DLOC Principles of Meteorological Doppler Radar:
Table of Contents
Web site owner:
Previous Page: Clear Air Returns
Data Collection Operations
The WSR-88D has two operational modes, Precipitation -- Mode A and Clear Air -- Mode B. The two modes employ nine Volume Coverage Patterns (VCPs): VCPs 11, 12, 21, 121, 211, 212, and 221 in Precipitation mode, and VCPs 31 and 32 in Clear Air mode. It should be noted that the term "Clear Air" does not imply "no-precipitation" mode. Even in Clear Air mode, the WSR-88D can detect and accumulate precipitation. Both VCPs 31 and 32 improve the accuracy of the base data estimate due to slower antenna rotation rates (more pulses per radial). VCP 31 employs long pulse, improving detection of lower reflectivity values, such as light rain or snow.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.
Antenna Rotation Rates
The antenna rotates at different speeds depending on the VCP. VCPs 31 and 32 have the slowest antenna speeds, ranging from 71 to 89 seconds for one rotation. In Clear Air mode, the antenna rotates slowly to maximize the number of pulses on target as possible which increases the statistical accuracy of the data. VCPs 12 and 121 have the fastest antenna speeds, ranging from 12 to 22 seconds for one rotation.
Next Page: WSR-88D Waveforms