Blending Climate-Chemistry Model Simulations and Observations for Calculating Global Radiative Forcing Due to Stratospheric Aerosols

In recent years, volcanic activity, including the powerful Hunga Tonga-Hunga Ha’api eruption, and extratropical wildfires have led to substantial variability in the total burden, size distribution, and composition of lower stratospheric aerosols. The perturbations to shortwave and longwave radiative fluxes due to these perturbations depend on the details of aerosol size distribution, morphology, and refractive index. Satellite measurements of stratospheric aerosols provide global coverage, and although they offer some constraints on aerosol type and/or size, the influence of size distribution, morphology, and refractive index cannot be independently constrained by satellite retrievals. In situ measurements from balloons and aircraft can provide much stronger and more detailed independent constraints on these aerosol properties, but they lack the comprehensive space and time coverage of satellites. A more complete picture of radiative perturbations due to stratospheric aerosol therefore requires integration of satellite and in situ observations.

Climate-chemistry models are capable of simulating the coupled chemical, dynamical, and radiative processes that determine the burden, spatiotemporal distribution, and microphysical properties of aerosols, provided sufficient process-level understanding exists. Although there are gaps in understanding of the relevant processes for stratospheric aerosols, particularly with respect to organic carbon in the stratosphere, these global models nonetheless reproduce many important aspects of observations of stratospheric aerosols. For this reason, model simulations can provide a physically and chemically complete representation of stratospheric aerosols suitable for interpreting and integrating satellite and in situ observations. One method of utilizing the model simulations is to extend the space and time coverage of in situ measurements to provide a more comprehensive set of constraints on aerosol refractive index and morphology. These quantities allow the computation radiative forcing efficiencies. The efficiencies can then be applied to satellite measurements of stratospheric aerosol optical depth for the computation of top of the atmosphere radiative forcing.

The viability of the calculation of radiative forcing at the top of the atmosphere by the aerosol forcing efficiency is established through its wide utilization in the literature. Other quantities associated with radiative fluxes are also of interest, such as changes in downwelling surface fluxes and the partitioning of the shortwave flux between direct and diffuse radiation. The suitability of an aerosol forcing efficiency framework will be evaluated for the calculation of surface flux quantities, including the influence of variations in gas and condensed phase species in the atmospheric column below the stratospheric aerosol layer on the radiative perturbations.