Airborne weather radar is an excellent tool on aircraft used to avoid dangerous clouds, such as Cbs. It is invented and developed to detect the position of the active and significant cloud cells and its intensity in order to provide information to the pilot for their avoidance. However, in order for pilots to successfully use weather radar to keep them out of trouble, they need to have a good understanding of how weather radar works, how to use the technology and how to interpret the information and display.
How does it work?
Weather radar works on a principle of sending radio waves and measuring the radio waves energy that returns, and also the time it takes to return. The reflectivity of water particles will depend on its size, concentration, and composition of the precipitation. The bigger the size of the water droplet and the intensity, the greater the reflectivity. The ‘wetter’ the precipitation, the greater the reflectivity. It needs to be noted that the frequency band of the radio waves (X band) is selected not to detect any cloud, small precipitation such as drizzle, fog or wind, as the droplets are too little or don’t exist. It also cannot detect windshear or clear air turbulence, as the precipitation do not exist, except in a microburst.
Ways to avoid troubles
Cb in New Zealand could easily extend up to Flight Level 250 or 25000 feet (or even higher in late summer in Auckland area.) Frontal Cbs usually form in a line. They often divert around. On the other hand, convective Cbs are random in allocation and always moving and changing. Moreover, thunderstorms are likely to travel in the same direction as the 10,000 foot wind indicated on the weather forecast report.
- Provide as much distance as practicable between aircraft and active Cb cells. For a rough idea, 20 NM laterally and 5000 feet vertically should be sufficient to avoid the chance of encountering severe turbulence
- Avoid areas of red and magenta as these colours represent very intense areas of rainfall and turbulence associated with Cbs
- Determine a sufficient heading change that will give the aircraft enough room to bypass a Cb by a safe lateral distance. Sometimes, it may be possible to climb above a Cb, but this will depend on the situation and other operating factors
- Detour the aircraft to the upwind side of a Cb as new cells often form on the downwind side of a thunderstorm, and turbulence could also be encountered in downwind eddies.
Interpretation of radar returns
*Green: light precipitation
*Yellow (or amber): medium precipitation
*Red: heavy precipitation
*Magenta: Extremely high precipitation
The pilots need to interpret the information from the display to obtain an indication of the danger presented. When interpreting, the pilots should take the following factors into consideration:
- Black hole: If there is a black hole area within the active cells, it should then be noted that the signal has been attenuated by the active cells which makes the return signal too weak to be seen. Pilot should take the black holes areas as potentially containing hazards.
- Weather forecast: The freezing level and weather forecast for the area associated to the flight will help the pilot to match the display colours to the expected weather. For instance, in a flight below the freezing level a large area of green on the display would show a stratiform cloud and light to moderate rain with no hazard. The pilot could then compare with the weather forecast to confirm the interpretation. On the other hand, in flight above freezing level an area of green could potentially shows an active cell and dry hail-a definite hazard. This sometimes could be difficult to detect because of poor reflectivity
- Shape: These following shapes are good indications of hazardous weather indication:
1.Close areas of different colours show a good indication of strong turbulence
2.The shapes of finger, hook, U-shape and scalloped edges show good indications of strong vertical draft and thus severe hail
3.Rapid-changing shapes show unstable weather
Weather Radar Controls
There are usually four main controls available to the pilots in order to operate the weather radar device.
1. Antenna tilt
Knowing the correct way to operate an antenna tilt will result in obtaining the optimum detection and visualisation of significant weather. The weather information showing on the display may not necessary be in front of the aircraft or at the flight level. However the information on the display is showing the returns that are cut by the radar beam. Pilots need to determine and adjust the angle between the centre of the beam and the horizon in order to obtain useful information on the display.
2. Display range
Most weather radars have the maximum range of 200 nm. Pilots are able to adjust the range in order to obtain weather information that he/she needed. For example, when a significant weather is detected, pilots should set a maximum range down to 80 nm or less in order to be able to avoid and just monitor this particular cell. On the other hand, if there are more than one active cells, a higher and lower range should both be set in order to get a ‘big picture’ of the situation.
3. Gain control
Changing the gain will adjust the sensitivity of the radar receiver. By selecting manual and decreasing the gain will:
- Show the relative intensity between two cells
- Draw attention to the stronger, hence more turbulent cells
- Be able to locate active cells in heavy rain from stratiform cloud
- Reduce black holes areas if they contain active cells
4. Radar Mode
- Weather only Mode (WX)
- Weather + Turbulent Mode( WX+T): The areas which have strong turbulence will be showing in magenta on the display. This area is formerly showed in green on WX only Mode.
Want to know more?