NASA GOES FLAT: Rain, snow, ice crystals, and hail are part

by Betty Flowers of Wallops Flight Facility

Rain, snow, ice crystals, and hail are part of a complex process that drives energy circulation in the atmosphere, which in turn helps regulate our planet's climate. A new NASA radar is helping scientists understand this process by measuring the characteristics of various forms of precipitation within rain and snowstorms.

The NASA Polarimetric Radar (NPOL), developed by a research team from NASA's Goddard Space Flight Center, Wallops Flight Facility, Wallops Island, VA, is unique. It is an ultra-modern flat panel radar antenna that is fully portable and self-contained. Hexagonal in shape and 18-feet (5.5 meters) across, it requires no special site preparation and can be set up on a basically flat area anywhere in the world.

"NPOL is NASA's only portable polarimetric precipitation research radar and one of the very few polarimetric weather radars in the world," said Dr. John Gerlach, NPOL Principal Investigator from Wallops. "This radar will allow us to conduct more extensive ground measurements to support orbiting satellites and enhance their data gathering capabilities."

The system is able to operate continuously. During a recent field campaign in Florida, it operated 24 hours a day, 7 days a week for almost three months allowing scientists and engineers to monitor and record all tropical rainfall within a 155 mile (250 kilometer) radius. They measured where and when rain fell and the vital details of the microphysical nature of the storms that produced the rain. Polarimetric weather radars measure both rainfall amounts and physical characteristics of raindrops, such as size.

A revolutionary flat panel radar "dish" is used to transmit and receive signals. Unlike more traditional radars and other polarimetric weather radars that use a parabolic shaped bowl to send and receive radar signals, this new system uses carefully sized engraved dipoles that have been etched into a specially prepared panel, much the same way a circuit board is manufactured. Each element reflects the incident energy, which is focused on a central feed horn, the small element in front of the radar panel that collects the energy and channels it to the receivers.

The flat panel has significant advantages over its traditional round-shaped cousin. First, it can be easily assembled and disassembled for shipping. Second, since hundreds of symmetrical squares of material between the dipole have been removed from the panels, by precision laser cutting, there is a dramatic reduction in the total mass of the panels. These holes also allow for operation in unusually windy conditions by permitting air to pass through the working structure of the system. In addition, the wind-friendly architecture does not require a radome - a dome shaped structure designed to shield a turning radar dish from the elements. And the need for a massive pedestal to support the entire structure is reduced, dramatically shrinking the total size of the system.

The radar site maintains Internet connectivity with a satellite uplink. Its sensitive computer and radar equipment are cooled with self-contained air conditioning units. Only one person, a radar operator, is required for full-scale operation at any one time, however safety considerations dictate the presence of a second person.

Radar data collected by researchers can be displayed in a variety of ways that use colors to designate specific characteristics. As it sweeps the sky in a circular pattern, the data is collected in a computer program which maps the sky in terms of precipitation, showing a variety of details such as how much water exists at one time in a given volume of sky and whether it is liquid or frozen.

This new, advanced radar system has been used in recent field research experiments and is the largest flat panel radar of its type in use in the world. From these experiments, the space agency is hopeful that new techniques will be developed to better understand the nature of precipitation leading to major insights about the Earth's climate.