May 22, 2006
Too much rain can be as big a problem as too little. A NASA instrument designed primarily to measure winds on the ocean surface is turning out to have other abilities over land that may help in both flood and drought situations.
Large hurricanes, such as Katrina and Rita, carry massive amounts of moisture deep inland, well beyond the coast where they come ashore. Weather radar shows rain in the atmosphere but not where it has fallen on the surface. While rain gauges measure how much rain has fallen in specific locations, their coverage is limited.
Working with data from the SeaWinds instrument on the QuikScat satellite, researchers have found a new way to make an immediate measurement of the amount of precipitation that has accumulated on the surface and its location. These are important factors in evaluating the flood potential of a particular region, especially when new storms hit areas already coping with large amounts of water from previous storms.
SeaWinds on QuikScat is a scatterometer--it sends out a pulse of radar that scatters back to the satellite from Earth's surface. Over the ocean, this backscatter reveals the speed and direction of ocean surface winds. SeaWinds data have become an important factor in improving hurricane forecasting. The satellite also collects data over land, and researchers have discovered that they can use radar backscatter from land to determine increases in surface soil moisture resulting from rainfall. By comparing the radar backscatter from before and after Hurricane Katrina made landfall, researchers were able to map rainwater distribution from the category-5 storm from the coastline to the Great Lakes. They also mapped the increase in surface water caused by Hurricane Rita.
"This is an innovative application for the QuikSCAT satellite," says Dr. Son Nghiem, a JPL specialist in remote sensing. "Many states are impacted from a large category-5 hurricane. For such a large area, it takes months to assemble final and consistently quality-controlled data from rain gauges. Now we can get information of rainwater on land from a single instrument. We can monitor the rainwater distribution on a continental scale."
"The capability of SeaWinds to monitor both the winds that move the moisture from the ocean and the water deposited over land makes the scatterometer a critical instrument in the characterization and understanding of the cause and effect of hurricane landfalls," says Dr. Timothy Liu, the QuikScat science team lead at JPL.
"Now that we've shown that SeaWinds is sensitive to surface water," says Nghiem, "we are going to be using it to monitor surface water from storms as well as in a new program for drought detection and monitoring." Nghiem is the principal investigator of a recently selected NASA proposal to help develop a national drought monitoring system for drought early warning using data from NASA satellites. Collaborating in the program are the University of Nebraska-Lincoln's National Drought Mitigation Center, the National Oceanic and Atmospheric Administration, the U.S. Geological Survey, and the Dartmouth College.
"We hope to improve our ability to monitor and forecast drought, information that is crucial for decision-makers in such areas as hazard management and mitigation, water management, agriculture, hydropower and forestry," says Nghiem.