The National Weather Service's (NWS) Weather Surveillance Doppler Radar (WSR-88D) is a highly sensitive, powerful, and invaluable technology. Key components of the system include the Radar Data Acquisition (RDA) and the Radar Products Generator (RPG). The RPG contains communications, software, and storage devices, which process data received from the RDA (antenna) located at Ft. Knox, KY to create many data fields and products that are viewed on our Advanced Weather Interactive Processing System (AWIPS) workstations. Basically, the RPG is the clearinghouse for NWS Doppler radar information. Data also can be archived which is useful in post-storm analysis studies and other research purposes.
NWS Doppler radar shows the location, intensity, and movement of precipitation, ranging from light snow flurries to very heavy rain and large hail. From the movement of precipitation and clouds, the radar also can sense motion (i.e., velocity) in the atmosphere directed toward and away from the radar (the Doppler effect). Velocity data helps in assessing atmospheric wind fields, as well as severe weather velocity signatures from thunderstorms. The radar system contains various software algorithms as well that produce a number of other radar products and alarms when threshold values of certain parameters are met. Data from numerous neighboring NWS Doppler radars can be ingested and displayed on AWIPS to assist in assessing more distant precipitation systems, including thunderstorms.
NWS Doppler radar greatly enhances the ability of NWS forecasters to issue short-term forecasts for any weather situation, as well as timely and accurate warnings during severe thunderstorm and other hazardous events. Forecasters undergo frequent recurring training in order to remain highly proficient and to properly interpret hazardous weather radar signatures. Besides training, frequent updates to the radar software, such as "super-resolution" and "dual pol" data, permit better detection and dissection of precipitation systems. Data available from the radar allows NWS meteorologists to thoroughly dissect and evaluate thunderstorms and their trends, all of which are extremely helpful in the severe weather warning decision process. Accurate spotter reports also are crucial in the storm analysis and verification process. Doppler radar pictures and example data fields and discussions are available on the NWS Louisville Science and Technology page. Also available are event summaries and NWS Doppler radar imagery from selective severe weather events in Kentucky and southern Indiana. The map below shows locations of WSR-88D radars across the United States.
Several of the important NWS Doppler radar products available to forecasters include:
Base Reflectivity:
Used extensively to identify the location, intensity, pattern, and movement of precipitation, from light snow flurries and drizzle to heavy rain and hail. Reflectivity data provide a wealth of crucial information needed to issue accurate hazardous weather warnings. High reflectivity values denote that heavy rain and possibly hail are occurring within thunderstorms. The radar generates reflectivity data at various altitudes, which permits an evaluation of thunderstorms in the lower, middle, and upper portions of the storms in order to determine their vertical structure. In winter storms, reflectivity data clearly can slow banded precipitation, including snow that can lead to narrow zones of heavier snowfall amounts. Example 1. Example 2. Example 3.
Composite Reflectivity:
A plan view display of maximum reflectivity values associated with any portion of (any altitude within) a thunderstorm or other precipitating weather entity. This product provides a quick reference as to the life cycle stage of various storms, their relative strength, and their potential for hail. Example.
Layer Maximum Reflectivity:
Provides maximum reflectivity values within discrete layers of the atmosphere. This product permits assessment of the middle and upper portions of thunderstorms, and is another important tool to determine hail and damaging downburst wind potential.
Base (Ground-Relative) Velocity:
Estimates actual atmospheric wind fields directed toward and away from the radar (i.e., "radial" winds). Base (ground-relative) velocity is very important for assessing rear inflow jets (RIJs) and other winds that can cause potential straight-line wind damage from severe thunderstorms, as well as overall wind flow patterns in the atmosphere. To better conceptualize ground-relative velocity, consider this example: if a person stood outside and did not move, the wind velocity that person felt would be "base" or "ground-relative," i.e., the actual wind in the atmosphere. Single Doppler radar only can assess wind components directed toward and away from the radar. However, from these radial winds, actual wind patterns often can be deduced. Example.
Storm-Relative Velocity:
Estimates winds relative to a moving precipitation entity (e.g., a thunderstorm). In other words, it is the wind that a thunderstorm actually "feels" as it moves through the environment. It is calculated by subtracting out the motion of the storm from the actual (ground-relative) wind. Storm-relative velocity is critical for detecting shear zones and mesocyclones/mesovortices within severe thunderstorms that could result in damaging surface winds, hail, and tornadoes. Converging and diverging wind fields can be detected using storm-relative data as well, which also have important implications on the structure and evolution of thunderstorms. To better conceptualize storm-relative velocity, consider this example: if a person rode a bike outside (i.e., was moving), the wind velocity that person felt while moving would be "storm-relative" or "system-relative" (combines the actual wind with the movement of the person), i.e., different from the actual or ground-relative wind. Example 1. Example 2.
Reflectivity and Velocity Vertical Cross-Sections:
Allows forecasters to assess the vertical structure of thunderstorms. Critical factors that can be analyzed include the echo top height, storm tilt, height and depth of high reflectivity values, height and depth of mesocyclones/mesovortices within a thunderstorm, airflow patterns within and near a storm, and the life cycle stage of a storm. Example.
Reflectivity and Velocity Four-Panels and All-Tilts:
Base reflectivity, base velocity, and storm-relative velocity data can be viewed in four-panel displays (Example 1. Example 2.), i.e., data from four radar elevation angles within the same volume scan can be viewed simultaneously on one screen to better visualize the vertical structure of a storm. One of the best ways (besides vertical cross-sections) to view and assess the full vertical depth and structure of a storm is to use the radar's "All-Tilts" capability (Example 1. Example 2.), where a meteorologist can very quickly loop through all elevation angles of data in time and space.
Digital Vertically Integrated Liquid (DVIL):
Represents a vertical integration of reflectivity values within the full depth of a thunderstorm, i.e., a type of measure of the amount of liquid and ice within storms, with values converted to equivalent liquid water. DVIL is useful for identifying storms containing hail and a large liquid water content (i.e., deep, tall storms). It also helps differentiate the relative strength and depth of storms. Example.
Precipitation Estimates (1-hour; 3-hour; user-defined; storm total):
The radar estimates precipitation amounts for 1-hour and 3-hour periods, and as a storm/weather system total. The user can also define the period for which he/she wants to display an estimated precipitation total. As an augment to ground-truth rain gauges, these radar products are crucial in determining the potential for flooding and flash flooding from precipitating weather systems, especially thunderstorms. There are a few accuracy limitations to these products, but overall the radar usually does a very good job in determining amounts and locations of precipitation. Example.
Velocity Azimuth Display (VAD) Wind Profile (VWP):
Based mainly on cloud and precipitation movement, the VWP is the radar's estimation of wind speeds at specific altitudes in the atmosphere as a function of time. The VWP product allows forecasters to assess vertical wind shear (speed and directional shear) in the environment, which is crucial to the organization and severity potential of thunderstorms. Vertical speed shear refers to increasing wind speeds with height. Directional shear refers to a change in wind direction with height. Example.
Dual-Polarization Data:
All NWS Doppler radars have been upgraded to include software and hardware that sends and receives both horizontal and vertical pulses of energy, providing a much more informative two-dimensional picture. Conventional Doppler radars only send out a horizontal pulse of energy that gives forecasters a one-dimensional picture of precipitation or non-precipitation entities. Dual-pol data helps forecasters clearly differentiate between rain, hail, snow, and ice pellets (precipitation type), and other flying objects, which conventional Doppler radar could not do. This ability improves forecasts and warnings for all types of weather. Another important benefit is that dual-pol more clearly can detect airborne tornado debris, allowing forecasters to confirm a tornado is on the ground and causing damage so they can more confidently warn communities in its path. This is especially helpful at night when ground spotters may be unable to see the tornado. More information is available on the dual-pol webpage of the NWS Louisville Science and Technology website.
Other products and product overlays are available on the NWS Doppler radar. The combination of extensive forecaster training, an excellent radar system, and spotter reports allow severe storms to be detected, evaluated, and warned for effectively to protect life and property. Some thunderstorms evolve and move very quickly; thus, timely warnings may not always be possible for every storm. However, the technology certainly is cutting edge, and is used daily.
