Quantifying drivers of the thermal-infrared radiative effect of Arctic low-level clouds in cold air outbreaks

Marine cold air outbreaks (CAOs) represent an important meridional transport mechanism out of the Arctic towards lower latitudes. The cloud field properties change with the air mass transformation, and the thermal-infrared all-sky cloud radiative effect (CRE) is increasing in the downstream direction during the initial stages of a CAO. These evolution processes are important to understand current and future CAOs in a warming Arctic, which will favor weaker events.

Here, we aim to identify the driving factors of this downstream increase for different CAO events of varying intensity, which were observed during the HALO-(AC)³ campaign in spring 2022. The High Altitude and LOng range research aircraft (HALO) sampled CAOs in a quasi-Lagrangian way with a remote sensing payload. The thermal-infrared imager VELOX (Video airbornE Longwave Observations within siX channels) provided 2D broadband (7.7 µm to 12.0 µm) brightness temperature fields of cloud tops and the surface with a spatial resolution of 10 m for a 10 km target distance. First, a cloud mask is applied to those brightness temperature fields to determine cloud fractions. In a next step, two types of CRE are calculated. A cloud-only CRE is derived for all identified cloud pixels while an all-sky CRE is calculated for cloud-free as well as cloud pixels. The comparison of the cloud-only and all-sky VELOX CREs enables a determination of the all-sky CRE driver, i.e., cloud top temperature or cloud fraction. In addition, lidar cloud top heights and a large-scale all-sky CRE, based on measurements by a broadband radiometer and radiative transfer simulations, are analyzed to provide further context for the analyzed cases. The results imply that the strength of the all-sky CRE increase depends on the CAO intensity and is in general driven by increasing cloud fraction. Thus, this analysis provides a TOA-like perspective on the thermal-infrared radiative impact of a low-level cloud field, which is (trans-)forming during the initial stages of a CAO.