The Skidaway Island Weather Center
Since 2005
What is Doppler Radar?
We have heard thousands of times the iconic term 'RADAR' used by hundreds of people throughout our lives. What does this term actually signify? The term is an acronym for RAdio Detection And Ranging. The earliest radar units were used by the military during World War II for detecting oncoming or outgoing enemy aircraft or, more blandly, projectiles. Refined by the National Weather Service during the years following the World War II crisis, radar was developed not to only detect projectiles and airborne objects, it also could detect precipitation that could impact the surface of the earth and the communities that were located in the radar unit's range. The earliest versions of radar by the military and the federal government were only used to detect the intensity of precipitation through the object's reflectivity, that is, the precipitation's willingness to reflect the initial energy sent from the radar back to the radar unit. Doppler radar, the Doppler term, was not incorporated into the radar's name and its hardware even though the concept and ideology was developed a century earlier.
There are several different types of Doppler radar. They can vary by power, coverage patterns, and wavelength. All of these different features determine how well the Doppler radar will work in a real-time, evolving environment. For instance, when dealing with a severe convective event, meteorologists use high-powered radar and fast-paced coverage patterns to (1) penetrate precipitation and other obstacles relatively easily without radar attenuation and (2) rapidly scan the atmospheric area around the radar. The National Weather Service has several different, distinct types of Volume Coverage Patterns (VCP) that are used to monitor constantly the volume of the atmospheric around the radar data acquisition unit, or RDA.
The first basic radar units were extremely primitive computer equipment that could detect simple precipitation within a defined area. While not revealing much useful information, it still gave meteorologists and weather enthusiasts the first foundation for modern Doppler radar that we know today. The illustration to the left shows how the early radar systems detected precipitation--and some birds and bugs too. Early computer technology could not differentiate between what was precipitation and what was not. Today, we still have some difficulty (with computer algorithms) trivially finding areas of false precipitation echos and those echoes that are true precipitation. Radar 'echos' are sometimes used to describe rain returns that are found in the radar's data. The basic schematic of the echo principle is described to the right. The simple example of one person at the top of large hole in the ground and yelling down into the tunnel shows the basic mechanics behind radar detection. The sound waves travel substantially slower than the waves in Doppler radar today; still though,
the echoes that are sent down the hole hit the bottom and bounce back up to the person who originally yelled the sound downward.
Some of the underlying questions you are now asking: how are those returning waves interpreted? How are the intensities detected? How is the distance calculated? All are very easy questions to answer because they all rely on the basic principles that have been the basis for modern weather radar. The term echo refers to the return "signal" after the energy sent outward from the radar returns to the unit allowing a complex computer calculation to determine the intensity and distance. Technically, the former is a simple physics equation, distance equals rate multiplied by time (d = rt). When performed hundreds of times per second, that once simple equation can become a nightmare. The micro-technology age helped overcome this obstacle, yielding the answers to complicated mathematical and physical equations in seconds.
While computer technology during the 1990s was still in its early stages, meteorologists with the National Weather Service and with numerous universities were investing time and resources developing and deploying new and proprietary technology (algorithms, hardware, software, etc.) to understand more clearly, to process more quickly, and to classify precipitation and projectiles accurately (now being called dual polarization radar by Baron Services, or "dual-pol" for short), for the good of the atmospheric science and radar meteorology communities. There is no question that this the onslaught of computer technology and the internet has led to more advancements in the radar field. Today most citizens of the United States have some understanding of RADAR. And while this type of technology continues to become more apparent every day (from the earliest storm tracking software for TV vendors in the 90s to today's three-dimensional storm interrogation techniques) the full meteorological potential of these apparatus has not yet been realized. Continued research has shown that while scientists have made great and momentous strides in this field, more work has to be completed. Simply observing how the weather radar operates reveals the elemental nature of the equipment.
How does it work?
When a RDA is scanning the sky, the radar satellite dish sends out extremely fast energy that continues outward until another force interacts or some objects blocks the signal. This energy and the frequency of those sound waves allow the radar to determine the distance, speed, and intensity. Because these variables can be calculated and are known, the raw radar data output can be displayed visually on a bin-oriented display in the form of four distinct products: base reflectivity, base velocity, storm relative velocity, and spectrum width. The first, reflectivity, is the raw intensity output of the radar using the returning 
radar echoes. Base velocity measures the true speed of projectiles that are in the atmosphere. This product does not provide accurate wind speeds for the surface. When a meteorologists makes predictions based upon some or all tilts of base velocity and velocity products, they are looking for specific signatures in data bins that are indicative of certain wind conditions (e.g. downburst winds, divergence/convergence, mesocyclonic rotation, etc.) that could occur on the surface.
The United States government's Department of Commerce oversees the National Oceanic and Atmospheric Administration which maintains the National Weather Service. This agency issues severe weather warnings, advisories, and watches. They currently maintain more than 150 WSR-88D, or Weather Service Radar 1988 Doppler. The first one of these advance Doppler radars was developed in 1988. The weather radars that are deployed by the National Weather Service are typically far superior to any television station's or media outlet's weather radar. The fact that the National Weather Service's Doppler radar can detect a shift and change in the energy that is sent outward from the radar data acquisition unit (RDA) allows the technology to detect the speed of the rainfall or object in the atmospheric space. However, there are some television radars that have the ability to detect and display data in the same manner and quality that the National Weather Service's WSR-88Ds do. The weather service radar WSR-88D have ample power to penetration precipitation and obstructions. On average, these RDA units output about 450 times the energy output of a typical home microwave, or approximately 450,000 watts. Sometimes, the weather service Doppler radar can reach a maximum energy output of one-million watts.
The Next Generation Weather Radar, or NEXRAD, of the National Weather Service does not scan in a continuous manner as some commercial Doppler radar units do. You may be familiar with the moving white (or some other color) beams that "sweep" on radar screens that you see on television. If that television station or media outlet uses National Weather Service radar data,
the beam is not real. These beams give the false impression that the radar data you are seeing from the National Weather Service WSR-88D is live data. This is not true. The NEXRAD gives a real-time image for those who are using an application that ingests radar data directly from the National Weather Service. The National Weather Service WSR-88D completes a full elevation scan and then interprets and disseminates the radar returns; therefore, the weather data can not update continuously. There are two types of radar data that are accessible by the public: Level II and Level III data. Are you asking how this data is related to sample scanning and data output? The two types of data have significantly different output but both have the same output increment. Level III data has sixteen data levels while Level II data has two-hundred fifty-six levels. Increasingly, with the distribution of high definition television, media outlets are turning to more detailed weather radar data for their viewers. This is especially true in highly competitive markets. The Level II data usually updates on a per-tilt basis giving a more real-time display of precipitation and atmospheric sampling, even though still not live. Level III data updates in the same fashion; however it only outputs the lowest four tilts of data regardless of the coverage pattern.
Christian Doppler: The Doppler Effect
Christian Andreas Doppler, born in the small town of Salzburg, Austria on November 29, 1803, is credited for discovering the Doppler Effect. His father owned a successful business, but because of Doppler's poor physical condition, he was unable to work there. After graduating high school, he attended Prague Polytechnic College,
now called Czech Technical University. While studying mathematics, astronomy, and physics, he developed his skills until being appointed to a professor position in 1841. Shortly thereafter in 1842, he published Über das farbige Licht der Doppelsterne or literally "Concering the Double Light of Colored Stars". He first observed the behavior, that was later called as the Doppler Effect, when observing stars. Doppler realized that a moving objects' pitch of sound varies for a non-moving observer and would in turn change what was perceived by that observer. He could illustrate and explain this behavior by observing stars in the sky which changed color based on their movement and their velocity relative to Earth. The Doppler Effect that came to be known from this paper was defined as a "shift in frequency and wavelength of waves which results from a source moving with respect to the medium, a receiver moving with respect to the medium, or even a moving medium".
The equation above expresses the relationship between the perceived frequency and the actual frequency of the sound waves emitted from a source where 'v' equals the velocity of the waves of the source, 'v of r' is the velocity of the receiver relative to the source, 'v of s' is the velocity of the source, 'f' is the observed frequency, and 'f sub-zero' is the actual frequency emitted. When all variables are inputted into the equation, the output will be the perceived frequency or any variable thereof. And while the Doppler principle was originally applied to light waves, the same can be said about sound waves. Sound and light waves behave in the same way to hold true with the Doppler Effect. Several other scientists during and after Doppler's life confirmed his theory; they include Buys Ballot in 1845 and Hippolyte Fizeau.
Using Doppler Weather Radar in Real-Time
As you can probably imagine, the advanced nature of Doppler radar allows meteorologists and the public to monitor and predict the future movement of severe thunderstorms and other weather phenomena. In fact, most weather warnings that you hear from television meteorologists or NOAA weather radios are issued based on Doppler radar signatures and data. The scanning nature of the WSR-88D provides volume scans where a three-dimensional sample of the atmosphere can be analyzed. Many media outlets argue that the National Weather Service weather radar is extremely slow when providing images of surface base reflectivity images (0.5º tilt or the image most likely to show ground-level rain intensities); while this is an accurate statement, the NEXRAD does not scan the lowest level of the atmosphere for five minutes (more or less depending on the VCP), it scans a dozen or more elevation tilts that provide a vertical picture of thunderstorm/precipitation structure before returning to scan the lowest elevation again. Other Doppler radars owned by various television stations only scan the lowest level of the atmosphere leaving valuable meteorological features not seen on the lowest elevation scan unknown. The nature of the NEXRAD's scanning strategies benefit the public, allowing meteorologists at the National Weather Service to issue timely severe weather warnings.
The volume coverage patterns (VCP) are what drive the NEXRAD's unparalleled scanning capabilities. When severe weather threatens, technicians switch the RDA into a coverage pattern that scans the atmosphere quickly and completely. As alluded earlier, the WSR-88D operate in a way that allows the vertical interrogation of weather phenomena, or more usually, severe thunderstorm convection. This vertical interrogation is particularly useful when trying to find hail cores or other radar features like bounded weak echo regions or bounded echo regions (BWER or BER respectively). This image shows an exceptionally large hail core in a supercell thunderstorm on March 16, 2008 during Saint Patrick's Day weekend. These types of images are possible because the WSR-88D scan an entire volume of the atmosphere. When this type of data is plotted in this fashion, other information used in tandem with these images allow even more conclusions to be postulated, including but not limited to estimated hail size and cloud-to-ground lightning stroke potential. The robust ability of computers to interpret and later plot that information allows mathematical algorithms to make binary predictions about weather phenomena potentials. And while these computer algorithms can give instant information, the ambiguous data that is in the raw NEXRAD output and the massive amount of data that has to be processed and run, among others, makes this technology an asset rather than a pass-all solution for Doppler radar limitations.
