BACKGROUND: An atmospheric scientist 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. Earlier warnings give pilots more time to take evasive action before a thunderstorm becomes violent, improving passenger safety and comfort, overall efficiency, and saving money. The new system can also help warn of possible turbulence above storms and downwind from mountain ranges.
HOW IT WORKS: The new forecasting system uses both visible and infrared images taken by NOAA's GOES and NASA's MODIS satellites. By merging these images, the forecast model gets new cloud-top temperature readings every 15 minutes. Using the new system, meteorologists can tell not only if a cloud is getting deeper, but also if the top of the cloud has turned from water droplets to ice, which could lead to lightning. Cumulus clouds that get colder from one reading to the next are building; cloud tops that get significantly colder are building rapidly.
DETECTING TURBULENCE: Wind blowing over the tops of mountains or billowing cloud tops can create a "wave" or rotor effect. Air is forced upwards and creates low pressure on the back side of the obstacle, which pulls the air into downdrafts. These can be quite powerful. The up-and-down airflow can create a turbulent wave pattern that will continue downwind. If the turbulence occurs in clear air, it can't be detected by the human eye, or by Doppler radar. But the infrared (heat) sensors on satellites in the new system can "see" the waves because rising air cools while sinking air warms, creating alternating bands of warmer and colder infrared radiation.
ABOUT INFRARED RADIATION: Infrared radiation is essentially heat. It falls just below the range for visible light on the electromagnetic spectrum. It's the reason a stovetop burner glows red: the atoms in the burner are excited by the influx of energy when the burner turns on, causing the emission of photons in that region of the electromagnetic spectrum. The energized atoms emit photons in the thermal IR regime as they return to their natural ground state. Thermal emissions can be detected and transmitted to computers to create a detailed pattern based on variations in temperature, called a thermogram. The thermogram is then translated into electrical impulses, and these are analyzed by a computer and sent to the display, which shows the data as various colors depending on the intensity of the IR emission.
WHAT IS DOPPLER RADAR: Doppler radar uses a well-known effect of light called the Doppler shift. Just as a train whistle will sound higher as it approaches a platform and then become lower in pitch as it moves away, light emitted by a moving object is perceived to increase in frequency (a blue shift) if it is moving toward the observer; if the object is moving away from us, it will be shifted toward the red end of the spectrum. Doppler radar sends out radio waves that bounce off objects in the air, such as raindrops or snow crystals, and then measures how much the frequency changes in returning radio waves to better determine wind direction and speed.
The American Meteorological Society contributed to the information contained in the TV portion of this report.