Abstract
Abstract. Here we describe an updated parameterization for prescribing stratospheric aerosol in the National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM1). The need for a new parameterization is motivated by the poor response of the CESM1 (formerly referred to as the Community Climate System Model, version 4, CCSM4) simulations contributed to the Coupled Model Intercomparison Project 5 (CMIP5) to colossal volcanic perturbations to the stratospheric aerosol layer (such as the 1991 Pinatubo eruption or the 1883 Krakatau eruption) in comparison to observations. In particular, the scheme used in the CMIP5 simulations by CESM1 simulated a global mean surface temperature decrease that was inconsistent with the GISS Surface Temperature Analysis (GISTEMP), NOAA's National Climatic Data Center, and the Hadley Centre of the UK Met Office (HADCRUT4). The new parameterization takes advantage of recent improvements in historical stratospheric aerosol databases to allow for variations in both the mass loading and size of the prescribed aerosol. An ensemble of simulations utilizing the old and new schemes shows CESM1's improved response to the 1991 Pinatubo eruption. Most significantly, the new scheme more accurately simulates the temperature response of the stratosphere due to local aerosol heating. Results also indicate that the new scheme decreases the global mean temperature response to the 1991 Pinatubo eruption by half of the observed temperature change, and modelled climate variability precludes statements as to the significance of this change.
Highlights
Volcanic perturbations to the stratospheric aerosol layer are an often ill-represented forcing in the climate model simulations (Solomon et al, 2011; Driscoll et al, 2012; Knutson et al, 2013; Zanchettin et al, 2015; Kremser et al, 2016)
Neale et al (2010) describe the scheme used to specify volcanic eruptions and the stratospheric aerosol layer in CCSM4 and how this interacts with the other parameterizations of the model
We summarize the main features of the volcanic prescription in CCSM4/CESM1(CAM4) that have been changed significantly in the updated scheme described below so that future studies utilizing CESM1 may account for changes in the model’s behaviour compared to simulations conducted for comparison Project 5 (CMIP5)
Summary
Volcanic perturbations to the stratospheric aerosol layer are an often ill-represented forcing in the climate model simulations (Solomon et al, 2011; Driscoll et al, 2012; Knutson et al, 2013; Zanchettin et al, 2015; Kremser et al, 2016). In CCSM4/CESM1(CAM4) the stratospheric aerosol mass is interpreted by the radiative transfer code via the predefined mass-specific extinctions, single scattering albedos, and asymmetry parameters These parameters are calculated using the constants defined above and are stored in lookup tables for the short-wave and long-wave radiative transfer schemes separately (the table has a single dimension that varies by spectral band) for use by each of the spectral bands in the Community Atmosphere Model Radiative Transfer (CAMRT) parameterization. To CCSM4/CESM1(CAM4), the mass from the Ammann et al (2003) data set is assumed to be comprised of 75 % sulfuric acid and 25 % water and have a constant log-normal size distribution with a wet effective radius of 0.426 μm and a standard deviation (σ (ln r)) of 1.8. The exact same optical property file for CCSM4/CESM1(CAM4) is by CESM1(CAM4-chem) and CESM1(WACCM4) and, again, is entitled “sulfuricacid_cam3_c080918.nc” and may be found in the CESM input data repository at “/glade/p/cesm/cseg/inputdata/atm/cam/physprops/”. In CAMRT in CESM1(WACCM4) and CESM1(CAM4chem) is the same as in CCSM4/CESM1(CAM4)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
