For example, observational studies suggest that volcanic ash leads to the increase of ice-containing clouds in the upper troposphere 6, 7. Human activities (e.g., transportation and energy production) and natural phenomena (e.g., volcanic eruptions and dust storms) that modify the concentration of atmospheric aerosol influence the properties of cirrus 4, impacting the hydrological balance of the atmosphere and ultimately Earth’s climate 2, 5, 6, 7. Outside of areas of strong convection, cirrus form by ice nucleation on aerosol particles. They control the hydrological balance of the upper troposphere and the longwave cloud feedback 2, 3. These features are in agreement with observations and suggest that the correct parameterization of σ w is critical to simulate realistic cirrus properties.Ĭirrus clouds, made of ice crystals and present at low temperatures (below 235 K) and high altitudes, cover approximately 17% of the Earth 1. Combining our estimate with reanalysis products and an advanced cloud formation scheme results in lower homogeneous ice nucleation frequency than previously reported, and a decreasing average ice crystal concentration with decreasing temperature. Globally about 90% of the simulated σ w values are below 0.1 m s −1 and about one in 10 4 cloud formation events occur in environments with σ w > 0.8 m s −1. The standard deviation in W, σ w, varies widely over the globe with the highest values resulting from orographic uplift and convection, and the lowest occurring in the Arctic. Using global atmospheric simulations with a spatial resolution of 7 km we obtain for the first time a direct estimate of the distribution of W at the scale relevant for cirrus formation, validated against long-term observations at two different ground sites. The representation of vertical wind velocity, W, in atmospheric models constitutes the largest source of uncertainty in the calculation of the cirrus formation rate. Cirrus clouds determine the radiative balance of the upper troposphere and the transport of water vapor across the tropopause.
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