Effects of Forbush decreases on clouds determined from PATMOS-x

Abstract

This study examines the relationship between cosmic rays and clouds during Forbush decreases (FDs) to understand the cause-effect relationships between cloud microphysics, cloud condensation nuclei (CCN), and ionisation in the atmosphere. The results of a Monte Carlo analysis of cloud parameters during FDs, which were obtained using newly calibrated satellite data (Pathfinder Atmospheres Extended (PATMOS-x)) from 1978 to 2018, show the connections between some cloud parameters and FDs. For context, FD is the event where the number of cosmic rays arriving in the atmosphere decreases and recovers over several days. Other studies have shown that FDs impacted the cloud fraction, aerosol optical depth, CCN, water content, and cloud effective radius (reff) in the atmosphere. Using the Monte Carlo analysis, nine atmospheric parameters from the dataset were evaluated and exhibited a significant response to FDs. Each added FD event (after the first event) reduces the noise, but only the strongest events add a significant signal (exceptionally when the 2nd and 5th rank FD data are added, the signal/noise ration dropped due to a change in the satellite version). We found that cloud fraction shows statistically significant signals following FDs at an achieved significance level of 0.33%. Cloud emissivity also showed highly significant signals from the analysis; however, these cannot be classified as physical causes of FDs since the response starts a week before the FDs. In contrast, the cloud optical depth, integrated total cloud water over the entire column, and reff did not show any significant signals in the frameworks of the applied methods. The top of the atmosphere brightness temperatures (TABTs) at nominal wavelengths of 3.75, 11.0, and 12.0 μm were analysed again along with surface BTs and showed significant signals. The estimated changes in the BT were determined using a radiative transfer model (Fu-Liou model) and showed consistent results with the observed changes in cloud parameters during FD events. Among the analysed several atmospheric/cloud/aerosol parameters, cloud fraction and TABT at nominal wavelengths of 3.75, 11.0, and 12.0 μm are the only parameters depicting a statistically significant and correct-phase response to FDs.