Surprising similarities in model and observational aerosol radiative forcing estimates

Abstract

<p><strong>Abstract.</strong> The radiative forcing from aerosols (particularly through their interaction with clouds) remains one of the most uncertain components of the human forcing of the climate. Observation-based studies have typically found a smaller aerosol effective radiative forcing than in model simulations and were given preferential weighting in the IPCC AR5 report. With their own sources of uncertainty, it is not clear that observation-based estimates are more reliable. Understanding the source of the model-observational difference is thus vital to reduce uncertainty in the impact of aerosols on the climate.</p> <p>These reported discrepancies arise from the different decompositions of the aerosol forcing used in model and observational studies. Applying the observational decomposition to global climate model output, the two different lines of evidence are surprisingly similar, with a much better agreement on the magnitude of aerosol impacts on cloud properties. Cloud adjustments remain a significant source of uncertainty, particularly for ice clouds. However, they are consistent with the uncertainty from observation-based methods, with the liquid water path adjustment usually enhancing the Twomey effect by less than 50 %. Depending on different sets of assumptions, this work suggests that model and observation-based estimates could be more equally weighted in future synthesis studies.</p>

Reducing the aerosol forcing uncertainty using observational constraints on warm rain processes

“…This leads to an increased planetary albedo through higher average q l and, thus, ℒ; indirectly, through the relative humidity-dependent cloud cover parameterization (62), f c increases as well. In this model, the ℒ adjustment (F ℒ = − 0.5 W m −2 ) is the greater of the two adjustments compared with the f c adjustment (F fc = − 0.3 W m −2 ) (41), in line with other state-of-the-art models (40,42). The radiative forcing F N d = − 0.5 W m −2 is of similar magnitude to F ℒ .…”

“…S7). First, in observations, the challenges in classifying precipitation intensity arise because of radar sensitivity to drizzle and because large contributions to F ℒ occur not only over ocean but also over land (40)(41)(42), where the heterogeneous surface properties substantially complicate intensity retrievals. Second, as specified earlier, longstanding structural problems cause the precipitation intensity to be biased low in models.…”

Bounding Global Aerosol Radiative Forcing of Climate Change

“…In any case, the size of the ‐ N d sensitivity component driven by precipitation suppression is still uncertain. Despite this, GCMs rarely produce an enhancement of the RFaci larger than 50% due to changes in (Gryspeerdt, Mülmenstädt, et al, ).…”

Separating radiative forcing by aerosol–cloud interactions and rapid cloud adjustments in the ECHAM–HAMMOZ aerosol–climate model using the method of partial radiative perturbations

“…Other adjustments, e.g., due to rapid changes in land surface temperatures or atmospheric temperature and humidity profiles, have been estimated as small in a previous study (Heyn et al, 2017). Each of these terms maps fairly well onto a parameterization in the GCM: RF ari is parameterized in the radiative transfer, F N d is parameterized in a droplet activation scheme, the ACI part of F L is parameterized in the precipitation microphysics (and, if enhanced evaporation becomes tractable in the future, that component will presumably be parameterized in the turbulence scheme; e.g., Guo et al, 2011;Neubauer et al, 2014), and F f c is parameterized in the cloud cover scheme (although in our model the response to the perturbation is indirect, via relative humidity changes subsequent to precipitation rate changes). The only component that emerges from the model dynamics rather than from an explicit parameterization is the adjustments of temperature and moisture profiles that entail further adjustments to aerosol-cloud interactions and to aerosol-radiation interactions (formerly known as "semi-direct effect"); both of these terms are small (Heyn et al, 2017;Stjern et al, 2017).…”