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Modelling Heterogeneous and Homogeneous Ice Nucleation and Growth at Cirrus Cloud Levels

English title Modelling Heterogeneous and Homogeneous Ice Nucleation and Growth at Cirrus Cloud Levels
Applicant Peter Thomas
Number 120175
Funding scheme Project funding (Div. I-III)
Research institution Institut für Atmosphäre und Klima ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Climatology. Atmospherical Chemistry, Aeronomy
Start/End 01.03.2009 - 28.02.2010
Approved amount 97'064.00
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Keywords (13)

supersaturation; dehydration / ice nucleation; cirrus; ice; heterogeneous nucleation; homogeneous nucleation; climate; mineral dust; organic aerosol; ammonium sulphate; trajectory modelling; box model; chemistry transport model

Lay Summary (English)

Lead
Lay summary
Many aspects of the global radiation budget are now well understood. The largest remaining uncertainty is how atmospheric aerosols modify the characteristics of clouds and how that affects the global radiation budget. This project will advance the current understanding of this indirect effect for the particularly climate-relevant cirrus clouds.Cirrus clouds are high altitude clouds, formed when atmospheric water freezes into ice crystals. They reflect infrared radiation as well as sunlight and can therefore warm or cool the surface of the Earth. Atmospheric aerosols are fine solid particles or liquid droplets in the atmosphere, examples being smoke, oceanic haze or simply liquid water. Some components of aerosols affect the temperature at which water droplets freeze and are thus capable of changing the radiative properties of cirrus clouds by influencing the number and size of the clouds' ice particles.Aerosol components which reduce the freezing temperature of water impede homogeneous nucleation, that is, the process of freezing of a droplet which is not in contact with a solid particle. Other types of aerosol, such as mineral dust, raise the freezing temperature of water by providing solid surface on which ice formation can begin (heterogeneous nucleation).Cirrus clouds are an important factor for the Earth's climate. Therefore, it is crucial that their representation in climate models can account for effects induced by anthropogenic changes in the number, size and composition of aerosols in the atmosphere. The aim of this project is to develop a representation of the formation and growth of ice particles for use in climate models.Firstly, we will develop mathematical descriptions of ice nucleation and growth rates, which account for the most important aerosol types, such as mineral dust or ammonium sulfate. An existing model will allow us to determine the effect of each of the examined substances on the properties of a cirrus cloud. The chemical and aerosol input for these experiments will be taken from a state of the art aerosol - chemistry transport model, the OsloCTM2. After developing a simplified cirrus cloud model, which will be incorporated into a global model in the last stage of the project, we can determine how and to what extent the aerosol composition influences the distribution and the radiative properties of cirrus clouds on a global scale.This project will develop the first physico-chemically based representation of cirrus cloud formation for chemistry climate models (CCMs), allowing the calculation of climate effects of changes in cirrus coverage due to anthropogenic modification of atmospheric aerosols.
Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants

Employees

Associated projects

Number Title Start Funding scheme
138039 Ice Freezing on Clay Minerals (IFClaM) 01.01.2012 Project funding (Div. I-III)
156251 Ice freezing on mineral dust samples 01.03.2015 Project funding (Div. I-III)
117987 Particle Backscatter and Relative Humidity in and around Cirrus Clouds Measured with a Lightweight Balloon Sonde 01.12.2007 Project funding (Div. I-III)

Abstract

The largest single remaining uncertainty in the global radiation budget is the indirect effect of aerosols, through the modification of cloud properties. Results of this project will help reduc-ing the uncertainty in this effect for cirrus clouds. Atmospheric aerosols have been shown to affect the radiative properties of cirrus clouds, through an influence on ice particle number density and size. In addition, the changes in ice particle sizes may possibly affect the sedimentation of ice crystals in the upper troposphere, and subsequently cirrus lifetime and cloudiness. Recent observations support the view that both heterogeneous and homogeneous ice nucleation are important in clouds below 5 km alti-tude. However, to what degree heterogeneous ice nucleation on solid inclusions, so-called ice nuclei (IN), plays a role for higher level cirrus clouds is less clear. Mineral dust particles have been shown to be present close to the tropopause, but only after transport through deep convection which might affect their suitability to act as IN. Conversely, aerosol droplet com-position may impede homogeneous nucleation, which may either suppress ice formation alto-gether (i.e., delay it to lower temperatures) or render heterogeneous nucleation more impor-tant. Given the significant role of cirrus clouds in the Earth’s climate system, their representations in climate models must be tested for sensitivities to changes in aerosol particle number den-sity, size and composition. The main objective of this project is the development of such a representation of ice particle formation and growth for use in climate models, and to provide sensitivity tests on these parameters. Using a hierarchy of models, we will approach this goal in a systematic manner, ranging from a description of the chemical aerosol precursors, via physico-chemical processes affecting heterogeneous and homogeneous ice nucleation, to the resulting number densities, sizes and fall speeds of ice particles. In the first stage of the project we will gather, and where necessary, develop, descriptions of homogeneous and heterogeneous nucleation and ice growth rates in the presence of those compounds currently considered most influential on cirrus properties (organic species, mineral dust, solid ammonium sulphate, soot, biological material). This information will then be used together with an existing microphysical trajectory box model to generate statistical data on the effect of each compound on cirrus properties. Chemical input fields for these experiments will come from the OsloCTM2, a state of the art aerosol - chemistry transport model. Using these modelling tools, sensitivity studies will be carried out and a simplified cirrus model will be developed, which will then be built back into and tested in global models in the last stage of the project, enabling us to calculate the global effect of aerosol composition on cirrus cloud distributions and radiative properties.
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