Model of optical response of marine aerosols to Forbush decreases

Abstract

Abstract. In order to elucidate the effect of galactic cosmic rays on cloud formation, we investigate the optical response of marine aerosols to Forbush decreases – abrupt decreases in galactic cosmic rays – by means of modeling. We vary the nucleation rate of new aerosols, in a sectional coagulation and condensation model, according to changes in ionization by the Forbush decrease. From the resulting size distribution we then calculate the aerosol optical thickness and Angstrom exponent, for the wavelength pairs 350, 450 nm and 550, 900 nm. For the shorter wavelength pair we observe a change in Angstrom exponent, following the Forbush Decrease, of −6 to +3% in the cases with atmospherically realistic output parameters. For some parameters we also observe a delay in the change of Angstrom exponent, compared to the maximum of the Forbush decrease, which is caused by different sensitivities of the probing wavelengths to changes in aerosol number concentration and size. For the long wavelengths these changes are generally smaller. The types and magnitude of change is investigated for a suite of nucleation rates, condensable gas production rates, and aerosol loss rates. Furthermore we compare the model output with observations of 5 of the largest Forbush decreases after year 2000. For the 350, 450 nm pair we use AERONET data and find a comparable change in signal while the Angstrom Exponent is lower in the model than in the data, due to AERONET being mainly sampled over land. For 550, 900 nm we compare with both AERONET and MODIS and find little to no response in both model and observations. In summary our study shows that the optical properties of aerosols show a distinct response to Forbush Decreases, assuming that the nucleation of fresh aerosols is driven by ions. Shorter wavelengths seem more favorable for observing these effects and great care should be taken when analyzing observations, in order to avoid the signal being drowned out by noise.

Clouds and Aerosols

“…Based on surface aerosol measurements at one site, Kulmala et al (2010) found no connection between the cosmic ray flux and new particle formation or any other aerosol property over a solar cycle (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008), although particles nucleated in the free troposphere are known to contribute to particle number and CCN concentrations in the boundary layer (Merikanto et al, 2009). Our understanding of the 'ion-aerosol clear air' mechanism as a whole relies on a few model investigations that simulate changes in cosmic ray flux over a solar cycle (Pierce and Adams, 2009b;Snow-Kropla et al, 2011;Kazil et al, 2012) or during strong Forbush decreases (Bondo et al, 2010;Snow-Kropla et al, 2011;Dunne et al, 2012). Changes in CCN concentrations due to variations in the cosmic ray flux appear too weak to cause a significant radiative effect because the aerosol system is insensitive to a small change in the nucleation rate in the presence of pre-existing aerosols (see also Section 7.3.2.2).…”

A multivariate study of Forbush decrease simultaneity

“…The Chree method (Chree, 1912(Chree, , 1913 of Superposed Epoch Analysis (SEA) dominates literature documenting the relationships of Fds with geomagnetic disturbance indexes (Kane, 2010), IMF parameters (Pankaj and Shukla, 1994), magnetic clouds (Badruddin et al, 1991;Ananth and Venkatesan, 1993), terrestrial clouds (Pudovkin and Veretenenko, 1995;Svensmark et al, 2009), marine aerosols (Bondo et al, 2010), atmospheric electricity (Marcz, 1997), solar flares (Belov et al, 2008) and CMEs (Pankaj and Singh, 2005). Some researchers (see, for example, Pankaj and Shukla (1994) and Kane (2010)) base the use of SEA on the assumption of global simultaneity of Fds at one or more stations.…”

The response of clouds and aerosols to cosmic ray decreases

“…However, from a modeling point of view, it is uncertain whether a variation in ion‐induced nucleation may translate into an observable change in cloud condensation nuclei (CCN) and thus in clouds. Bondo et al [] suggest that an aerosol effect could be observable under atmospheric conditions while general circulation modeling gives rise to much smaller responses in the CCNs [ Kazil et al , ; Pierce and Adams , ; Snow‐Kropla et al , ; Yu and Luo , ]. Of these studies, Yu and Luo [] find a response almost an order of magnitude larger than previous estimates but still insufficient to explain the large observed variations in the ocean heat content over the solar cycle [ Shaviv , ; Howard et al , ].…”