Rain Rates from Space: Past & Future (Invited Presentation)

The idea of measuring rain rates from spacecraft has been around since the 1970s when the first passive microwave radiometer was flown on the Nimbus 4 satellite. The group at Goddard Space Flight Center led by William Nordberg was among the first to notice that the upwelling microwave flux showed a colder apparent temperature over open-ocean than that over areas of rainfall. The difference was so large that the good signal-to-noise suggested that it might be possible to use this effect to estimate rain rates quantitatively. Tom Wilheit’s 1979 paper showed us how by providing a formula connecting rain rates over the ocean to the apparent temperature.

Tom, Otto Thiele and I proposed the Tropical Rainfall Mission (TRMM) in 1984. We were given the go-ahead with the mission after a keen competition with other candidates. The proposed configuration was for a microwave radiometer, a Vis-IR sensor for energy fluxes, a lightning detector, and finally for the first time in space an electrically scanning radar that would yield the vertical distribution of hydrometeors. After some inquiries we approached the Japanese as partners in the venture. Dr. Naboyoshi Fugono was the leader of the program on the Japanese side. The work was divided nearly evenly: US supplied the sensors except for the radar to be built by the Japanese. The US was to assemble and integrate the satellite components and the Japanese were to supply the launch vehicle, an H1 Rocket.

My role was promoting the mission to the hydrology community and to the climate science community. I also led the statistical studies, which were very important because so little was known about the rather peculiar statistics of rain rates. Our goal was a data set of month-long averages over 500km squares. I will spend a little time on some of these methods along with some issues in the retrieval algorithms. The TRMM satellite was launched in 1997, into a tropical orbit (spanning from 35S to 35N) and flew for 17 years, collecting the first global rainfall climatology from space. The vertical patterns of rain were also measured in tropical storms for the first time from space.

When the life of TRMM ended, Global Precipitation Mission (GPM), also a US-Japanese partnership, took its place. The main satellite of GPM is in an orbit that covers most of the mid-latitudes with a precipitation radar sacrificing some sample precision in the tropics. GPM includes more sun-synchronous satellites as well and continues to fly, extending the global rainfall climatology.