The radar continuously scans the atmosphere by completing volume coverage patterns (VCP). A VCP consists of the radar making multiple 360° scans of the atmosphere, sampling a set of increasing elevation angles.
There are two main operating states of the WSR-88D; Clear Air Mode and Precipitation Mode. Within these two operating states there are several VCPs the NWS forecasters can utilize to help analyze the atmosphere around the radar. These different VCPs have varying numbers of elevation tilts and rotation speeds of the radar itself. Each VCP therefore can provide a different perspective of the atmosphere.
Common among all VCPs (except for VCP 12) is the tilt elevation of the lowest five elevation angles. The scanning begins with 0.5° elevation meaning the centerline the radar beam antenna is angled 0.5° above the ground. Since the the beam itself is 1° wide, it returns information about what it "sees" between 0° and 1° above the horizon. As it completes that elevation scan the radar is tilted another degree with the center line of the beam now at 1.5° and the process of observing the atmosphere begins again then continues through the 2.4°, 3.4° and 4.3° elevation angles.
Clear Air Mode
Clear Air mode is used when there is no rain within the range of the radar. In this mode, the radar is in its most sensitive operation state. This mode has the slowest antenna rotation rate which permits the radar to sample a given volume of the atmosphere longer. This increased sampling increases the radar's sensitivity and ability to detect smaller objects in the atmosphere than in precipitation mode.
A typical radar image in clear air mode will not reveal much. Generally, the only returned energy to the radar will be very close to the radar's location. A lot of what is seen will be airborne dust, bugs, and particulate matter (image at right).
However, snow does not reflect energy sent from the radar very well. So clear air mode will occasionally be used for the detection of light snow as well. Also, this mode is helpful in detecting discontinuities in the air mass, such as a frontal boundary, and in monitoring the onset of precipitation.
There are two clear mode VCPs; VCP 31 and VCP 32. Both VCPs complete a volume scan using five elevation angles in 10 minutes. For both VCP's the radar makes two 360° scans of the atmosphere at both the 0.5° and 1.5° elevation angles. During the first scan at each elevation the radar is in surveillance mode and is looking for objects. During the second sweep at each of these two lowest elevation angles the radar is determining the velocity of the wind. In the remaining three elevation angles, the radar conducts both surveillance and velocity operations together.
The difference between VCP 31 and VCP 32 is how defined by the pulse mode. VCP 31 uses a "long pulse" mode meaning the time the radar is transmitting each pulse is 4.7x10-6 seconds. This is repeated 314 times a second (the pulse repetition frequency is at its lowest). Therefore, the wavelength is the much longer in this mode increasing the radar's sensitivity. But this comes at a cost which is a decrease in the range of the winds velocity radar can determine.
VCP 32 has a higher PRF (more pulses per second) so it is not quite as sensitive as VCP 31 but it can now detect a wider range of the wind's velocity. For this reason, most NWS Doppler radars will be in VCP 32 during the Clear Air Mode.
When precipitation is occurring, the radar does not need to be as sensitive as in clear air mode as rain provides plenty of returning signals. At the same time, meteorologists want to see higher in the atmosphere when precipitation is occurring to analyze the vertical structure of the storms. This is when the meteorologists switch the radar to precipitation mode.
Currently, there are four precipitation mode VCPs. VCP 11 has 14 elevations slices and completes 16 360° scans in 5 minutes, up to 19.5°, to provide better sampling of the vertical structure of storm clouds and to produce images at a much quicker pace. For several years, VCP 11 was the most common operating mode during severe weather. This mode provides rapid updates as well as the ability to see high into the atmosphere.
VCP 21, while it also tilts up to 19.5° to see high into the atmosphere, operates at a slower rotation speed and eliminates some of the upper elevation tilts. In this mode, the radar takes 6 minutes to move though these 9 elevation tilts. This is used primarily for "strato-form" precipitation where vertical features of rain clouds are not as important as during the convective, thunderstorm-type of rain.
The two newest VCP's available to the NWS forecasters are VCP 12 and 121. VCP 12 also has 14 elevations slices, like VCP 11, but performs 17 360° scans in a very fast 4 minutes 6 seconds. Instead of 1° elevation tilt increments seen in all other VCP's, the elevation tilt increase in VCP 12 range from 0.4° to 0.9° up to 4°. In other words, the radar beams overlap each over.
This provides a denser vertical sampling at lower elevation angles which means better vertical definition of storms, improved detection capability of radars impacted by terrain blockage, better rainfall and snowfall estimates, and resulting in more storms being identified, in addition to the quicker update cycle.
VCP 121 addresses velocity aliasing or the ability of the radar to determine wind velocity and problems caused by "second trip echoes". With the same nine elevation tilts as VCP 21, VCP 121 completes 20 rotations in five minutes. The difference is the radar makes several elevations scans at the same elevation tilt but at different pulse durations (called "pulse repetition frequency" or PRF).
This gives the radar the ability to minimize "range folding" The radar normally determines the range to an object based on the time it transmits a pulse until the time it receives a returned signal. However, depending upon how fast the radar is transmitting pulses, the returned signal may be associated with one of the previous pulses, known as second (or third) trip echoes.
If the PRF is low (longer time between transmission of pules) the signal can travel farther to the more distant objects and reduces second trip echoes. However, the ability to determine velocity is greatly reduced. High PRF's (less listening time between pulses) greatly improve the radar's ability to determine velocity. Yet, it also increases the number of second or third trip echoes. This tradeoff between distance and velocity is known as the Doppler dilemma.
VCP 121 combines varying PRF's and different antenna dish rotation speeds to help decrease range folding.