M. Wendisch, J. Kretzschmar, J. Stapf

Airborne pyrgeometer and pyranometer measurements of broadband solar and thermal-infrared upward and downward irradiances are analyzed. The data were collected along low-level flights (average flight altitude of 80 m above ground) over sea ice, the marginal sea ice zone, and the open (ice-free) ocean during two aircraft campaigns conducted North of Svalbard (Arctic) in 2017 and 2019. Corresponding collocated simulations with the atmospheric Icosahedral nonhydrostatic (ICON) numerical weather prediction model of the German Weather Service with a horizontal resolution of approximately 1.2 km are compared with the airborne measurements along the low-level flight sections. The measured data show several modes in the number of occurrence of net irradiances, which are characteristic for cloudy and cloudless conditions above the three surface types. The modes reveal a distinct transition from late winter towards the melting season. This multi-mode structure of the solar and terrestrial net irradiance field close to the surface extends the common picture of two modes during polar night with no solar effects, which were observed in ground-based observations over one specific surface type (sea ice/snow). The modes also appear in the ICON simulations. It is shown that the agreement between the measured and the simulated modes is significantly reduced when respective aircraft measurements are implemented into the ICON simulations. Finally, from the net irradiance measurements and corresponding radiative transfer simulations, the warming or cooling effects due to Arctic low-level clouds over the three investigated surface types is quantified.