The build-up of ice on aeroplanes in flight is a major winter hazard for small and commuter planes. But scientists at the US National Centre for Atmospheric Research (NCAR) are testing a new system that may pinpoint water droplets in clouds that cause icing, potentially enabling pilots to avoid dangerous areas.
The system, known as S-Polka, is said to combine two existing radars that use different wavelengths. By studying the differences between the images that are reflected back to each radar, scientists hope to find tiny water droplets that are difficult to distinguish using either radar alone.
‘This will take out a lot of the guess work,’ explained Marcia Politovich, director of NCAR’s icing program. ‘We think it will show exactly where the water is. That information could ultimately turn into an important warning system for pilots.’
Scientists and engineers at NCAR are deploying S-Polka at NCAR’s Marshall facility southeast of Boulder, Colorado. The system consists of a powerful polarised radar, known as S-Pol, which operates at a frequency of 3,000 MHz, and a polarised Ka-band radar, which operates at 35,000 MHz.
The S-Pol radar produces detailed images of clouds and precipitation, whereas the Ka-band radar can detect weaker clouds that are not precipitating. By comparing the images from each radar, researchers hope to find areas in clouds that contain water droplets.
Finding cloud water droplets has long posed a scientific challenge, as the droplets are 50 microns or less in diameter. They may remain in liquid form even when the surrounding air temperature drops below freezing. The droplets are most dangerous at that time because they adhere to aircraft wings and then freeze, reducing the plane’s aerodynamic properties.
Existing radar often cannot detect the droplets if they are surrounded by larger raindrops or snow. Even if small cloud particles are detected, a radar signal cannot indicate whether they are droplets or ice crystals.
‘When it comes to cloud particles, we can’t interpret the standard radar echo,’ explains NCAR’s Jothiram Vivekanandan, the lead scientist on the project. ‘This research is very challenging.’
The two radars have been mounted on a single pedestal at the Marshall facility. They are aligned to look at the same defined area at the same time. Researchers will compare the radar images with data collected from a University of North Dakota Citation research aeroplane flying in the test area to determine whether the radar system is pinpointing water droplets.
After data are collected this month, the researchers will focus on identifying and measuring droplets within the radar images accurately. If all goes well, the instrument will undergo final tests in a couple of years before being made available to airports.