THOR science: thickness retrievals for clouds and for snow and ice

THOR's most distinguishing feature is that its multiple wide field-of-views provide spatial and temporal information on multiple scattering processes that occur deep inside thick media such as thick clouds, snow, or sea ice. In contrast, most other lidars use a narrow field-of-view in order to reduce observational noise caused by background illumination, and consequently provide information only about a thin skin layer, because multiple scattering quickly spreads most photons outside their narrow field-of-view.

The initial focus of THOR data analysis has been retrieval of geometrical thickness. Such retrievals are possible because photons can reach distant parts of thick clouds, whereas they tend to leak out through the base of thin clouds (see figure below). As a result, THOR observes a larger bright halo for thicker clouds. In addition, the observed signal extends to longer times for thicker clouds, because photons spend more time traveling inside the cloud.

Thickness retrievals have already been validated for clouds, and theoretical simulations indicate that snow and sea ice thickness retrievals are also feasible.

Initial results also suggest that THOR holds promise for measuring the internal structure of the observed medium. We plan to combine THOR data with coinciding observations by a cloud radar and a microwave radiometer in order to estimate vertical profiles of droplet concentration and size in stratiform clouds.