Using a multiphysics ensemble for exploring diversity in cloud-shortwave feedback in GCMs

Abstract

This study proposes a systematic approach to investigate cloud-radiative feedbacks to climate change induced by an increase of CO2 concentrations in global climatemodels (GCMs). Based on two versions of theModel for Interdisciplinary Research on Climate (MIROC), which have opposite signs for cloud-shortwave feedback (DSWcld) and hence different equilibrium climate sensitivities (ECSs), hybrid models are constructed by replacing one or more parameterization schemes for cumulus convection, cloud, and turbulence between them. An ensemble of climate change simulations using a suite of eightmodels, called amultiphysics ensemble(MPE), is generated. TheMPE provides a range of ECS as wide as the CoupledModel Intercomparison Project phase 3 (CMIP3) multimodel ensemble and reveals a different magnitude and sign of DSWcld over the tropics, which is crucial for determining ECS. It is found that no single process controls DSWcld, but that the coupling of two processes does. Namely, changing the cloud and turbulence schemes greatly alters the mean and the response of low clouds, whereas replacing the convection and cloud schemes affects low and middle clouds over the convective region. For each of the circulation regimes, DSWcld and cloud changes in theMPE have a nonlinear, but systematic, relationship with the mean cloud amount, which can be constrained from satellite estimates. The analysis suggests a positive feedback over the subsidence regime and a near-neutral or weak negative DSWcld over the convective regime in these model configurations, which, however, may not be carried into other models. © 2012 American Meteorological Society.