Radar Imagery

Met radar operates using the same principle as any other radar by directing a beam of electromagnetic energy from a focused antenna. If the beam encounters any precipitation, either water or ice particles, some of the energy of the beam is returned to the radar. The antenna also acts as a receiver, and by timing how long it takes for the pulse of electromagnetic energy to return to the radar, the distance to the object (precipitation) can be determined.  There is a mathematical relationship between the particles' characteristics and the power of the returned beam, and this is used to plot a colour-coded image which shows precipitation intensity.  Generally, the stronger the echo, the heavier the precipitation.  

There are a few key facts about weather radar that will help you make the best use of the information.

  • The radar rotates through a full 360° cone, and the entire scan takes approximately 7 ½ minutes.
  • This means that any image you look at will be at least 7 ½ minutes old.
  • Because of the Earth's curvature, the further the beam is from the radar the higher it is looking in the atmosphere - at 250 km, the lowest beam elevation is approximately 6 km (about 19,700 feet). 
  • Precipitation below the beam will be undetected, and there is no guarantee any precipitation detected by the radar at those elevations is reaching the ground.

Invercargill

Above mean sea level

TCu

Towering Cumulus

agl

above ground level

Cb

Cumulonimbus

Flight Information Service communications

ATS

Air Traffic Service

Royal New Zealand Air Force

in vicinity of aerodrome Thunderstorm

ATC

Air Traffic Control

VC

in vicinity of aerodrome

Probabilityxx%

TIL

Until

FM

From

Becoming

Temporarily

Routine air report from aircraft in flight

FIR

Flight Information Region

Internet Flight Information Service

VFR

Visual flight rules

Automatic Terminal Information Service

BWR

Basic weather report

New Zealand Flight Information Region

Aerodrome special routine meteorological report

Automatic aerodrome routine meteorological report

Aerodrome routine meteorological report

TAF

Aerodrome forecast

Area forecast

Significant meteorological information

Drizzle

Drizzle is associated with cloud in the lower levels of the atmosphere. There could be drizzle below the beam of the radar but it’s physically impossible for the radar to detect it. Compounding the problem is the very small size of drizzle droplets; any energy returned to the radar will be very weak, making detection difficult. However, the visibility associated with drizzle is typically very low so just because you don’t see much on the radar, doesn’t mean flying conditions are good!

Orographic Screening

Basically, like any other primary radar, the beam can’t go through mountains. It can be raining heavily on the West Coast, but the Christchurch radar won’t pick this up. This image shows that the beam reaching into some of the large river valleys.

Orographic Screening

Attenuation

In the image on the right, the strong echoes southwest of the radar (located at the green X) are blocking the beam in the area indicated by the red arrow. There would still be rain in that area, but none of the beam’s energy can penetrate the heavy rain. Some newer radars can correct for this phenomena using algorithmic techniques.

Attenuation of the radar signal

Icing and Virga

Virga is precipitation not reaching the ground.  If you only looked at the image on the right, you might think it’s raining in Whanganui.  METAR AUTO reports during this time didn’t report any rain.  However, there was a report of moderate icing in the area.  The radar is detecting ice in the cloud and/or virga below the cloud.  Below the freezing level, the ice will melt and any precipitation (ice or rain) falling from the cloud must have evaporated before reaching the ground. 

Use other information, such as METAR AUTOs, to help complete your mental picture.

Ducting

If there is an inversion, the radar beam can be trapped below it, enhancing ground clutter and the range of the radar may be increased by 500%.  Ground clutter is when the radar detects a signal from objects such as trees or terrain, and it is usually filtered out.  However, because of the increased range under ducting conditions, the radar algorithms can fail to detect that a signal is ground clutter.

Ducting is most likely overnight when the surface based inversion is at its strongest.

If you only looked at the radar image to the right, you might think there’s an area of heavy precipitation off the North Canterbury coast (Pegasus Bay area) – perhaps even a CB!

This satellite image from the same time (the radar scan area is indicated by the darker circle) shows some cloud in the area, but it appears quite smooth-looking (stratiform).  Directly over where the radar echoes appear is a clear area (black in the image).

The Christchurch METAR AUTO at the time reported BKN250.

Putting all this information together, it is safe to assume that the radar echo that appears in the image isn’t real precipitation. 

Sea Clutter

Sea clutter occasionally appears in radar imagery when the radar detects the changes in sea state often associated with a wind change. This is particularly pronounced with southerly changes moving through Cook Strait, as shown by the red arrow below. In this example, there was no precipitation associated with the wind change. Sea clutter is generally removed automatically, so won’t usually appear in the image that gets displayed on MetFlight.

Meteorological radar can detect insects, particularly around sunset and even smoke from fires can be detected. Radar returns from these can be used by forecasters to track wind shifts, in the absence of precipitation.

The key point is that you need to use radar in conjunction with other information, such as satellite imagery, reports and forecasts.