Atmospheric Adjustments to In Situ Cirrus Formation in Ice Supersaturated Regions

Abstract High‐level ice clouds exert a net warming on the climate system because their greenhouse effect outweighs their albedo effect. Focusing on in‐situ cirrus, their formation involves the conversion of upper‐tropospheric water vapor into ice crystals. Anthropogenic perturbations to the upper‐tropospheric water budget—such as aviation‐induced cloudiness or the proposed cirrus cloud thinning concept—also trigger atmospheric adjustments that contribute to the total effective radiative forcing. This study presents an idealized climate‐model pulse experiment in which water exceeding saturation within cloud‐free, ice‐supersaturated regions is instantaneously condensed to form cirrus clouds. In contrast to sustained perturbations, this transient modification of cirrus cloud cover enables the direct isolation and examination of atmospheric adjustments. We further introduce a novel ensemble‐based framework that suppresses the statistical impact of atmospheric variability, allowing for a robust assessment of the comparatively weak atmospheric response. Cirrus cloud formation initially increases high‐level cirrus cloud cover, followed by a decay as ice crystals sediment and sublimate. This process induces an immediate reduction in upper‐atmospheric specific and relative humidity, with recovery occurring on substantially longer timescales. The combined cloud and humidity responses generate an initially positive radiative perturbation that transitions to a negative signal after a couple of hours. Notably, relative humidity requires up to four days to return to its equilibrium value. Adjustments in high‐level cloud fraction and top‐of‐atmosphere longwave radiation exhibit non‐linear behavior. Together, these findings elucidate how the radiative effects of cirrus clouds—whether natural or anthropogenic—are partially counteracted by atmospheric adjustments, with important implications for the climate efficacy of cirrus‐related forcings.