Spotting Signal Enhancement Using Rain Radar

I was first alerted to the fact that you can detect strong surface conditions by observing the Meteorology Rain Radar service by Jim VK3ZYC.

I had not been observing this phenomena until we had extremely strong duct conditions in the south east of Australia in Feb 2004, where I heard people commenting about the rain on the rain radar, yet as far as I could see it was a clear day. All of a sudden all the info I had heard Jim speaking about suddenly fitted into place. The rain being detected by the radar was in fact not rain at all, but it was "Phantom Rain" which is sometimes detected and plotted by the radars when strong signal enhancement conditions are around radar area.

These surface duct conditions will usually be quite strong on 144 MHz and even provide signals into the microwave bands.

I have included a classic example below of what to look for when your checking rain radars for enhanced signal areas.

Basically what your looking for are areas indicating rainfall on the radar, but on the cloud maps there are no corresponding clouds. You know then that its obviously "phantom rainfall".

To give you an idea of where the possibility of there being enhanced conditions is, have a look at the Hepburn Charts.

Personally I think its best to monitor the Weather Zone site and look at the rain indications on their Lightning Tracker Map. You can then look across at the Cloud/MSLP Map to see if there is cloud cover at a certain location. If you then want more detail and you want to find out what the strength of the enhancement is like, you can go to the BoM Radar Page and open the maps of the area your looking at. The colour's of the reflections give an indication of the strength of the conditions.

Above: But then looking at the rain radar image from BoM at the same time, this would show under normal circumstances substantial rainfall. This is actually "Phantom Rainfall" and indicates a very strong surface enhanced condition, surface ducting or trapping.

Above: A zoomed in image indicates that the conditions are strong and is very wide spread.

Rain radars usually appear to mostly detect surface based enhancement conditions which are normally on a more localised scale. Though, given the right conditions, surface based ducts can extend a great distance with no obstruction. Sticking by the Propagation Logger and seeing stations logging different conditions and reception reports helps you to understand what is going on.

An interesting explanation of what is happening has been provided by David VK3HZ -

The radar sends out a periodic pulse of RF and listens for returns from reflecting objects (like rainfall) in between times - the more distant the object, the later the return. The time between pulses is generally set to correspond to just over the maximum operating range of the radar in normal conditions. However, when there is anomalous propagation, reflections may come back from objects at much greater than normal operating range. These returns will be received in the time period after the next pulse has been sent and so the radar "thinks" the returns are much closer than they actually are.

Also, during normal operation, the radar builds up a "clutter map" of the surroundings so that fixed objects (like mountains) can be cancelled out of the results. Thus only transient objects - normally rain clouds - are displayed. However, when anomalous returns occur from terrain much more distant, the radar subtracts the close-in clutter map and, naturally, comes up with lots of transient objects displayed as phantom rainfall.

If you trace out from the phantom rainfall an additional distance corresponding to the maximum range of the radar, you'll probably find mountains or similar at that location. This is particularly evident on the Melbourne radar to the south when there's an opening to VK7. You'll see "clouds" in the middle of Bass Strait which are caused by anomalous returns from the hills in northern Tasmania. END

All Radar images below are of 'Radar Enhancement'which generally indicates strong local and semi local signals. Depending upon the consistency and extent of the same conditions it is possible to make contacts in excess of 1000 Km, usually across water.