Juts a quick question regaring rain radar such as the one at
http://www.raintoday.co.uk

I've noticed quite often that rain is shown above where I live in large
quantities, but on the ground many times it is just very overcast. Or
there will be heavy rain shown, but only light rain falling on the
ground.

I realise that the radar images are low resolution and can't show very
local conditions all that well, but sometimes the radar shows great
swathes of rain, and still nothing much on the ground in reality.

Is the radar merely picking up on rain in the atmosphere no matter what
the height, as opposed to actually showing rain that is reaching the
ground?

Lastly, is there a decent high resolution precipitation radar service
on the web (subscription or otherwise)?

Simon
--
http://www.5ep.co.uk

Simon Wyndham wrote in message
...
| Juts a quick question regaring rain radar such as the one at
| http://www.raintoday.co.uk
|
| I've noticed quite often that rain is shown above where I live in large
| quantities, but on the ground many times it is just very overcast. Or
| there will be heavy rain shown, but only light rain falling on the
| ground.
|
| I realise that the radar images are low resolution and can't show very
| local conditions all that well, but sometimes the radar shows great
| swathes of rain, and still nothing much on the ground in reality.
|
| Is the radar merely picking up on rain in the atmosphere no matter what
| the height, as opposed to actually showing rain that is reaching the
| ground?
|

When you think about it, the radar cannot pick up the rain reaching the
ground, unless it is very close to the radar. It normally sends a roughly
horizontal beam out in all directions, so samples a layer of air.

The curvature of the earth means that the further away from the radar the
beam goes, the higher in the atmosphere it is sampling no matter how close
to the horizon you point the radar. In practice, the radar has to be angled
slightly upwards in order to miss nearby obstacles and actually sample the
atmosphere.

So if you live some distance from the nearest radar site, the radar will
indeed be measuring precipitation some way above the ground.

This brings two further effects into play:

1) If the air below cloudbase is dry, the rain will evaporate on the way
down and this will mean the rainfall rate according to the radar will indeed
be higher than that reaching the ground. Recently we have had very dry air
about and this both means the cloud base is higher and the rain evaporates
more quickly on the way down. It is possible in these circumstances that
very little precipitation can reach the ground from quite active showers.

2) Snow and rain reflect radar differently, and a mix of rain and snow
reflects more than either alone. So if the rainfall radar is sampling air
near the freezing level it will again suggest a higher rainfall rate than
actually reaches the ground. However, I believe modern radars can be set up
to at least partly compensate for this. In the old days you could sometimes
see a bright ring on the radar display marking the distance away from the
radar where the beam angle intercepted the freezing (or rather melting)
level.

In addition to the above, certain atmospheric conditions can produce false
echoes, making it appear rain is falling when in fact there is none. These
are reasonable well known and, although they may show in raw data, any
source which uses "processed" data should largely eliminate these.

I must admit my knowledge of weather radar is not up to date - I graduated
many years ago. Perhaps someone else can rise to the challenge and update /
correct the comments I have made.
--
- Yokel -

"Yokel" posts via a spam-trap account which is not read.