613 SEMINAR SERIES: Ryan Kramer

Climate & Radiation Laboratory Seminar Series

Wednesday, February 19, 2020

Building 33, Room H114



Ryan Kramer
NPP/USRA and the Climate & Radiation Laboratory

What makes radiative forcing different across climate models and what can observations teach us?

Radiative forcing is the radiative perturbation caused by a change in atmospheric composition from which all other climate changes stem. Although a fundamental metric in climate science, it has not always been clearly defined. In this talk I will draw some clear lines about what radiative forcing is and use these definitions to identify the root causes of why radiative forcing differs across climate models. In particular, I will show that instantaneous radiative forcing is the largest contributor to inter-model spread in total, or effective, radiative forcing. Instantaneous radiative forcing is dependent on the base state and radiative transfer code of the model. I will quantify these two sources of spread and highlight that radiative transfer error in models serves as a low hanging fruit for reducing spread in climate sensitivity overall. 

Observations are a useful place to turn for addressing the dependence of radiative forcing on base state.  Making use of radiative kernels, I will introduce a new technique for extracting the observed radiative forcing from CERES radiative flux measurements and will highlight how this can be used to a) constrain radiative forcing in climate models, b) quantify the sensitivity of radiative forcing to base state and c) offer some insight about the accuracy of our satellite measurements of the base state. Finally, using MODIS Cloud Regimes, I will offer a method for identifying how specific cloud types contribute to these observed radiative flux perturbations.

Seminar Series Coordinators