Abstract

Abstract. Clouds and aerosols contribute the largest uncertainty to current estimates and interpretations of the Earth’s changing energy budget. Here we use a new-generation large-domain large-eddy model, ICON-LEM (ICOsahedral Non-hydrostatic Large Eddy Model), to simulate the response of clouds to realistic anthropogenic perturbations in aerosols serving as cloud condensation nuclei (CCN). The novelty compared to previous studies is that (i) the LEM is run in weather prediction mode and with fully interactive land surface over a large domain and (ii) a large range of data from various sources are used for the detection and attribution. The aerosol perturbation was chosen as peak-aerosol conditions over Europe in 1985, with more than fivefold more sulfate than in 2013. Observational data from various satellite and ground-based remote sensing instruments are used, aiming at the detection and attribution of this response. The simulation was run for a selected day (2 May 2013) in which a large variety of cloud regimes was present over the selected domain of central Europe. It is first demonstrated that the aerosol fields used in the model are consistent with corresponding satellite aerosol optical depth retrievals for both 1985 (perturbed) and 2013 (reference) conditions. In comparison to retrievals from ground-based lidar for 2013, CCN profiles for the reference conditions were consistent with the observations, while the ones for the 1985 conditions were not. Similarly, the detection and attribution process was successful for droplet number concentrations: the ones simulated for the 2013 conditions were consistent with satellite as well as new ground-based lidar retrievals, while the ones for the 1985 conditions were outside the observational range. For other cloud quantities, including cloud fraction, liquid water path, cloud base altitude and cloud lifetime, the aerosol response was small compared to their natural variability. Also, large uncertainties in satellite and ground-based observations make the detection and attribution difficult for these quantities. An exception to this is the fact that at a large liquid water path value (LWP > 200 g m−2), the control simulation matches the observations, while the perturbed one shows an LWP which is too large. The model simulations allowed for quantifying the radiative forcing due to aerosol–cloud interactions, as well as the adjustments to this forcing. The latter were small compared to the variability and showed overall a small positive radiative effect. The overall effective radiative forcing (ERF) due to aerosol–cloud interactions (ERFaci) in the simulation was dominated thus by the Twomey effect and yielded for this day, region and aerosol perturbation −2.6 W m−2. Using general circulation models to scale this to a global-mean present-day vs. pre-industrial ERFaci yields a global ERFaci of −0.8 W m−2.

Highlights

  • According to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), clouds and aerosols are the largest contributors to uncertainty estimations and interpretations of the Earth’s changing energy budget (Boucher et al, 2013)

  • Note that since Advanced Very High Resolution Radiometer (AVHRR) retrieves aerosol optical depth (AOD) only in cloudless cases over sea, AOD is only available over the North Sea and Baltic Sea region for a small fraction of the time

  • Since not very many data points go into the average from the satellite, the distribution is more noisy compared to the simulation output that is available anytime

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Summary

Introduction

According to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), clouds and aerosols are the largest contributors to uncertainty estimations and interpretations of the Earth’s changing energy budget (Boucher et al, 2013). Cloud adjustments to aerosol–cloud interactions are manifested as changes in horizontal (cloud fraction) and vertical extent (manifested as liquid water path; LWP) of cloudiness, with consequent impact on the Earth’s radiation budget and, climate. One hypothesis for a subsequent adjustment is that the precipitation rates are reduced, implying that the cloud lifetime increases (Albrecht, 1989), and LWP and cloud fraction increase. At the same time, other adjustment processes, such as responses of the cloud mixing and evaporation (Ackerman et al, 2004), occur that partly act in the opposite direction (Stevens and Feingold, 2009; Mülmenstädt and Feingold, 2018; Gryspeerdt et al, 2019). Because different effects can compensate each other (Stevens and Feingold, 2009; Mülmenstädt and Feingold, 2018), it is difficult to observe isolated cloud effects

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