A Tight Gradient and Storm Surge

Written by Geoff

Tuesday, 21 February 2012 23:46

A TIGHT PRESSURE GRADIENT AND STORM SURGE:

If the distance between the isobars on one area of the chart is 200 km apart, then the pressure gradient force is 4 hPa per 200km or 1 hPa per 50km. At latitude of around 33deg south (Sydney) this would give light wind speeds. Imagine how a low pressure system developing near this location with a pressure gradient of 4hPa per 50km or 1 hPa per 15km. The wind speeds in this case would be in very strong.

A GUST is a momentary increase in wind speed.

GUSTS MAY BE UP TO 40% STRONGER THAN THE AVERAGE SPEED

A LULL is a sudden dropping off of the wind speed.

A SQUALL is an abrupt and large increase of wind speed by at least 16 knots to a mean of 22 knots and lasts for at least one minute. This is common with the passage of a cold front, outflow from a thunderstorm or a heavy shower. The Bureau of Meteorology uses degrees TRUE for wind direction.

FURTHER INFORMATION: (These are all 10 minute averages).

STRONG WIND: 25 TO 33 knots.

GALE FORCE: 34 TO 47 knots.

STORM FORCE: MORE THAN 47 KNOTS

HURRICANE FORCE: USED ONLY IN TROPICAL AREAS; MORE THAN 63 KNOTS

If the barometer is falling 3 hPa in 3 hours expect strong winds.

A 6 hPa fall in 3 hours expect gale force winds to develop within the next 6 hours. SEEK SHELTER OR PREPARE FOR A BLOW.

A 9 hPa fall in 3 hours expect gale to storm force winds within the next 3 hours or so. THIS WILL BE A BIG BLOW. SEEK SHELTER AND REDUCE TO STORM SAILS EARLY.

The biggest danger is with the development of Tropical cyclones but when NSW experiences an East Coast Low or even worse a 'meteorological bomb' the tight pressure gradients form destructive winds and huge seas cause major damage.

STORM SURGE: Firstly, in the vicinity of the centre of deepening low pressure system or a Cyclone, the sea level rises approximately 1cm for every 1 hPa drop in pressure. This means that the pressure drop alone may account for a 70cm rise in sea level near the eye.

Secondly, and most importantly, the stress of wind on the sea's surface raises sea and swell that many produce individual waves of 10-20 metres in height. As these run onto the coast, coupled with pressure induced rise, and coinciding with high tides, the sea can stream a long way inland.

With thanks to Geoff Smith – lecturer in meteorology