Comment on egusphere-2023-155

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Natural cirrus clouds and contrails cover about 30 % of the Earth's mid-latitudes and up to 70 % of its Tropics. Due to their widespread occurrence, cirrus have a considerable impact on the Earth energy budget, which, on average, leads to a warming net radiative effect (solar + thermal-infrared). However, whether the instantaneous radiative effect (RE) of natural cirrus or contrails is positive or negative depends on their microphysical, macrophysical, and optical, as well as radiative properties of the environment. This is further complicated by the fact that the actual ice crystal shape is often unknown and thus ice clouds remain one of the components that are least understood in the Earth's radiative budget. The present study aims to separate the effect on cirrus RE of eight parameters: solar zenith angle, ice water content, ice crystal effective radius, cirrus temperature, surface albedo, surface temperature, liquid water cloud optical thickness of an underlying cloud, and three ice crystal shapes. In total, 94,500 radiative transfer simulations have been performed, spanning the parameter ranges that are typically associated with natural cirrus and contrails. The multi-dimensionality and complexity of the 8-dimension parameter space makes it impractical to discuss all potential configurations in detail. Therefore, specific cases are selected and discussed. The ice crystal effective radius has the largest impact on solar, thermal-infrared (TIR), and net RE. The second most important parameter is the ice water content, which impacts the solar and terrestrial RE equally. Solar and TIR RE have opposite signs, meaning that the ice water content has a relatively small impact on net RE. Beyond the ice crystal effective radius and the ice water content, the solar RE of cirrus is determined by solar zenith angle, surface albedo, liquid cloud optical thickness, and the ice crystal shape in descending priority. RE in the TIR spectrum is dominated by the surface temperature, the ice cloud temperature, the liquid water cloud optical thickness, and the ice crystal shape. Net RE is controlled by the surface albedo, the solar zenith angle, and the surface albedo in decreasing importance. The relative importance of the studied parameters differs depending on the ambient conditions and during nighttime the net RE is equal to the TIR RE. The data set generated in this work is publicly available. It can be used to compute the radiative effect of cirrus clouds, contrails, and contrail cirrus instead of full radiative transfer calculations.