs for Moorabbin and Point
Wilson (below), show a more gradual increase in wind speed,
and are more in line with what would have been expected
from the forecasts. I was interested in understanding the
meteorology behind such a dramatic wind speed increase within
an apparently benign synoptic pattern. Several colleagues
suggested explanations but I am drawn to that provided by Ted
Williams of the Bureau's Melbourne Office who pointed out that
the wind at the 750 metre level was from the north at 37 knots.
He suggested:
“the sudden increase in wind speed is due to mixing
down of the stronger winds just above the inversion
as things heat up. I think the suddenness would be
less pronounced at Point Wilson and St Kilda due the
effect of the land/friction stirring things up compared
to Fawkner Beacon. A cool, stable pool of air over
the cold water would gradually heat up (relatively
uniformly) until the temperature trace sits along dry
adiabatic lapse rate and reaches those winds at about
2000ft.”
Wind speed and direction for 11 June 2018 at Fawkner Beacon (top left), Point Wilson (top right) and Moorabbin Airport
(bottom middle).
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