Doppler Weather Radar - METNET - India Meteorological Department

Table D.1. PPI and RHI scanning types
NOTE: Generally in meteorology, radars usually employ one of two or both following scanning
techniques:
Figure D.4. Plan Position Indicator
Figure D.5. Range Height Indicator
Radar Scanning in two orientations
Plan Position Indicator (PPI): The radar holds it elevation angle constant but varies its
azimuth angle. If the radar rotates through 360 degrees, the scan is called a “surveillance scan”.
If the radar rotates through less than 360 degrees, the scan is called a “sector scan”. It’s good
surveillance scan in operational setting. Plan Position Indicator scanning is shown in Figure.D.4
Range Height Indicator (RHI): The radar holds its azimuth angle constant, but varies its
elevation angle. The elevation normally is rotated from near the horizon to near the zenith (the
point in the sky directly overhead). It’s good for determining the vertical structure of the storm.
Range Height Indicator scanning is shown in Figure.D.5
D.2. Scanning Strategies
Doppler Weather Radar, being new generation radar incorporating digital, modular
sophisticated technology, is capable of working in full automatic and continuous mode, 24 hours
for 365 days. It probes the atmosphere according to the schedules/parameters defined to it. The
parameters for scan are defined in its controlling workstation and are loaded in to the onboard
computer of radar. The radar runs the same schedules and scans until it is stopped or modified.
The DWRs network data in IMD is being used by forecasters to issue more accurate and
effective weather forecast especially in Nowcasting.
DWR scans are designed so as to suit the prevailing weather situations and data
requirements. The scans done by radars should satisfy following needs in general:
1. No important event/phenomena/happening in the atmosphere is missed
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