Radiative Forcing and Stratospheric Heating by Stratospheric Aerosols: Sensitivity to Microphysics, Cloud Radiative Properties, and Radiative Parameterizations

Stratospheric aerosols have been proposed as a deliberate climate intervention to partially, imperfectly, and temporarily offset radiative forcing due to well-mixed greenhouse gases. This hypothetical intervention relies on the ability of stratospheric aerosols to produce a negative radiative forcing. This radiative forcing is intended to reduce solar radiation reaching the surface, causing a reduction in global surface temperatures. Besides this influence at the surface, stratospheric aerosols modify stratospheric radiative heating profiles. This combination of radiative forcing and radiative heating influences important aspects of atmospheric dynamics and chemistry.

Here we present a sensitivity study of the radiative forcing by stratospheric aerosols to variations in aerosol microphysics, cloud radiative properties, and radiative transfer parameterizations. Global chemistry-climate models variously employ modal and sectional schemes to represent stratospheric aerosols. Understanding the differences among models attributable to microphysics is a topic currently of keen interest in the modeling community. Stratospheric aerosol radiative heating and forcing is influenced by the radiative properties of clouds. Clouds that efficiently scatter radiation back to space will reduce the cooling capacity of stratospheric aerosols relative to more absorbing atmospheric conditions. Clouds that are opaque in the infrared will modify radiative heating by stratospheric aerosols relative to more transparent atmospheric conditions. It is known that estimates aerosol radiative forcing depend on choice of scattering phase representation. We will examine the dependence of aerosol radiative quantities on further details of scattering parameterization.

A number of additional quantities may be computed from the radiative forcing and heating. These quantities relate to climate, atmospheric dynamics, and atmospheric chemistry. Changes in stratospheric temperature and in stratosphere-troposphere temperature contrast lead to dynamical changes, altering the stratospheric thermal wind, and perturbing the Quasi-Biennial Oscillation (QBO) and the Brewer-Dobson Circulation (BDC). Changes to stratospheric temperature also change the tropical tropopause temperature, which has a key role in controlling entry of water vapor into the stratosphere. Changes in stratospheric water vapor change stratospheric chemistry. Additionally, stratospheric water vapor is itself a direct radiative forcing agent. Changes to aerosol scattering alter the production of diffuse light, which influences photochemical reaction rates and the productivity of natural ecosystems. We will estimate plausible changes to these climate, dynamical, and chemical quantities ensuing from the sensitivity of the radiative properties of stratospheric aerosols to uncertainties and assumptions regarding microphysics, clouds, and radiative transfer.