Abstract
Abstract. In this work we present downscaling experiments with the Weather Research and Forecasting model (WRF) to test the sensitivity to resolving aerosol–radiation and aerosol–cloud interactions on simulated regional climate for the EURO-CORDEX domain. The sensitivities mainly focus on the aerosol–radiation interactions (direct and semi-direct effects) with four different aerosol optical depth datasets (Tegen, MAC-v1, MACC, GOCART) being used and changes to the aerosol absorptivity (single scattering albedo) being examined. Moreover, part of the sensitivities also investigates aerosol–cloud interactions (indirect effect). Simulations have a resolution of 0.44∘ and are forced by the ERA-Interim reanalysis. A basic evaluation is performed in the context of seasonal-mean comparisons to ground-based (E-OBS) and satellite-based (CM SAF SARAH, CLARA) benchmark observational datasets. The impact of aerosols is calculated by comparing it against a simulation that has no aerosol effects. The implementation of aerosol–radiation interactions reduces the direct component of the incoming surface solar radiation by 20 %–30 % in all seasons, due to enhanced aerosol scattering and absorption. Moreover the aerosol–radiation interactions increase the diffuse component of surface solar radiation in both summer (30 %–40 %) and winter (5 %–8 %), whereas the overall downward solar radiation at the surface is attenuated by 3 %–8 %. The resulting aerosol radiative effect is negative and is comprised of the net effect from the combination of the highly negative direct aerosol effect (−17 to −5 W m−2) and the small positive changes in the cloud radiative effect (+5 W m−2), attributed to the semi-direct effect. The aerosol radiative effect is also stronger in summer (−12 W m−2) than in winter (−2 W m−2). We also show that modelling aerosol–radiation and aerosol–cloud interactions can lead to small changes in cloudiness, mainly regarding low-level clouds, and circulation anomalies in the lower and mid-troposphere, which in some cases, mainly close to the Black Sea in autumn, can be of statistical significance. Precipitation is not affected in a consistent pattern throughout the year by the aerosol implementation, and changes do not exceed ±5 % except for the case of unrealistically absorbing aerosol. Temperature, on the other hand, systematically decreases by −0.1 to −0.5 ∘C due to aerosol–radiation interactions with regional changes that can be up to −1.5 ∘C.
Highlights
Aerosols play an important role in the Earth’s climate system due to their substantial effects on the radiation budget and cloud properties (Ramanathan et al, 2001)
We have explored the sensitivity of resolving aerosol interactions within downscaling regional climate model experiments over Europe
The different experiments and configurations applied in our model simulations allow for (i) the quantification of the direct and semi-direct aerosol effect over Europe and (ii) the assessment of the impact of aerosol parameterisation (AOD, asymmetry factor (ASY), single scattering albedo (SSA)) and type on regional climate
Summary
Aerosols play an important role in the Earth’s climate system due to their substantial effects on the radiation budget and cloud properties (Ramanathan et al, 2001). Similar results were documented by Jimenez et al (2016) by implementing aerosol–cloud–radiation feedbacks into WRF with the use of the new Thompson aerosol–cloud interacting (aerosol-aware) cloud microphysics scheme (Thompson and Eidhammer, 2014) that is computationally inexpensive enough to support operational weather and solar forecasting. Da Silva et al (2018) used this aerosol–cloud interacting cloud microphysics scheme in WRF to estimate the aerosol indirect effect and its impact on summer precipitation over the Euro-Mediterranean region, concluding that higher aerosol loads lead to decreased precipitation amounts. 3.2) and examine the impact of aerosol–radiation interactions on the European climate, including different aerosol parameterisations and model configurations as well as aerosol climatologies We examine various radiation components, which are commonly not examined in RCM simulations (total, clear sky, direct and diffuse radiation), clouds, temperature and precipitation
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