Weather radar

AIRBORNE WEATHER RADAR

Hal Stoen
©2001

first release: 27 November, 2001

PURPOSE OF THIS TUTORIAL

To give the sim. pilot operating information and some insighton how airborne weather radar works, its limitations, and howto interpret the displays.

BACKGROUND

During my aviation career I attended many courses on how toutilize and interpret aircraft weather radar. I walked away fromthese with a "singular" thought:
Nothing is the same, every situation is different, never sit thereand just watch, be proactive.

A SHORT STORY

One day, cruising along with strong convective (thunderstorm)activity in the area, I heard this conversation: "Big Air531, Center. You have your weather radar on?" "That'saffirmative Center, we're picking our way through this stuff,Big Air 531." "Well Sir, you might consider making a180 degree turn, you're heading up a blind canyon out there thatcomes to an end in about 30 miles." (Pause......) "AhCenter, Big Air 531, requesting a right 180."

THE BIG PICTURE

What happened to "Big Air 531" could happen to anyof us- they were too close to the trees to see the Forest. Whenworking an air mass with convective activity, you have to be constantlyaware of The Big Picture. You have to know, before "goingin", that there is an exit to the area that you are entering.That may sound simple enough, but your eyes for this meteorologicaldance is the airborne weather radar, and this set of eyes hassome limitations.

It is entirely possible to enter an area of what appears to be"level 2" precipitation, only to stumble into "level3" stuff that will beat the hell out of you. This happenedsome years ago, down South, to a DC-9. With the windshields smashedby hail, engines choked by heavy rain, they crash landed on ahighway. The level 3 stuff that got them never showed up on theirweather radar- it was hiding behind a large band of level 2 precipitationthat fooled their radar because of attenuation- a subject coveredlater 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, rightthere in the narrow, pointed end of the bird. The smaller theaircraft, the smaller the radome, the smaller the radome, thesmaller 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 displayedon the CRT that is mounted in the panel. Green is light precipitation(level 1), yellow is moderate (level 2), and red is heavy (level3). Some units have a magenta color for the really heavy stuff.Generally speaking, precipitation levels and turbulence go handin 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 prettyiffy, depending on the density of the media. Generally speaking,snow does not show up well on radar, even on the ground unitsthat have large antennas.

PERSPECTIVE

Interpreting this information is an art, based on knowledgeand, more importantly, experience. Gradients, the distance fromone color band to another, can indicate turbulence. The closerthe bands are together the stronger the turbulence- usually. Alsothe location of the cell in relationship to the frontal line canbe an indicator of severity- sometimes. Geographic location playsa roll. A level three storm in Iowa is stronger than a level threestorm in Florida- most of the time. Time of the year is also afactor- 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.

TILT!

One of the least understood aspects of airborne weather radaris the subject of antenna tilt. The display on the panel has acontrol that allows the pilot to tilt the antenna up or down.This can be the most critical adjustment of all. (The radar antennaplatform up in the nose is stabilized in the roll mode. The antennaplatform is tied into the horizontal gyro circuit so that theplatform remains level in reference to the Earth's horizon asthe aircraft turns.) Proper antenna tilt, when taking a read ona thunderstorm, makes the difference between valuable information,and no information.

A: Aimed too high, above the freezing level, no returns. Thissituation could also happen in a climb if the tilt is not adjusteddownwards.

B: Still aimed too high, above the freezing level with mostlyfrozen (dry) precipitation.

C: Proper tilt. The maximum available data is being displayed.

D: Tilted too low. The screen is full of what is termed "groundreturn". This could also happen in a descent if the tiltis not adjusted upwards.

MAPPING

As an aside, you can also use the radar for ground mappingby adjusting the tilt for the best ground mapping display. Whendoing this, lakes and mountains can be picked out. In the CentralPlains of the United States, where the ground is flat for mileupon mile, you will actually be able to pick up a grid effectfrom 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 groundreturn is being displayed. Then tilt up until ground return isat a minimum, but still present. This will usually give the pilothis optimum setting for precipitation display. In actual usage,the tilt feature is used to "scan" the storm in a verticalfashion, allowing the pilot to receive as much information aspossible.

ATTENUATION

This is the critical one, right along side of tilt setting.The airborne weather radar depends on the signal that it emitshitting something and returning to the antenna. It is possible,due to a large area of precipitation, for the signal to be bouncedaround all over the place, and become "absorbed" bythe moisture. This results in the display not accurately showingwhat lies within the area of precip. Take a look above at Figure"C". See that area of green on the backside? Most likelythat is the strongest part of the thunderstorm.

"But," you say, "it's green! How can that areabe the strongest part of the storm?" Ah, Grasshopper, attenuation!So much of the antenna's radiated signal has been absorbed andscattered by the "near" precipitation that the "far"precipitation, the green stuff, only shows up as level 2. Shouldyou decide to penetrate this fellow by diverting to the right,then angling back to the left to pass between the level 2 andlevel 3 returns, you will, most likely, find yourself eyeballto eyeball with the meanest, nastiest, meteorological phenomenathat you have ever encountered. And, very likely, you and yourpassengers will not live to describe the experience.

SUMMATION

There's a whole lot more to airborne weather radar than thisshort tutorial presents, but hopefully enough information hasbeen provided to permit the sim. pilot to gaze at that CRT inhis panel and have a new found respect for the information thatit displays.

This tutorial is available on aCD

This tutorial, along with additional content, is availableon a CD. Click here formore information.

If you have any questions, or comments, please contact me.

Hal Stoen

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