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Understanding RADAR and rain

September 11, 2014

How well do you understand what you see on a “radar image” ?
Do you really know how far away you are from the radar dome ?

~~~~ click to enlarge ~~~~~~~~~~~

Radar locations near Warren

Radar locations near Warren

It is best to use the TDWR radar in conjunction with
the traditional NWS NEXRAD doppler radar to insure nothing is missed.

Detroit Metro Airport – Composite Reflectivity –
ENHANCED Radar Image from National Weather Service
http://radar.weather.gov/radar.php?rid=dtx&overlays=11101111&product=N0R&loop=no

Weather Underground – Detroit Metro Airport –
TDWR High Definition – Composite Reflectivity
http://www.wunderground.com/radar/radblast.asp?ID=DTW

The National Weather Service has around 155 NEXRAD
doppler radar stations across the USA (usually near airports)

These NEXRAD stations can detect most precipitation
within approximately 90 mi of the radar,
and intense rain or snow within approximately 155 mi.

Radar and reflection

Radar and reflection


 
Radar on the web is usually from the local
NWS Doppler radar (NEXRAD).
The image you look at online is made up of
several different images from a series of radars.

Radar at various tilt levels

Radar at various tilt levels


 
When a radar scans the atmosphere for storms,
it tilts at different elevations.
The Doppler Radar’s maximum tilt elevation is 19.5 degrees.

— So when a storm is near or over the radar site, data is unavailable

Radar Cone of silence

Radar Cone of silence

Base reflectivity only shows reflected energy
at a single elevation scan of the radar.

Composite reflectivity displays the highest reflectivity
of ALL elevations scans.

So, if heavier precipitation, is higher in the atmosphere,
– over an area of lighter precipitation
(the heavier rain that has yet to reach the ground),
the composite reflectivity image will display
the stronger dBZ level.

This occurs often with severe thunderstorms.
The updraft, which feeds the thunderstorm with moist air,
is strong enough to keep a large amount of water aloft.
Once the updraft can no longer support the
weight of suspended water , then the rain intensity at the surface
increases as the rain falls from the cloud.

~~~~ click to enlarge ~~~~~~~~~~~

TDWR radar

TDWR radar

The Terminal Doppler Weather Radar (TDWR) is an
advanced technology weather radar deployed
near 45 of the larger airports in the U.S.A

TDWR at Detroit Airport KDTW
(Simply scroll down until you see Detroit DTW)
http://somdweather.com/radartdwr.shtml

The newer Terminal Doppler Weather Radars are higher resolution,
and can “see” details in much finer detail close to the radar.
Thanks to a collaboration between the National Weather Service (NWS)
and the FAA, the data for all 45 TDWRs is now available in
real time via a free satellite broadcast (NOAAPORT).
Some systems call them “High-Def” stations.

The Weather Underground maintains a NOAAPORT satellite dish
which continuously ingests the Level III NEXRAD radar data
directly from the National Weather Service Doppler radars.
http://www.wunderground.com/radar/help.asp?

* The TDWR rainfall products generally underestimate precipitation,
due to attenuation problems.
TDWRs use shorter wavelengths to see details as small as 150 meters
along the beam, at the radar’s regular range.
This is nearly twice the resolution of the NEXRAD doppler radars.

*The most serious drawback to using the TDWRs is the
attenuation of the signal due to heavy precipitation falling near the radar.
Since the TDWRs use the shorter 5 cm wavelength,
which is closer to the size of a raindrop
than the 10 cm wavelength used by the traditional radar,
the TDWR beam is more easily absorbed
and scattered away by precipitation.
*This attenuation means that the radar cannot “see” very far
through heavy rain.

It is often the case that a TDWR will completely miss
seeing tornado signatures when there is heavy rain
falling between the radar and the tornado.
Hail causes even more trouble.

It is best to use the TDWR radar in conjunction with
the traditional NWS doppler radar to insure nothing is missed.

Detroit Metro Airport – Composite Reflectivity –
ENHANCED Radar Image from National Weather Service
http://radar.weather.gov/radar.php?rid=dtx&overlays=11101111&product=N0R&loop=no

Weather Underground – Detroit Metro Airport –
TDWR High Definition – Composite Reflectivity
http://www.wunderground.com/radar/radblast.asp?ID=DTW

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