Thursday, 4 July 2013

BoM radar seminar Canberra 4 July

Cutting to the chase, this was an absolutely excellent event.  Masses of information and presented in a very user-friendly way.  In making that comment, the presenters, Sean Carson and Simon Louis, did allow for the likelihood that most of the audience were already users of the service so had a rough idea - at least - about the weather and radar so didn't waste time on absolute basics.  The questions from the audience definitely showed good knowledge to be the case!

They were a bit unlucky with the first announcement of the seminar in that it coincided with a spell that the new improved radar wasn't working.
I guess one thinks about the meaning of Murphy's Law and moves on!

The event started by putting up a chart of the number of hits per month on the BoM website.  Thus far the peak month was January 2013 with an astounding 6 billion (and I mean the 'b') hits.  A lot of this traffic is to the radar page!

What follows are some of the points that I thought particularly interesting.  There was a lot more stuff and many very interesting loops that I haven't got details for.

The starting point was that the radar doesn't pick up clouds.  It picks up raindrops or snow/ice.  The various forms of watery material reflect differently:

  • raindrops - each an accumulation of water droplets  - reflect very well;
  • snow, hail or ice pellets do not reflect well;
  • supercooled water drops (essentially snowflakes with an external coating of water - used t be called freezing rain in Canada) reflects very well,  This is what caused the strange event over the Captains Flat radar on 24 June.

The operation of radar is a mixture of the resolution desired and the range of the beam.  The core of the process is firing off a pulse for 10 milliseconds and then listening for 990 milliseconds for a response.  The scanner rotates, taking 20 seconds for each circle and also changes the angle of the beam 14 times from 0.5 degrees above horizontal up to 32 degrees above horizontal.  It thus takes (14*20 = ) 280 seconds to complete each set of pulses.  Due to the curvature of the Earth by the time the lowest beam gets about 250km out it is 9000m above sea level (so will not pick up any rain below that level.

Rain is rainfall lasting 30 minutes or more at a point.  Showers are under convective clouds and last less than 30 minutes.  They use words such as "isolated" when showers are patchy!

In answer to a question about why the forecast for Townsville came from Brisbane the response was that forecasters don't need to be on the ground: they just need more information.  Later it was suggested that there is beginning to be so much information around the problem is to  pick out the crucial bits!

The information form the radar is mosaiced together to give an image based on what is happening at about 3000m above sea level.   This explains two complaints:

  • "The radar says it is raining here but it isn't" is explained by it raining at 3000m but the heat rising from the ground causes the rain to evaporate before it hits.  This is verga!
  • "Its raining here but the radar says it isn't"  The rain is falling below 3000m - ie the cloud is very low.

A related point is that if the rain is below the highest beam, depending on the distance from the transmitter it may give lower intensity.
Case A fills the beam while case B doesn't.

There was some explanation of things which can interfere with the reliability of the radars:

  • Hills can block what is behind them (these are generally noted on the BoM site);
  • Where thunderstorms form a line relative to the station the nearer ones can block the information from the further one;
  • De-roosting bats can cause strange effects (as can birds - the best documented example is the Radar Angels of London and this later but related comment);
  • Military aircraft on maneuvers where they deploy tinsel (aka chaff) to obfuscate other radars can produce weird, but ephemeral effects on the weather radar.
  • Wind farms seem to produce permanent false positives, as exemplified by the white dots in this image (they were much more obvious in the full-resolution images shown by Sean and Simon).

A major aspect of the upgrade to Captains Flat was the inclusion of a Doppler radar.  This measures radial wind velocity (still by bouncing a beam off raindrops) and the colour palette chosen follows that of astronomers, who use Doppler effects to measure how fast galaxies are moving.  This palette goes to red for wind away from the station and blue for wind towards the station.  If the wind direction is tangential to the scanning circle the velocity towards the station is zero, and this is shown as white.

As soon as I got home I looked at the Doppler image - get them here
which showed a very strong NW wind.  (The tangent to a NW -> SE  flow runs NE-SW.)
The highest wind shown for The Flat radar is 104 kph: if the wind is stronger than that the palette cycles so starts again at red.  Sean thought that might happen in the coming 48 hours.  (It hasn't so far, 8am on 5 July, but there is some very dark blue out to the West!)

The session concluded with some publicity for Meteye: a very complete series of forecasts and information presentation tools.  It should certainly replace the Elders weather site I have used in the past.  Get it here!
Here is an example of one of the standard reports (I suggest click on the image to get a larger version.)
It also enables the reader to get very local forecasts (based on GPS for localities and 6km sq grids. Here is the (brilliant) offering for Carwoola.
If it seems that this post is nauseatingly positive then so be it. I call 'em as I see 'em and in this case the Bureau and particularly Sean and Simon deserve great big bouquets!

No comments: