Introduction to the new airborne thermal infrared imager VELOX for remote sensing of cloud and surface properties

The new airborne thermal infrared imager VELOX (Video airbornE Longwave Observations within siX channels) with six spectral bands in the thermal infrared wavelength range from 7.7 µm to 12 µm is introduced. The imager is operated on board the German High Altitude and Long Range Research Aircraft (HALO), measuring two-dimensional (2D) fields of brightness temperature. VELOX was used for the first time during the ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte (EUREC4A) campaign. Technical specifications, information on the calibration procedure, and first results from the VELOX broadband channel are presented. VELOX is used to estimate cloud masks/fractions along the flight tracks. For the cloudy parts, 2D fields of the cloud top altitude are derived and analysed. First analysis reveal that the cloud top brightness temperature can be resolved with a thermal resolution of about 0.1 K, which means that the cloud top altitude can be resolved with a vertical resolution of about 40 m.