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At higher altitudes, hazardous weather is progressively defined by the amount of precipitation below the aircraft. It is important to remember that frozen precipitation is not nearly as reflective as liquid droplets. Since weather radar (in general) assumes liquid precipitation when depicting intensity, precipitation above the freezing level tends to underestimate the hazard ahead. As such, green (low in- tensity) returns for precipitation above 20,000 feet indicate hazardous weather. Confirming this analysis is easy: Tilt the antenna down to see what is below you; in many cases, once the radar beam is pointed towards the liquid part of the cell it will paint red and magenta returns – the exact sort of thing that you do not want to be flying above.
Hazardous turbulence can exist many thousands of feet above those red and magenta areas – not to mention dry hail, which is the precipitation form of a stealth fighter (i.e., dangerous and resistant to radar). The goal should be to tilt towards the area of the sky that gives the greatest indica- tion of hazardous weather, and that area almost exclusively exists below 20,000 feet.
A simple solution above 20,000 feet is to tilt down in order to paint the outer third of the radar display with ground returns. This will keep the radar focused at the lower, liquid portion of storm cells. In combination with this, routinely vary the range that the radar is displaying. To paint the ground on the outer third of the display at shorter ranges, you will have to tilt the antenna down even further. This will ensure that you are not inadvertently overflying a rapidly growing cumulonimbus. (We had a passenger not long ago get seriously injured following an encounter with severe turbulence due to a cell rapidly growing beneath the radar beam as the aircraft approached it. The invariable tilt of the radar resulted in the hazardous weather never being presented to the flight crew).
One last advantage of painting the outer portion of the display with the ground is that it makes for a foolproof means to identify truly strong weather. A ground shadow behind a cell (i.e., a conspicuous area where no ground is being painted) indicates that all the radar energy is being absorbed by the storm cell itself. This is a cell to avoid by a minimum of 20 nm, every time.
Ground-Based Radar
The size and capabilities of ground-based radar far exceed that found onboard aircraft, though they are not without limitations as it relates to inflight decision making. There are three common ways to access these ground-based re- sources: on the ground via internet connection, in the air via data-link, and verbally via communication with Air Traffic controllers. All of these methods provide a more complete picture than onboard radar alone is capable of. It is the combination of these radar resources that ensure the saf- est path of flight for a given situation (sometimes the safest path of flight is to be tied down and chocked).
Air Traffic Control as a resource varies by location. In the United States, Center and Approach controllers have weather radar data available at their stations. They are re- quired by rule to provide weather alerts, but their primary
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September 2020 / TWIN & TURBINE • 9