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Abstract

Cirrus clouds play an important role in the radiation budget of the Earth. Despite recent progress in remote sensing observations of cirrus in general, the radiative impact of thin cirrus clouds in the tropopause and lowermost stratosphere remains poorly constrained. This is due to their small vertical extent and optical depth, which make them very difficult to observe for most instruments. In addition, their shortwave (cooling) and longwave (warming) radiative effects (RE) are often just in balance, which together with existing uncertainties regarding their shape and size, make their overall effect difficult to quantify. In this study the SOCRATES radiative transfer model was used to calculate the shortwave and longwave RE for observed thin cirrus from CRISTA2 infrared limb sounder measurements. Using sensitivity simulations with different ice particle sizes and shapes, we provide an uncertainty range for their RE during both summer and winter months. Cloud top height and ice water content are based on CRISTA2 retrievals, while the cloud vertical thicknesses were assumed to be 0.5 or 2 km. Our results indicate that if the ice crystals of these thin cirrus clouds are assumed to be spherical, then their net RE is generally positive (warming). In contrast, if they are assumed to be aggregates, then their net RE is generally negative (cooling) during summer months and positive (warming) during winter months.