Observations of clouds encountered during THOR’s flight on March 25, 2002.
(a) The top left panel shows some THOR observations taken during a flight over Oklahoma on March 25, 2002. The panel’s RGB coloring scheme reveals the laser signal’s 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.
(b) 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 THORs measurements.
(c) 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 THOR’s overflights.
(d) The lower right panel shows measurements of a gorund-based cloud radar operating at the ARM site. The radar data were used to examine the internal structure of clouds and to detect drizzle.
