AIRBORNE WEATHER RADAR
first release: 27 November, 2001
PURPOSE OF THIS TUTORIAL
To give the sim. pilot operating information and some insight on how airborne weather radar works, its limitations, and how to interpret the displays.
During my aviation career I attended many courses on how to
utilize and interpret aircraft weather radar. I walked away from
these with a "singular" thought:
Nothing is the same, every situation is different, never sit there and just watch, be proactive.
A SHORT STORY
One day, cruising along with strong convective (thunderstorm) activity in the area, I heard this conversation: "Big Air 531, Center. You have your weather radar on?" "That's affirmative Center, we're picking our way through this stuff, Big Air 531." "Well Sir, you might consider making a 180 degree turn, you're heading up a blind canyon out there that comes to an end in about 30 miles." (Pause......) "Ah Center, Big Air 531, requesting a right 180."
THE BIG PICTURE
What happened to "Big Air 531" could happen to any of us- they were too close to the trees to see the Forest. When working an air mass with convective activity, you have to be constantly aware of The Big Picture. You have to know, before "going in", that there is an exit to the area that you are entering. That may sound simple enough, but your eyes for this meteorological dance is the airborne weather radar, and this set of eyes has some limitations.
It is entirely possible to enter an area of what appears to be "level 2" precipitation, only to stumble into "level 3" stuff that will beat the hell out of you. This happened some years ago, down South, to a DC-9. With the windshields smashed by hail, engines choked by heavy rain, they crash landed on a highway. The level 3 stuff that got them never showed up on their weather radar- it was hiding behind a large band of level 2 precipitation that fooled their radar because of attenuation- a subject covered later in this tutorial.
I'LL STILL RESPECT YOU IN THE MORNING
Airborne weather radar is not for penetrating thunderstorms, it's for avoiding thunderstorms.
SIZE DOES MATTER
When it comes to weather radar antennas, bigger is better. And just where is the aircraft's antenna located? Of course, right there in the narrow, pointed end of the bird. The smaller the aircraft, the smaller the radome, the smaller the radome, the smaller the antenna, the smaller the antenna the worse the data. Weather radar sends out a signal, it bounces off of a target, and some of that reflected energy is received back by the antenna. This returned energy is interpreted by the unit and displayed on the CRT that is mounted in the panel. Green is light precipitation (level 1), yellow is moderate (level 2), and red is heavy (level 3). Some units have a magenta color for the really heavy stuff. Generally speaking, precipitation levels and turbulence go hand in hand- the heavier the precipitation, the heavier the turbulence.
WET AND DRY
Weather radar gets the best returns off of wet precipitation. Huh? Isn't all precipitation "wet"? Nope, it's not. Hail and snow are two forms of "dry" precipitation. And, to muck it up a bit, there can be "wet" hail and "wet" snow. Airborne weather radar will display "wet" hail and snow just fine- the "dry" forms can be pretty iffy, depending on the density of the media. Generally speaking, snow does not show up well on radar, even on the ground units that have large antennas.
Interpreting this information is an art, based on knowledge and, more importantly, experience. Gradients, the distance from one color band to another, can indicate turbulence. The closer the bands are together the stronger the turbulence- usually. Also the location of the cell in relationship to the frontal line can be an indicator of severity- sometimes. Geographic location plays a roll. A level three storm in Iowa is stronger than a level three storm in Florida- most of the time. Time of the year is also a factor- normally.
I think that you're getting the picture. Words like "usually," "sometimes", "most of the time", "normally", are clues that weather- especially convective weather- is a dynamic, ever changing medium.
One of the least understood aspects of airborne weather radar is the subject of antenna tilt. The display on the panel has a control that allows the pilot to tilt the antenna up or down. This can be the most critical adjustment of all. (The radar antenna platform up in the nose is stabilized in the roll mode. The antenna platform is tied into the horizontal gyro circuit so that the platform remains level in reference to the Earth's horizon as the aircraft turns.) Proper antenna tilt, when taking a read on a thunderstorm, makes the difference between valuable information, and no information.
A: Aimed too high, above the freezing level, no returns. This situation could also happen in a climb if the tilt is not adjusted downwards.
B: Still aimed too high, above the freezing level with mostly frozen (dry) precipitation.
C: Proper tilt. The maximum available data is being displayed.
D: Tilted too low. The screen is full of what is termed "ground return". This could also happen in a descent if the tilt is not adjusted upwards.
As an aside, you can also use the radar for ground mapping
by adjusting the tilt for the best ground mapping display. When
doing this, lakes and mountains can be picked out. In the Central
Plains of the United States, where the ground is flat for mile
upon mile, you will actually be able to pick up a grid effect
from the roads and fields all being at right angles to one another.
This is often referred to as "The fence post effect".
To properly adjust the tilt, tilt the antenna down until ground return is being displayed. Then tilt up until ground return is at a minimum, but still present. This will usually give the pilot his optimum setting for precipitation display. In actual usage, the tilt feature is used to "scan" the storm in a vertical fashion, allowing the pilot to receive as much information as possible.
This is the critical one, right along side of tilt setting. The airborne weather radar depends on the signal that it emits hitting something and returning to the antenna. It is possible, due to a large area of precipitation, for the signal to be bounced around all over the place, and become "absorbed" by the moisture. This results in the display not accurately showing what lies within the area of precip. Take a look above at Figure "C". See that area of green on the backside? Most likely that is the strongest part of the thunderstorm.
"But," you say, "it's green! How can that area be the strongest part of the storm?" Ah, Grasshopper, attenuation! So much of the antenna's radiated signal has been absorbed and scattered by the "near" precipitation that the "far" precipitation, the green stuff, only shows up as level 2. Should you decide to penetrate this fellow by diverting to the right, then angling back to the left to pass between the level 2 and level 3 returns, you will, most likely, find yourself eyeball to eyeball with the meanest, nastiest, meteorological phenomena that you have ever encountered. And, very likely, you and your passengers will not live to describe the experience.
There's a whole lot more to airborne weather radar than this short tutorial presents, but hopefully enough information has been provided to permit the sim. pilot to gaze at that CRT in his panel and have a new found respect for the information that it displays.
This narrative, along with aditional content, is available as a CD or an eBook.
For CD information click here. For eBook information click here.
If you have any questions, or comments, please contact me.
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This tutorial ©2001
Hal Stoen (Hal@stoenworks.com)