The impact of southern African biomass burning aerosols on temperature tendencies in the GEOS-5 Earth System Model

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 aerosols over southern Africa. Biomass burning aerosols dominate aerosol loading over much of southern Africa from August through October. These aerosols both warm the atmosphere and cool the surface through the direct effect. They also have a thermodynamic effect on cloud cover through the semi-direct effect. Heating of the atmospheric 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. Over land, there is generally sparse cloud cover during this season. Despite a persistent stratocumulous cloud deck off the western coast of the continent, the transported biomass burning aerosol layer tends to be well-separated from the clouds below. Microphysical interactions between aerosols and clouds (aerosol indirect effects) are thus limited in this season and region.

Here, the GEOS-5 DAS analysis update (AU) -- the difference between the meteorological analysis and the model forecast -- provides an observational constraint on our simulations.  The AU represents the complex combination of all model errors due to inadequate representation of physical processes (model parametrizations), numerical errors, and processes that have been omitted from the model. By examining the AU for temperature we demonstrate that there is a clear correlation between the AU and AOD over southern Africa during the biomass burning season, indicating that the effects of aerosols are contributing to the model “error” for temperature. In a series of sensitivity studies in which we vary aerosol parameters (amount, absorption), we examine the effect of aerosols on temperature tendencies and the AU.  We show that, in combination with comparisons of aerosol properties to observations (e.g. from MODIS), analysis of these tendencies can be a useful metric for tuning aerosol properties to minimize the AU.