Atmospheric ionization and cloud radiative forcing

Abstract

Atmospheric ionization produced by cosmic rays has been suspected to influence aerosols and clouds, but its actual importance has been questioned. If changes in atmospheric ionization have a substantial impact on clouds, one would expect to observe significant responses in Earth's energy budget. Here it is shown that the average of the five strongest week-long decreases in atmospheric ionization coincides with changes in the average net radiative balance of 1.7 W/m[Formula: see text] (median value: 1.2 W/m[Formula: see text]) using CERES satellite observations. Simultaneous satellite observations of clouds show that these variations are mainly caused by changes in the short-wave radiation of low liquid clouds along with small changes in the long-wave radiation, and are almost exclusively located over the pristine areas of the oceans. These observed radiation and cloud changes are consistent with a link in which atmospheric ionization modulates aerosol's formation and growth, which survive to cloud condensation nuclei and ultimately affect cloud formation and thereby temporarily the radiative balance of Earth.

Figure 1

The top of atmosphere radiative balance (relative to the mean of the time series, left column) from CERES data superposed for the five strongest FDs since 2000. The panel (a) is the change in net radiative forcing for three situations: geographical areas with predominantly, (1) liquid clouds (red curve), (2) areas with high clouds (cyan curve), (3) and global averages (purple curve). The gray area signifies the period used to integrate the response (day 0–10). The blue curve is the variation in cosmic rays as measured by the Oulu neutron monitor. Right-hand columns are bootstrap distribution functions of the integrated signal. The red, cyan, and purple lines denote the significance of the FD signal obtained from the three distribution functions. Panel (b) left-hand side shows changes in TOA short wave radiation for areas with liquid clouds (red curve), high clouds (cyan curve), and globally (purple curve). Results of the Fu-Liou radiative transfer model of changes in TOA SW radiative forcing obtained using the observed changes in MODIS cloud parameters are described by the gray curve. The right-hand panel (b) shows the achieved significance of the FD signals. Finally, panel (c) is the change in TOA LW radiative forcing similarly as for SW in panel (b).

Figure 2

The response of CERES short-wave radiation as a function of FD strength. The left panel shows the most substantial deviation between day 0 and day 10 in short wave radiation globaly averaged for each Forbush decrease as a function of the FD strength (dots). The black line is a linear fit to the 13 data points and the dashed lines and dotted lines represent two and four standard deviations of the fit, respectively. The dash-dotted line is the mean of the most substantial deviation between day 0 and day 10 using 10${}^4$ Monte Carlo simulations with random FD dates. The black circles are the 13 FDs. The two panels to the right display the distribution functions of the slope and intercept of the linear fit, based on 10${}^4$ Monte Carlo simulations using CERES SW data with random dates for the FD. Note that the significance of the above linear slope is $\sim$3$\sigma$, and that the Monte Carlo simulations using random dates for the FD events result in an average zero slope as expected.

Figure 3

Maps depicting changes in clouds and TOA radiative balance as a function of latitude and time (centered on the FD minimum). The maps are superposed data from the five strongest FDs after 2000. (a) The left panel is based on liquid cloud fraction observations from MODIS in (%). The right panel in all rows displays the change in cosmic rays during the same period (black curve). (b) Left panel is ice-cloud-fraction observations from MODIS in (%). (c) Left panel is TOA CERES NET radiation in (W/m${}^2$). (d) Left panel is TOA CERES SW radiation in (W/m${}^2$). (e) Shows the LW similar to the SW in (d).

Figure 4

The left-hand column represents variations in the average radiative state before the FD (for details see Methods) averaged over the first nine days before the FD minimum for the NET, SW, and LW, respectively, and the right-hand column is the variations in the average radiative state after the FD averaged over nine days after the FD starting on day 3. Top panels are NET radiation. Middle panels are for SW radiation, and finally, the bottom panel is LW radiation.

Figure 5

Same figure as Fig. 4 but without the strong FD2 event. Note that the overall patterns are similar to Fig. 4 but as expected a little weaker.

Figure 6

Global maps of three cloud types based on MODIS observations. The top panel is regions with liquid clouds (mainly the green regions). The middle panel is high clouds. The bottom panel is the ice-cloud-fraction. The gray contour lines on all three maps show threshold cloud fraction 0.25 used for the cloud masks. The fractional area of Earth with cloud types exceeding the threshold are: a (Liquid clouds) = 0.52, a (High clouds) = 0.44 and, a (Ice clouds) = 0.40. Superposed on all three maps are the (orange hatched) regions with large responses found in CERES TOA net radiation following the minimum in cosmic rays (black contour lines are 6 W/m${}^2$). Note that these regions are almost exclusively confined to areas dominated by liquid clouds.