Cloud droplet size distribution and optical properties only weakly linked to aerosol size

Abstract

Changes in aerosol concentrations can modify cloud brightness, producing a strong but poorly constrained influence on Earth's energy balance. Because cloud reflectivity depends on the size distribution of cloud droplets, and aerosol size strongly governs activation into droplets, one might expect cloud properties to be sensitive to aerosol size distributions. Here we show, through a combination of cloud chamber experiments and high-resolution simulations, that cloud microphysical and optical properties are often insensitive to aerosol size. Detectable impacts on cloud optical properties occur only under weak convective forcing and high aerosol concentrations. These results indicate that, in most conditions, cloud reflectivity can be predicted from aerosol number alone without detailed knowledge of aerosol size distributions, providing new constraints on how aerosol perturbations affect climate.

Figures (2)

• Figure 1: Cloud droplet size distributions and supersaturation fields are generally insensitive to aerosol size, except under polluted, weakly forced conditions. For each experiment (rows), the (a) observed and (b) simulated size distributions within the Pi Chamber domain are only weakly sensitive to the size of injected particles (colors) under most conditions, and the observations and simulations for each experiment are broadly consistent for each experiment (comparison between columns a and b). This weak sensitivity in the droplet size distribution to aerosol size corresponds to a (c) weak sensitivity in the simulated water vFigureapor supersaturation. Only under conditions of weak forcing and high aerosol loadings (bottom panel) did we find significant variation in the size distribution or supersaturation with aerosol size. Under these conditions of weak forcing and high aerosol concentrations, the LES reveals high concentrations of small particles that are below the detection limit of the Welas (shown by grey shading in column b). Box plots (c) shows median (vertical line), inner-quartile-range (IQR) (boxes), and ranges of 1.5 IQR (whiskers); supersaturation is shown on a symmetric logarithmic log scale, with values below zero shown by the grey shading in column c.
• Figure 2: Cloud optical properties show a strong dependence on aerosol size only under polluted, weakly forced regimes, consistent across experiments and LES. The weak response in the droplet size distribution to aerosol size shown in Fig. \ref{['fig:size_distribution']} leads to a similarly weak sensitivity in the (a) observed extinction coefficient, $\beta_{\text{ext}}$, except under conditions of weak forcing and high aerosol loadings. To quantify how the environmental regime impacts the response in cloud optical properties to aerosol size, we preformed LES across a wide range of chamber conditions and quantified the relative response in extinction to a relative change in aerosol size, $d\ln\beta_{\text{ext}}/d\ln D_{\text{a}}$, shown in panel (b); predictions were interpolated from the LES output using a Gaussian Process Regressor with the Matern kernel for moderate smoothness. Convective forcing in the Pi Chamber is controlled by the temperature difference ($\Delta T$) between the chamber's floor and ceiling. Observations of $d\ln\beta_{\text{ext}}/d\ln D_{\text{a}}$ agree well with model predictions, as shown by markers in (b), where edge colors indicate strong vs. weak forcing and marker shape indicates different injection rates. The spread in cloud droplet effective radius $r_{\text{eff}}$ and $\beta_{\text{ext}}$ with aerosol size is shown by the shading in panel (c), further illustrating that cloud microphysical responses to aerosol size depend on environmental and pollution regime; model predictions shown by the lines and shading are broadly consistent with observations (markers).