Space Lasers Take To The Winds

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."