The National Weather Service's Doppler Weather Radar
The NWS Forecast Office in Tallahassee monitors weather over North Florida, Southwest Georgia, and Southeast Alabama with our state-of-the-art Doppler Weather Radar. We also routinely access the Department of Defense Doppler radars at Eglin AFB (KEVX) in the western Florida Panhandle, Fort Rucker (KEOX) in Southeast Alabama, and Moody AFB (KVAX) in South Central Georgia. NWS Tallahassee's Doppler radar (KTLH) was commissioned in 1995. This radar and others like it around the region provide extended weather radar coverage of the tri-state area.
The National Weather Service (NWS) has recently completed a major modernization program that has already improved the quality and reliability of its products and services. The technological keystone of this modernization, and one of the first phases to be completed, was the Weather Surveillance Radar - 1988 Doppler (WSR-88D). The WSR-88D excels at detecting the severe weather events that threaten life and property, from early detection of damaging winds to estimating rainfall amounts for use in river and flood forecasting. Moreover, the WSR-88D can increase the lead time for warnings, and the specificity of such warnings, for short-lived, often catastrophic events such as tornadoes, damaging thunderstorm winds and flash floods.
In a cooperative effort with the Department of Defense and the Federal Aviation Administration, the NWS has deployed over 150 Doppler radars nationwide. Through an integrated network spanning the entire United States and its island territories, from Guam to Puerto Rico, the WSR-88D has dramatically enhanced our ability to safeguard life, property and commerce.
The term WSR-88D is simple to explain: WSR is Weather Surveillance Radar; 88 is the year the first NEXRAD was commissioned for use (1988); and the D means it is a Doppler radar.
Radar requires three integral parts to work: (1) an antenna/receiver, (2) computers that process the raw radar data, and (3) an interactive workstation that the forecasters can use to display the processed radar data.
In a basic way, radar works like sonar. It all starts at the Radar Data Acquisition unit (RDA), which is the tower pictured at left located a mile east of the Tallahasee Regional Airport terminal. The radar dish is protected by a covering that resembles a giant soccer ball. The transmitter at the antenna within this protective dome sends out a pulse of energy. When this energy hits an object, like a raindrop or snowflake, it is reflected in all directions. Part of the energy will be reflected back to the antenna, where the dish focuses the returned energy.
The returned energy, called base data, is sent from the RDA site to the processing computer located here in the office. The processing computer, known as the Radar Product Generator (RPG), performs various data quality checks of the raw radar data and creates radar images and products. The Advanced Weather Interactive Processing System (AWIPS) workstation (left), allows forecasters to display radar data.
There are many types of radar images. The most common is a reflectivity image, which shows the areal extent of precipitation. Unfortunately, the radar can display "echoes" that may or may not represent meteorological targets. The radar is so sensitive that it can detect clouds, dust and aerosol particles, insects, and birds, not to mention airplanes and ground based objects near the RDA itself. In addition, radar alone cannot determine if precipitation is rain, sleet, hail or snow. Forecasters use other information at their disposal to make this determination. Click here to see the latest national composite of WSR-88D reflectivity images. Local radar data for Tallahassee and other areas in the U.S. can be obtained from our Radar Imagery page.
One of the main capabilities that distinguishes Doppler radar from older conventional radars is its ability to determine wind direction and wind speed. More accurately, NEXRAD determines the movement of wind-driven precipitation using a principle known as the "Doppler effect". The Doppler effect is the alteration of the frequency of the radar-transmitted pulse of energy at the moment it is reflected off of a moving target (precipitation). If wind is moving precipitation toward the radar, the frequency of reflected energy picked up by NEXRAD increases. If the wind is moving precipitation away, the frequency of reflected energy decreases. The speed is determined by the amount of this frequency shift. The sophisticated computers within NEXRAD analyze these frequency shifts which can then be displayed as color imagery on the PUP workstation.
The reflectivity and velocity data from the WSR-88D are the primary data sources that National Weather Service meteorologists use to issue short-fused warnings for tornadoes, severe thunderstorms, and flash floods.
The following WSR-88D products are also available to NWS forecasters:
The National Weather Service is proud to announce our new on-line network of national Doppler radars. Use the interactive national interface to access Doppler radar data for anywhere in the 50 states, Puerto Rico or Guam. Our locally maintained radar page has been revamped to access local radars in the network. Four products are available: base reflectivity, composite reflectivity, one-hour radar estimated precipitation, and storm-total radar estimated precipitation. Each local radar page has a zoom feature and can be looped. There is also a handy interface in the upper left hand corner that allows you to jump easily between adjacent radars.
If you would like to learn more about radar and NEXRAD, check out these links.