JammedStab

I am reading a report(from the '70's) which has this quote about thunderstorms...

"Airplane or ground radars may show the gust front as a thin line echo caused by differences between the high air density behind the front and the relatively low air density ahead of it".

Have never heard of this before. Has anyone seen this on their radar?

hikoushi

Scroll down to post #6 on the following thread to see a textbook US Midwest image from NEXRAD (the link in post #1 is dead, and as its poster indicates the photo is an XM satellite paint that filters out weak returns, including gust fronts most of the time). Have never noticed one on airborne radar (thank you Lord for small favors), and if anyone has, would be interested to know at what altitude / tilt setting / etc it was observed.

Nice example of a gust front along a squall line - Pilots of America Message Board

Machinbird

No need to scroll down. Here is the referenced picture. Interesting subject.

BOAC

JS - I am not familiar with weather radar detecting air mass factors, and I would have thought that unless the difference in 'air density' produced water vapour concentrations along the line it would not show.

FullWings

I've heard some met. men say that it's due to birds/insects/dust ahead of the gust front giving radar returns...

JammedStab

I really can't see how strictly just a change in air density at a gust front would show up on a radar.

Machinbird

Think of the air-water interface. What enables you to see that? Could it be the change in refractive index?

Same thing across the gust front it appears. And since the gust front is an irregular surface, some of it is oriented correctly to reflect energy back towards a radar transmitter. It is only a weak return though, so the change in refractive index is no where near as big as the water-air interface change. At least, that is my theory on the subject.

What interests me is how far the gust front appears to be ahead of the related clouds and moisture.

Brian Abraham

Happen to live where there are large plantations of pine forest. On occasions you would see a large yellow cloud of pollen preceding the gust front. Wonder if that would show on radar?

MurphyWasRight

Machinbird:I noticed the gap behind the line first, before re-reading the earlier posts
I saw that as the interesting part.

My instinct is that the refractive effect due to air density changes would not be enough to cause phantom returns it -might- be sufficient to deflect weak returns.

Could it be that the clear area behind the line actually has precipitation that is hidden by the gust front?

Note: I am not at all a weather expert so don't know if this fits at all.

EEngr

Here's a Navy pub on this topic.

http://www.weather.nps.navy.mil/~psg...f_EM_Waves.doc

They seem to concur with the opinion that atmospheric pressure (density) alone don't significantly affect radar propagation. A temperature difference and/or change in moisture will have a greater effect.

However, even absent rainfall, a change in pressure and/or temperature can have a significant effect on condensation (cloud formation, droplet size) which can produce more pronounced effects on radar.

FullWings

I doubt it has much to do with differences in air density. I'd be more inclined to the theory that it was reflections from particulate matter, insects etc. carried up into the air at the frontal boundary.

I say this as an ex- radar engineer and also as someone who has a fair amount of experience flying in and around gust fronts.