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Space Lasers Take To The Winds
Scientists hope to use lasers to provide higher quality snapshots of the winds that travel the globe.

by Annie Strickler

Scientists at the Global Hydrology and Climate Centre are studying a type of radar that uses laser light instead of microwaves to provide high-quality snapshots of the winds that travel the globe. Knowing the wind's speed and direction over large areas could help meteorologists answer the riddle of tomorrow's weather further in advance, saving lives and benefiting many areas of the world's economy -- particularly air travel.

"Lidar," or Light Detection and Ranging, is a technique that's been used for years to study the atmosphere from the ground. But now researchers are touting the benefits of lidar from space.

Dr. Michael Kavaya, a scientist at the Global Hydrology and Climate Centre (GHCC) in Huntsville, USA, champions space-based laser sensing of the Earth's winds because of the "immediate benefit of wind sensors orbiting the Earth."

"Orbiting wind sensors could help airline pilots avoid headwinds, leading to savings on fuel," explains Kavaya. According to one study, these savings could amount to $100 million to $200 million per year for airlines.

Fuel savings just scratch the surface of what lidar can accomplish.

Graphic showing sea-surface wind speeds

"On-board lidar sensors on each airplane would be valuable for microburst wind shear and clear-air turbulence warnings. These are significant threats to passengers' safety," says Kavaya.

"Clear air turbulence occasionally hurts people and even kills them. There have been a few deaths in the past couple of years - people don't have their seat belts on and they're thrown into the ceiling, or the food cart is thrown at them. Microburst wind shears can cause accidents on landing or take off "

Lidar may also prove invaluable for monitoring the flow of water through rivers, for improving weather forecasts, and even for understanding the complex El Niño/La Niña phenomenon.

Light Detection and Ranging

Lidar works by beaming pulses of laser light through the atmosphere and detecting the light reflected back by dust and other small particles in the air, called aerosols. The time between the pulse and the echo determines the distance, and the shift in the colour of the light determines the velocity of the particles along the laser's line of sight. True wind speed and direction can be calculated from these results.

The Laser Radar (lidar) can fly on normal DC-8 Aircraft to detect wind direction and speed.

An earth based lidar shooting up a sodium resonance beam over the night sky of Puerto Rico.

If this sounds a lot like Doppler radar, that's because it is. But there are some important differences between the two technologies.

"Radar excels at piercing bad weather, but it needs raindrops or hydrometeors (hail or snow) to get a signal," Kavaya said. "Lidar struggles to go through thick clouds or heavy rain, but it can get you wind (measurements) in clear air, because it relies on aerosols."

The reason for this difference is the frequency of the radiation that each technology uses. Both emit electromagnetic waves, but while radar typically uses frequencies in the range of microwaves, lidar uses higher frequencies in the visible or near-visible light range. Higher frequency radiation (light) will be reflected by smaller particles than lower frequency radiation (microwaves).

Lidar also emits a narrower beam than radar, which minimise interference from ground clutter and improves the resolution of the data. The disadvantage is that lidar has more trouble covering large areas than does radar.

Rivers are fair game, too

The narrower beam opens up another possible application for lidar: measuring water flow in rivers.

Scientists at the Marshall Space Flight Centre and the GHCC have been working with the U.S. Geological Survey to see if lidar may be able to replace the manual stream-flow measuring technique that the USGS currently uses.

"It turns out that some of their standard methods for measuring stream current involved an element of risk to personnel," said Dr. Jeff Rothermel, a NASA scientist at the GHCC. "In fact, one USGS employee lost his life in the line of duty while making measurements. So there is an interest there to determine whether lidar can be used to measure stream current."

Mounted either at the side of the stream or on a satellite in space, a lidar system would measure the speed of the water's surface at several points across the width of the river. Knowing the shape of the river's bottom would allow the volume of water flowing in the river to be calculated from those measurements.

"[In addition to US rivers] I imagine that we could contribute to the study of the Earth's hydrology greatly by having improved river flow (data) worldwide," Kavaya said.

River flow measurement is a newer application for lidar than wind measurement, and it is only in the early stages of development.

"We've attempted to do a proof of concept experiment along the Tennessee River," Rothermel said, "and so far the results are encouraging."

Lidar's Crystal Ball

Predicting hazardous or inclement weather could benefit many sectors of the US economy. One study estimates savings of about $110 billion annually if reliable weather forecasts could be extended to seven days in advance.

Image credit:NASA/Goddard

Water, water, everywhere - but where does it go? This image of the Earth depicts water vapour in the atmosphere.Predicting where it will fall days ahead of time is a challenge for weather researchers.

Scientists working on lidar believe that a lidar-equipped satellite in a polar orbit could bring about such an improvement in weather forecasting.

Why are wind measurements so important for weather forecasting?

"The wind carries heat, moisture, momentum, radiatively-active trace gases, and aerosols," answers Rothermel. "The wind interacts with clouds and radiation to produce weather and climate, and variations thereof. Moreover, numerical model simulations indicate the addition of new wind observations may improve forecasts more than the addition of new temperature or humidity data."

Better wind data could also help refine mathematical models of large scale weather patterns such as the El Niño-Southern Oscillation (ENSO).

"We need better wind information to measure how well we're doing or how bad we're doing in modelling the [El Niño] situation," said Pete Robertson, a scientist at the Global Hydrology and Climate Centre in Huntsville, Alabama, who works on modelling ENSO.

Robertson noted that the patchy wind data provided by radiosondes, ground stations, airplanes and cloud-watching satellites leave data gaps in the tropics, which are particularly important for his research.

By helping "validate how the forecast model compares to reality," data from a satellite-based lidar system could allow Robertson's team to improve their mathematical models of ENSO, which in turn could improve medium-range forecasts -- those between seven and 10 days in advance, Robertson said. On those time scales, pressure waves in the air over the Pacific (where ENSO occurs) have time to spread to the North American continent and affect US weather patterns.

More wind data may help extend weather forecasts, but Robertson cautioned that there is a theoretical limit to how far into the future accurate forecasts will ever be possible.

"No matter how good you know your initial conditions, there's a certain amount of chaotic behaviour in the atmosphere," Robertson said. "So ... a really deterministic weather forecast is only going to be possible for maybe up to two weeks."

Dr. James Keesling, a professor of mathematics at the University of Florida who specialises in chaos theory, commented on this theoretical limit.

"The lidar system may provide us with unprecedented detailed information about the direction and intensity of winds throughout the globe," Keesling said. "However, we know that unless this data is perfect and the computers using that data in their computations use an impossible number of digits, we will not be able to predict very far into the future. The problem is in the mathematics itself, not the accuracy of the data."

The Butterfly Effect

Chaos theory predicts that systems such as the world's weather that involve chaotic behaviour (in the mathematical sense of "chaotic") will exhibit a property sometimes called the "butterfly effect." First identified by a meteorologist named Edward Lorenz in 1963, the butterfly effect refers to a situation when very small differences can lead to very large differences over time. Hence the famous example of a butterfly flapping its wings in New York City's Central Park and causing a tornado in Texas.

Lidar might not be able to sense the gentle breeze of a butterfly in flight, but by forecasting storms and detecting turbulence the technology could save millions of dollars and even human lives.

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First Science 2014