Dropsondes--Work Horses in Hurricane Forecasting

Small cylinders dropped from airplanes gather atmospheric data on their way down

Inside a cylinder that is about the size of a roll of paper towels lives a circuit board filled with sensors. It's called a dropsonde, or "sonde" for short. It's a work horse of hurricane forecasting, dropping out of "Hurricane Hunter" airplanes right into raging storms. As the sonde falls through the air, its sensors gather data about the atmosphere to help us better understand climate and other atmospheric conditions.

"Dropsondes have a huge impact on our understanding of hurricanes and our ability to predict hurricanes," explains electrical engineer Terry Hock at the Earth Observing Laboratory in the National Center for Atmospheric Research (NCAR), located in Boulder, Colo.

With support from the National Science Foundation (NSF), Hock and his colleagues at NCAR have been designing, building and improving dropsonde technology for more than 30 years. "Our most current development is a fully automated dropsonde system for NASA's unmanned Global Hawk aircraft," says Hock.

Compared to earlier models, today's sondes are lighter weight, relatively inexpensive and loaded with sensors.

"We have a lot of electronics and, on the back side, a battery pack to operate the sonde. We have a temperature and two humidity sensors, and we have a GPS receiver," explains Hock, as he points out the different circuit board components. "As the sonde moves, we're using that GPS receiver to track the sonde's movements very precisely, which is then telling us the wind speed and wind direction. At the top of the sonde is a parachute which slows down the descent."

Electrical engineer Dean Lauritsen, a member of Hock's team, developed the system software on the aircraft, which controls the aircraft data system and process, and also displays dropsonde data during the sondes free fall to earth. There's such a system on the HIAPER, the NSF/NCAR Gulfstream V Research Aircraft, which uses sondes for scientific research, and a similar system used by the U.S. Air Force Reserve Hurricane Hunters in Biloxi, Miss., and the NOAA Hurricane Hunters in Tampa, Fla. On board each aircraft are a computer and a rack of electronic equipment to monitor and receive information from sondes. "The system is capable of tracking as many as eight dropsondes in the air at the same time. Each one of them is transmitting data on a separate frequency as it falls." says Lauritsen.

From the time the sonde leaves the aircraft, it is checking surroundings two times a second and sending information back to the aircraft, including pressure, temperature, humidity, wind speed, and wind direction. Future developments are expected to include sensors for chemicals such as ozone.

"We're taking vertical slices of the atmosphere constantly as the sonde falls," says Hock. "We're seeing very precise single measurements show up immediately on the computer screen."

Researchers process the information using NCAR-developed custom software, and then send it to weather forecasters and researchers around the world. In the case of the Hurricane Hunters, the information goes to the National Hurricane Center in Miami.

NCAR software engineer Charlie Martin develops custom software called ASPEN, which stands for Atmospheric Sounding Processing Environment. ASPEN helps make sense of all the dropsonde data. "Once the dropsonde has fallen through the atmosphere and the data has come back to the aircraft, that raw data needs a little more treatment before we send it to weather services around the world," explains Martin.

Martin points to a map showing a compilation of dropsonde wind data collected in August 2011, as Hurricane Irene was churning its way toward the Florida coast. "The winds are in a circular pattern," says Martin, as he identifies small triangles on the map that represent the wind and wind direction. "The center of the hurricane is clearly depicted in the center of the circular pattern. The National Hurricane Center uses this data along with other data to classify the hurricane and assign a category to it."

Hock and his team also custom fit aircraft with launchers to deploy the sondes, including one system for helium-filled balloons. In 2010, American and French researchers deployed balloons over Antarctica that dropped 600 sondes over a four-month period to study atmospheric conditions and the shifting ozone layer. "There is now a very dense set of measurements that came out of this project that has mapped the Antarctic atmosphere like it has never been done before," notes Martin.

"Atmospheric conditions above the Antarctic continent are hard to study since only a handful of sounding stations are regularly maintained there," says Peter Milne, program manager for ocean and atmospheric sciences within NSF's Office of Polar Programs. "Fortunately, the Antarctic polar vortex, a huge cyclone that sets up above the entire continent, is like the NASCAR of long distance ballooning, with balloons sweeping around the continent for as long as they stay aloft. Using these drifting platforms provided a unique data set."

Such "inside information" is helping scientists learn more about climate and hurricanes. Data from dropsondes is also giving scientists a better understanding about atmospheric conditions that spawn any number of weather conditions. Hock expects this will help forecasters make earlier and more precise hurricane predictions, giving people in the path of a killer storm more time to get out of harm's way.

Miles O'Brien, Science Nation Correspondent

Ann Kellan, Science Nation Producer

The Depths of Winter: How Much Snow Is In Fact On the Ground?

Equipped with specialized lasers and GPS technology, scientists are working to address a critical wintertime weather challenge: how to accurately measure the amount of snow on the ground.

Transportation crews, water managers and others who make vital safety decisions need precise measurements of how snow depth varies across wide areas.

But traditional measuring devices such as snow gauges and yardsticks are often inadequate for capturing snow totals that may vary even within a single field or neighborhood.

Now scientists at the National Center for Atmospheric Research (NCAR) in Boulder, Colo., and at other institutions are finding that prototype devices that use light pulses, satellite signals and other technologies offer the potential to almost instantly measure large areas of snow.

In time, such devices might provide a global picture of snow depth.

"We've been measuring rain accurately for centuries, but snow is much harder because of the way it's affected by wind and sun and other factors," says NCAR researcher Ethan Gutmann.

"It looks like new technology, however, will finally give us the ability to say exactly how much snow is on the ground."

NCAR is conducting the effort with several collaborating organizations, including the National Oceanic and Atmospheric Administration (NOAA) and the University of Colorado Boulder.

The work is supported by NCAR's sponsor, the National Science Foundation (NSF).

"Snow represents both a hazard and a water resource in the western states," says Thomas Torgersen, NSF program director for hydrologic sciences. "Both require detailed assessments of snow amounts and depth. This technology will provide new and important guidance."

Emergency managers rely on snowfall measurements when mobilizing snow plows or deciding whether to shut down highways and airports during major storms.

They also use snow totals when determining whether a region qualifies for disaster assistance.

In mountainous areas, officials need accurate reports of snowpack depth to assess the threat of avalanches or floods, and to anticipate the amount of water available from spring and summer runoff.

But traditional approaches to measuring snow can greatly underreport or overreport snow totals, especially in severe conditions.

Snow gauges may miss almost a third of the snow in a windy storm, even when they are protected by specialized fencing designed to cut down on the wind's effects.

Snow probes or yardsticks can reveal snow depth within limited areas. But such tools require numerous in-person measurements at different locations, a method that may not keep up with totals during heavy snowfalls.

Weather experts also sometimes monitor the amount of snow that collects on flat, white pieces of wood known as snow boards, but this is a time-intensive approach that requires people to check the boards and clear them off every few hours.

The nation's two largest volunteer efforts--the National Weather Service's Cooperative Observer Program, and the Community Collaborative Rain, Hail, and Snow Network (CoCoRaHS)--each involve thousands of participants nationwide using snow boards, but their reports are usually filed just once a day.

More recently, ultrasonic devices have been deployed in some of the world's most wintry regions.

Much like radar, these devices measure the length of time needed for a pulse of ultrasonic energy to bounce off the surface of the snow and return to the transmitter.

However, the signal may be affected by shifting atmospheric conditions, including temperature, humidity and winds.

The specialized laser instruments under development at NCAR can correct for such problems.

Once set up at a location, they can automatically measure snow depth across large areas. Unlike ultrasonic instruments, lasers rely on light pulses that are not affected by atmospheric conditions.

New tests by Gutmann indicate that a laser instrument installed high above treeline in the Rocky Mountains west of Boulder can measure 10 feet or more of snow with an accuracy as fine as half an inch or better.

In a little more than an hour, the instrument measures snow at more than 1,000 points across an area almost the size of a football field to produce a three-dimensional image of the snowpack and its variations in depth.

Gutmann's next step will be to build and test a laser instrument that can measure snow over several square miles. Tracking such a large area would require a new instrument capable of taking more than 12,000 measurements per second.

"If we're successful, these types of instruments will reveal a continually-updated picture of snow across an entire basin," he says.

One limitation for the lasers, however, is that light pulses cannot penetrate through objects such as trees and buildings.

This could require development of networks of low-cost laser installations that would each record snow depths within a confined area.

Alternatively, future satellites equipped with such lasers might be capable of mapping the entire world from above.

Gutmann and Kristine Larson, a scientist at the University of Colorado, are also exploring how to use GPS sensors for snowfall measurements.

GPS sensors record satellite signals that reach them directly and signals that bounce off the ground.

When there is snow on the ground, the GPS signal bounces off the snow with a different frequency than when it bounces off bare soil, enabling scientists to determine how high the surface of the snow is above the ground.

Such units could be a cost-effective way of measuring snow totals; meteorologists could tap into the existing global network of ground-based GPS receivers.

However, researchers are seeking to fully understand how the density of the snow and the roughness of its surface alter GPS signals.

"Our hope is to develop a set of high-tech tools that will enable officials to continually monitor snow depth, even during an intense storm," Larson says.

"While we still have our work cut out for us, the technology is very promising."

-NSF-

Broken Glass Yields Clues to Climate Change

Clues to future climate may be found in the way an ordinary drinking glass shatters.

Results of a study published this week in the journal Proceedings of the National Academy of Sciences find that microscopic particles of dust can break apart in patterns that are similar to the fragment patterns of broken glass and other brittle objects.

The research, by National Center for Atmospheric Research (NCAR) scientist Jasper Kok, suggests there are several times more dust particles in the atmosphere than previously believed, since shattered dust appears to produce an unexpectedly high number of large fragments.

The finding has implications for understanding future climate change because dust plays a significant role in controlling the amount of solar energy in the atmosphere.

Depending on their size and other characteristics, some dust particles reflect solar energy and cool the planet, while others trap energy as heat.

"As small as they are, conglomerates of dust particles in soils behave the same way on impact as a glass dropped on a kitchen floor," Kok says. "Knowing this pattern can help us put together a clearer picture of what our future climate will look like."

The study may also improve the accuracy of weather forecasting, especially in dust-prone regions. Dust particles affect clouds and precipitation, as well as temperature.

"This research provides valuable new information on the nature and distribution of dust aerosols in the atmosphere," says Sarah Ruth, program director in the National Science Foundation (NSF)'s Division of Atmospheric and Geospace Sciences, which funds NCAR. 

"The results may lead to improvements in our ability to model and predict both weather and climate."

Kok's research focused on a type of airborne particle known as mineral dust.

These particles are usually emitted when grains of sand are blown into soil, shattering dirt and sending fragments into the air.

The fragments can be as large as about 50 microns in diameter, or about the thickness of a fine strand of human hair.

The smallest particles, which are classified as clay and are as tiny as 2 microns in diameter, remain in the atmosphere for about a week, circling much of the globe and exerting a cooling influence by reflecting heat from the Sun back into space.

Larger particles, classified as silt, fall out of the atmosphere after a few days. The larger the particle, the more it will tend to have a heating effect on the atmosphere.

Kok's research indicates that the ratio of silt particles to clay particles is two to eight times greater than represented in climate models.

Since climate scientists carefully calibrate the models to simulate the actual number of clay particles in the atmosphere, the paper suggests that models most likely err when it comes to silt particles.

Most of these larger particles swirl in the atmosphere within about 1,000 miles of desert regions, so adjusting their quantity in computer models should generate better projections of future climate in desert regions, such as the southwestern United States and northern Africa.

Additional research will be needed to determine whether future temperatures in those regions will increase as much or more than currently indicated by computer models.

The study results also suggest that marine ecosystems, which draw down carbon dioxide from the atmosphere, may receive substantially more iron from airborne particles than previously estimated.

The iron enhances biological activity, benefiting ocean food webs, including plants that take up carbon during photosynthesis.

In addition to influencing the amount of solar heat in the atmosphere, dust particles also are deposited on mountain snowpacks, where they absorb heat and accelerate snowmelt.

Physicists have long known that certain brittle objects, such as glass, rocks, or even atomic nuclei, fracture in predictable patterns. The resulting fragments follow a certain range of sizes, with a predictable distribution of small, medium, and large pieces.

Scientists refer to this type of pattern as scale invariance or self-similarity.

Physicists have devised mathematical formulas for the process by which cracks propagate in predictable ways as a brittle object breaks.

Kok theorized that it would be possible to use these formulas to estimate the range of dust particle sizes. By applying the formulas for fracture patterns of brittle objects to soil measurements, Kok determined the size distribution of emitted dust particles.

To his surprise, the formulas described measurements of dust particle sizes almost exactly.

"The idea that all these objects shatter in the same way is a beautiful thing, actually," Kok says. "It's nature's way of creating order in chaos."

-NSF-