Radiative and Chemical Response to Interactive Stratospheric Sulfate Aerosols in Fully Coupled CESM1(WACCM)

Abstract

We present new insights into the evolution and interactions of stratospheric aerosol using an updated version of the Whole Atmosphere Community Climate Model (WACCM). Improved horizontal resolution, dynamics, and chemistry now produce an internally generated quasi‐biennial oscillation and significant improvements to stratospheric temperatures and ozone compared to observations. We present a validation of WACCM column ozone and climate calculations against observations. The prognostic treatment of stratospheric sulfate aerosols accurately represents the evolution of stratospheric aerosol optical depth and perturbations to solar and longwave radiation following the June 1991 eruption of Mount Pinatubo. We confirm the inclusion of interactive OH chemistry as an important factor in the formation and initial distribution of aerosol following large inputs of sulfur dioxide (SO2) to the stratosphere. We calculate that depletion of OH levels within the dense SO2 cloud in the first weeks following the Pinatubo eruption significantly prolonged the average initial e‐folding decay time for SO2 oxidation to 47 days. Previous observational and model studies showing a 30 day decay time have not accounted for the large (30–55%) losses of SO2 on ash and ice within 7–9 days posteruption and have not correctly accounted for OH depletion. We examine the variability of aerosol evolution in free‐running climate simulations due to meteorology, with comparison to simulations nudged with specified dynamics. We assess calculated impacts of volcanic aerosols on ozone loss with comparisons to observations. The completeness of the chemistry, dynamics, and aerosol microphysics in WACCM qualify it for studies of stratospheric sulfate aerosol geoengineering.

CESM1(WACCM) Stratospheric Aerosol Geoengineering Large Ensemble Project

“…Some significant changes in the climate response are identified when using the updated CLM4.5 compared to CLM4.0, including seasonal changes in surface temperatures over the Arctic. A detailed evaluation of the model (using CLM4.0) is provided by Mills et al (2017).…”

“…These experiments require model capabilities beyond those in solar reduction simulations. The stratospheric aerosol distribution resulting from such injections depends on the model's aerosol microphysical scheme, as well as interactions with chemical, dynamical, and radiative processes (Pitari et al 2014;Mills et al 2017). Aerosol size and sedimentation are increased with the injection amount and the efficiency of the sulfates to affect the top of the atmosphere radiative imbalance is reduced (Niemeier and Timmreck 2015;Kleinschmitt et al 2017).…”

“…The RCP8.5 simulations are started in 2010 using atmospheric initial conditions from a CESM1(WACCM) free-running historical simulation, following the RCP8.5 emission scenario (after 2005), as described by Mills et al (2017). Initial conditions for the land and river model are taken from a 10-yr spinup simulation and 2012 of the reference simulation and a round-off (order of 10 -14 K) air temperature perturbation, while the land, sea ice, and ocean start from the same initial conditions for each ensemble member.…”

“…Some significant changes in the climate response are identified when using the updated CLM4.5 compared to CLM4.0, including seasonal changes in surface temperatures over the Arctic. A detailed evaluation of the model (using CLM4.0) is provided by Mills et al (2017).The model compares favorably with present-day observations in terms of its climatology and the variability of the ocean and atmosphere. It produces a reasonably good representation of the QBO and an excellent representation of stratospheric ozone column and water vapor concentrations .…”

Multi-model comparison of the volcanic sulfate deposition from the 1815 eruption of Mt. Tambora

“…Conversely, the models differ in the amount Table 1. Description of models, adapted from Kremser et al (2016) of emitted SO 2 required to achieve good comparisons to observations with the mass of SO 2 emitted by the four models ranging from 10 Tg for UM-UKCA (Dhomse et al, 2014) and CESM1(WACCM) (Mills et al, , 2017 to 12-14 Tg for SOCOL-AER (Sheng et al, 2015b) to 17 Tg for MAECHAM5-HAM (Niemeier et al, 2009;Toohey et al, 2011). For this reason, the use of a common protocol in this study (Sect.…”