The world's first space mission dedicated to observing and understanding tropical rainfall has successfully completed its first year of continuous data-gathering. Launched last fall, the Tropical Rainfall Measuring Mission (TRMM) spacecraft continues to provide exciting new insight into cloud and precipitation systems over the tropics.
TRMM is a joint U.S.-Japanese mission that was launched on Nov. 27, 1997, from the National Space Development Agency at Japan's Tanegashima Space Center. The TRMM satellite has produced continuous data since Dec. 8, 1997. Tropical rainfall -- that which falls within 35 degrees north and 35 degrees south of the equator -- comprises more than two-thirds of the rainfall on Earth. Changes in wind patterns generated by these tropical systems spread across the globe to impact weather patterns everywhere.
Launched to provide a validation for poorly known rainfall datasets generated by global climate models, TRMM has demonstrated its utility by reducing uncertainties in global rainfall measurements by a factor of two -- from approximately 50 percent to 25 percent. While pleased with the results to date, "there is clearly an aspect of tropical rainfall which does not fit our conceptual models," said Dr. Christian Kummerow, TRMM project scientist at NASA's Goddard Space Flight Center, Greenbelt, MD.
"At the moment, all fingers are pointing at the possibility that raindrops are significantly smaller than we used to believe. Looking 'under the hood,' of clouds with radars and radiometers has given us a unique perspective on the rain and ice processes. As soon as we make sense of all these new and sometimes contradictory observations, a whole new improved way of viewing and modeling rainfall processes should emerge. These particle sizes have the potential effect of regulating the amount of water vapor and ice being pumped into the upper atmosphere, which plays a key role in global climate change studies," added Kummerow.
"The cloud types and area coverage generated by the rainfall process can directly alter the heat balance of the atmosphere," said Arthur Hou, deputy TRMM project scientist at Goddard. "The combined view of this process from all the TRMM sensors is offering an unprecedented insight here." Observations of cloud droplets near the cloud tops of thunderstorms have also yielded surprises. "The darker appearance of raining clouds and the unexpected suppression of rain in polluted atmospheres might be explained by the presence or absence of large raindrops near the cloud top," said Danny Rosenfeld, an Israeli scientist who is a member of the TRMM science team.
Scientists long have theorized that convection, or heat transfer, is different over land than over the ocean. TRMM's sensors provided direct observational evidence that faster and stronger convective updrafts over land are contributing to the formation of "taller" continental storms with more lightning. This is in contrast to the almost complete absence of lightning over the world's tropical oceans.
One unexpected phenomenon observed by TRMM was the massive tall chimney clouds in Hurricane Bonnie. While monitoring the progress of one of this year's most dramatic hurricanes, NASA researchers obtained compelling images of Hurricane Bonnie showing a (cumulonimbus) storm cloud, towering like a sky scraper, 59,000 feet into the sky from the storm's eyewall. This new view of "hot towers" in hurricanes could help forecasters predict hurricane intensity earlier, and identify those storms that will proceed to a stronger category.
Last July, TRMM shed new light on the phenomenon known as La Nina. TRMM research team members successfully retrieved sea- surface temperature data from the TRMM Microwave Imager (TMI) instrument aboard the spacecraft. This temperature data, obtained by the TMI, gives scientists the ability to obtain observations even in cloudy conditions. The coincidence of having both an El Nino and a La Nina event is giving scientists a rare opportunity to study the evolution of these events and the transition from one to another.
La Nina is essentially the opposite of the El Nino phenomenon and is characterized by unusually cold ocean temperatures in the eastern equatorial Pacific. An El Nino occurs when ocean temperatures are warmer than normal. La Nina and El Nino often are spoken of together and termed the El Nino/Southern Oscillations, or "ENSO." La Nina sometimes is referred to as the cold phase of the ENSO. An unexpected benefit from TRMM has been the almost immediate impact the data have had in improving the understanding of atmospheric water and energy cycle in assimilated global data sets. While still early, scientists are very encouraged that this improvement will lead directly to enhanced research efforts as well as better weather forecasts.
TRMM is part of NASA's Earth Science Enterprise, a long-term research program designed to study the Earth's land, oceans, air, ice and life as a total system. Images from the TRMM mission are available on the Internet at URL:
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