Orbiting detectors map lightning

17 Jun 2002

Optical sensors on satellites have produced the first planet-wide view of where lightning strikes.

Strike sites

Using two orbiting optical sensors, NASA researchers based at the Marshall Space Flight Center claim to have produced the most accurate map detailing the history of lightning strikes. The maps show, for the first time, the global distribution of lightning as a function of latitude, longitude and time of year.

Lightning sensor

Two satellite-based detectors, each containing a high-speed array of 128 x 128 charge-coupled devices (CCD), are responsible for providing this new insight into lightning strikes.

The prototype Optical Transient Detector (OTD) and its successor the Lightning Imaging Sensor (LIS) both acquire in excess of 500 frames per second and look for changes in the top of clouds.

The OTD and LIS analyze a narrow waveband centered at 777 nm, the peak wavelength emitted by lightning, allowing them to monitor flashes even under daytime conditions. Software measures the background cloud level and records only sudden, transient changes, enabling the system to detect even weak lightning with an efficiency of 90%.

The LIS uses a wide field-of-view expanded optics lens and can detect individual lightning storms occurring on a scale of 4 - 7 km over a region of 600 x 600 km on the Earth's surface. The spatial resolution of the camera is between 3 - 6 km.

Travelling a distance of nearly 16 000 miles per hour in a low earth orbit of 350 km, the same point stays in the LIS field-of-view for 90 seconds. This is sufficient time to detect a single storm should it occur.

These instruments form part of the Tropical Rainfall Measurement Mission (TRMM) satellite. Using results from other TRMM instruments, the researchers gain vital information on aspects of storms such as updrafts, total ice content, cloud-top height and total rainfall.

The LIS has allowed scientists to convert lightning flash rate measurement into useful forecasting, scientific and diagnostic tools. Because of this success, NASA now plans to undertake an advanced study and is currently designing a detector called the Lightning Map Sensor (LMS). The LMS will provide valuable information allowing forecasters to determine whether a storm will become severe.