101: Weather Radar (Radar History)

It began with WWII. As the world plunged into its' second great war, the significance of radar technology became immediately apparent. Soon, the armed forces of the United States, Russia, and Germany had all developed and fielded their first generation of aircraft surveillance radar. However, there was a problem: weather often cluttered the signal and inhibited the ability to reliably track enemy activity. There were some though, who saw this annoyance and realized a way to turn these lemons into lemonade. Thus, by the end of the war, dedicated weather radar had become an important tool in the meteorologist's arsenal.



The History of Weather Radar


First Generation:

Radar, which is actually an acronym for RAdio Detection And Ranging, began its meteorological career as a secretive and highly expensive technology. Although purpose build units were constructed in the United States (figure 1), mainly for research, the first true application of radar as an operational tool occurred in the Pacific towards the end of the World War II. Perhaps the most notable of these early applications was the tracking of Typhoon Cobra by US Task Force 38 while en route to the Philippines (figure 2). For the first time, the intricate structure of an intense tropical cyclone was able to be observed. Unfortunately, those ships that observed the storm were also struck by it, resulting in 790 fatalities, several sunken ships, and hundreds of planes lost. It was events like this, and another typhoon incident less than a year later (involving the same fleet), that caused radar to be seen as an integral part of operational meteorology, instead of just some technological curiosity.

Figure 1: An early radar image depicting a cold front approaching Boston.

Figure 2: Typhoon Cobra as seen on radar from aboard one of the Task Force 38 ships on 18 December, 1944. 

As soon as the war ended and radar technology became declassified, the Weather Bureau acquired 25 ex-navy units for use in weather research. The first radar designed specifically for weather surveillance was the CPS-9, which was installed in military bases around the US in the mid-1950s. This first generation of weather radar was significantly hindered by the Weather Bureau's policy of providing only daily forecasts, thus radar's primary function, as a early warning aid, was seriously neglected. This shortcoming was not lost on the public, especially since the US military's weather service had been issuing successful storm warnings for years. With mounting pressure from the public and congress, the Bureau changed their position and began issuing warnings in 1952. However, for them to be effective, they would need a new radar network.


Second Generation:

Beginning in 1961, a new radar network was deployed across the US east of the Rockies consisting of the newly developed WSR-57 (Weather Surveillance Radar - 1957) units (figure 3). Unlike the digital, automated operation of today's radars, the WSR-57s required an operator to be physically stationed at the site (figures 4 and 5). The operator would manually adjust the unit's direction and tilt to interrogate storms and other weather phenomena. Then, they would note the general location of weather echoes and send this information off to Washington, D.C. where a national mosaic of radar descriptions would be produced once an hour. Eventually this process became more and more automated to the point that raw data could be sent directly to forecasters by the late 1970s. While useful in identifying storms, the early radar units could only detect reflectivity and discriminate between just a few different values of intensity.

Figure 3: An old retired WSR-57

Figure 4: The WSR-57 workstation at which a radar operator would make observations. The cathode ray tube display in the center is the plan position indicator (PPI), which displays the radar image as two-dimensional with north in the y-direction and east in the x-direction

Figure 5: An example of a WSR-57's range height indicator (RHI), which acted as a cross-section radiating out from the radar unit.

Eventually, a slightly updated unit, the WSR-74, was deployed to fill in some of the gaps left by the existing network. This model came in two varieties, the WSR-74C and the WSR-74S. The C-series used the C-band wavelength, which provides higher resolution at the cost of range and attenuation due to heavy precipitation. The S-series, like the WSR-57 before it, used S-band signal to provide much larger range with less attenuation, although the resolution is significantly lower. Many of the now retired WSR-74 units have been sold to other countries (figure 6).

Figure 6: A WSR-74C that now operates overseas.

Third Generation:

As far back as WWII, engineers realized the potential for radars to measure Doppler shift (the change in signal frequency due to a target's velocity relative to the radar). However, this remained the realm of theory and a few research dedicated units. By the 1970s, technology advances in computer processing and high resolution displays brought the possibility of an operational network of Doppler radars into consideration. Unfortunately, a study in 1976 found that it was not feasible to update the existing network with Doppler capability. The answer to this issue was to create an entirely new network of radar units, a system that acquired the name NEXRAD (next generation radar) in 1979. After years of design and testing, the production unit was finalized in 1988 and named WSR-88D (D for Doppler). The first prototype unit began operation in Norman, Oklahoma in 1990 (figure 7), with the rest of the original planned units being deployed between 1993 and 1997, while the WSR-57 and -74 units were retired. By 2012, a total of 154 WSR-88D units were operating across the United States and 5 at Department of Defense locations in Okinawa, South Korea, Guam, and the Azores (figures 8 and 9).

Figure 7: The first operational WSR-88D in Norman, Oklahoma.

Figure 8: Map of the coverage of all of the WSR-88D units in the contiguous United States.

Figure 9: Map of the coverage of all of the WSR-88D units outside the contiguous United States.

The operation of the NEXRAD system is a radical departure from previous radar networks. Instead of being manually controlled by an operator, the WSR-88D units conduct pre-programmed "volume scans". Each of these scans includes the full 360 degree sweep of the beam, at all of the pre-programmed tilts (figure 10). The number of tilts in a scan, along with rotational speed of the antenna itself varies depending on what mode, called volume coverage patterns (VCP), the radar is set to. This ranges from "storm modes", such as VCP 12, which uses 14 tilts and completes a scan in about 4.5 minutes, to "clear air modes", such as VCP 31, which only uses 5 tilts and takes roughly 10 minutes to complete a scan. Since the radar transmits the entire volume scan in one transmission, there is no rotating dial that updates the image as seen on older radar units. For some reason, television stations and other popular radar displays add this sweeping dial, despite being only for aesthetic purposes.

Figure 10: Diagram of  the tilts available in VCP 12

Fourth Generation:

As long ago as 1945, it was realized that the shape of objects targeted by radar could affect how well different angles of the polarization of the beam were reflected back. For example, large rain drops tend to flatten out as they fall, making them much larger in the horizontal than in the vertical. Therefore, the horizontally polarized portion of a radar beam would return a much stronger signal than the vertically polarized portion when the radar is scanning a area of heavy precipitation. Thus, given enough information on the polarity of a target's radar echo, it is possible to estimate what kind of object the radar is detecting, such as snow, rain, hail, or even insects. While this was known in theory, it was not until the 1990s that computers became fast enough to process the data in real time. The potential for these dual-pol (dual polarity) radars was quickly realized and a decision was made in 2003 to upgrade all of the existing WSR-88D units to have dual-pol capability. The first upgrade was completed in 2011 at Vance Air Force Base in Oklahoma, and rest of the NEXRAD network was upgraded by the end of 2013.


Terminal Doppler Weather Radar:

In response to a series of fatal aircraft accidents caused by heavy wind shear events, such as microbursts, in the 1980s, the Federal Aviation Administration decided to develop a network of radar units that were dedicated for use near airports. The TDWR (Terminal Doppler Weather Radar) was designed in 1988 and the first was deployed in Memphis in 1992. Since then, 48 units have been installed across the United States, mainly near large cities east of the Rockies (figure 11). In order to get a clear view of storms that is free from ground clutter and able to view entire storms with its' available tilts, WSR-88D units are intentionally placed about 30 miles from major cities. Thus, the much closer proximity of the TDWR units allows them to obtain higher resolution images. Furthermore, TDWR are C-band radars, which allows for higher resolution than the NEXRAD S-band units (figure 12). The downside is that the beam is easily attenuated by precipitation, so it might underestimate the intensity of distant precipitation if there is heavy precipitation closer to the radar. The TDWR have just two modes: the clear air mode has 17 tilts while the precipitation mode has 23 tilts, with the highest tilt being 60 degrees, over three times higher than the WSR-88D's highest tilt (19.5 degrees), providing a more complete view of activity near the radar.

Figure 11: Map of the coverage of all of the TDWR units in the contiguous United States.

Figure 12: The difference in resolution between a TDWR (left) and a WSR-88D (right).