Direct estimation of the global distribution of vertical velocity within cirrus clouds

Donifan Barahona,Heike Kalesse,Andrea Molod

doi:10.1038/s41598-017-07038-6

Donifan Barahona, Heike Kalesse + Show 1 more

Open Access

https://doi.org/10.1038/s41598-017-07038-6

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Abstract

Cirrus clouds determine the radiative balance of the upper troposphere and the transport of water vapor across the tropopause. The representation of vertical wind velocity, W, in atmospheric models constitutes the largest source of uncertainty in the calculation of the cirrus formation rate. 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 standard deviation in W, σw, varies widely over the globe with the highest values resulting from orographic uplift and convection, and the lowest occurring in the Arctic. Globally about 90% of the simulated σw values are below 0.1 m s−1 and about one in 104 cloud formation events occur in environments with σw > 0.8 m s−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. These features are in agreement with observations and suggest that the correct parameterization of σw is critical to simulate realistic cirrus properties.

Highlights

Cirrus clouds, made of ice crystals and present at low temperatures and high altitudes, cover approximately 17% of the Earth[1]
Ice formation is realized by two different processes: homogeneous ice nucleation (HOM), i.e., the spontaneous freezing of supercooled liquid droplets, and, heterogeneous ice nucleation (HET), which requires the presence of ice nucleating particles (INP)
By implementing our estimates in a global model and running experiments constrained by observations we show that the global distribution of σw largely determines the balance between homogeneous and heterogeneous ice nucleation during the formation of cirrus

Summary

Introduction

Made of ice crystals and present at low temperatures (below 235 K) and high altitudes, cover approximately 17% of the Earth[1]. Because of the separation between the relevant scale for ice nucleation and the scale resolved by the GCMs, it is likely that several cloud formation events occur within a model grid cell This unresolved variability is characterized by a “subgrid” distribution of vertical velocity, Φ(W, σw), largely determined by its standard deviation, σw, since the mean resolved vertical transport is slow compared to cloud-scale motions[13, 14, 29]. Field campaign analyses and cloud modeling studies[24, 29, 38] suggest a strong relation between the effect of aerosol emissions on cloud properties and W These highlight the importance of improving the representation of subgrid W variability in GCMs. In this work we develop a method, using ultra high resolution global simulations to directly calculate the global distribution of subgrid vertical velocity affecting cirrus formation. By implementing our estimates in a global model and running experiments constrained by observations we show that the global distribution of σw largely determines the balance between homogeneous and heterogeneous ice nucleation during the formation of cirrus

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