The National Weather Service maintains a network of weather radar units at various locations across the United States for continuous weather surveillance and warning purposes. Most of the conventional weather radar units are being replaced by Doppler (WSR-88D) radar units as part of the modernization of the National Weather Service.
A radar (an acronym for radio direction and range) unit operates on the principle that a signal consisting of a directed pulsed beam of electromagnetic radiation is sent out toward a target and returned to the unit if the beam were reflected back from an object. The signal emitted from the radar unit is in the upper radio range of the microwave region of the electromagnetic spectrum. The target represents any foreign object that reflects (backscatters) the microwave signal.
Currently, two basic types of radar systems are available to the meteorologist. The conventional weather radar units used since the 1950's detect the location and extent of the precipitation and its intensity. The newer Doppler weather radar additionally detects the lateral motion of precipitation particles. In weather applications, several types of hydrometeors (including raindrops, snowflakes or hail), serve as targets for the emitted pulsed signal from a weather radar and reflect a portion of that signal back to the receiver.
The time elapsed between the broadcast of the signal and the receipt of a return signal is directly related to the distance between the target precipitation and the radar unit. The reflectivity (or the amount of power reflected back to the unit) provides additional information about the target. In the case of weather radar, the precipitation intensity and precipitation types can be estimated from the reflectivity. A discussion of the features and capabilities of the WSR-88D Doppler radar system is found elsewhere.
Current radar imagery is available in several formats. An overlay of the radar summary indicating precipitation areas and intensity is often placed over the surface analysis . A separate national radar summary chart is also available. These composite radar summary charts are prepared hourly at 35 minutes past the hour from the data supplied by National Weather Service radar reports originating from the network of primary weather radar units, not inclusive of the WSR-88D Doppler radars.
The intent of the Radar Summary chart is to graphically depict large scale distribution of precipitation. Precipitation intensities are indicated on the chart by color codes. Some adjustment of the data may be needed, especially if precipitation echoes are reported by more than one radar site. Additional information such as echo tops/bases, cell and area movement, precipitation type, and intensity may be depicted. When applicable, current severe thunderstorm and tornado watch areas ("watch boxes") are plotted on the radar summary.
The reflectivity or intensity of the reflected radar signal depends upon several factors to include the size, shape, density (the number of targets per unit volume), and state (rain, snow or mixture) of the hydrometeor target. Usually, the intensity of the reflected echo is assumed proportional to the rain drop density, thus indicating the precipitation rate or intensity.
Contours of echo reflectivity, also known as "VIP levels" (for Video Integrator and Processor), are plotted on the radar charts as a color coded display. The colors within these contours provide an indication of the precipitation intensity and the size depicts the areal extent of the detected precipitation. Based on empirical studies, six standard VIP levels are related to the rainfall rate for the steady "stratiform" precipitation typical of winter time and for showery "convective" precipitation.
RADAR INTENSITY LEVELS
COLOR VIP RAINFALL RATE PRECIPITATION LEVEL [inches per hour] INTENSITY STRATIFORM CONVECTIVE Blue 1 < 0.1 < 0.2 Light - Cyan 2 0.1 to 0.5 0.2 to 1.1 Moderate Green 3 0.5 to 1.0 1.1 to 2.2 Strong + Yellow 4 1.0 to 2.0 2.2 to 4.5 Very Strong ++ Magenta 5 2.0 to 5.0 4.5 to 7.1 Intense X Red 6 > 5.0 > 7.1 Extreme XX White Unknown
Typically, the blue colors indicate light rain or snow; the green colors identify light thunderstorms/moderate rain showers; yellows identify moderate thunderstorms, while the magenta grading to red are reserved for the severe thunderstorms that can cause flooding rains. On a local scale (not possible on the national summary), an echo with red can also indicate highly reflective water-coated hail.
Echo movements are plotted on the radar summary chart as arrows to indicate the direction and speed of the echo. Arrows with tail feathers indicate organized area or line movement. The number of tail feathers indicates the speed; each half barb identifies a 5 knot increment, one barb is used for each 10 knot increment and a pennant represents 50 knots.
The precipitation types and changes in echo intensity are plotted on the radar summary charts.
TRW Thunderstorms SW Snowshowers RW Rainshowers S Snow HAIL Hail IP Ice pellets (sleet) R Rain ZR Freezing rain
The status of each radar unit is plotted over the location of the radar site where appropriate. Special status abbreviations are used on the charts at those radar sites not reporting precipitation:
NE No Echoes/precipitation within the radar range NA Radar data Not Available OM Radar Out for Maintenance
The areas encompassed by an official severe weather watch issued by the Storm Prediction Center are plotted on the radar summary chart. These areas appear as a rectangular "box" outlined by dashed lines, conforming to the map coordinates of the affected area. Weather watch numbers associated with each box and the expiration or "valid" time (the time when the watch will expire) are placed in the box.
The weather watch numbers are sequentially numbered throughout the year and have a letter prefix to identify the type of watch: a letter "S" indicates severe thunderstorm watch, while a "T" indicates tornado watch. As an example:
to 5Z A tornado watch #335 that expires at 0500 UTC.
to 8Z A severe thunderstorm watch #336 expiring at 0800 UTC.
This chart helps fill in the precipitation regions between surface observation stations. Hence, the meteorologist can obtain an additional perspective of precipitation types, intensities and movements not provided by the conventional surface analysis. However, care must be exercised and surface observations inspected, because radar summaries may contain echoes of precipitation that does not reach the ground. This condition, called "virga" exists because the air is too dry and evaporation of the rain droplets has taken place. The inclusion of current severe weather watch boxes helps identify potentially conducive for severe weather.
Last revision 10 June 1996© Copyright, 1996 Edward J. Hopkins, Ph.D. firstname.lastname@example.org
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