Stratospheric Aerosol Backscatter and Depolarization Ratio Observed With Ground-Based Lidar at Tsukuba, Japan, and Lauder, New Zealand

Abstract

Vertical distributions of stratospheric aerosol backscatter and depolarization ratio (nonsphericity) have been measured using ground-based lidars at Tsukuba, Japan and Lauder, New Zealand. The observational results after 2003 show that the aerosol increased several times after large volcanic eruptions and wildfires. The largest increases in vertically integrated stratospheric aerosol backscattering coefficient (IBC) above 16.5 km (IBC_16.5) were observed after the Raikoke volcanic eruption in 2019 at Tsukuba and the Australian wildfire in 2019/20 at Lauder. The increased IBC_16.5 returned to normal levels in 1–2 years. After the volcanic eruptions, the particle depolarization ratio (PDR) increased for several days and then decreased in a few dozen days in most cases. In contrast, the increased PDR after the large wildfires gradually decreased in 1–2 years. These suggest that large volcanic ash was quickly removed to the troposphere or dissolved in or coated with liquid to become non-depolarizing. In contrast, the wildfire smoke stays in the stratosphere for a few years due to its small size and mass density. We compare the lidar-derived stratospheric aerosol extinction coefficient profiles and the optical depth (stratospheric aerosol optical depth (SAOD)) with those obtained with a balloon-borne optical particle counter (OPC), satellite-borne instruments (SAGE-II, Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP), and Global Space-based Stratospheric Aerosol Climatology (GloSSAC)), and the Meteorological Research Institute Earth System Model (MRI-ESM2) for the validation. The mean differences from lidar-derived SAOD were +15% for SAGE-II, −11% for GloSSAC, +32% for CALIOP, and −44% for MRI-ESM2 at Tsukuba, and +19% for the SAGE-II, +28% for OPC, +7% for CALIOP, −15% for GloSSAC, and −72% from MRI-ESM2 at Lauder.