High-Resolution Image

These images illustrate cloud geometrical thickness measurements by the THOR (THickness from Offbeam Returns) system. THOR is uniquely well-suited for such measurements because it can observe the horizontal spreading of laser pulses inside a cloud. In a thicker cloud, many laser photons that would quickly leak out the cloud base become delayed by multiple scattering inside. This results in a larger halo for thicker clouds. Parts of the halo further from the entry point of the laser pulse correspond to higher orders of scattering, and thus are observable only after longer time delays.

The top left panel shows some raw THOR observations taken during a flight over Oklahoma in March 2002. The panelÕs RGB coloring scheme reveals the laser signalÕs spread: red indicates direct backscatter, while green and blue indicate the signals returning from increasingly wide rings around the illuminated spot. The Cirrus layer around 5.5 km appears red, because it contains too few particles to spread photons and create a significant halo. The very top of the Stratus layer around 1.6 km also appears bright red due to an intense direct backscatter. The signal received at longer time-delays (displayed as if it had returned from lower altitudes) comes from outer parts of the halo, and so the red signal turns white at first, then green, and eventually blue. So red means single or low order scattering, while blue indicates multiple high order scattering.

The sudden jumps in altitude occur when the airplane turns, because the laser pulse needs more time to travel to the cloud and back along slanted paths. Some photons even appear to return from below the surface, because they spend more time bouncing among cloud droplets than the time they would need to travel to the ground and back along a straight path.

The lower left panel shows measurements of a ground-based micropulse lidar with the conventional narrow field-of-view, operated by the Department of Energy Atmospheric Radiation Measurement program. Though this lidar cannot see through the thick Stratus cloud, it can easily detect the cloud base. These cloud base values were used to validate THOR cloud retrievals during THORÕs overflights.

The upper right panel shows the vertical profile of temperature (blue) and also dew point temperature (red), as obtained from balloon soundings at the nearby ARM SGP site. The plot reveals the cloud base and top, as the two temperatures are very close in the humid air inside the cloud, whereas they are far apart in the surrounding dry air. Such meteorological observations were also used to corroborate THORÕs measurements.

The lower right panel compares estimates of the physical thickness (in meters) of the low optically thick Stratus cloud for six overflights (numbered sequentially) when the Cirrus cloud was relatively optically thin. Vertical axis estimates are based on THORÕs halo method, and horizontal axis estimates are based on subtracting the cloud base seen by ground-based lidar from the cloud top observed by THOR. This panel shows that THORÕs cloud base estimates agree to within 40 meters with the ground-based values, showing that THOR can "see through" thick clouds. (Submitted by R. Cahalan, T. Varnai, K. Yetzer.)

See the related THOR paper.