Background on Weather Radar
What does Radar mean?
Radio Detection and Ranging
During World War II, this Radio Detection and Ranging technique was developed to track enemy ship and aircraft. However, it was soon noted that precipitation, of any kind, would obstruct this remote detection. At first this was a problem, but the potential benefits were soon seen. This was the birth of weather Radar.
Today's radars are so advanced they can even identify types of precipitation, detect important weather features that make a storm severe, and track and predict the motion of storms.
How does a Radar work
Radar uses electromagnetic radiation to sense precipitation. It sends out a microwave pulse, 4-10 cm, and listens for a return echo. This microwave radiation is scattered by precipitation particles, both frozen and non-frozen, energy is sent in
all directions. The radar has a listening period where it detects the radiation scattered in it's direction, this radiation is called the echo.
The Radar beam is typically inclined 0.5° above the horizon and is 1.5° wide. The Radar itself also rotates to see a full circle or 'sweep' in order to get the full picture of the precipitation field in its area. Typically, a Radar beam can reach
about 200 miles.
The precipitation intensity is measured by the strength of the echo in the units of decibels dbZ. Larger/numerous particles reflect waves with greater intensity than smaller/fewer particles. An image showing precipitation intensity is called a "reflectivity image."
Click on the image for a better quality view:
A computer will interpret the intensities, an color code them on an image. Blues are lower reflectivity and Reds are more intense precipitation and higher reflectivity.
Types of Radar
There are two types of Radar
- Conventional Radar
Echoes are displayed on radar screen.
Only produces reflectivity images.
Not only sweeps in circles, but also up and down to look at different levels and individual storms
Base reflectivity is the reflectivity at the base level, usually 0.5°
Composite reflectivity is the maximum reflectivity, from any level.
Conventional radar Good for:
Seeing bands/location of precip and their intensity
Ground clutter, bouncing off things other than precipitation
Overestimation/Underestimation of precip
Cannot tell type of precipitation by radar alone!! (Have to use temperatures, actual observations, etc.
- Doppler Radar
One of the most advanced versions of radar
Does everything a conventional radar can do, PLUS more...
In addition to conventional techniques, the Doppler Radar has a scan that operates on principle of the Doppler Effect
Usually described using sound waves
Definition: the change in the observed frequency of waves produced by the motion of the wave source and/or wave receive.
- One example of how we are exposed to the Doppler Effect:
- This is how the Doppler Effect is used in meteorology:
Doppler radar give us BOTH reflectivity and wind velocity images, as opposed to only reflectivity images with a conventional radar, the Doppler component measures the actual speed/direction of the wind NOT the actual amount of signal reflected. Image will be labeled with some kind of VELOCITY term, like 'STORM RELATIVE VELOCITY'
Greens/blues = winds moving toward the radar (i.e. inbound)
Reds/oranges = winds moving away from the radar (i.e. outbound)
Note: Most Doppler Radars can operate in either a conventional or Doppler mode
The weather community shows reflectivities (a.k.a. conventional data) to the general public
Therefore, as a civilian, you will almost never see a velocity (a.k.a. Doppler wind field image)
Radar and Tornados
Before Doppler Radar, tornado warnings could not be issued until the tornado was on the ground.
Now, we know that potentially tornadic thunderstorms often have characteristics recognizable using Doppler Radar. Thus, we can give the general public an advanced warning before the tornado hits.
What is the structure of a tornatic thunderstorm? What are we looking for?
Click here to see
This (the link above) structure is observed in the reflectivity images. Curling of the reflectivity in back corner of storm is called a Hook Echo and is the most likely place for tornadoes to form.
Tornado Vortex Signature
This is an "image" of a tornado on a Doppler velocity image. It shows up as a small region of rapidly changing wind speeds inside a supercell thunderstorm.
Difference between max inbound and outbound velocities (shear) greater than or equal to 90 knots at less than 30 nmi, or greater than or equal to 70 knots between 30 and 55 nmi
Background on Weather Satellites
In 1957 Russia launched the first satellite, Sputnik.
By 1959 scientists at the Space Science and Engineering Center (SSEC)
at UW-Madison conducted pioneering meteorological satellite research,
revealing the vast benefits of meteorological satellites.
On April 1, 1960 the first satellite completely dedicated to satellite meteorology, named TIROS was launched.
TIROS = Television and InfraRed Observational Satellite. It had a life span of TIROS was 79 days.
Types of Satellites
There are two classifications of satellites defined by their orbital characteristics:
- GOES Geostationary Operational Environmental Satellites.
Geostationary satellites orbit as fast as the earth spins. They maintain a constant altitude and momentum over a single point. Approximate altitude = 36,000 km (22,300 mi). In order to maintain their location, they must be located over the equator.
- Good Temporal Resolution: Imagery is obtained and displayed approximately every 15 minutes.
In the case of severe weather, or hurricanes, passes over smaller areas are able to be obtained every 2-5 minutes.
- Poor Spatial Resolution: At a high altitude and fixed point, geostationary satellites can view a large, fixed area.
- Equatorial regions are covered well, polar regions are covered poorly.
- POES Polar Operational Environmental Satellites (also referred to as "LEO," Low Earth
- Polar orbiting satellites travel in a circular orbit moving from pole to pole.
Significantly closer to the earth (879 km ~500 miles) than geostationary.
Sees the entire planet twice in a 24 hour period.
Good Spatial Resolution: Lower altitude results in higher resolution images and atmospheric profiles.
- Poor Temporal Resolution: Over any point on Earth, the satellite only captures two images per day.
Types of Satellite Images
Both of these types of satellites take measurements of different wavelengths of radiation. There are
three widely used atmospheric windows (channels) That allow radiation from the lower atmosphere to
- Visible (~0.6 μm)
Visible images record visible light from the sun reflected back to the satellite by cloud
tops, land, and sea surfaces. Equivalently a black and white photograph from space.
Visible images can only be made during daylight.
- Dark areas: Regions where small amounts of visible light are reflected back to space. i.e.
- Bright areas: Regions where large amounts of visible light are reflected back to space. i.e.
snow, thick clouds
Current Visible Image
- Infrared or IR (10 to 12 μm)
Infrared images record infrared radiation emitted directly by cloud tops, land, or ocean surfaces.
- Cooler temperatures shown as light gray tones.
- Warmer temperatures shown as dark gray tones.
Current IR Image
- Water vapor (6.5 to 6.7 μm)
Water vapor images record infrared radiation emitted by water vapor in the atmosphere.
- Bright, white shades represent radiation from a moist layer or cloud in the upper troposphere (cold
- Dark, gray/black shades represent radiation from the Earth or a dry layer in the middle troposphere
(warm brightness temperature).
Current WV Image
There are also images of combined wavelenghts
Modis: Moderate Resolution Imaging Spectroradiometer.
Key instrument on TERRA and AQUA polar orbiter satellites.
Acquires data in 36 spectral bands (groups of wavelengths).
Combination of three separate channels by taking red (channel 1), green (4), and blue (3) components from
Creates a true color image useful in many ways, including:
- Showing change in vegetation color during fall.
- Showing smoke plumes from volcanoes, fires.
Satellites and Radar
Now we can put it all together, what can satellites and radar tell us about weather? Lets looks at an
extreme weather example from, September 15, 2004: