Weather radar / dry hail

[Mach trim]

1)For those of you who are familiar with the older Bendix weather radar. When should one be in the NORM mode and when should one be in CONT ?

I take it CONT is better at painting bigger cells but the NORM has a more accurate narrow beam for the light stuff.... ?

2) Could someone please tell me more about dry hail.... Which one cannot pick up on weather radar ?


Thanks in advance and blue side up

[Tinstaafl]

'Cont' refers to 'Iso-contour'. The radar unit measures the strength of the return signal and, if it's above a certain threshold, inverts that part of the signal. The effect is to give an apparant extra 'step' in the display to improve the clarity of what you see. Colour radars do this using colour coding ie green not very strong return, yellow stronger, red very strong etc.

So,looking at a typical TS cell without ISO, you would see a splodge of green (returned signal strength above the display threshold, including all the really strong returns) surrounded by black (ie signal weak enough it's below the display threshold).

Switching on the ISO function causes the display to turn black wherever the signal is above the ISO strength. This gives the effect of turning the really strong returns black. Looking at the screen then you'd see an area of black (the really strong return), surrounded by green (where signal strength is between the display threshold & the ISO threshold), in turn surrounded by an area of black (signal below the display threshold).

Bear in mind that the radar frequency is optimised for a return from rain drops above a certain size, not cloud droplets or frozen water (ie 'dry' water eg snow etc. If the objects aren't reflecting the signal efficiently they must be absorbing it. This means a much weakened signal finally received back at the radar. Hence the 'bloom' effect eg flying through cloud but painting a 'weak' return ahead which apparantly gains strength as you get nearer.

The correlation between turbulence & the signal is based on the idea that the larger the water droplets, the stronger the updrafts needed to form &//or support them. A strong return therefore is associated with strong updraughts. Updrafts also mean downdrafts. The smaller the separation over distance between a strong updraught then the greater the shear between them ie the worse the turbulance.

Things to look for on the radar are areas where there is a rapid change from one signal strenth to another. Shapes such as 'hooks' & 'scallops' tend to indicate this. This is where ISO helps. It allows you to discriminate between an additional level of return.

[Hudson]

Re airborne dry ice crystals. With most weather radars it is wise to tune the Gain control from "auto" to "Max" when flying at high altitudes in IMC. Because the tops of CB consist of dry ice crystals, they do not reflect on the radar screen.

Max gain gives a more powerful beam (not a very accurate description) and providing the tilt is set accurately, a CB top may show up as a tiny echo instead of no echo in Gain auto. Once the CB top is detected in this manner, then a judicious combination of gain control and tilt and range will allow the crew to "see" the weather problem.

During the investigation into the Mount Erebus (Antartic) Air New Zealand DC10 disaster, some criticism was made of the perceived failure by the crew to use their weather radar to pick up the slopes of Mt Erebus. Investigation revealed that even if the crew did have their radar operating, the mountain would not have been displayed on radar because the mountain was covered by solid ice and therefore zero reflection.

[Mach trim]

Thank you gentlemen.

It is interesting what shows up on these old Bendix wx. radars

[compressor stall]

Further to the GPWS useage of wx radar, at McMurdo base in the Antarctic, the US have put in 44 gallon drums along the approach strip for use in very bad wx (Pre GPS) to give returns off the wx radar.

Rumour has it that after the drums were buried in a storm, crews use the fuselage of a wrecked herc short of the runway for their radar positioning in whiteout..... :ooh: