![]() ![]() The morphologies of various cirrus cloud types and the high degree of variability in their physical properties are largely imparted by the meteorological environment in which they form. ![]() This distinction is meaningful owing to the widespread global occurrence and unique role of high ice clouds in radiative-convective feedbacks. Glaciated liquid-phase clouds in the mid- and lower troposphere are not classified as cirrus. Aircraft-produced contrail cirrus-the only man-made type of clouds-comprise line-shaped contrails and irregularly shaped cirrus clouds evolving from them. Ice crystals in cirrus are detrained from convective (anvil cirrus) or frontal cloud outflow or they form at low temperatures (<230 K) in situ in the upper troposphere and tropopause region not directly associated with convection. Some climate intervention studies suggest a potential cooling effect of deliberately perturbed cirrus, but at the risk of modifying precipitation inadvertently.Ĭirrus are high-altitude (>8 km) ice clouds lacking a liquid cloud water phase. Microphysical and macrophysical representation of cirrus in global models must first be advanced before we can predict changes in climate with fewer uncertainties. These uncertainties keep us from drawing robust conclusions about anthropogenic influences on cirrus. Two main uncertain factors contribute to the current inability to constrain background cirrus formation: small-scale variability in dynamical forcings that drive ice nucleation parameterizations and the ability of airborne particles to act as efficient heterogeneous ice nuclei. Changes in physical properties and chemical composition of liquid aerosol particles will unlikely affect cirrus significantly, but anthropogenic influences may occur through changes in heterogeneous ice nuclei. This review assesses recent observational and modeling evidence of how anthropogenic activities might affect cirrus. Cirrus clouds are ubiquitous, long-lived, high-level ice clouds that exert a considerable global radiative effect on the climate system. ![]()
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