Satellite data warns of storms and clear air turbulence

Dr. John Mecikalski, assistant professor of atmospheric science at the University of Alabama, with colleagues from the University of Wisconsin and NASA's Langley Research Centre. Dr Mecikalski, has developed a system that is about 65 percent accurate in giving a one-hour warning before heavy rain starts to fall within a thunderstorm. The results of their recent work were published in the January edition of "Monthly Weather Review" and will be presented at the annual winter meeting of the American Meteorological Society in Atlanta.

"Our goal is to take existing, real-time satellite instruments and predict aviation hazards due to thunderstorms and severe weather," says Dr Mecikalski. "the National Oceanic and Atmospheric Administration (NOAA) is evaluating our tool, the FAA is testing it and the Huntsville National Weather Service office used it this summer."

Using both both visible and infrared images taken by NOAA's Geostationary Satellites (GOES) (see images here in real time http://www.goes.noaa.gov/) and NASA's Moderate-resolution Imaging Spectroradiometer (MODIS) satellites the system merges the visible and infrared data, and the forecast model gets new cloud top temperature readings every 15 minutes. Cumulus clouds that get colder from one reading to the next are building; cloud tops that get significantly colder are building rapidly.

"Not only can you tell if a cloud is getting deeper, you can also tell if the top has turned from water droplets to ice, which is related to lightning formation," Mecikalski said.

As the system can give warnings an hour in advance it can help airline and private pilots take evasive action before a thunderstorm becomes violent improving safety and comfort, it also improves efficiency and so cut costs.

In addition helping avoid storms it can also help warn of possible turbulence above storms and downwind from mountain ranges. Wind blowing over the tops of either mountains or billowing cloud tops can create a "wave" or rotor effect. Air being forced upward as it blows over mountains or storms creates low pressure on the back side of the obstacle, which pulls the air into sometimes powerful downdrafts. This vertical airflow can create a turbulent wave pattern that will continue downwind. In clear air, it is invisible to the human eye and can't be detected by Doppler radar.

These waves, however, can be seen by the satellite sensors, says Mecikalski. "When one of these waves breaks like it does on a beach, that's when you can have a problem for aviation. It acts like a wave on a beach, but it's invisible. The satellites let you see these phenomenon."

The satellite infrared sensors "see" these waves because rising air cools while sinking air warms, creating alternating bands of warmer and colder infrared radiation.

One advantage of using satellite information is that it is available over most of the globe, so it might be used to provide hazard warnings over areas not covered by Doppler or aviation radar systems.

In addition to studying turbulence and convective storms, Mecikalski and his colleagues are also developing techniques for using satellite data to detect and warn of volcanic ash in the atmosphere, and of possible icing conditions.

Pic is a colour enhanced Water Vapour plot of West Coast USA from NOAA GOES feed.