Radiative impact of thin cirrus clouds in the lowermost stratosphere and tropopause region

Abstract

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 region and in the 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 in approximate balance, which together with uncertainties regarding the shape and size of cirrus particles, make their overall impact on climate difficult to quantify. In this study the SOCRATES (Suite Of Community RAdiative Transfer codes based on Edwards and Slingo) radiative transfer model was used to calculate the shortwave and longwave RE for observed thin cirrus during the second space shuttle mission by the CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA-2) instrument. Unusual high cloud top heights with respect to the tropopause and rather high occurrence rates have been retrieved in earlier studies. However, the question remained open if these optically ultra thin cirrus clouds (UTC), so far not considered in global model calculation, are of importance for the Earth's radiation budget. Using sensitivity simulations with different ice effective particle size and shape, we provide an uncertainty range for the RE of UTCs in the lowermost stratosphere and tropopause region during both summer and winter months. Cloud top height and ice water content are based on CRISTA-2 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, a less likely habit for this high altitude cirrus type, then their net RE is generally negative (cooling) during summer months and positive (warming) during winter months. The cooling or warming RE is in the order of ±(0.1–0.01) W/m2 for a realistic global cloud coverage of 10 %, similar to the magnitude of the contrail cirrus radiative forcing best estimate of ~0.1 W/m2. RE is also dependent on the cloud vertical extent and consequently the optically thickness and effective radius (Reff) of the particle size distribution (e.g. Reff increase from 10 to 30 µm results in a factor ≃ 3 smaller short and longwave effects). We argue that the radiative impact of UTC clouds in the lowermost stratosphere and tropopause region needs to be better addressed in observations and need to be taken into account in climate simulations.