Weather RADAR
(RAdio Detecting
And Ranging)
Weather RADAR is a very complex
and detailed subject. Research is ongoing in federal agencies,
universities, and private enterprise on how to improve it. The subject
matter here is a brief introduction into how weather RADAR operates and the
function that it serves for the public. It is not intended to be a
complete summary of this topic and the reader is encouraged to review the
plethora of information on the Internet. Additionally, this topic is not
intended to cover the use of RADAR for aviation purposes.
Overview -
In the US the National Weather
Service (NWS), along with the FAA and Department of Defense, maintain a network
of weather RADAR sites. There are approximately 158 of these sites.
The current RADAR used for weather purposes is called the WSR-88D or Weather
Surveillance RADAR 1988 Doppler. The name is pretty much self explanatory,
the 1988 is when the RADAR development was pretty much completed and it uses the
Doppler affect for some of its products.
Weather RADAR is an active
sensing device, meaning that it actively scans for its measurement. In
contrast a temperature sensor is a passive sensing device. The RADAR
transmits a signal at a specific wavelength, power, and tilt level. The
intensity of the return signal and wavelength may be processed to determined the
strength of a storm, possibility of hail, wind direction, precipitation amounts,
and the elevation of storm tops.
Types of Weather RADAR's -
There are primarily two types
of RADAR used for weather. One is used by the NWS and the other is used by
private entities, such as TV Stations. There are many differences between
these systems. The wavelength, power, beam width, dish size, and
processing all differentiate these systems. WSR-88D, also called NEXRAD
(Next Generation Weather RADAR) in its development days, has a wavelength around
10cm, power output @750 kilowatts, and a dish size around 10 meters. In
contrast, private RADAR's are commonly 5cm wavelength, power output @250
kilowatts, and dish sizes vary.
The main limitation of any
RADAR is the curvature of the Earth. As the beam emits from its source,
two things happen 1) the beam's elevation increases as it continues a straight
line and the Earth curves beneath it and 2) the beam's width increases.
Additionally, since the RADAR emits the beam, it must "listen" for a time for
the signal back. The amount of time for that occur also affects the range.
The best range is around 120 miles from the RADAR site, with data available out
to 240 miles.
Uses -
Weather RADAR are primarily
used for public warning and information. A secondary use is research,
pulled from archives. The NWS uses RADAR for a variety of purposes:
severe thunderstorm, tornado, and flash flood warnings; snowfall; atmospheric
wind profile; forecast; and boundary detection to name a few. TV Stations
use RADAR for similar purposes, although they do not have warning authority.
They will typically relay information to the public based on their experiences.
RADAR is but one component in a complex system that makes up the public warning
and forecast system through the NWS.
Products -
WSR-88D is different from TV
RADAR in another way, it scans several layers of the atmosphere in what is
called a volume scan. The scan starts at a elevation angle (tilt) of 0.5
degrees off the horizontal (surface to make it easy). Although, these
RADAR's are typically 100 feet above the surface. The RADAR turns 360
degrees and then adjusts its tilt to 1.5, it continues this process until the
volume scan is complete. At the end it adjusts back to 0.5 degrees and
starts the scan over. Now, the volume scan process can be adjusted through
what is called the VCP or Volume Coverage Pattern. This is essentially a
set of macros that control what elevation angles the RADAR samples and the speed
that it turns. TV RADAR's traditionally only scanned the 0.5 elevation
angle. However, unlike WSR-88D they can be stopped and aimed at a storm or
point of interest for detailed analysis.
TV RADAR is typically
real-time, meaning that the data are displayed as it is received and processed
during the elevation scan. WSR-88D produces a product once the elevation
scan is complete. Common products are reflectivity, velocity, and
storm-relative velocity. Once the volume scan is complete, additional
products are produced, common products are: VIL - Vertically Integrated
Liquid (useful for determining hail), precipitation rates, echo (storm) tops,
and composite reflectivity. This is by no means a comprehensive list of
WSR-88D products.
Data Distribution -
In the early days, around 1992,
the NWS contracted with four vendors to disseminate RADAR data to companies and
the public. The initial vendors were: Alden, Kavorus, Unisys, and
WSI. These were called the NIDS - NEXRAD Information Dissemination Service,
vendors. These companies would provide data, for a fee, to those that
wanted it. In 2001, the NWS did not renew the NIDS contract. Before
this point many companies and universities could not provide data on the
Internet. With the demise of the contract, RADAR data are now available on
the Internet to the general public. Since many universities and the NWS
already have systems in place to receive WSR-88D products (data stream), adding
an Internet interface was the next step. Additionally, the NWS and others
typically archive RADAR data for future research.
Emerging Technology -
Research is ongoing that would
develop a new type of weather RADAR, called phased array. This technology
has been in use by the military for many years. Unfortunately, like all
research projects, it will be several years in development. A prototype is
under construction in Norman, Oklahoma, and should be completed in a few months.
http://www.nssl.noaa.gov/rrdd/par/index.shtml
Sources for free RADAR Data -
College of DuPage
National Weather Service
Additionally, many TV Stations
put their RADAR displays on their websites.
Reference Links -
RADAR Operations Center
WSR-88D
Transmit/Receive Status
NWS WSR-88D FAQ's - A great guide on weather
RADAR.
