R.Manasseh Fluid Dyn Papers
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It is likely that the offshore flow noted on the first circuit was a low-level
land-breeze, in effect a drainage flow. Observations of slightly lower
temperatures near the mouths of metropolitan river valleys are consistent with
this. These temperature variations were small, less than 
, and
superimposed on the rising ambient temperature due to morning insolation. The
presence on the first circuit of north-westerly winds on the northern leg,
together with south-westerly wind on the southern leg, is consistent with
katabatic air flows off the high ground to the north and south of central
Sydney. The magnitudes of wind speed noted are consistent with the small
temperature differences observed and the height of the offshore flow. A rough
check using the gravity-current relationship
for the propagation speed c of the flow (Simpson
1987), where g is the acceleration due to gravity, 
is
the buoyancy difference corresponding to the temperature difference and h is
the height of the flow, gives c in the range 
, much as
observed. In reality the feeder current in a gravity current, which is what
would be experienced as the local wind, moves faster than c; moreover, a
katabatic gravity current will have added momentum owing to its downslope
acceleration. In addition, the temperatures over land where the katabatic flows
began are likely to have been cooler than those measured over sea, where some
mixing would already have occurred. On the other hand, friction should reduce
the theoretical speed. Thus, all factors considered, the observed values of
offshore flows seem quite reasonable.
The zone of pollutant indicators over the ocean was centred near the entrance to Sydney Harbour, extended no more than 20 km out to sea and was below 200 m. It is likely that it was advected out to sea by the light katabatic morning winds. Given the presence of pollutant indicators prior to 0730, it is probable that they represented residual pollution from the previous day's motor traffic. The depth of the katabatic flows being no more than 200 m is consistent with previous studies (Carras and Johnson 1982) of Sydney nocturnal katabatic flows.
It appears that by about 1000 these compounds had either begun to disperse, undergo conversions to other species, or had been advected further out to sea.
By the second circuit, which began at about 0900, the conditions had changed, with southerly winds on the southern leg being replaced by northerly winds. Possibly the inland drainage flows were abating by then. Warming and convection inland lead to the formation of a sea-breeze cell; the yacht observations indicated that it began offshore by about 0925. The cumulus cloud bank seen 50-70 km offshore may have marked the seawards limit of the sea-breeze cell. The sea breeze suppresses cloud formations over the ocean because the descending air in the sea-breeze cell inhibits convection (Simpson 1994). The sea breeze seemed to have arrived by 1012, when the sea leg of the second circuit began. This is consistent with the arrival of the sea breeze front at Fort Denison at 1015. Furthermore, divergence of the wind vectors over the mouth of the Hawkesbury is consistent with the behaviour of sea breezes encountering a curved coastline (Abbs and Physick 1992). It is likely that this run occurred just as the sea breeze was crossing the coastline and interacting with the residual katabatic outflows. The variations in wind structure, with onshore and offshore flows occurring simultaneously along various parts of the coastline, shows the complexity of this interaction. The line of cloud along the coast, noted on the sea leg of the second circuit, were probably caused by the updrafts at the sea-breeze front.
The sea-breeze front had completely crossed the coast by the time of the ocean
leg of the third circuit (1228), although there was a 
difference in
wind direction from the northern to the southern boundaries of the metropolitan
area. The front was probably located about 10 km inland on the northern leg,
where wind and temperature changes were noted at 1224. However, at 400 m
the flow over the region where the sea-breeze cell was thought to exist had a
large alongshore (northerly) component, whilst it was mainly onshore at 100 m.
At the front itself there was a southerly component at 400 m. On the southern
leg of the third circuit, it was found that the sea-breeze flow was below
450 m. Therefore, circulation in the sea-breeze cell is likely to have been
bounded below 400 m, which is typical for most temperate sea-breeze regimes
(Abbs and Physick 1992), although the head of the front itself extended above
400 m.
The offshore zone of pollutant indicators, originally located off the centre of Sydney, had been advected to the south of the metropolitan area by 1245. The sea-breeze front was located about 15 km inland at 1315; the coincidence of the frontal location with the inland extension of the pollutant indicators makes it likely that it had advected some of the offshore pollutant indicators inland.
R.Manasseh Fluid Dyn Papers
R.Manasseh Fluid Dyn Home