If it rains in the Pacific will Kansas have a drought?

Kwajalein experiment will support tropical rainfall measuring mission

Aug. 24, 1999: Who cares if it rains in the middle of the ocean? Farmers, forestry managers, and landowners, among others. While the weather forecast over a particular part of the ocean may be of little interest, it has implications for everyone.

"Rain over water affects the climate of the whole planet," said Robbie Hood of the Global Hydrology and Climate Center in Huntsville (GHCC). "It releases energy into the atmosphere. What happens in the tropics is extremely important to the climate of the whole Earth. If you can't measure rain over the ocean, you're missing a lot of activity and you'll never truly understand what is happening to the weather across the world."

The problem is, almost no instruments are located there, leaving a large gap in our understanding of the world's weather systems. Weather satellites with microwave sensors can measure moisture in the air, but they see the entire volume and cannot distinguish what will fall or move on to another location.

In November 1997, NASA and Japan's National Space Development Agency (NASDA) launched the Tropical Rainfall Measuring Mission (TRMM) satellite to develop methods to plug those holes. But we first have to know if we are correctly interpreting what TRMM is telling us. The most important data are the sizes and quantities of raindrops, and whether they actually hit the surface or get carried away by vertical air movement.

"The TRMM team wanted field experiments from the very beginning because it knew there would be issues about validating the results from the satellite," said Dr. Eric Smith director of the GHCC. To date, TRMM has been supported by three experiment campaigns (see box).

The fourth experiment got under way on July 23 - at least 18 aircraft missions have been flown so far (see list at end of story) - and runs through Sept. 15 in the Central Pacific Ocean. Operating out of the Kwajalein Atoll, a U.S. Army missile test site, scientists with instruments aboard three aircraft, a ship, and TRMM are conducting the Kwajelein Experiment (KWAJEX) to measure rain physics in and under thunderstorms over the open ocean. Their sorties will be timed to coincide (when possible) with overflights by TRMM which is equipped with a Precipitation Radar, a Lightning Imaging Sensor, and other instruments that could be used on operational satellites in the future.

"We can improve our global computer models of how weather and our long-term climate might be affected not only by human activities, but by natural phenomena such as El Nin&tildeo as well," said Dr. Ramesh Kakar, the TRMM program scientist at NASA Headquarters, Washington, DC.

"With its radar and microwave instruments, the satellite obtains a large-scale view of precipitation, but with less detail than many surface-based instruments. In contrast, ground and airborne measurements allow us to really understand the three-dimensional structure and evolution of tropical storm systems," said Steve Hipskind, Chief of the Atmospheric Chemistry and Dynamics Branch at NASA's Ames Research Center, Moffett Field, Calif. "Even without the satellite, these field experiments are leading to significant scientific progress in understanding precipitation processes."

"KWAJEX has always been the central field experiment since TRMM was designed for rain over open oceans," Smith explained. "When the TRMM ground radar validation network was set up in Darwin (Australia), Melbourne (Fla.), Houston, Texas, and Kwajalein, it was clear that Kwajalein was the only open-ocean radar site."

Smith noted that the Kwajalein area, the world's largest atoll and part of the Republic of Marshall Islands, is representative of much of the open ocean, and that the atolls are so small they should have no influence on rain systems. Various surrounding islands will launch radiosondes, instrument-bearing balloons that radio back readings of temperature, moisture, and winds as they ascend through clouds. Another balloon will be tethered at Meck Island. To the southwest of Kwajalein Island, the NOAA research vessel Ronald H. Brown will watch with a Doppler radar similar to a unit on Kwajalein, allowing scientists to assemble true 3D models of rain motion.

Smith is one of four KWAJEX research managers. Hood, Anthony Guillory, and Michael Goodman, all from the GHCC, will share duties as mission scientist aboard NASA's DC-8 research jet. Dr. Hugh Christian and members of his lightning research team will measure lightning activity in open-ocean storms.

KWAJEX is markedly different from last-year's well-known CAMEX-3. Where CAMEX looked at hurricanes with a fisheye lens, KWAJEX puts ocean storms under a microscope.

"The goals for this experiment are more focused on droplet size distribution and the relationships of the drops and ice crystals and how they relate to measurements by satellites," Hood said.

"Microphysically speaking, everything you measure is really a function of the distribution and size of rain drops and cloud particles, the index of refraction of these media (that is, how much they bend radio waves), and pockets of air and water trapped in ice," Smith explained.

As a result, when radar looks at a cloud over the ocean, it may seem to get the same picture as over land, but the microphysics may be completely different .

In addition to balloons and radar on Kwajelein and the R/V Ron Brown, KWAJEX will employ the University of Washington's Convair 580, and the University of North Dakota's Cessna Citation. The ER-2 high-altitude aircraft used in other TRMM experiments will not be used because it would involve overwater flights too long for safe operation by a single-engine jet.

"We are bringing in microphysics aircraft with special instruments," Smith said. "In that same context, we're going to do vertical profiles, the Holy Grail for cloud researchers for years, to get vertical slices of cloud microphysics. We've never had an experiment designed to do that over the deep ocean."

Because storms cannot be scheduled to coincide with TRMM's overflights, NASA has equipped the DC-8 with the GHCC's Advanced Microwave Precipitation Radiometer (AMPR) that matches the TRMM Microwave Imager. The DC-8 will also carry the Airborne Rain Mapping Radar, from the Jet Propulsion Laboratory, which matches TRMM's Precipitation Radar. This will let the DC-8 stand in for TRMM when necessary.

The only way to correlate remote-sensing measurements with what's actually happening inside the storm is to get inside the storm. So the Convair 580, Citation, and DC-8 will carry instruments into the storms to take a census of rain drops and ice crystals: how many are there in what sizes, and what are the winds doing to them. The answer will be important to refining computer models of rain.

Christian of the GHCC will measure electrical activity in clouds and storms. He is the principal investigator for the Lightning Imaging Sensor aboard TRMM, but on this mission he will use electric field sensors aboard the Cessna Citation since little lightning is expected inside the storms.

Anvils (so called for the characteristic shape of these storm clouds) in oceanic storms tend to be very long-lived and affect the energy of the storm system a great deal. Improved understanding is needed so weather forecasters can provide more accurate predictions of what will happen around launch areas. For example, launch of the Chandra X-ray Observatory was postponed on July 21 because of thunderstorms in the area. One of the launch commit criteria prohibits a launch if lightning has struck within 20 miles of the launch pad in the last 30 minutes. The criterion probably is too cautious, but not enough is known to revise it to reduce postponements while not increasing the risk of loosing a rocket.

"Anvils were thought to be fairly well understood for a long time," Smith said. But it turns out that the microphysics are poorly known, meaning the anvil's life cycle also is poorly known. For example, they last days when they shouldn't even last hours.

"Anvils over the ocean are produced by independent complexes of convection," Smith added. "They form and then they sit there for many hours and even days at a time. This is a big problem in precipitating clouds." Lightweight microphysics aircraft cannot reach the higher altitudes in anvils, so radar and electric field sensors are needed.

Web links:

KWAJEX home page at the TRMM web site includes operational data and links to other sites including:
KWAJEX home page at the Earth Observing System web site.
KWAJEX logistics page details key personnel, operational information, and other factoids.
University of Washington web site includes information on science to be conducted aboard the Convair 580.
Tropical Rainfall Measuring Mission home page at Goddard Space Flight Center.
R/V Ronald Brown home page includes details about the ship.
Scientists analyze immense data haul from hurricanes, a wrapup of 1998's Convection and Moisture Experiment (CAMEX 3).