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!
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.
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.
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.
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 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.