Storm Tracking
Storm conditions approaching from onboard view. Credit: Kevin Green
This feature article originally appeared in AFLOAT Magazine and is the latest edition of Weather with Malcolm Riley exploring marine weather, storm tracking and offshore sailing conditions.
There have been all kinds of wild weather through March. Strong tropical cyclones travelled about 6,600 kilometres through the Coral Sea, across Cape York, through the Gulf of Carpentaria, across the Top End and northern parts of WA and out into the Indian Ocean (off the Pilbara coast). It made landfall again near Exmouth and then down through WA and exited into the Great Australian Bight and parts further south. There were also unrelated severe thunderstorms in NSW and Victoria and a low pressure off the coast of NSW with very strong winds that generated very large waves. Add snowfalls in March into the mix and it has been an interesting month weather wise.
Here in Tasmania, it was not that bad but we did have a rain event that I thought I would be useful to explain how to look at weather situations and use the radar and satellite in combination. In this article it is all Bureau of Meteorology Data and thanks to them for all the images.
One of the first places to look is the actual weather map, old school I know but still gives a good picture of what is happening. Firstly, you can see it is not a particularly deep low and from last month’s article the isobars are not bunched, so not incredibly windy. There is a cold front (blue line with triangles) moving to the east at 20 knots as well as some troughs (blue dashed lines) in the vicinity. There is also a warm front (red line) to the south of Tasmania.
Satellite imagery
Next thing to view is the satellite image. There are several different layers you can look at. The default is the day/night which looks the same but is often a mixture of a visible image (like from a normal camera) and an infrared image which are basically readings of temperature. The higher the cloud, the lower the temperature. You can tell which one is which by the colour of the land. If it is a visible part of the image, the land is green or brown. If it is infrared, the land is a faint grey colour. There is a “Layers” selection and if you go there you can choose between visible and infrared images.
The image I chose to show of the event was from the “Layers” menu and was “Infrared + Zehr”. What this image highlights is some of the colder temperatures in the atmosphere which can give an indication of deep convection, which could lead to quite heavy rainfall. In the image below you can see that there is a thick cloud band over Tasmania with the dark blue areas which indicate the coldest temperatures of the cloud tops as they are higher and the areas where the cloud would be thickest. There is a scale at the bottom of the website where the cloud tops get into the reds, yellows and above and these are very cold temperatures so the clouds tops producing them are very high and the cloud would be having a very large vertical extent.
Radar imagery
Next place to look is the weather radar but beware there are some traps. In the case of the rain event the cloud (and rain) was moving to the south then southeast down from Victoria, across Bass Strait and over Tasmania to Hobart where I was. One of the advantages of the new BOM radar viewer is that all radar images are merged onto the same map. On the old site you would need to navigate to the 512 composite image to see part of your local/nearest radar.
If you just looked at the 256 km radar you would not see the rainfall extending over NW Tasmania and Bass Strait, so you are likely to assume that the extent of the rain band was less than what it actually was. This is not the case with the BOM’s new viewer. All the radars are showing their information on the one page and you zoom in to the area you are interested in.
There are some aspects of the weather radar that can result in differences between the actual weather and what is recorded on the radar. To give an accurate reading the beam of the radar needs to be filled – the further the radar signal gets from the radar the wider the beam becomes.
Above: The “beam” of the radar spreads out as a cone the further it travels from the radar. In this case there are two identical rain showers, the closest fills the beam of the radar and gives a good return reflection. However, the shower further away from the radar does not fill the beam so not as much energy is returned and the radar will “see” (and show) this as a weaker echo despite the shower being identical. Also, as not all of the radar energy passes through the first shower the return from the more distant shower will be weaker again.
Above: The “beam” of the radar travels in a straight line. Due to the curvature of the earth the further out the beam travels the higher above ground it becomes. In this case there are two identical rain showers, the closest fills the beam of the radar and gives a good return reflection. However, the shower further away from the radar is completely missed by the beam and will not show up at all. The larger shower with the heavier rainfall only partially fills the beam so will show up as a weaker echo than it should.
Radars are complex pieces of equipment and in the main are reliable. However, there can be breakdowns and periods of scheduled maintenance when the radar is off the air. On the BOM’s old website if the radar was down (for whatever reason) a notice appeared on where the radar image would be with outage information. On the new BOM merged radar site the outage information appears when you visit a forecast location within the individual radar’s operating area – not on the radar screen. If you just go to the radar page (maybe you have bookmarked) and it is not showing any rain, it could be there is no rain or it could be the radar is unserviceable – it is not shown if it is down. This is not ideal, some kind of marker on the map where the radar is not working would be better.
Above: Occasionally there can be “hiccups” in the system. This image is from the new BOM radar page. It shows TC Narelle which crossed the coast near Exmouth. You can see the gap between the radar from near Exmouth and the radar based near Carnarvon. What is actually happening in this image I do not know – some kind of hiccup between the two radars. Coincidently, the Exmouth radar went off the air after TC Narelle’s passage, it was likely damaged by the strong winds.
This feature article originally appeared in AFLOAT Magazine. To explore more stories like this, browse the AFLOAT Magazine archive or view the latest edition of AFLOAT Magazine.
