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Radar Corner - The use, and misuse of weather radar – Part 3

Radar Corner

The use, and misuse of weather radar – Part 3

This article will look at another of the radar questions that we address on a regular basis. Adjusting gain is straightforward, unfortunately differences between systems and how it is shown on displays complicates the issue immensely. Adding to the confusion is that some radars have fixed gain in weather, some can only reduce gain, while others can increase or decrease gain. In this article we’ll look at several topics including:

1.    What is the purpose of the AUTO or CAL gain position?

2.    How do I know if I can increase or decrease gain?

3.    What do the display annunciations mean?

4.    And when do I use increased or reduced gain?

Figure 1 – Some Common Radar Displays and Controllers

All radars have an AUTO or calibrated gain position. The only time the colors on the display correlate to the rainfall rate table found in the pilot’s guides (Figure 2) is when the gain control is in the AUTO or calibrated position. To say it another way – when you are not in the AUTO or calibrated gain position the colors on the display do not correspond to the rainfall rates in the table. The reason that we have this calibrated position is so that you can get into any aircraft with any manufacturer’s radar and the colors will represent the same thing. 

Figure 2 – Rainfall Rate Table

So why are the colors important? Based on studies we know that with higher reflectivity comes higher probabilities of turbulence and hail size. Both good reasons to detect the maximum reflectivity and look at the colors. If we enter the turbulence probability chart (Figure 3) at the beginning of the red color, we could say that we have a 55% chance of light turbulence, a 40% chance of moderate turbulence, and a 5% chance of severe turbulence. Another way to say it would be that we have a 45% chance of experiencing less than light turbulence, a 60% chance of encountering less than moderate turbulence, and a 95% chance of experiencing less than severe turbulence. So why would we want to have the gain in any other position? We’ll get to that in a minute. 

Figure 3 – Rainfall Rate vs. Probability of Turbulence & Hail

As mentioned earlier some radars can increase gain, some can decrease gain and others can do both. So how do you know what your radar does? If it tells you in the pilot’s guide that’s great, but most don’t. The next time that you’re at cruise altitude, there is weather, and it’s safe try this quick little experiment. First look at the displayed weather and notice the color levels or take a picture of the display. Now move the gain control out of the AUTO or CAL position and set it to whatever the maximum gain is on your system (MAX, 100%, full clockwise, etc.) and note the display again. If the display looks exactly the same, then CAL is the maximum, and you can only reduce gain. If the returns are more intense then you have increased gain capability. On some systems it might only be a slight increase and on others it could be as much as one-color level (+10dB). Now do the same thing in the opposite direction. Put it in CAL, observe the display and then observe what happens to the display as you reduce gain. If you see an area of green turn to black, and the yellow turn to green you reduced the gain 10dB. Most systems will not allow you to reduce the gain such that red returns will completely disappear. Get an idea of how much you can turn the gain down. Gain reduction is a very useful feature so it’s good to get an idea of how much you can reduce the gain.

After completing the above experiment, you should have a better understanding of what the gain display (MIN, MAX, CAL, 80%, 100%, etc.) is telling you.

That brings us to the final and probably the most important question. When do I use increased or decreased gain? To answer that we’ll start by looking at a simple model of a storm cell (Figure 4). Radar is designed to reflect off of water droplets of sufficient size and quantity. Below the freezing level raindrops are good radar reflectors. The reflectivity of the cell increases to a maximum around the freezing level. In this area wet hail and wet snow look like large raindrops to the radar and reflect a lot of energy. This is what is typically referred to as the bright band. 

Figure 4 – Generalized Storm Cell Model

Above the freezing level are mainly less reflective frozen storm tops (Figure 5). If we use a raindrop as our standard, we can see that wet hail and wet snow are also good reflectors. But dry hail and dry snow are poor reflectors returning only about 3% of the energy that a raindrop does. So that answers part of our question. Increased gain should be used when looking at less reflective particles above the freezing level. The gain control is like a volume control on your TV or stereo. If the volume is very loud (or reflective) then you would only want to turn it down. If the volume was very low (less reflective) all you would want to do is turn it up. 

Figure 5 – Reflectivity Characteristics

The ability to reduce gain is extremely useful. One example is reducing gain to minimum and observing the display. In Figure 6 below the display on the right has reduced the gain to minimum (MIN) and what is shown on the display are the strongest returns. In addition, turbulence detection isn’t affected by the gain control so what remains are the strongest cells and any turbulence indications. Both good things to avoid. That was a qualitative comparison. For a quantitative comparison reduce the gain until areas of yellow turn green and areas of green turn black. You have just turned the gain down 10dB or one-color level. If you refer back to Figures 2 and 3 you can see you are dealing with reflectivity greater than 50dB and are at a higher risk of experiencing turbulence and larger hail.

Figure 5 – Reflectivity Characteristics

Program Pilot Stephen Hammack supports Honeywell Apex and radar for Flight Technical Services. He can be reached via email at stephen.hammack@Honeywell.com.