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NEWS LINKS to CYCLONE MARSHA AND CYCLONE LAM:
From the Bureau of Meteorology
The trough and approach of Marcia is producing heavy rain and locally damaging wind gusts about the Wide Bay and Burnett and Southeast Coast districts today. Currently, heaviest rain is located on radar near coastal parts from the Sunshine Coast to the Gold Coast. These conditions should extend inland to the eastern parts of the Darling Downs and Granite Belt district later today or tonight. This heavy rain may lead to flash flooding, with some 24 hour totals in excess of 300mm. Damaging wind gusts may reach around 90 km/hr, mostly near the coast and ranges.
Water levels on the high tide are likely to exceed the highest tide of the year tomorrow. Dangerous surf conditions and beach erosion are also expected on exposed beaches.
Moderate flood warning for the Stanley and Mary rivers, minor flood warning for the Maroochy and Mooloolah rivers and Coochin creek.
A flood watch is current for the Wide Bay and Burnett, Southeast Coast and the Darling Downs & Granite Belt District forecast districts.
Locations which may be affected include Warwick, Gold Coast, Toowoomba, the Lockyer Valley, Ipswich, Brisbane, Caboolture, the Sunshine Coast, Kingaroy and Gympie.
Many locations near the coast from Brisbane up to the Sunshine Coast have had over 100mm of rainfall since 9am this morning with several locations having had over 150mm since 9am. Beachmere and Caboolture have both had over 180mm sonce 9am.
|map as Marsha approaches|
South Brisbane Storms
OZ Cyclone Chasers
|Satellite image as Marcia crosses coast makes land fall at|
Australia uses a five category system based on wind speeds to grade cyclones.
Category 1: wind speeds up to 125 kilometres per hour, causing only slight damage.
Category 2: wind speeds up to 169 kilometres per hour, causing minor damage to buildings, severe damage to signs and trees, and heavy damage to crops.
Category 3: wind speeds up to 224 kilometres per hour, very destructive, causing structural damage to roofs and bringing down power lines.
Category 4: wind speeds of 279 kilometres per hour causing significant roofing and structural damage with a lot of airborne debris and widespread power failures.
Category 5: wind speeds over 280 kilometres per hour and capable of causing almost total destruction with buildings flattened and cars overturned.
Winds are measured by official weather bureau anemometers on the ground , although good estimates can also be made from satellite data.
Perhaps the most important impact of the Coriolis effect is in the large-scale dynamics of the oceans and the atmosphere. In meteorology and oceanography, it is convenient to postulate a rotating frame of reference wherein the Earth is stationary. In accommodation of that provisional postulation, the centrifugal and Coriolis forces are introduced. Their relative importance is determined by the applicable Rossby numbers.Newton's laws of motion describe the motion of an object in a (non-accelerating) inertial frame of reference.
Tornadoes have high Rossby numbers, so, while tornado-associated centrifugal forces are quite substantial, Coriolis forces associated with tornadoes are for practical purposes negligible.
When Newton's laws are transformed to a uniformly rotating frame of reference, the Coriolis and centrifugal forces appear. Both forces are proportional to the mass of the object. The Coriolis force is proportional to the rotation rate and the centrifugal force is proportional to its square.
The Coriolis force acts in a direction perpendicular to the rotation axis and to the velocity of the body in the rotating frame and is proportional to the object's speed in the rotating frame. The centrifugal force acts outwards in the radial direction and is proportional to the distance of the body from the axis of the rotating frame. These additional forces are termed inertial forces, fictitious forces or pseudo forces.
They allow the application of Newton's laws to a rotating system. They are correction factors that do not exist in a non-accelerating or inertial reference frame.
A commonly encountered rotating reference frame is the Earth. The Coriolis effect is caused by the rotation of the Earth and the inertia of the mass experiencing the effect.
Because the Earth completes only one rotation per day, the Coriolis force is quite small, and its effects generally become noticeable only for motions occurring over large distances and long periods of time, such as large-scale movement of air in the atmosphere or water in the ocean.
Such motions are constrained by the surface of the earth, so only the horizontal component of the Coriolis force is generally important. This force causes moving objects on the surface of the Earth to be deflected to the right (with respect to the direction of travel) in the Northern Hemisphere and to the left in the Southern Hemisphere.
Rather than flowing directly from areas of high pressure to low pressure, as they would in a non-rotating system, winds and currents tend to flow to the right of this direction north of the equator and to the left of this direction south of it. This effect is responsible for the rotation of large cyclones (see Coriolis effects in meteorology).
To explain this intuitively, consider how an object that moves northwards from the equator has a tendency to maintain its greater speed at the equator (rotating around towards the right as you look at the sphere of the Earth), where the "horizontal diameter" is larger, and therefore tends to move towards the right as it passed northwards where the "horizontal diameter" of the Earth (the rings of latitude) are smaller, and the speed of local objects around the central axis of the Earth is slower.
Because currents are driven by the movement of wind across the water of the ocean, the Coriolis effect also affects the movement of the ocean’s currents and therefore hurricanes as well. Many of the ocean's largest currents circulate around warm, high-pressure areas called gyres. Though the circulation is not as significant as that in the air, the deflection caused by the Coriolis effect is what creates the spiraling pattern in these gyres. The spiraling wind pattern helps the hurricane form. The stronger the force from the Coriolis effect, the faster the wind will spin and pick up additional energy, increasing the strength of the hurricane.
Air within high-pressure systems rotates in a direction such that the Coriolis force will be directed radially inwards, and nearly balanced by the outwardly radial pressure gradient. As a result, air travels clockwise around high pressure in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere. Air within low-pressure systems rotate in the opposite direction, so that the Coriolis force is directed radially outward and nearly balances an inwardly radial pressure gradient.
Images by Eminpee Fotography