The value of remote marine aerosol measurements for constraining radiative forcing uncertainty

Abstract

<p><strong>Abstract.</strong> Aerosol measurements over the Southern Ocean are used to constrain aerosol-cloud interaction radiative forcing uncertainty in a global climate model. Aerosol forcing uncertainty is quantified using one million climate model variants that sample the uncertainty in nearly 30 model parameters. Ship-based measurements of cloud condensation nuclei, particle number concentrations and sulfate mass concentrations from the Antarctic Circumnavigation Expedition: Study of Preindustrial-like Aerosols and Their Climate Effects (ACE-SPACE) are used to identify observationally implausible variants and thereby reduce the spread in the simulated forcing. Southern Ocean measurements strongly constrain natural aerosol emissions: default sea spray emissions in the model need to be increased by around a factor of 3 to be consistent with measurements. Aerosol forcing uncertainty is reduced by around 7 % using these measurements, which is comparable to the 8 % reduction achieved using an extensive set of over 9000 predominantly Northern Hemisphere measurements. The radiative forcing due to aerosol–cloud interactions (RF<sub>aci</sub>) is constrained to −2.61 to −1.10 W m<sup>−2</sup> (95 % confidence) and the effective radiative forcing from aerosol-cloud interactions (ERF<sub>aci</sub>) is constrained to −2.43 to −0.54 W m<sup>−2</sup>. When Southern Ocean and Northern Hemisphere measurements are combined, the uncertainty in RF<sub>aci</sub> is reduced by 21 % and the strongest 20 % of forcing values are ruled out as implausible. In this combined constraint the observationally plausible RF<sub>aci</sub> is around 0.17 W m<sup>−2</sup> weaker (less negative) with credible values ranging from −2.51 to −1.17 W m<sup>−2</sup> and from −2.18 to −1.46 W m<sup>−2</sup> when using one standard deviation to quantify the uncertainty. The Southern Ocean and Northern Hemisphere measurement datasets are complementary because they constrain different processes. These results highlight the value of remote marine aerosol measurements.</p>

The hemispheric contrast in cloud microphysical properties constrains aerosol forcing

“…One caveat to our constraint on RF aci and ∆N d(PD-PI) is that our methodology may suffer from the same limitations that all single-observation constraints suffer from, which is producing an overly tight constraint ( 45 ). However, combining this methodology with other observational constraints may avoid these single observation issues [e.g., as in the multiobservation constraint on ERF aci in Johnson et al ( 45 ) and Regayre et al ( 46 )] as well as help to constrain uncertainty associated with other processes in the models not captured by ∆ N d(NH-SH) (e.g., the aerosol optical depth constraint on RF ari presented in ref. 32 ).…”

“…Creation of the emulator assumes trapezoidal priors developed using expert solicitation ( 23 ). This makes the sample members more centralized in the multidimensional parameter space compared to the uniform priors assumed in earlier works ( 42 , 45 , 46 , 94 ). If these uniform priors, which assume the entire range for all parameters are equally likely, are used in our analysis instead, the range of possible RF aci consistent with estimated ∆ N d(NH-SH) is between −1.4 W⋅m −2 and −0.5 W⋅m −2 ( SI Appendix , Fig.…”

The dual-field-of-view polarization lidar technique: A new concept in monitoring aerosol effects in liquid-water clouds – Case studies

“…The Doppler lidar of LACROS showed the occurrence of gravity wave structures with a pronounced updraft period (45 minutes), from about 8:45 to 9:30 UTC on 22 March 2019 (when a cloud layer formed after the cloud-free period) and vertical winds mostly between 0.5 and 1 m s −1 , and partly exceeded 1 m s −1 , so that the water supersaturation was probably clearly higher than 0.5%. At these higher supersaturation conditions the CCN concentration is higher by a factor of about 2 than the value for the 0.2% supersaturation level as discussed in Mamouri and Ansmann (2016) and recently in Regayre et al (2019) for Southern Ocean marine CCN conditions.…”

“…In this model-based study, simulated N CCN values (for a supersaturation of 0.2%) were in the range from 50-80 cm −3 during the late summer season (February and March) and N d showed values from 30-50 cm −3 . According to the recent publication of Regayre et al (2019), N CCN is usually underestimated by a factor of 2 in models focusing on aerosols and clouds in the Southern Ocean. Our lidar-derived N CCN values are in very good agreement with the CCN numbers presented by of usually 100-200 cm −3 .…”

Constraints on global aerosol number concentration, SO<sub>2</sub> and condensation sink in UKESM1 using ATom measurements

“…Even though simulations with the Metzger boundary layer nucleation scheme (Metzger et al, 2010) helped reduce this bias, this nucleation scheme is primarily dependent on the concentration of organic vapors from terrestrial sources, which are low over marine regions. The biases in the boundary layer high latitudes could also be due to uncertainties associated with the sea spray parametrisation in the model (Regayre et al, 2020).…”