Volume: 02, Issue: 13 07/21/2004 
Southeast Asia has been under siege by raging monsoon rains.  This TRMM image indicates areas of the heaviest rainfall with dark red coloring. Image produced by Hal Pierce, courtesy SSAI/NASA GSFC.
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Eye of Hurricane Isabel, September 13, 2003.  Photo courtesy NASA/JSC/ES&IA.
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Landslide in Slovenia in 2001.  Photo courtesy SRC SASA/ESA.
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Other Articles in This Issue:
Chasing the Storm
Aircraft Hunt for Weather Clues
Scientist Sees Lightning like Superman
Enhance Your Storm Experience
Chase Storms in Your Classroom
 

Satellites and Radar Track Devastating Storms

Severe weather has long been a mystery to scientists. As technology improves, we can begin to more accurately predict and prepare for severe weather. Satellites and radar are important tools being used by scientists around the world to research destructive thunderstorms, hurricanes, and more.

Monsoons

This July, devastating floods have swept southern Asia as summer monsoon rains continue to cause flash floods and mudslides. More than 300 people have been killed and many millions displaced from their homes in Nepal, India, and Bangladesh since the rains began.

Summer monsoons are seasonal reversals of wind direction that occur in response to temperature differences between the land and sea, accompanied by increases and decreases in precipitation. They can occur over all low-latitude continental regions, often becoming especially severe in Asia.

The current flooding in southern Asia is being closely studied by the Tropical Rainfall Measuring Mission (TRMM), a joint mission between NASA and the Japan Aerospace Exploration Agency (JAXA) designed to monitor and study tropical rainfall. TRMM uses several instruments in its research, including precipitation radar, microwave imager, visible infrared radiometer, cloud and Earth radiant energy sensor, and lightning imaging sensor. Experts hope the findings from these instruments can help them understand more about monsoon conditions and prevent such widespread destruction in the future.

Monsoons are also being studied in North America, as the National Oceanic and Atmospheric Administration (NOAA) and Mexico’s weather service, the Servicio Meteorologico Nacionale, recently joined forces to develop improved monsoon season forecasts. The initial phase of the North American Monsoon Experiment (NAME) aims to improve long-range precipitation forecasts during the North American monsoon season of June through September.

The eight-year program has set far-reaching goals in operational climate forecasting. This year, the NAME 2004 Field Campaign will gather atmospheric, oceanic, and land-surface observations in the core region of North American monsoons: northwest Mexico, southwest U.S., and adjacent oceanic areas. The Field Campaign involves scientists from more than 30 universities, government laboratories, and federal agencies in the U.S., Mexico, and Central America. It also includes more than 25 scientists from the NOAA National Weather Service and involves the National Science Foundation, NASA, and the U.S. departments of Agriculture and Defense.

“The 2004 Field Campaign will improve our understanding of the daily cycle of precipitation in the complex terrain of the core monsoon region,” said Dr. Wayne Higgins, lead scientist for NAME and the principal climate scientist at the NOAA Climate Prediction Center.

The North American monsoon, though much weaker than the Asian monsoon, exerts a strong influence on the precipitation, temperature and wind patterns in the core monsoon region and much of the western half of North America and adjacent ocean areas.

“In many countries, the arrival of the summer monsoon rainfall is good news since it replenishes the waterways and provides a critical supply of water for agriculture and other economic concerns,” said Higgins. “But every few years, excessive rains cause serious flooding.”

A weaker monsoon season can worsen water shortages, leaving fields and waterways parched and dry and the region more susceptible to large-scale forest fires. Improved forecasts would give communities more time to plan for dangerous conditions.

Hurricanes

The Atlantic Ocean becomes a meteorological mixing bowl from June 1 to November 30, replete with all needed ingredients for a hurricane. Scientists turn to a cadre of satellites around the globe to serve up a feast of information to the forecasters who seek to monitor and understand these capricious storms. If a hurricane forms, it is critical to know how strong it may be and which coastal communities or sea lanes will be at risk.

Thirty years ago, meteorologists were unable to see the factors in hurricane formation and could only spot a hurricane with still pictures from the TIROS-N satellite. Over the past 10 years, visible and infrared satellite sensors were the workhorses for monitoring hurricanes. Today, multiple satellites exploit everything from radar pulses to microwaves to enhance forecasts, providing data to researchers several times a day.

The first ingredient in the hurricane recipe is sea surface temperature of at least 82 F. Unlike traditional infrared satellite instruments, the Aqua satellite's Advanced Microwave Scanning Radiometer (AMSR-E) and the TRMM’s Microwave Imager can detect sea surface temperatures through clouds. This information can help determine if a tropical cyclone is likely to strengthen or weaken. The Jason-1 satellite altimeter provides data on sea surface height, a key measurement of ocean energy available to encourage and sustain hurricanes.

Another necessary ingredient is rotating winds over the ocean's surface, precursors to tropical cyclone development. The SeaWinds instruments aboard Japan's Midori 2 and NASA's QuikSCAT satellites can detect these winds before other instruments, providing even earlier notice of developing storms to forecasters and scientists.

Air temperature and humidity are also important factors. The Atmospheric Infrared Sounder (AIRS) experiment suite aboard the Aqua satellite obtains measurements of global temperature and humidity throughout the atmosphere.

Rainfall intensity is the final ingredient, and the Precipitation Radar provided by Japan for the TRMM satellite provides CAT scan-like views of rainfall in the massive thunderstorms of hurricanes. TRMM also sees "hot towers," or vertical columns of rapidly rising air, which indicate very strong thunderstorms. These towers are like powerful pistons that convert energy from water vapor into a powerful wind and rain-producing engine. Once a storm develops, TRMM provides an inside view of how organized and tightly spiraled rain bands are, key indicators of storm intensity.

Europe also has space-bound “eyes” searching for possible hurricanes. The European Centre for Medium-Range Weather Forecasts routinely assimilates data from an instrument aboard ERS-2 that “sees” wind fields over the ocean.

“Our analysis of offline test runs showed, on average, a positive impact on global forecasts and the ability to correct for the positions of tropical cyclones,” said Dr. Hans Hersbach of the European Centre for Medium-Range Weather Forecasts (ECMWF). The ECMWF is an international organization supported by 25 European states charged with using powerful supercomputers to prepare medium-range weather forecasts for up to 10 days ahead.

Next month the European Space Agency’s (ESA) ERS-2 satellite celebrates its ninth anniversary in orbit. Its payload includes a radar scatterometer that works by firing a trio of high-frequency radar beams down to the ocean, then analyzing the pattern of backscatter reflected up again. Wind-driven ripples on the ocean surface modify the radar backscatter, and as the energy in these ripples increases with wind velocity, backscatter increases as well. Scatterometer results enable the measurements of wind speed and direction across the water surface.

What makes ERS-2’s scatterometer especially valuable is that its C-band radar frequency is almost unaffected by heavy rain, so it can return useful wind data even from the heart of the fiercest storms – and is the sole scatterometer of this type currently in orbit. ERS-2 provided a unique view of the pressure system at the center of Hurricane Isabel as it descended upon the United States in September 2003.

“What we obtain from the scatterometer is not the actual wind speed and direction but measurements of radar pulses scattered from the water surface,” Hersbach explained. “Some 20 million scatterometer observations were compared to our model winds, using five months of ERS-2 data, and [we] developed a new geophysical model function that gives us much greater accuracy than before, particularly at higher wind speeds.”

Satellites are crucial to scientists’ understanding of hurricanes. The data provided can lead to improved weather forecasts; improved determination of cyclone intensity, location, and tracks; and the severe weather associated with storms, such as damaging winds.

Landslides

As heavy rains befall Europe, thousands of square kilometers at the continent’s heart face a looming threat: steep slopes and waterlogged soils combine to trigger landslides. A build-up of groundwater within a slope increases its weight and decreases its cohesiveness, weakening the slope’s ability to resist the remorseless pull of gravity. The heavy earth flows downward. For all in the path of a landslide, the results are devastating and frequently lethal.

“In Italy, landslides have claimed an average of 54 victims per year during the last half century,” says Nicola Casagli of Italy’s National Group for Hydro-geological Disaster Prevention (GNDCI), a research network working with Italy’s Civil Protection Department. “The extreme rainfall of our climate, our mountainous geography, and recent uncontrolled urbanization of unstable land makes us one of the countries most affected by landslide hazards. The total cost of direct damage done by Italian landslides is estimated at between one and two thousand million Euro per year.”

Very gradual ground shifts are known to precede more major landslides. Often these are on a scale of millimeters too slight to even be noticed by local observers, but enough to be detected via satellite using a powerful technique called radar interferometry. It involves mathematically combining multiple radar images of the same site - acquired using instruments such as the Synthetic Aperture Radar (SAR) aboard ESA’s ERS spacecraft - in such a way that tiny changes in the landscape occurring between images are highlighted.

This technique is the basis of a project called Service for Landslide Monitoring (SLAM), enabling landslide susceptibility mapping across parts of Italy and Switzerland, two of the European countries most under threat. GNDCI is one of three national-level users working with SLAM, along with Italy’s Ministry of the Environment and Switzerland’s Federal Office for Water and Geology (FOWG).

“Surface movements assessed over wide areas are one of the best indicators of landslide activity, and can be employed for risk forecasting,” added Casagli. “Extremely slow movements usually occur for several weeks or months before a sudden collapse.”

Three different service products are available: a large-scale Landslide Motion Survey identifying areas affected by landslides across an entire river basin, a reduced-scale Landslide Displacement Monitoring measuring ground deformation over particular sites of interest, and Landslide Susceptibility Mapping, which merges the previous data products with thematic maps of land use, slope, geomorphology, and other relevant parameters to provide geological hazard maps.

Resources:
NASA: http://www.nasa.gov
NOAA: http://www.noaa.gov
NOAA Satellites and Information Service: http://www.nesdis.noaa.gov
NOAA National Weather Service: http://www.nws.noaa.gov/
TRMM: http://trmm.gsfc.nasa.gov/
ESA: http://www.esa.int

    
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