Atmospheric Polarimetric Lidar Applied to Glacial Melt Water Measurements

Lidar instruments configured with polarimetric observing capabilities have been widely used by the atmospheric science community to study the thermodynamic phase of clouds as well as shape and composition of atmospheric aerosols. These instruments have opened new opportunities for similar polarimetric instruments to be applied to other prominent science endeavours such as measuring glacial melt water in the Polar Regions. Warming temperatures over the GrIS (Greenland Ice Sheet) have significantly increased the surface melt flowing through the supraglacial hydrologic system and surface melt now represents more than half of the sea level contribution from the GrIS. Scientific requirements established by the cryospheric community call for hydrographic lidar measurements with water depth accuracy better than ±10 cm over meter-scale depths during the melt season. Lakes observed in Southwest, Greenland were on average 2-3 m deep, with maxima near 8 m, and stream depths ranging from 0.6 to 3.4 m with a mean depth of 2 m.

Recently, polarization principles have been applied to observing and characterizing semi-transparent media to address these observational needs. A technique developed, dubbed INPHAMIS (INtrapulse PHAse Modification Induced by Scattering), has led to breakthroughs in addressing severe signal dynamic range limitations introduced by strong specular surface reflections and weak diffuse subsurface scatter – a similar issue with zenith observations of cirrus clouds has been described by many – and the ability to operate in shallow water regimes (<2 m), where prior complex systems still struggle. Utilization of this technique limits the precision to the timing resolution of the time-to-digital converter instead of laser pulse width, with demonstrated precisions of 4 mm with a lab demonstrator. Furthering the instrument capability by utilizing multi-stop FIFO (first-in first-out) sparse storage data acquisition techniques, this next generation of instrument is able to provide an alternative to the traditional multi-channel scalar approach that operates at significantly lower data rates, but yields no drop in information. Employing fast and stable timing electronics, +/-1 cm accuracy is achieved in determining water depth and subsurface topography. These two breakthrough capabilities were accomplished by applying polarimetric lidar configurations similar to those used in the atmosphere while exploiting polarization attributes of water surface and bottom surface scattering. This technology provides the means to observe shallow glacial melt water and the subsequent small-scale subsurface features at a very high precision, all of which are not currently identifiable from satellite imagery and current airborne lidar instruments.

This presentation will display atmospheric and topographic/bathymetric results from polarimetric lidar instruments developed at the University of Colorado Boulder. Emphasis will be put on novel techniques developed, the resulting measurement accuracy and precision, and their applicability to cryospheric science objectives.