UC San Diego

Mineral aerosols (i.e. dust) perturb the Earth’s energy budget by scattering and absorbing shortwave (SW) and longwave (LW) radiation. Generally dust has a net (SW + LW) negative direct radiative effect (DRE) at the surface and top of the atmosphere (i.e. cooling of the Earth’s climate system). A few studies found that the net top of the atmosphere (TOA) DRE of dust, the difference between the net radiative flux in clear-sky (cloud-free) and pristine-sky (aerosol and cloud free) conditions, can be near zero or positive over desert regions. It is important to constrain the net DRE of dust over source regions since biases in this parameter can lead to errors in estimates of the global average. In Chapter 2 an observation-based method to estimate the dust SW forcing efficiency (FE), the DRE normalized by the aerosol optical depth, is developed and validated. Associated uncertainties in this method are further investigated. By accounting for the relationship between dust and water vapor, biases in the TOA SW FE of dust are reduced and estimates of the TOA SW F E of dust over the Sahara Desert range from −26.4±17.6 to 2.35±139 W m^-2 per optical depth. In Chapter 3 this method is applied to observations and retrievals from a small dust source region in southeastern California to estimate the surface and TOA dust SW FE. These results are compared with the dust SW FE estimated with the output from a radiative transfer model. After validating the model, modeled and observed estimates of the instantaneous SW FE are statistically similar and therefore the model is used to estimate the instantaneous dust LW FE and the diurnally averaged net dust FE. Values of the net FE of dust are −29±10, −10±11, and 19±15 W m^-2 per optical depth at the surface, TOA, and atmosphere, respectively, over the small dust source region. The findings presented in this dissertation contribute towards the breadth of observational estimates of the SW dust FE over dust source regions which is critical to constrain model-based estimates of regional and global values of the dust SW FE.