Doppler Weather Radar Overview
8.1 In 1842, the Austrian physicist Johann Christian Doppler developed the theory and supporting math equations to explain the relationship between motion and frequency changes in sound. Application of doppler theory was first recorded in the late 1950s; however, it wasnt until the early 1990s that doppler technology became the standard mechanism to measure radar imagery. Doppler provides a rich suite of automated algorithms (internal computer programs) allowing meteorologists to more definitively identity and track thunderstorm/precipitation intensity, tops, severe weather signatures, microbursts, windshear and precipitation accumulation. This chapter will acquaint you with doppler theory, basic NEXRAD operations and radar signature identification.
The Doppler Effect
"The observed change in the frequency of sound or electromagnetic waves due to the relative motions of the source and observer."
8.2 The Doppler effect is best described by the change in pitch of a trains whistle as the train passes by your position. The pitch of the trains whistle changes because the frequency/wavelengths of the sound increases/decreases as the train moves toward you and decreases/increases as the train moves away from you. The same concept applies to meteorological targets. Doppler radar is able to detect the changes in frequency/wavelength resulting from storm motions toward or away from the radar. This concept is the basis for all velocity products.
8.3 RADAR is the acronym for RAdio Detection And Ranging. NEXRAD uses these radio waves to determine target/object size, stength and location. Radio waves are a form of electromagnetic energy. If we could see these radio waves, they would appear as a sine wave (figure 7-2). Shorter wavelengths allow the detection of small droplets, while longer wavelengths are suitable for heavier precipitation such as rain showers and thunderstorms. The ground transmitter emits short powerful bursts of electromagnetic energy called "pulses", similar to a machine-gun firing bullets. The pulses travel along a focused path called a beam. Once this beam strikes an object large enough to be detected, a fraction of the transmitted energy is returned to the antenna. Silent periods occur between pulses (figure 7-3), allowing the antenna time to listen for their return. The listening time and energy return characteristics are used to determine precipitation size, location and intensity.m The radar calculates the round trip of the beam by measuring the elapsed time from transmission time until return.
8.3.1. The "beam" is transmitted from the radar location and the image is displayed relative to the radar site. Products are displayed using a 360° azimuth color image. The center of the image display is usually the ground-station location. Your location relative to the radar station is significant; i.e., the primary radar site and subsequent center-field location may be different from your briefing location. The pixel resolution determines the radar range. Identifying the range is very significant as a 124 nm range will be interpreted differently from a 32 or 248 nm range (i.e. the range markers and subsequent relative echo location will be closer or further from the image center) The available ranges for the base and velocity products are as follows:
Figure 8-4 Pulse Characteristics
The NEXRAD System
8.4. The Doppler System consists of 5 components; all are relevant to how you receive your data. The crew member does not manipulate the radar controls but is reliant upon the selection of the radar technician or service provider. The system components include the Radar Data Acquisition (RDA), Wideband Communication, Radar Product Generator, Pup Workstation, Application Terminal, and Principle or Other Users.
8.4.1. Radar Data Acquisition (RDA)- The RDA unit consists of the antenna, transmitter, receiver and signal processor. These components generate/transmit the energy pulses, receive the reflected energy and process the received energy into base data.
8.4.2. Wideband Communication Link - A Wideband full-duplex (2 way-communication) Communication link provides the data transmission between the RPG and the RDA.
8.4.3. Radar Product Generator (RPG)- The RPG serves as the command center for the entire system. The RPG processes the digital data and creates the Base and Derived Products, providing clutter filtering and other functions.
8.4.5. Principal User Processor (PUP)- The PUP is used to access, process and store the products received from the RPG. It is usually located in a computer equipment room of the base weather shop.
8.4.6. Applications Terminal -The Applications terminal is where the technician manipulates the data and is usually located near the briefing counter. The Application terminal has a keyboard, mouse and graphic monitors.
8.4.7. Principle and Other Users - Principle Users include the Department of Defense, Department of Commerce (NWS) and the Department of Transportation (FAA). Aircrew view doppler information in-person at the weather shop or via web-based technology. If using web based or other remote-access technology, products are predetermined and normally, additional products cannot be requested.
You are simply viewing what is available at the time and cannot manipulate the antenna elevation angle range or other characteristics of the image.
The NEXRAD Network
8.5. The NEXRAD network is comprised of military weather stations, NWS and FAA flight service stations. Those DoD stations with NEXRAD capability are listed in the appropriate en route supplement. Figure 7-7-1 shows an example listing from an US DoD flight Information Handbook. At stations where the PMSV is collocated with the radar equipment, the forecaster can provide a real time radar report to airborne crews within radio range. The weather forecaster can advise the aircrew of the location, movement, intensity and tops of the precipitation (not necessarily the tops of the clouds), but the forecaster cannot vector the aircraft around weather returns.
8.5.1. A few FAA flight service stations (FSS) located near radar-equipped National Weather Service (NWS) stations have Terminal Drop Weather Radar (TDWR). FSS specialists at these locations are certified to make interpretations of weather displayed from the radar display. They can brief the airborne crew on the displayed weather pattern, including the area covered and the weather movement, including the area covered and the weather movement. However, the FSS pilot briefer does not analyze the precipitation echo but merely reports the analysis performed by the NES forecaster.
8.6 The Product Legend is located to the right of all NEXRAD products. Useful information to include the current date and time, station ID or RDA, field elevation, maximum data value, product name, range resolution, elevation slice (angle), center of display, data values and color levels. Always check the PRODUCT NAME to properly identify the product type. Of course, the product type will affect interpretation. Any other information such as operational mode (should read "A" for precipitation mode), ID number, mnemonic, VCP is for weather technician use.
Base and Derived Products
8.7 There are approximately 40 basic meteorological products available to meteorologists; however, this chapter will cover those most useful to you, the crewmember. A Base Product is the result of digital data received after initial processing ("one step removed "raw data) and is differentiated from Derived products. Base products are not manipulated outside of the initial processing and are preferred by technicians for analyzing significant meteorological features. Derived products are generated through the use of meteorological algorithms (internal computer programs).
Base Reflectivity Product
8.8. Base Reflectivity (sometimes referred to as the Precipitation Display) presents the backscattered electromagnetic energy from targets illuminated by the radar beam (figure 8-9). Reflectivity strength is measured in decibels "DBZ." The color bar on the right side of the image indicates the associated reflectivity strength (DBZ). The maximum reflectivity is entered above the scale. Stronger precipitation levels are farther down the color scale and should be avoided. The National Composite Display is a derived, computer-generated compilation of reflectivity data input by multiple users across the country (figure 8-1).
8.8.1. The weather technician can display an image using different elevation angles, which can also affect the display presentation. Each elevation angle will provide a different view of the storm structure. The greater the angle, the higher the beam scan; thus, the higher the view of the storm. The standard elevation angle is .54° and is selected by the radar technician. As a result, the pilot views the angle selected. Reflectivity products are not useful in readily determining severe weather signatures.
8.9. Occasionally you may encounter a Composite Reflectivity Product. The Composite Reflectivity product is used primarily by technicians as a METWATCH- tool to alert for developing storms. Upon first glance, the Base and Composite Reflectivity imagery appear very similar. However, the Composite Reflectivity (figure 8.10) product is differentiated from the Base Reflectivity in that the Composite Reflectivity displays a synopsis of the maximum reflectivity collected from all elevation scans, in the entire radar coverage area, providing the highest Dbz value. The technician interrogates further based upon these reflectivity returns.
Echo Identification using NEXRAD
8.10. Precipitation Intensity, Movement and Trends- Time lapse or "loop" of base reflectivity is an excellent tool for determining the movement of precipitation and to determine storm structure. Technicians use reflectivity to determine the onset, and end of precipitation. Other useful information include intensity increase/decrease which indicative if the storm is growing or dissipating.
8.10.1. Severe Weather Signatures Reflectivity products are useful in identifying severe weather signatures such a general thunderstorms, hail, hook echoes, squall lines, Line Echo Wave Patterns (LEWP), Bow Echo. microbursts, outflow boundries, gust fronts. A pilot looking at a NEXRAD display should plan to circumnavigate areas of red on the color scale, as these are generally considered danger areas and should be avoided.
8.10.2. Hail - Very high reflectivity values (over 55 DBZ) may indicate the presence of hail. Thunderstorms with strong updrafts, extensive vertical height, high water content, and large cloud drop sizes are favorable for the formation of hail
8.10.3. Squall Line A squall line (figure 7-11) or a line of thunderstorms are easily identified by a line of highly reflective echoes. Squall lines are a strong indicator of potential severe weather.
8.10.4. Line Echo Wave Pattern - A Line Echo Wave Pattern (LEWP) (figure 7.12) is a squall line that has developed into a wave-like pattern due to acceleration at one end of the line and deceleration along the portion immediately adjacent. A Line Echo Wave Pattern (LEWP) indicates possible tornadoes, large hail and high winds. The faster the movement of the entire LEWP structure, the higher the potential for severe weather.
8.10.5. Microbursts and Outflow boundaries are best detected using the velocity mode (For a complete discussion of the causes of low level wind shear, microbursts, outflow boundries and gust fronts, see AFH 11-203 Vol I, Chapter 10). Due to the shallow vertical extent, microbursts are mostly seen at low levels and usually can not be detected beyond 20 nm from the ground station. Microbursts are recognized by a strong velocity flow toward the radar matched by an opposite pattern of strong flow away from the radar. Using reflectivity, like any other boundary it will appear as a narrow line of enhanced values as high as 10 dBz. Outflow boundaries occur on a much larger in scale than microbursts. These lines depicting cooler, denser air sinking and spreading across the ground are easily discernible and can be tracked before reaching the ground station.
8.11. Non-precipitation phenomena The sensitivity of the WSR-88D allows technicians to see non precipitation echoes such as ground clutter, birds, solar effects birds, insects, fire and smoke plumes. Always verify the presence of non-precipitation phenomena with a certified weather technician.8.11.1. Ground Clutter - Ground clutter is the return from a stationary non-weather related ground object such as a large building, city, highway over-pass or water tower. Semi-stationary objects such as fluttering leaves and cars also produce ground clutter returns. Ground returns are highly reflective due to the high density of the target and therefore easily identified. Depending upon the object, most ground clutter returns are stationary. If the viewer is not aware of the presence of ground clutter, over-interpretation can result (ie. echoes may be mistaken to be stronger than actual). Clutter suppression algorithms provide a filter; however, any signal not removed by the algorithm is considered "residual clutter." Anomolous Propogation (radar beam propogating through a non-standard atmosphere) can produce a ground clutter echo.
Birds and insects
8.11.2. The identification of insects can be difficult, except in extreme cases such as a locust swarm. However, bird flocks produce a strong signal for the radar (figure 8-18). The echo will expand outward into a ring, as the birds fly out towards their feeding areas. At night the process may reverse as the birds return.
8.11.3. Solar Effects Echoes from the sunrise are narrow and similar to a "baseball bat" in shape. Solar effects can occur near sunrise or sunset, because the sun and NEXRAD radiates energy in the same microwave region of the electromagnetic spectrum
Fire and Smoke
8.11.4. Fire and smoke phenomena can be seen using NEXRAD (figure 8-19). The smoke source is primarily stationary with the plume blowing toward the direction of the wind-flow. In the example, the smoke plume grows and extends downstream (to the east-southeast) indicating a west-northwesterly flow.
Base Velocity Products
8.12. Velocity derived products, provides exact location and forecast movement of wind speeds, windshear and microburst activity. This information allows ATC shift supervisors to anticipate hazards and best manage airfield traffic flow. With base velocity products, negative values indicate inbound values and positive values indicate outbound values from your position. High winds are located by matching the display to the numerical code.
Vertial Azimuth Display (VAD) Wind Profile Product
8.12.1. NEXRAD is capable of displaying vertical wind profiles within 20 nm of the station (figure 7-21). The VAD Product is an effective briefing tool to show real-time winds at multiple flight altitudes provides effective real-time winds When the Wind Display is selected, "KT" replaces the "DBZ" units, because the units of measure are "knots." Wind shafts depict direction, barbs depict the speed. VAD products are available during in-person briefings where there is an applications terminal.
8.12.2. There are several display data analysis options available to detect windshear such as Low Level Wind Shear Alert Systems (LLWAS) and Terminal Doppler Weather Radar (TDWR). The pilot is briefed by technicians if the presence of windshear is detected or suspected. Regardless of airfield capability, pilot awareness and coordination are crucial for timely windshear recognition and recovery.