Sensing the rain

Anne L. Fischer, contributing editor, anne.fischer@photonics.com

The Tropical Rainfall Measuring Mission (TRMM) is a project run jointly by NASA and the Japan Aerospace Exploration Agency to monitor and study tropical rainfall. When tropical cyclone Laurence hit Australia in December 2009, TRMM determined that total rainfall exceeded 150 mm.

The three primary instruments onboard the satellite are precipitation radar, a microwave imager, and a visible and infrared scanner. Although radar devices have been used since World War II for meteorological applications on the ground, TRMM is the first attempt to use radar to measure rainfall quantitatively from space. To construct high-resolution, three-dimensional maps during the short period of time the satellite is flying over a storm, Japan’s Communications Research Laboratory used a radar frequency about three times higher than that of typical ground-based radar, and the team designed a “phased array” antenna that would narrow the beam as well as steer it to the target area.

Precipitation analysis done aboard TRMM shows that tropical cyclone Magda was dropping about 2 in. of rainfall per hour west of the eye, and some of the intense thunderstorms near the eye were as high as 16 km. Credit: NASA/SSAI/Hal Pierce.

The microwave scanner onboard is an updated design of an instrument that dates back more than 20 years. It has an additional 10.7-GHz channel, which adds the sensitivity needed to record the higher rates of rainfall common in tropical regions. The visible and infrared scanner measures radiation coming up from the Earth in five spectral regions from 0.63 to 12 µm. The instrument’s rotating mirror picks out individual cloud features as small as 2.4 km as it scans across an area 833 km wide.

Another instrument onboard is a lightning sensor that combines optical and electronic elements. One of these components is an imager that stares at one point on Earth for 80 seconds to find out whether a storm is growing or decaying, determined by how fast the lightning is flashing. The imager’s expanded optics lens system provides a wide field of view and a narrowband filter that minimizes background light. It has a high-speed CCD detector array that creates an image of the lightning; then a real-time event processor extracts the signal, calculating when the lightning occurred. The system is designed to distinguish lightning even against bright sunlight.

The sensor was developed by the Global Hydrology Center at NASA’s Marshall Space Flight Center in Huntsville, Ala., in conjunction with Lockheed Martin of Palo Alto and Kaiser Electro Optics of Carlsbad, both in California. Continued development may result in an instrument that helps weather forecasters see where lightning is striking Earth within 30 seconds of an occurrence.

The data generated by the instruments aboard the TRMM research satellite are intended to help increase our knowledge of the rainfall and heat release associated with weather in tropical and subtropical regions. It’s understood that the atmosphere is propelled around the globe by solar energy as well as by evaporating water, so, by studying tropical rainfall, much can be learned about the global climate.