Determining aerosol radiative adjustments from UKESM1 with Partial Radiative Perturbation

Radiative adjustments are additional contributions to instantaneous radiative forcing. They have the potential to strongly enhance the initial forcing, for example in the case of aerosol interactions with clouds. We investigate aerosol radiative adjustments in an Earth System model using an offline partial radiative perturbation (PRP) technique.

Radiative adjustments occur via many mechanisms. To understand them requires a variety of modelling techniques to separate individual adjustments. In PRP radiatively important variables simulated by the online model are input to an offline radiative transfer code to calculate the radiative effects of their adjustments. We apply the PRP method to adjustments arising from anthropogenic sulphate and black carbon industrial-era emission perturbations simulated by the UK Earth System Model 1 (UKESM1) using its offline radiative transfer code (SOCRATES) with settings closely matching the online simulations.

This method reduces errors introduced when using PRP with, or radiative kernels generated from, different settings or radiative transfer models to that used in the online climate model. We assess radiative adjustments arising from several factors, including cloud fields, and compare with their adjustments in the online simulations.