Precipitation field experiment wraps up in western NC
Posted June 19, 2014
A research group is pulling up stakes from an outpost in western North Carolina this week, but not before collecting what NASA research fellow Piotr Domaszczynski calls a “unique dataset.” The outpost – located in a rural part of North Carolina along the South Carolina line west of Charlotte — was part of the Integrated Precipitation and Hydrology Experiment, or IPHEx, a multiple-million-dollar joint research project between researchers from NASA, Duke University, and NOAA’s Hydrometeorological Testbed.
Don’t let the budget fool you: Field research isn’t glamorous. Their setup amounted to a half dozen shipping containers, a pair of research radars, and a port-a-potty poking up above the surrounding landscape of ranch land NC/SC line west of Charlotte. But their mission has them looking up, way up. Their goal of the overall project is a ground validation of instruments aboard a newly-launched satellite called the GPM Core Observatory. GPM stands for “Global Precipitation Measurement Mission,” a larger effort to deploy satellites to measure rain, snow, and other precipitation across the globe.
Validating Better Rainfall
Rainfall can be a particularly localized phenomenon, as anyone who’s had it raining on aside of the road and not on the other can tell you. And there are few perfect ways to observe just how much rain falls across a large area. Rain gauges are the gold standard, of course, but they are not particularly representative. In other words, you do not have to get very far away from the gauge before the actual amount of rain likely changes from what you measured.
The next step beyond gauges is collecting radar data. Radar systems can estimate how much rain falls across a larger area, but radar estimates are subject to errors. For one, hail and other non-liquid “targets” can contaminate the returns, causing the estimates to be too high or too low. Dual-polarization radar — the same technology that powers the DUALDoppler5000 — offers an improved ability to estimate rainfall amounts based on the size and shape of those targets, allowing the radar to compensate for hail and other issues.
But all radar systems – and the data they collect – suffer from a range of limitations. For example, the quality of rainfall estimates suffers with increasing distance from the radar site due to a number of basic issues, including the curvature of the earth and limitations of radar physics, such as beam spreading and attenuation. Further, radars cannot see through mountains, so in areas of high terrain, mountains can block the radar beams, preventing the radar from seeing the other side of the mountain from the radar.
In spite of those limitations, the best rainfall data in the United States are typically a combination of gauge-corrected radar data. However, there are many areas where installing a dense network of rain gauges or radar systems is impractical or even impossible. It’s many of these same areas that would benefit from more accurate estimates of rainfall patterns, such as mountainous areas subject to flooding.
That’s one reason why the GPM Core Observatory was launched.
GPM: An International Mission
The mission, initiated by NASA and the Japan Aerospace Exploration Agency (JAXA), includes a consortium of defense, meteorological and space agencies from the US, India and Europe. The GPM Core Observatory serves as the reference standard in a constellation of satellites providing a global picture of rain and snow every three hours through measurements of precipitation intensity, 3-D structure of storm systems and microphysics of the ice and liquid within clouds.
The GPM mission builds on the success of Tropical Rainfall Measuring Mission (TRMM) in operation since 1997. Tropical storms don’t just stay in the tropics and TRMM’s orbit limits it to latitudes between 35º north (Fayetteville, NC) and 35º south (southern tip of Africa). The orbit of GPM core provides significantly more coverage, between the Artic and Antarctic circles (65º north latitude and 65º south latitude). This not only gives researchers a more complete view of the lifecycle of storms but also allows operational meteorologists to better monitor the position and internal structure approaching storms.
“With GPM's more sensitive instruments and wider coverage of the globe, we can more accurately profile a tropical cyclone, predicting where they're likely to form, how intense they're likely to become and tracking the path they'll take so that agencies can make better decisions to help get people out of harm's way,” says TRMM project scientist Scott Braun.
The GPM Core Observatory carries two instruments: a microwave imager and a dual-frequency radar. The imager measures energy radiated by precipitation across 13 channels. Lower frequencies detect moderate rain, middle frequencies for rain/snow mix, and higher frequencies falling snow and ice. Meanwhile, the radar measures the structure of the precipitation from within the cloud down to the surface; dual frequencies allow it to do this in 3-D and to account for attenuation of the signal.
“A Unique Dataset”
Validating the GPM instruments has two purposes: To make sure the data we get from the satellite are of similar or better quality than the data we already have and to understand any limitations with the dataset. A comprehensive dataset is needed to accomplish both. The field campaign spent weeks collecting such a dataset across the western Carolinas. Researchers deployed almost half a dozen radar systems and tapped into a network of more than a hundred rain gauges to collect the standard dataset for such an analysis.
But they did not stop there.
The project also flew two different aircraft, including an ER-2, the research version of the Air Force’s U-2 high-altitude spy plane, to collect data in and around the various showers and thunderstorms over the survey area. The ER-2 is actually simulating the data collected by the satellite while a specially-configured Cessna Citation flies around and through the storms to study the physics inside the storm.
On the ground, the research team also has a number of soil moisture sensors, stream gauges, and disdrometers – instruments that measure the sizes of individual raindrops – to round out the data set. These data will not only help to validate the GPM instrumentation, but they will go to improve computer models used to estimate both how much rain has fallen over a given area and how the rain will flow into streams and rivers once it falls.
The outpost was part of a focused field campaign to observe precipitation over the mountains of North Carolina. The North Carolina field campaign follows similar exercises in Oklahoma and Iowa and was designed to improve rainfall estimation and hydrological efforts over the mountains after two campaigns over relatively flat terrain. The team will move to Washington state to collect similar datasets around the Cascades.