Using Model Nudging to Isolate Rapid Adjustment Mechanisms Caused by Anthropogenic Short-lived Pollutants

Abstract:

Here we present results from using a novel technique of model nudging to isolate different rapid adjustment mechanisms in response to anthropogenic short-lived climate pollutants in an earth system model. Rapid adjustments are changes in climate variables (e.g. cloud fraction, specific humidity) in response to a climate forcing agent (e.g. aerosol emission changes) that cause a further radiative forcing alongside the instantaneous radiative forcing of the forcing agent itself. The nudging techniques employed can separate out adjustments resulting from changes in circulation, tropospheric temperatures, and stratospheric temperatures. This complements existing techniques for quantifying radiative forcing caused by rapid adjustments, such as diagnostic radiation calls in models and radiative kernels, which typically separate adjustments in different climate variables, but do not separate the contributions of different mechanisms to those adjustments. We investigate the effectiveness of these model nudging techniques for separating out the rapid adjustment mechanisms resulting from anthropogenic aerosol emissions.

We conduct global perturbation experiments with the UK Earth System Model (UKESM1). Several pairs of simulations are conducted, each pair including a control simulation with conditions representing 2014, and a perturbed simulation with the emission rate of the species of interest representing 1850, but otherwise identical conditions to the control. Horizontal winds, tropospheric temperature and stratospheric temperature are nudged progressively across the simulation pairs, causing adjustment mechanisms to be progressively suppressed across the perturbed simulations. For example, nudging horizontal winds in the troposphere should suppress any adjustments in circulation, and hence, suppress any cloud adjustments that would result from the circulation changes.

Here we present the differences in radiative forcing and adjustments to climate variables, such as cloud fraction, across the simulations. We also consider the sensitivity of the results to the nudging setup, such as the nudging strength and model levels over which nudging is present, and how effectively the nudging suppresses adjustment mechanisms.