Cloud mask derived from airborne KT19 measurements during the EUREC4A field campaign.

A cloud mask was derived from measurements of the Kelvin infrared radiation Thermometer (KT19, version KT19.85II) onboard of HALO during EUREC4A.
The KT19 was operated in a nadir-viewing configuration within the temperature and pressure stabilized housing of the
Video airbornE Longwave Observations within siX channels (VELOX) system.
The KT19 measures brightness temperatures for the wavelength range of 9.6 to 11.5 μm and covers a field of view of 2.3°,
corresponding to an along-track spatial resolution of 400 m (at 10 km flight altitude).

The cloud mask is derived by comparing the measured brightness temperature with simulations assuming cloud-free conditions.
Three different thresholds (0.7 K, 1.0 K, and 2.0 K) for the difference between measurements and simulations are applied.
The final cloud mask logically combines the different thresholds for each time step.
If all thresholds are exceeded, the measurement is denoted as "most likely cloudy", if only one of the thresholds is exceeded,
the measurement is denoted as "probably cloudy". "cloud free" is classified, when no threshold is reached.
"unknown" is classified, when no estimation was possible, e.g., due to condensation at the instruments window.