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Abstract Arctic single‐layer mixed‐phase clouds were studied using a one‐dimensional model that incorporated the adaptive habit growth model for ice microphysics. The base case was from the Indirect and Semidirect Aerosol Campaign, and it was perturbed over a range of cloud‐average temperatures, maximum (per model run) ice nuclei (IN) concentrations, and large‐scale subsidence velocities. For each parameter combination, the model was iterated out to 48 hr, and the time, called the glaciation time, to complete disappearance of liquid recorded if this occurred within the 48 hr. Dependence of glaciation times on cloud‐average temperatures from −30°C to −5°C, maximum IN concentrations from 0.10 to 30 L−1, and strong–no subsidence, with both isometric and habit‐dependent ice crystal growth, were investigated. For isometric crystal growth, the relationship between the critical maximum IN concentration (INcrit), the maximum (per model run) IN concentration above which a mixed‐phase cloud glaciated within a fixed model runtime, and cloud‐average temperature was monotonic. INcritdecreased with decreasing cloud‐average temperature. Strengthening of subsidence led to a further decrease in INcritfor every cloud‐average temperature. For habit‐dependent ice crystal growth, the relationship between INcritand cloud‐average temperature was nonmonotonic. Ice crystals develop dendritic and columnar habits near −15°C and −7°C, respectively, and at these two temperatures, ice crystals grew and depleted supercooled liquid water faster than the case when ice crystals grew isometrically. This led to deep local minima in INcritaround these two temperatures in the model runs. Habit‐dependent ice crystal growth, coupled with changes in cloud‐average temperature, INcrit, and subsidence strength, led to significant changes in Arctic single‐layer mixed‐phase cloud lifetimes.
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Additional global climate cooling by clouds due to ice crystal complexity
Abstract. Ice crystal submicron structures have a large impact on the opticalproperties of cirrus clouds and consequently on their radiative effect.Although there is growing evidence that atmospheric ice crystals are rarelypristine, direct in situ observations of the degree of ice crystal complexityare largely missing. Here we show a comprehensive in situ data set of icecrystal complexity coupled with measurements of the cloud angular scatteringfunctions collected during a number of observational airborne campaigns atdiverse geographical locations. Our results demonstrate that an overwhelmingfraction (between 61 % and 81 %) of atmospheric ice crystals sampledin the different regions contain mesoscopic deformations and, as aconsequence, a similar flat and featureless angular scattering function isobserved. A comparison between the measurements and a database of opticalparticle properties showed that severely roughened hexagonal aggregatesoptimally represent the measurements in the observed angular range. Based onthis optical model, a new parameterization of the cloud bulk asymmetry factorwas introduced and its effects were tested in a global climate model. Themodelling results suggest that, due to ice crystal complexity, ice-containingclouds can induce an additional short-wave cooling effect of−1.12 W m2 on the top-of-the-atmosphere radiative budget that hasnot yet been considered.
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- Award ID(s):
- 1762096
- PAR ID:
- 10234574
- Date Published:
- Journal Name:
- Atmospheric Chemistry and Physics
- Volume:
- 18
- Issue:
- 21
- ISSN:
- 1680-7324
- Page Range / eLocation ID:
- 15767 to 15781
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.5194/acp-18-15767-2018
