Aerosol effects on clouds are concealed by natural cloud heterogeneity and satellite retrieval errors

Abstract

One major source of uncertainty in the cloud-mediated aerosol forcing arises from the magnitude of the cloud liquid water path (LWP) adjustment to aerosol-cloud interactions, which is poorly constrained by observations. Many of the recent satellite-based studies have observed a decreasing LWP as a function of cloud droplet number concentration (CDNC) as the dominating behavior. Estimating the LWP response to the CDNC changes is a complex task since various confounding factors need to be isolated. However, an important aspect has not been sufficiently considered: the propagation of natural spatial variability and errors in satellite retrievals of cloud optical depth and cloud effective radius to estimates of CDNC and LWP. Here we use satellite and simulated measurements to demonstrate that, because of this propagation, even a positive LWP adjustment is likely to be misinterpreted as negative. This biasing effect therefore leads to an underestimate of the aerosol-cloud-climate cooling and must be properly considered in future studies.

Impacts of Mesoscale Cloud Organization on Aerosol‐Induced Cloud Water Adjustment and Cloud Brightness

“…While strongly suggestive, the empirical analysis of the observational record in this study does not prove causality. For example, the retrieval errors of τ and r e could potentially introduce a significant bias in L 0 (Arola et al., 2022). Future studies are encouraged in this regard.…”

Diurnal Patterns in the Observed Cloud Liquid Water Path Response to Droplet Number Perturbations

“…These corrections adjust the all-sky LWP (including clear and cloudy pixels) over a 1°× 1°area to that which would be observed by that of microwave imagers, that do not suffer scattering biases. Smalley and Lebsock (2023a) found that the overall errors in ABI LWP relative to microwave observations are 2-3 times smaller after applying the corrections, with a rootmean-squared error of 20-30 g m 2 , partially alleviates concerns made by Arola et al (2022) that changes in λ could be misinterpreted due to errors in the satellite retrievals. Overall, they demonstrate that the corrected ABI LWP is able to reproduce the diurnal cycle of LWP observed by the fleet of microwave imagers but with the benefit of the 10-min temporal resolution of ABI.…”

Frontiers in Satellite‐Based Estimates of Cloud‐Mediated Aerosol Forcing

“…The bias due to broken clouds is overcome to a large extent by retrieving the N d of only the cores of the clouds (Zhu et al., 2018). As both N d and LWP retrievals rely on the r e ‐retrieval, even random biases in r e can produce spurious relationships between N d and LWP, giving the appearance of an LWP adjustment (Arola et al., 2022). The use of an independent LWP retrieval (e.g., passive microwave) can reduce the magnitude of this bias (Gryspeerdt et al., 2019), but it may still have an important impact on observation‐based assessments of the ERF aci .…”

“…This may be a reason for LWP increasing with N d for N d < 30 cm −3 , as seen in satellite statistics (e.g., Gryspeerdt et al., 2019). Large uncertainties in such quantifications remain (e.g., Arola et al., 2022). The cloud cover effect : Cloud horizontal extent also adjusts to N d perturbations.…”