Volume Coverage Patterns
(in progress)

Unlike older radars where the radar operator could directly control the environmental scanning pattern of the antenna, including performing vertical or horizontal scans, NEXRAD has a variety of so-called Volume Coverage Patterns (VCPs) that it uses to interrogate the environment.

A Volume Coverage Pattern or VCP is a set of instructions that controls various aspects of NEXRAD antenna functioning including the rotation rate, Pulse Repetition Frequency (PRF), pulse time, and the elevation angles, to maximize the information-gathering for different weather situations.

NEXRAD VCPs are described in detail in the Federal Meteorological Handbook 11 FMH-11-Part C WSR-88D Products and Algorithms--Chapter 5 .

VCP Classification and Naming Convention

There are 9 total VCPs--7 for the precip. mode, and 2 for the clear-air mode.

Three precipitation mode groups covering 5 VCPs and one clear-air mode group of 2 VCPs were identified in FMH-11, and since then, two more VCPs have been added.

The Warning Decision Training Branch groups the current 9 VCP's in the following groupings:
VCP Grouping:

"The VCP numbers come from several groups. Each group name implies an intended use, but there is some overlap in applications for each VCP.

1. Convection Group -- VCPs 11 and 12 are in this group

2. Shallow Precipitation Group -- VCP 21 is in this group

3. Clear Air Group -- VCPs 31 and 32 are in this group

4. Range Folding Mitigation Group -- VCPs 121, 211, 212, and 221 are in this group

The Multiple PRF Dealiasing Algorithm (MPDA) is part of VCP 121 processing. MPDA reduced range folding by processing additional Doppler rotations at lower elevation angles.

The Sachinananda-Zrnic (SZ-2) technique is implemented for the lower two or three elevations for VCPs 211, 221, and 212. When echoes are overlaid, SZ-2 can usually recover velocity data for two of the overlaid range bins. SZ-2 is also used for one of the Doppler rotations at 0.5 and 1.5 degrees with VCP 121."

The following "Quick Reference VCP Comparison Table for RPG [Radar Product Generator] Operators" from http://www.wdtb.noaa.gov/tools/RPS/VCPCompTable.pdf summarizes VCP characteristics.  The "legacy" VCPs have been superceded by the three-digit VCP's which use more sophisticated processing to reduce range-folding in the velocity data; sometimes these "legacy" VCP's are still used anyway.

This table is from Wikipedia's NEXRAD page.
VCP Scan Time (min) Elevation scans Elevation angles () Usage Special attributes
11 5 14 0.5, 1.5, 2.4, 3.4, 4.3, 5.3, 6.2, 7.5, 8.7, 10, 12, 14, 16.7, 19.5 Convection, especially when close to the radar Has the best overall volume coverage.
211 Convection, especially when close to the radar Improves range-obscured velocity data over VCP 11
12 4.5 14 0.5, 0.9, 1.3, 1.8, 2.4, 3.1, 4.0, 5.1, 6.4, 8.0, 10.0, 12.5, 15.6, 19.5 Convection, especially activity at longer ranges Focuses on lower elevations to better sample the lower levels of storms.
212 Widespread severe convective events Improves range-obscured velocity data over VCP 12
121 6 9 0.5, 1.5, 2.4, 3.4, 4.3, 6.0, 9.9, 14.6, 19.5 Large number of rotating storms, tropical systems, or when better velocity data is needed. Scans lower cuts multiple times with varying pulse repetitions to greatly enhance velocity data.
21 5 Shallow precipitation Rarely used for convection due to sparse elevation data and long completion time.
221 Widespread precipitation with embedded convection. (i.e., tropical systems) Improves range-obscured velocity data over VCP 121
31 10 5 0.5, 1.5, 2.5, 3.5, 4.5 Detecting subtle boundaries or wintry precipitation Long-pulse
32 Slow rotation speed allows for increased sensitivity. Default clear-air mode, reduces wear on antenna. Short-pulse

The WSR-88D initiates data aquisition in the Clear-Air Mode and automatically switches to Precipitation Mode when it detects precipitation above a certain threshold.  It can also be manually switched to precip. mode.

WSR-88D Sampling Terminology:

Split Cut--scans a particular elevation slice two or more times using a different PRF for each full scan. Used for the lowest elevation angles (those below 1.65o) where efficient clutter suppression is required and velocity range folding is likely.
    Contiguous Surveillance (CS)--The low PRF scan of the split cut.  Gives a high Rmax value to determine proper target location and intensity, but a low Vmax value limits the velocities that can be measured.
    Contiguous Doppler (CD)--The high PRF scan of the split cut.  Gives a low
Rmax value causing more range folded (multiple trip) echoes, but a high Vmax value to get higher, more accurate velocity values.

Batch Mode--uses alternating low and high PRFs on each radial for one full rotation at each elevation angle. The two resulting data sets (low PRF and high PRF) are combined to resolve range ambiguity. Used in the middle elevation angles.

Contiguous Doppler X (CDX)(or Contiguous Doppler with no range unfolding)--combines a high PRF and a rapid antenna rotation rate to obtain all base data in the higher elevation slices >
7o).  Does not need to maximize the range since, for example, at 7.5o the radar beam is already at an elevation of ~50,000 feet at 62 nm range from the radar.

Sachinananda-Zrnic (SZ-2) Split Cut (SZCS/SZCD) (quoted from Warning Decision Training Branch training materials)

SZ-2 is a different range unfolding technique applied to the Split Cuts of VCPs 121, 211, 212, and 221. SZ-2 is able to reveal far more velocity data compared to the other range unfolding technique. Details on both of these techniques are presented in later sections of Topic 3.

NOTE: As with the standard Split Cut mode, dual-polarization base data are processed from the SZ-2 Contiguous Surveillance rotation.

Multiple PRF Dealiasing Algorithm (MPDA) (quoted from Warning Decision Training Branch training materials)

The MPDA is implemented solely by invoking VCP 121. Though MPDA is not a waveform, it uses a unique combination of the SZCS, SZCD, CS, CD, and B waveforms. In the lower elevations of VCP 121, there are multiple Doppler rotations at each angle, each of which uses a different PRF.

Illustration of the elevations angles scanned and waveforms used during VCP 121.

Figure 21: Volume Coverage Pattern (VCP) 121 samples nine elevation angles in about 5.75 minutes. The lowest two angles apply the SZ-2 technique as part of MPDA, the lowest five angles apply the MPDA technique, while 6.0 deg uses Batch (B), and the higher angles use Contiguous Doppler without range unfolding (CD/WO) mode.

As an example, the link below shows an animation of VCP 11, originally from the Warning Decision Training Branch:
VCP 11 Animation -- Click on "Actual" to see the actual rotation rate of the beam in real time.

Brief description of VCPs.

Volume Coverage Pattern Quick Reference Guide