Icing conditions pose a hazard for air travel and disrupt air traffic operations over wide areas. The most serious icing conditions result from supercooled large droplets (SLD), which are chilled to temperatures below 0 degrees Celsius without freezing. Although freezing rain can certainly be a hazard, evidence from cloud physics research using aircraft and wind tunnels shows that the danger posed by SLD can be more serious.
SLD are dangerous because they can collect and freeze almost anywhere on an aircraft's wing—creating a rough ice surface, which quickly increases drag and can cause a relatively serious degradation in aircraft performance. Furthermore, unlike freezing rain, SLD can form anywhere in a cloud making them harder to detect. Because SLD creates a primary icing hazard for in-flight aircraft, a SLD remote sensing detection technology was needed—and NOAA had the solution.
The importance of SLD was first recognized in the 1980's from measurements made by research aircraft. By the mid-1990's, SLD became a focus of a FAA-sponsored international conference of aircraft in-flight icing. Based on this conference, there was consensus that the development of remote-sensing methods to detect SLD was a high-priority. Fortunately, NOAA was already on track to address this challenge.
Specifically, NOAA was able to combine several technologies to address this problem. First, it had already developed the "microwave radiometer" technology needed to continuously measure the amount of liquid water in clouds overhead. Secondly, it developed radar techniques to distinguish between harmless ice crystals and hazardous SLD for the FAA in a series of Winter Icing and Storms Projects (WISP) that culminated in 1999 with the Mount Washington Icing Sensors Project (MWISP). Because ice crystals form with diverse shapes, they settle through the air with preferred orientations. SLD and the smaller cloud droplets, on the other hand, are spherical and have no preferred orientation. Knowing this, NOAA scientists were able to use dual-polarization radar to measure the differences between cloud populations of the highly "oriented," harmless ice crystals and the hazardous SLD—a distinction that cannot be made by measuring ordinary radar reflectivity.
NOAA demonstrated the success of these techniques during MWISP and validated their results directly with in-cloud measurements. These experiments laid the foundation for the development of an extremely sensitive radar to continuously monitor clouds for icing hazards within the airspace of airports. This radar allows for real-time warnings to air traffic controllers and pilots and makes flying safer for everyone.