Radiative Influence of Horizontally Oriented Ice Crystals over Summit, Greenland.

Ice crystals commonly adopt a horizontal orientation under certain aerodynamic and electrodynamic conditions that occur in the atmosphere. While the radiative impact of horizontally oriented ice crystals (HOIC) has been theoretically studied with respect to their impact on shortwave cloud albedo, the longwave impact remains unexplored. This work analyzes the occurrence of HOIC at Summit, Greenland, from July 2015 to June 2017. Using polarization lidar and ancillary atmospheric sensors, ice crystal orientations are identified and used to interpret cloud radiative impact on the surface radiation budget. We find HOIC occur in at least 25.6% of all ice‐only column observations. We find that the shortwave impact of HOIC is to increase cloud radiative effect by approximately 22% for a given solar zenith angle. We also find that the longwave impact of HOIC compared to randomly oriented ice crystals are statistically different at the p < 0.01 significance level, increasing the surface radiative effect by approximately 8% for clouds with infrared optical depths < ~1. We suggest that the observed difference between the surface radiative effect for clouds containing randomly oriented ice crystals and HOIC may be due to enhanced scattering, but this hypothesis needs to be further explored with more detailed observations and modeling. Plain Language Summary: Because of their shape, ice crystals can fall from clouds with a preferential horizontal orientation due to drag forces acting on their surfaces. Using novel remote sensing techniques, we identify the occurrence of preferentially oriented ice crystals and attempt to quantify the impact of ice crystal orientation on the surface environment at a field site in the Arctic. We observe, to our knowledge, for the first time, statistically significant differences in the radiative properties of ice clouds containing preferentially oriented ice crystals from those containing no detectable preferentially oriented ice crystals. We suggest a hypothesis that may explain this difference, but note it requires further testing to fully analyze and define methods to explore the mechanism further. Key Points: First long‐term observational study of the radiative importance of ice crystal orientation in the polar regions is presentedIce clouds containing oriented crystals show greater radiative effect than clouds without oriented crystals in the shortwave and longwaveObservations suggesting enhanced radiative effect of oriented ice crystals occurring in the longwave require further investigation [ABSTRACT FROM AUTHOR]