Quantifying the semi-direct effect of smoke aerosol over southern Africa using the NASA GEOS-5 Incremental Analysis Update (IAU)

The aerosol direct effect, caused by scattering and absorption of radiation, alters the energy distribution of the atmospheric column. Purely scattering aerosols, such as sulfate and sea salt, exert a radiative cooling effect on climate at the top-of-the atmosphere (TOA) and the surface. More absorbing aerosols, such as anthropogenic black carbon and smoke, can have a radiative cooling or warming effect depending on the brightness of the clouds or surface beneath them. These absorbing aerosols warm the atmospheric column, and by doing so they have a thermodynamic impact on climate known as the semi-direct effect. Heating of the column tends to increase the saturation water vapor pressure and reduce the ambient relative humidity (RH), effects that in turn tend to increase low-level stability and act to reduce cloud formation. Removal of reflective clouds tend to contribute to a radiative warming of the climate system. Accompanying the direct and semi-direct effects of absorbing aerosols are regional to global changes in circulation that can have additional impacts on cloud distributions. In general circulation models (GCMs), it is difficult to separate the effects of direct aerosol atmospheric heating from these circulation changes in determining the impact of aerosol heating on clouds. The problem becomes even more complex if microphysical interactions of aerosols and clouds are taken into account (the aerosol indirect effect).

In this study we use the NASA GEOS-5 Earth System Model (ESM) and Data Assimilation System (DAS) to examine the direct and semi-direct impacts of absorbing biomass burning (“smoke”) aerosol over southern Africa. We perform an ensemble of short-term (5 day) forecasts during the peak of the burning season (August). Constrained by initial conditions imposed by the assimilation, the short-term nature of the forecast serves to separate the direct effects of aerosol absorption (and atmospheric heating) from the changes in circulation that can accrue over longer free-running GCM simulations. We are thus able to more accurately estimate direct aerosol forcing and resulting changes in cloud radiative forcing, a measure of the semi-direct effect.

One of the unique features of the GEOS-5 DAS is the Incremental Analysis Update (IAU) technique in which the observational constraint is introduced gradually, preventing shocks in model physical parameterizations. The IAU is defined as the difference between the model forecast and analysis in a data assimilation cycle. It represents the complex combination of all model errors due to inadequate representation of physical processes (model parameterizations), numerical errors, and processes that have been omitted from the model. The difference in the IAU generated in model run with and without biomass burning aerosols, or with varying aerosol parameters, is a useful metric with which to assess the effect of aerosols on climate and cloud distributions. We perform a series of sensitivity simulations in which we vary biomass burning aerosol properties (e.g. increased emissions, increased absorption), and we examine the impact of changes in these properties on clouds and the IAU. The set of aerosol properties which tend to minimize the IAU (i.e. the model error) and agree with available observations (e.g. from satellite sensors) are expected to yield the best estimate of both the biomass burning direct and semi-direct effects. Based on these sensitivity studies, we present our best estimate of the aerosol direct radiative forcing and the aerosol semi-direct effect, quantified as a difference in cloud radiative forcing between a simulation with biomass burning aerosols and one without.