Low cloud properties influenced by cosmic rays

The influence of solar variability on climate is currently uncertain. Recent observations have indicated a possible mechanism via the influence of solar modulated cosmic rays on global cloud cover. Surprisingly the influence of solar variability is strongest in low clouds (<= 3km), which points to a microphysical mechanism involving aerosol formation that is enhanced by ionisation due to cosmic rays. If confirmed it suggests that the average state of the Heliosphere is important for climate on Earth.Comment: 10 pages, 2 figure

The response of clouds and aerosols to cosmic ray decreases

A method is developed to rank Forbush decreases (FDs) in the galactic cosmic ray radiation according to their expected impact on the ionization of the lower atmosphere. Then a Monte Carlo bootstrap-based statistical test is formulated to estimate the significance of the apparent response in physical and microphysical cloud parameters to FDs. The test is subsequently applied to one ground-based and three satellite-based data sets. Responses (&gt;95%) to FDs are found in the following parameters of the analyzed data sets. AERONET: &#xC5;ngstr&#xF6;m exponent (cloud condensation nuclei changes), SSM/I: liquid water content, International Satellite Cloud Climate Project (ISCCP): total, high, and middle, IR-detected clouds over the oceans, Moderate Resolution Imaging Spectroradiometer (MODIS): cloud effective emissivity, cloud optical thickness, liquid water, cloud fraction, liquid water path, and liquid cloud effective radius. Moreover, the responses in MODIS are found to correlate positively with the strength of the FDs, and the signs and magnitudes of the responses agree with model-based expectations. The effect is mainly seen in liquid clouds. An impact through changes in UV-driven photo chemistry is shown to be negligible and an impact via UV absorption in the stratosphere is found to have no effect on clouds. The total solar irradiance has a relative decrease in connection with FDs of the order of 10&#x2212;3, which is too small to have a thermodynamic impact on timescales of a few&#xA0;days. The results demonstrate that there is a real influence of FDs on clouds probably through ions.</p

The Ion and Charged Aerosol Growth Enhancement (ION-CAGE) code: A numerical model for the growth of charged and neutral aerosols

The presence of small ions influences the growth dynamics of a size distribution of aerosols. Specifically the often neglected mass of small ions influences the aerosol growth rate, which may be important for terrestrial cloud formation. To this end, we develop a numerical model to calculate the growth of a species of aerosols in the presence of charge, which explicitly includes terms for ion-condensation. It is shown that a positive contribution to aerosol growth rate is obtained by increasing the ion-pair concentration through this effect, consistent with recent experimental findings. The ion-condensation effect is then compared to aerosol growth from charged aerosol coagulation, which is seen to be independent of ion-pair concentration. The model source code is made available through a public repository.Comment: 29 pages, 12 figure

The role of atmospheric ions in aerosol nucleation:a review

Atmospheric aerosols affect climate and yet the reason for many observed events of new aerosol formation is not understood. One of the theories put forward to explain these events is that the presence of ions can enhance the formation of aerosols. The theory is called Ion Induced Nucleation and in this paper the state of observations, theory and experiments within the field will be reviewed. While evidence for Ion Induced Nucleation is accumulating the exact mechanism is still not known and more research is required to understand and quantify the effect