Rain blanketed most of Australia this week, even in the dry inland.
The most violent waterfall hits the southeastern part of the continent, and a great flood has just swept in .
Other parts of the East Coast are gearing up for more flooding in the next few days.
So what is actually causing this rain?
It all started last week when the unusually warm seas of northwest Australia released large amounts of moist air.
This air rose to form huge clouds, propelled by the wind, and carried billions of tons of water across the continent.
Clouds may look fluffy and insubstantial, but they carry enormous amounts of water.
Let’s take a look at the nearly 100mm of rain that has fallen in Sydney’s inner city (about 25 square kilometers) so far this week. That’s about 2.5 billion liters of water.
On the wettest days, the continent averages more than 4 mm of rain.
This equates to about 30 trillion liters of water. Or, using colloquial Australian measurements, 60 Sydney Harbor values (1 Sydharb = 500 gigaliters).
Why does it rain in the first place?
A major rainfall event requires two major components: atmospheric moisture and uplift.
Most of that moisture is due to evaporation from the ocean, but some is due to evaporation from the land, especially when it is wet.
Climbing occurs when surfaces are heated, air is forced to rise over obstacles (such as mountains), or there is a weather system that causes the air to rise.
A mass of moist air rising from the surface expands as it moves higher in the atmosphere. This is because atmospheric pressure drops rapidly with altitude.
This is why the balloon eventually pops as it rises into the sky. You don’t see this blob rising. It’s not white and fluffy yet.
Expansion of this moist air mass requires work, so energy must be found somewhere.
Air cools because energy comes from the motion of air and water molecules inside the blob, and temperature is a measure of the motion of the molecules.
As the air cools and the water molecules slow down, they tend to stick together and form droplets.
This is the process of condensation, forming clouds.
Clouds vary in size, but the largest cumulonimbus clouds (dark, towering storm clouds) can reach 10 kilometers or more above the surface.
Even small clouds contain a lot of water. A cloud covering 1 cubic kilometer contains about 500 tons of water.
You may wonder why the entire cloud doesn’t fall off at once with this weight.
The answer is that moisture is spread throughout the cloud, and the air underneath the cloud is denser.
At some point, enough water will condense into droplets to overcome gravity and pull the water to the ground as rain.
So why is it raining so much now? We now have an abundance of moisture in the air.
The weather is poised to lift atmospheric moisture via low pressure systems and cold fronts moving from west to east.
A low-pressure system means that the air pressure is lower than the surrounding area.
Nature likes to level things out, so surface air tries to counteract pressure differences, but the Earth’s rotation forces air into it in a spiral instead of directly entering it.
This creates winds that move towards the center of the low and then have to move upwards, carrying moisture.
That is why low pressure systems are associated with wind and rain.
A cold front is characterized by a rising mass of air as it marks the boundary between cold and warm air.
Warm air is less dense and is pushed up on top of cold air.
Why is there so much moisture in the air?
This is related to rising sea temperatures in northern Australia and more water evaporating from the surface.
The La Niña events we have been experiencing for three consecutive years have cooled the waters of the central and eastern Pacific near the equator and warmed above average sea surface temperatures in the western Pacific, including around Australia.
But La Niña has friends.
In addition, the so-called negative Indian Ocean Dipole is also occurring, with stronger westerly winds warming the waters off Indonesia and northwest Australia.
The intersection of these two climate cycles is adding more moisture to the air around Australia.
When a low pressure system forms, it attracts moisture over the continents, causing the air to rise and form heavy clouds.
Heavy rain is possible even without a La Niña, but a La Niña loads the dice, making it more likely that a more intense and more widespread rainfall event will occur.
For example, a third of Australia is more than twice as likely to receive rain during a La Niña period as under neutral conditions, and more than five times as likely as during an El Niño event.
Only a few parts of Australia receive more than 1mm of rain on most spring days.
Occasionally, however, more than a third of the continent will experience rain.
Rain, rain, go away
With the devastating floods of February and March still fresh in our minds, most Australians are hoping the rains will stop.
But the Deluge isn’t over yet.
As the La Niña continues, heavy rains are expected to become more widespread.
In addition, soils in eastern Australia are saturated in many areas, creating new flood potential.
By the beginning of next year, most forecast models predict a weakening La Niña. However, it is likely to be a wet summer.
Keep your eyes on the horizon and look for clouds.
https://www.sbs.com.au/news/article/rain-rain-go-away-whats-behind-australias-rain-event/o4qmirfbo Australian Weather: What’s Behind the National Rain Event?
