Radar - Products
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Radar Products Offered
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. Included in the Level III NEXRAD data are the following products, all updated every 6 minutes if the radar is in Precipitation Mode or every 10 minutes if the radar is in Clear Air Mode.
Note that zooming is currently allowed only into the center of the radar image. Eventually, we hope to add an option to zoom into any region of the image.
This is a display of echo intensity (reflectivity) measured in dBZ. The base reflectivity images in Precipitation Mode are available at four radar "tilt" angles, 0.5°, 1.45°, 2.40°, and 3.35° (these tilt angles are slightly higher when the radar is operated in Clear Air Mode). A tilt angle of 0.5° means that the radar's antenna is tilted 0.5° above the horizon. Viewing multiple tilt angles can help one detect precipitation, evaluate storm structure, locate atmospheric boundaries, and determine hail potential.
The maximum range of the "short range" base reflectivity product is 124 nautical miles (about 143 miles) from the radar location. This view will not display echoes that are more distant than 124 nm, even though precipitation may be occurring at greater distances.
This display is of maximum echo intensity (reflectivity) measured in dBZ from all four radar "tilt" angles, 0.5°, 1.45°, 2.40°, and 3.35°. This product is used to reveal the highest reflectivity in all echoes. When compared with Base Reflectivity, the Composite Reflectivity can reveal important storm structure features and intensity trends of storms.
The maximum range of the "short range" composite reflectivity product is 124 nm (about 143 miles) from the radar location. This view will not display echoes that are more distant than 124 nm, even though precipitation may be occurring at greater distances.
Base Radial Velocity
This is the velocity of the precipitation either toward or away from the radar (in a radial direction). No information about the strength of the precipitation is given. This product is available for just two radar "tilt" angles, 0.5° and 1.45°. Precipitation moving toward the radar has negative velocity (blues and greens). Precipitation moving away from the radar has positive velocity (yellows and oranges). Precipitation moving perpendicular to the radar beam (in a circle around the radar) will have a radial velocity of zero, and will be colored grey. The velocity is given in knots (10 knots = 11.5 mph).
Where the display is colored pink (coded as "RF" on the color legend on the left side), the radar detected an echo but was unable to determine the wind velocity, due to inherent limitations in the Doppler radar technology. RF stands for "Range Folding".
- Determining True Wind Direction The true wind direction can be determined on a radial velocity plot only where the radial velocity is zero (grey colors). Where you see a grey area, draw an arrow from negative velocities (greens and blues) to positive velocities (yellows and oranges) so that the arrow is perpendicular to the radar beam. The radar beam can be envisioned as a line connecting the grey point with the center of the radar. To think of it another way, draw the wind direction line so that the wind will be blowing in a circle around the radar (no radial velocity, only tangential velocity). In order to determine the wind direction everywhere on the plot, a second Doppler radar positioned in a different location would be required. Research programs frequently use such "dual Doppler" techniques to generate a full 3-D picture of the winds over a large area.
- Finding Tornadoes If you see a small area of strong positive velocities (yellows and oranges) right next to a small area of strong negative velocities (greens and blues), this may be the signature of a mesocyclone--a rotating thunderstorm. Approximately 40% of all mesocyclones produce tornadoes. 90% of the time, the mesocyclone (and tornado) will be spinning counter-clockwise. If the thunderstorm is moving rapidly toward or away from you, the mesocyclone may be harder to detect. In these cases, it is better to subtract off the mean velocity of the storm center, and look at the Storm Relative Mean Radial Velocity.
Storm Relative Mean Radial Velocity
This is the same as the Base Radial Velocity, but with the mean motion of the storm subtracted out. This product is available for four radar "tilt" angles, 0.5°, 1.45°, 2.40°, and 3.35°.
Vertically Integrated Liquid Water (VIL)
VIL is the amount of liquid water that the radar detects in a vertical column of the atmosphere for an area of precipitation. High values are associated with heavy rain or hail. VIL values are computed for each 2.2x2.2 nm grid box for each elevation angle within 124 nm radius of the radar, then vertically integrated. VIL units are in kilograms per square meter--the total mass of water above a given area of the surface. VIL is useful for:
- Finding the presence and approximate size of hail (used in conjunction with spotter reports). VIL is computed assuming that all the echoes are due to liquid water. Since hail has a much higher reflectivity than a rain drop, abnormally high VIL levels are typically indicative of hail. The following example shows high VIL levels associated with a thunderstorm that produced 1.5 inch hail and an F1 tornado near Lansing, Michigan in August 2003.
- Locating the most significant thunderstorms or areas of possible heavy rainfall.
- Predicting the onset of wind damage. Rapid decreases in VIL values frequently indicate wind damage may be occurring.
A handy VIL interpretation guide is available from the Oklahoma Climatological Survey.
The Echo Tops image shows the maximum height of precipitation echoes. The radar will not report echo tops below 5,000 feet or above 70,000 feet, and will only report those tops that are at a reflectivity of 18.5 dBZ or higher. In addition, the radar will not be able to see the tops of some storms very close to the radar. For very tall storms close to the radar, the maximum tilt angle of the radar (19.5 degrees) is not high enough to let the radar beam reach the top of the storm. For example, the radar beam at a distance 30 miles from the radar can only see echo tops up to 58,000 feet. The following example taken from Hurricane Claudette in 2003 shows this limitation.
Echo top information is useful for identifying areas of strong thunderstorm updrafts. In addition, a sudden decrease in the echo tops inside a thunderstorm can signal the onset of a downburst--a severe weather event where the thunderstorm downdraft rushes down to the ground at high velocities and causes tornado-intensity wind damage.
Storm Total Precipitation
The Storm Total Precipitation image is of estimated accumulated rainfall, continuously updated, since the last one-hour break in precipitation. This product is used to locate flood potential over urban or rural areas, estimate total basin runoff and provide rainfall accumulations for the duration of the event.
1 Hour Running Total Precipitation
The 1 Hour Running Total Precipitation image is an estimate of one-hour precipitation accumulation on a 1.1x1.1 nm grid. This product is useful for assessing rainfall intensities for flash flood warnings, urban flood statements and special weather statements.
Velocity Azimuth Display (VAD) Wind Profile
The VAD Wind Profile image presents snapshots of the horizontal winds blowing at different altitudes above the radar. These wind profiles will be spaced 6 to 10 minutes apart in time, with the most recent snapshot at the far right. If there is no precipitation above the radar to bounce off, a "ND" (Non-Detection) value will be plotted. Wind are plotted in knots using the standard station model.
Altitudes are given in thousands of feet (KFT), and the time is GMT (5 hours ahead of EST). The colors of the wind barbs are coded by how confident the radar was that it measured a correct value. High values of the RMS (Root Mean Square) error (in knots) mean that the radar was not very confident that the wind it is displaying is accurate--there was a lot of change in the wind during the measurement.
Storm Attributes Table
The Storm Attributes Table is a NEXRAD derived product which attempts to identify storm cells.
The table contains the following fields:
- ID - This is the ID of the cell. The ID is also printed on the radar image to enable you to reference the table with storms on the radar image. If a triangle is shown in this field, it indicates NEXRAD detection of a possible tornadic cell (this "detection" is called the tornado vortex signature). If a diamond appears in this field, NEXRAD algorithms detect the storm is a mesocyclone. If a yellow-filled square appears, the storm has a 70% or greater chance of containing hail.
- Max DBZ - This is the highest reflectivity found within the storm cell.
- Top (ft) - Storm top elevation in feet.
- VIL (kg/m²) - Vertically Integrated Water. This is an estimation of the mass of water suspended in the storm per square meter.
- Probability of severe hail - Probability that the storm contains severe hail.
- Probability of hail - Probability that the storm contains hail.
- Max hail size (in) - Maximum hail stone diameter.
- Speed (knots) - Speed of the storm movement in knots.
- Direction - Direction of storm movement.
On the radar image, arrows show the forcast movement of storm cells. Each tick mark indicates 20 minutes of time. The arrow length indicates where the cells are forecast to be in 60 minutes.
When choosing the top 5 or top 10 storms from the "Show Storms" select box, the top storms are based on Max DBZ.