The initial focus of THOR cloud data analysis has been retrieval of cloud geometrical thickness. Such retrievals are desirable because cloud thickness and internal structure influence the vertical profiles of latent and radiative heating, thus affecting cloud development and atmospheric circulation as well as surface warming by greenhouse effect. Cloud thickness is also a diagnostic of cloud formation processes and atmospheric circulation.

Comparison to other instruments

• Unlike conventional narrow field-of-view lidars, THOR can estimate the thickness of highly opaque, thick clouds.
• Unlike radars, THOR can detect clouds consisting of small droplets and cannot confuse clouds with drizzle.
• Because the background solar illumination causes large observational noise in THOR's wide field-of-views, THOR can provide cloud observations only at night.

Retrieval methodology

Cloud thickness is retrieved by comparing THOR observations to simulated data generated by a three-dimensional Monte Carlo radiative transfer model for a wide range of cloud conditions.

Validation

NASA P3-B flight track on March 25, 2002 around the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) program’s Southern Great Plain (SGP) site in central Oklahoma (marked by a letter "c"). White coloring indicates flight segments where breaks in an overlying cirrus layer allowed retrievals of stratus cloud thickness without cirrus correction.

Evaluation of stratus thicknesses retrieved at the 14 cirrus-free flight segments. THOR retrievals based on multi-view halo observations are compared to thicknesses obtained as the difference between the cloud top observed by THOR's central channel and the cloud base provided by ground-based instruments, including the ARM Micro Pulse Lidar (MPL). The mean difference between the two estimates is 30 m.

A summary view of observations used for THOR validation is also available.