Global observations of aerosol-cloud-precipitation-climate interactions

Abstract

Cloud drop condensation nuclei (CCN) and ice nuclei (IN) particles determine to a large extent cloud microstructure and, consequently, cloud albedo and the dynamic response of clouds to aerosol-induced changes to precipitation. This can modify the reflected solar radiation and the thermal radiation emitted to space. Measurements of tropospheric CCN and IN over large areas have not been possible and can be only roughly approximated from satellite-sensor-based estimates of optical properties of aerosols. Our lack of ability to measure both CCN and cloud updrafts precludes disentangling the effects of meteorology from those of aerosols and represents the largest component in our uncertainty in anthropogenic climate forcing. Ways to improve the retrieval accuracy include multiangle and multipolarimetric passive measurements of the optical signal and multispectral lidar polarimetric measurements. Indirect methods include proxies of trace gases, as retrieved by hyperspectral sensors. Perhaps the most promising emerging direction is retrieving the CCN properties by simultaneously retrieving convective cloud drop number concentrations and updraft speeds, which amounts to using clouds as natural CCN chambers. These satellite observations have to be constrained by in situ observations of aerosol-cloud-precipitation-climate (ACPC) interactions, which in turn constrain a hierarchy of model simulations of ACPC. Since the essence of a general circulation model is an accurate quantification of the energy and mass fluxes in all forms between the surface, atmosphere and outer space, a route to progress is proposed here in the form of a series of box flux closure experiments in the various climate regimes. A roadmap is provided for quantifying the ACPC interactions and thereby reducing the uncertainty in anthropogenic climate forcing.

What do we learn from long-term cloud condensation nuclei number concentration, particle number size distribution, and chemical composition measurements at regionally representative observatories?

“…Cloud droplets are formed by activation of a subset of aerosol particles called cloud condensation nuclei (CCN), which affect the radiative properties of clouds through modifying the cloud droplet number concentrations (CDNC), cloud droplet size, 5 cloud lifetime and precipitation processes (Rosenfeld et al, 2014). To date, radiative forcing through aerosol-cloud interactions constitutes the least understood anthropogenic influence on climate (IPCC, 2013): the uncertainty in aerosolinduced radiative forcing of ± 0.70 W m -2 (from a mean of -0.55 W m -2 ) is twice the uncertainty for CO 2 (± 0.35, mean +1.68 W m -2 ).…”

“…Reutter et al (2009) found that cloud droplet formation can either be limited by the presence of CCN (CCN-limited regime), by the updraft velocity (updraftlimited regime), or both (transition regime). Globally, however, the CCN-limited regime prevails (Rosenfeld et al, 2014). 15 Among the main factors driving the uncertainty in simulating CCN abundance are the aerosol particle number size distributions, size-dependent removal processes, the contribution of boundary layer new particle formation events to particle number concentration and their size, the particle number size distribution of emitted primary particles, the particle activation diameter, the formation of biogenic and anthropogenic secondary organic aerosol (SOA) as well as the processing of SO 2 in clouds into particulate sulfate (e.g., Croft et al, 2009;Lee et al, 2013;Wilcox et al, 2015).…”

“…To improve model performance, data from measurements of particle number size distribution, CCN number concentrations, aerosol particle chemical composition and hygroscopicity are needed (Carslaw et al, 2013;Ghan and Schwartz, 2007;Rosenfeld et al, 2014;Seinfeld et al, 2016). Satellite observations, covering large scales and longtime horizons, can provide proxies of these variables.…”

Relative influence of meteorological conditions and aerosols on the lifetime of mesoscale convective systems

“…Although these aerosol effects have been seen in observations from field campaigns (9, 10), they have been undetectable on large spatial and multiyear scales. Rosenfeld et al (11) have attributed this lack of detectability on the large scale of aerosol invigoration of convection to its variation with meteorological conditions and to the lack of knowledge of the relative humidity (RH) outside the clouds.…”

“…The variation of aerosol effects on convection with meteorological parameters has been studied previously (11). For example, model simulation has shown that an increase in aerosol concentrations up to an optimal level can invigorate the MCSs under weak vertical wind shear (VWS) and higher RH but suppress the MCSs under strong VWS in a dry environment (12,13).…”