Clear Air Mode
In this mode, the radar is in its most sensitive operation. 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 lot of what you will see in clear air mode will be airborne dust and particulate matter. Also, snow does not reflect energy sent from the radar very well. Therefore, clear air mode will occasionally be used for the detection of light snow.
The radar continuously scans the atmosphere by completing volume coverage patterns (VCP). A VCP consists of the radar making several 360° scans of the atmosphere, sampling a set of increasing elevation angles. There are two clear mode VCPs.
In clear air mode, the radar begins a volume scan at the 0.5° elevation angle (i.e., the radar antenna is angled 0.5° above the ground). Once it makes two full sweeps (a surveillance/reflectivity sweep and a Doppler/velocity sweep) at the 0.5° elevation angle, it increases to 1.5° and makes two more 360° rotations. For one of the clear air mode VCPs, two full sweeps are also made at 2.5°. Otherwise, at the higher elevations (2.5°, 3.5°, and 4.5°) a single sweep is made (reflectivity and velocity data are collected together).
This process is repeated at 2.5°, 3.5°, and 4.5°. Then the radar returns to the 0.5° elevation angle to begin the next volume scan which will repeat the same sequence of elevation angles. In clear air mode, the complete scan of the atmosphere takes about 10 minutes at 5 different elevation angles.
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 using one of two volume coverage patterns.
Both precipitation VCP's begin like the clear air mode mentioned above with the same evaluations scans as in the clear air mode. The difference is the radar continues looking higher in the atmosphere, up to 19.5° to complete the volume scan. The time it takes to complete the entire volume scan is also less. In the slower VCP, the radar completes the volume scan of nine different elevations in six minutes. In the faster VCP, the radar completes 14 different elevation scans in five minutes.
Differences in the quality of radar images between the two precipitation mode VCPs are relatively minor. Therefore, during severe weather, the faster VCP is almost always used as it provides the meteorologists with the quickest updates and most elevation slices through the storms.
In summary, when the radar is in clear air mode, radar images will be updated approximately every ten minutes. In precipitation mode, the updates will occur around five to six minutes apart.