Two-dimensional cloud mask and cloud fraction with 1 Hz temporal resolution derived from VELOX during the EUREC4A field campaign.

Two-dimensional cloud masks with 1 Hz temporal resolution were derived from measurements of the
Video airbornE Longwave Observations within siX channels (VELOX) system onboard of HALO during EUREC4A.
VELOX is a thermal imager with a field of view of 35.5° by 28.7° (640 by 512 spatial pixels) and measures brightness temperature
in six spectral bands in the thermal infrared wavelength range from 7.7 to 12.0 µm.

The cloud mask is derived with 1 Hz temporal resolution from the broadband channel of VELOX by comparing the measured brightness temperature
with simulations assuming cloud-free conditions. Four different thresholds (0.5 K, 1.0 K, 1.5 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 spatial pixel and 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.