Regional radiative impact of volcanic aerosol from the 2009 eruption of Redoubt volcano
Authors: Young C. L. , Sokolik I. N. , Dufek J. 
Published: September 2011

Abstract. High northern latitude eruptions have the potential to release volcanic aerosol into the Arctic environment, perturbing the Arctic's climate system. In this study, we present assessments of shortwave (SW), longwave (LW) and net direct aerosol radiative forcings (DARFs) and atmospheric heating/cooling rates caused by volcanic aerosol from the 2009 eruption of Redoubt Volcano by performing radiative transfer modeling constrained by NASA A-Train satellite data. The Ozone Monitoring Instrument (OMI), the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model for volcanic ash were used to characterize aerosol across the region. A representative range of aerosol optical depths (AODs) at 550 nm were obtained from MODIS, and the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) was used to determine the altitude and thickness of the plumes. The optical properties of volcanic aerosol were calculated using a compositionally resolved microphysical model developed for both ash and sulfates. Two compositions of volcanic aerosol were considered in order to examine a fresh, ash rich plume and an older, ash poor plume. Optical models were incorporated into a modified version of the Santa Barbara Disort Atmospheric Radiative Transfer (SBDART) model. Radiative transfer calculations were made for a range of surface albedos and solar zenith angles (SZA) representative of the region. We find that the total DARF caused by a fresh, thin plume (~2.5–7 km) at an AOD (550 nm) range of 0.16–0.58 and SZA = 55° is –46 W m−2AOD−1 at the top of the atmosphere (TOA), 110 W m−2AOD−1 in the aerosol layer, and – 150 W m−2AOD−1 at the surface over seawater. However, the total DARF for the same plume over snow and at the same SZA at TOA, in the layer, and at the surface is 170, 170, and −2 W m−2AOD−1, respectively. We also see that the total DARF when SZA = 75° for the same layer over snow is 35 W m−2AOD−1 at TOA, 64 W m−2AOD−1 in the layer, and 11 W m−2AOD−1 at the surface. These results indicate that environmental conditions, such as surface albedo and SZA, control the sign of the radiative forcing at TOA and at the surface and the magnitude of the forcing in the aerosol layer. An older plume over snow at SZA = 55° would have total DARFs of 25, 31, and −5 W m−2AOD−1 at TOA, in the layer, and at the surface, respectively. Our results demonstrate that plume aging can alter the magnitude of the radiative forcing. We also compare results for the thin plume to those for a thick plume (~3–20 km) with an AOD (550 nm) range of 1 to 3. The fresh, thin plume with AOD = 0.58, over seawater, and SZA = 55° will heat the atmosphere in the SW by ~2.5 K day−1 and cool the atmosphere in the LW by ~0.3 Kday−1. The fresh, thick plume with AOD = 3 under the same environmental conditions will produce SW heating in the atmosphere by ~31 Kday−1 and atmospheric LW cooling of ~6.7 K day−1. These calculations convey the importance of vertical plume structure in determining the magnitudes of the radiative effects. We compare our assessments with those reported for other aerosols typical to the Arctic environment (smoke from wildfires, Arctic haze, and dust) to demonstrate the importance of volcanic aerosols.