Abstract
Abstract. Climate models continue to exhibit strong sensitivity to the representation of aerosol effects on cloud reflectance and cloud amount. This paper evaluates a proposed method to constrain modeled cloud liquid water path (LWP) adjustments in response to changes in aerosol concentration Na using observations of precipitation susceptibility. Recent climate modeling has suggested a linear relationship between relative LWP responses to relative changes in Na, i.e., dln LWP / dln Na, and the precipitation frequency susceptibility Spop, which is defined as the relative change in the probability of precipitation for a relative change in Na. Using large-eddy simulations (LES) of marine stratocumulus and trade wind cumulus clouds, we show that these two cloud regimes exhibit qualitatively different relationships between λ and Spop; in stratocumulus clouds, λ increases with Spop, while in trade wind cumulus, λ decreases with Spop. The LES-derived relationship for marine stratocumulus is qualitatively similar but quantitatively different than that derived from climate model simulations of oceanic clouds aggregated over much larger spatial scales. We explore possible reasons for variability in these relationships, including the selected precipitation threshold and the various definitions of precipitation susceptibility that are currently in use. Because aerosol–cloud–precipitation interactions are inherently small-scale processes, we recommend that when deriving the relationship between λ and Spop, careful attention be given to the cloud regime, the scale, and the extent of aggregation of the model output or the observed data.
Highlights
Like its predecessors, the IPCC Fifth Assessment Report (AR5; IPCC 2013) continues to point to aerosol effects on clouds as a major source of uncertainty in our predictive climate-modeling capability
Recognizing that cloud systems constantly adjust to aerosol perturbations, AR5 chose to combine both cloud albedo and liquid water path (LWP) responses to aerosol changes into one term, i.e., the effective radiative forcing associated with aerosol–cloud interactions (ERFaci)
Given the difficulty in observationally constraining the LWP response to an increase in aerosol loading λ, Wang et al (2012) explored the relationship between λ and the precipitation frequency susceptibility Spop based on a set of climate model simulations
Summary
The IPCC Fifth Assessment Report (AR5; IPCC 2013) continues to point to aerosol effects on clouds as a major source of uncertainty in our predictive climate-modeling capability. Efforts (e.g., Quaas et al, 2006, 2009) used satellite-based measurements of drop concentration (or size) responses to changes in aerosol (Bréon et al, 2002) to constrain the albedo effect (Twomey, 1977). More detailed analysis using surface-based remote sensing and proxy data from cloud-resolving models pointed to the scale dependence of these relationships (McComiskey and Feingold, 2008, 2012) and called for a clear distinction between the cloud process scale and the satellite data aggregation scale before such observational constraints are applied
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