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Weather Radar: Clear Air Mode

by Darren Smith, CFII/MEI
IFR Checkride Reviewer, May 2009
CFIDarren Newsletter, November 8, 2011
Navigation:  Standard Weather Briefing | Weather Briefing Form | Weather Contractions | Graphical Weather | Textual Weather | METAR Practice | TAF Practice | Winds Aloft Practice | Mysteries of the METAR | Radar

One of the preflight actions that is among the most critical is taking a look at the weather radar.  Hours before the flight, a quick look at the weather radar from the TV will suffice, but as the take off time nears, a look at the weather display at the airport is critical to me.  For pilots in the South East, this is especially critical because there always seems to be convective activity.  Soon after I adopted this practice, I discovered there were two modes of radar -- "Clear Air" and "Precipitation" mode.  If you're a CFI, no doubt a student has asked you about this. 

In explaining the difference between Clear Air Mode and Precipitation mode, we need to set the stage by discussing something called Volume Coverage Patterns.  As you know, ground based weather radar has an antenna that turns in order to get a 360
° view of the sky. The WSR-88D employs scanning strategies in which the antenna automatically raises to higher and higher preset angles, or elevation slices, as it turns. These elevation slices comprise a volume coverage pattern or VCP.  It always starts a 0.5° tilt above the horizon.  On the second pass, it goes to 1.5° tilt above the horizon, then 2.4, 3.4, and 4.3.  Once the radar has swept through all of the preset elevation angles a "Volume Scan" is completed.  A collection of preset elevation slices that the radar sweeps through is called a Volume Coverage Pattern.

As you can see from this diagram above, as the beam slices through the sky, each time at a different elevation (or angle), the weather radar can get a three dimensional view of any convective activity that is might be present.  Not only can it determine range (distance from the antenna), it can also determine altitude and intensity.  The radar is also limited close in by its inability to scan directly overhead. Therefore, close to the radar, returns are not possible due to the radar's maximum tilt elevation of 19.5°. This area is commonly referred to as the radar's "Cone of Silence" (see graphic to the left).

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.  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).  In the immediate area of the radar, "ground clutter" generally appears within a radius of 20 nm. This appears as a roughly circular region with echoes that show little spatial continuity. Returns from aerial targets are also rather common. Echoes from migrating birds regularly appear during nighttime hours between late February and late May, and again from August through early November. Return from insects is sometimes apparent during July and August.
It also results from radio energy reflected back to the radar from outside the central radar beam, from the earth's surface or buildings.

Because Clear Air Mode can detect objects and wind movement, it is also used to detect the onset of precipitation (including light snow) and disturbances in an air mass such as a frontal boundary. 

You can think of the Clear Air Mode as a high quality scan of the sky.  In fact, it takes 10 minutes to fully scan the sky at the five different elevations described above. 

Precipitation Mode

Precipitation Mode is used when there is rain within the operating range of the radar.   The US National Weather Service's WSR-88D radar does not determine raindrop size.  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.

Depending on the version of
Precipitation Mode used (VCP11 or VCP21), it takes between 15 to 19 seconds to make that 360° sweep of the sky.  Instead of the five slices Clear Air Mode takes, Precipitation Mode takes those five slices plus 4 or 9 additional slices depending on which Precipitation Mode is used. 

The following diagram depicts VCP11 and takes 14 slices of the sky:

According to the US National Weather Service, Precipitation Mode VCP 11 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.

This diagram depicts VCP21 and takes 9 slices of the sky:

According to the US National Weather Service, Precipitation Mode VCP 21, while it also tilts up to 19.5° to see high into the atmosphere, but 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 future of weather radar includes networked weather radar antennas providing a three dimensional view of convective activity which could be uplinked to the flight deck and onboard computers which would allow you to plot a course and a series of altitudes through an area of convection.  Additional weather radar antennas would provide
an effective scan rate  of an updated 3D view of the nation's weather each minute.  With phased array technology, stationary antennas will be able to complete a volume scan of the sky in under 30 seconds.  Add to this, the sharing of private weather data streams from TV stations and airline meteorologists, you'll have a stunning variety of visual display of convective weather never before seen in the flight deck.  It's an amazing future, and the exponential advances will lead to better situational awareness and increased safety. 

For more information, see:
The Future of Weather Radar
History of Weather Radar
The VORTEX Project
NOAA Celebrates 200 Years
Storm Chasing
Basic Weather Lessons - Introduction
Weather Phenomena (tornadoes, thunderstorms, lightning, hurricanes, floods, winter weather and hail)
NOAA Photo Library
Tornadoes...Nature's Most Violent Storms

 "One learns by doing a thing; for though you think you know it, you have no certainty until you try." — Sophocles

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