Probing the Skies for Grimsvötn's Ashes with an EPFL Raman Lidar

Little over one year has passed since the EPFL Raman lidar last tracked volcanic ash concentrations over the Swiss sky, blasted into the atmosphere by the Eyjafjallayökull volcano in southern Iceland. Since the beginning of this week, the lidar is once again being called upon to measure aerosol concentrations with the aim of tracking the ash cloud formed by the eruption of the latest Icelandic volcano to make headlines, the Grimsvötn.

A lidar, short for LIght Detection And Ranging, is an instrument that works somewhat like a radar (Radio detection And Ranging). But rather than detecting large objects using radio waves, it uses pulsed laser light to infer concentration profiles of atmospheric quantities. Its main components are (i) a laser that emits short light pulses (a few billionths of a second), and (ii) a powerful telescope that receives incoming light, both aimed straight up into the sky. When a pulse of laser light travels through the air, it loses intensity while at the same time being deflected by molecules and particles in a process known as scattering. Some of light is actually backscattered and can be captured by the lidar's receiver telescope. Analyzing the power of the backscattered light, its wavelength, and the time between the emission of the light pulse and the reception of the backscatter allows to determine vertical concentration profiles of water vapor and temperature, as well as of airborne aerosols such as those found in volcanic ash clouds.

Since 2008, the EPFL Raman lidar has been generating water vapor and temperature profiles at the Meteo Suisse meteorological observatory in Payerne that are routinely fed into weather forecasting models. But because of its capacity to detect and accurately determine the altitude of aerosols in the atmosphere, the EPFL lidar, as well similar instruments deployed throughout Europe, play an important role in the event of a volcanic eruption like that of the Grimsvötn volcano earlier this week.

The ash cloud formed during the eruption of a volcano is rich in silicate, a tiny particle that can be hazardous to jet engines, so aviation safety authorities need reliable data on its location. Typically this is obtained by inserting a simulated volcanic plume into a meteorological forecasting model, and modeling the dispersion of the ashes in the atmosphere. Remote sensing techniques using satellite imagery provide an additional source of information on the geographic location of an ash cloud. But the numerical data obtained from the meteorological forecasting model are only as accurate as the weather forecast, and the satellite data fails to provide accurate data on the exact altitude of the ash cloud.

According to Dr. Valentin Simeonov, principal investigator for the lidar project at EFLUM, EPFL, the lidar is useful to determine the time evolution and the mass evolution in of the passing ash cloud, as well as its altitude. Despite the fact that lidar measurements alone do not allow to distinguish between water vapor, ice clouds, or aerosols, and that measurements are limited to the lower boundary of the cloud cover, the lidar has the potential to provide complementary information to aviation safety authorities, as it proved during the volcanic event in 2010.