Cloud fraction driven thermal-infrared radiative effects of Arctic low-level clouds in marine cold air outbreaks

Abstract. Arctic marine cold air outbreaks (MCAOs) are important meridional air mass transformation events accompanied by spatially extensive cloud formation. Here, the top-of-atmosphere radiative effect of these MCAO clouds is analyzed using airborne thermal-infrared imaging observations conducted during the HALO-(AC)3 research campaign. Based on the capabilities of the high-resolution (∼ 10 m) imaging, a fine-scale cloud mask is derived. Furthermore, two cloud radiative effects (CREs), an all-sky CRE (including both cloudy and cloud-free areas) and a cloud-only CRE, are quantified. This way, the effects of cloud fraction and cloud properties are separated, and the drivers of the all-sky CRE are evaluated. It is found that the increase of the all-sky CRE downstream in an MCAO is primarily driven by cloud fraction, in particular for a strong MCAO event. In a next step, the approach applicability and its sensitivity to coarser spatial sampling is evaluated by scaling the airborne images down to lower resolutions of satellite observations provided by EarthCARE MSI (500 m) and Terra/Aqua MODIS (1000 m). This coarsening results in an underestimation of the CREs, with maximum values found in the strong MCAO case and at MODIS resolution with −1.9 W m−2 (all-sky CRE) and −2.9 W m−2 (cloud-only CRE). The relations between CREs and cloud fraction can still be identified for lower spatial resolutions but lack in robustness. This highlights the importance of high-resolution airborne thermal-infrared imaging in support of large-scale spaceborne cloud observations to determine CREs.