GPS To Monitor Developing Southwestern Monsoon

With only days to spare before the start of the 2003 monsoon season, UA atmospheric scientists deployed the last of six, state-of-the-art sensor systems at sites in southern Arizona and northwestern Mexico. The sensors give them precise measurements of water vapor in the atmosphere every half hour. They are using Global Positioning System (GPS) receivers and other devices to accurately monitor changing humidity in near real time.

Near-real-time information on atmospheric moisture will aid wildfire-fighters, flash-flood and severe-storm forecasters.

"Accurate predictions of rainfall require accurate knowledge of the amount of moisture in the atmosphere and how it changes in time and space," said E. Robert Kursinski, associate professor of atmospheric sciences. "The GPS moisture observations will add to our understanding of moisture flow throughout the year but particularly during the monsoon season. These will lead to improved ability to predict precipitation in our region from hourly to seasonal timescales."

Kursinski, Andrea N. Hahmann, a research scientist professor in atmospheric sciences, graduate student Carlos Minjarez, and others worked to link southern Arizona and Sonora, Mexico, to the larger "SuomiNet" network of moisture-sensing stations. SuomiNet, a collaborative effort that started with the University Corporation for Atmospheric Research (NCAR) in Boulder, Colo., is sponsored by the National Science Foundation (NSF).

Last year the UA team established two moisture-sensing systems, one atop the Gould-Simpson Science Building on the UA campus, the other at Cochise College in Douglas, Ariz. This year, collaborators at the Salt River Project provided a Phoenix site, and University of Sonora colleagues located sensors at their campus in Hermosillo and at a joint university research facility in Puerto Peñasco.

The scientists also recently installed a receiver on Mount Hopkins. "This receiver will aid astronomers making observations (at the 6.5-meter UA/Smithsonian MMTO telescope) as well as give atmospheric scientists detailed measurements of how moisture interacts with mountains to produce monsoonal thunderstorms here in the Southwest," Kursinski said.

The GPS receiver is the primary instrument at each location. Each receiver is supplemented by a package of instruments that measure surface temperature, pressure and humidity.

GPS was extensively used by the military in the Iraq war, Kursinski noted. But the receivers deployed here use GPS signals to precisely measure the amount of humidity in the atmosphere. As humidity builds, additional water molecules make the air denser, so it takes the GPS signal longer to travel from one of the 24 GPS Earth-orbiting satellites to the GPS receiver on the ground. That is, the more water vapor in the atmosphere, the longer the delay in the GPS signal. The researchers can measure the time delay equivalent to roughly one-tenth millimeter extra path length, which translates to about a half-millimeter to one-millimeter precipitation.

One of the great advantages to GPS is its ability to detect moisture through clouds.

"When the monsoon arrives, we get so many clouds which limit the utility of the infrared satellite data," Kursinski said. "The infrared satellite data tells you there's a cloud there, but it can't tell you how much water is there under the clouds. The GPS can tell you that."

"In the past five years, people have realized the need for more frequent weather data, like information we get from this system," Hahmann said. "It is important for planning water resources, for so many applications here in the Southwest. If we knew in advance what kind of monsoon we will get, where it will come from, and when it will get here ˆ that would be very, very useful."

GPS provides crucial observations for monitoring and predicting weather, particularly severe weather, Hahmann said. Scientists recently discovered that GPS moisture measurements significantly improve lightning predictions at NASA's Cape Canaveral launch site, for example.

"We expect the GPS measurements will improve our ability to predict flash floods by identifying rapid 'waves' of water vapor, known among area meteorologists as 'surges', moving up from the Gulf of California, well before they reach us," Hahmann added.

Kursinski, Hahmann and Minjarez plan to further extend the network into Mexico next summer, more than doubling its size in time for the North American Monsoon Experiment (NAME), a June ˆ September 2004 experiment funded by the National Oceanic and Atmospheric Administration and NSF.

NAME objectives are to better understand the basic features of the monsoon and how they vary, to understand how the important our monsoon is to the global water cycle, to provide better measurements of the monsoon, and to better predict monthly-to-seasonal precipitation and water resources.

For NAME, greater coverage by GPS observations will be combined with other radar and balloon-borne measurements together with substantial modeling efforts. The research will yield significant new insights into how our monsoon works and better predictions of its behavior, the UA scientists said.