UCLA

Aerosols come from both natural and anthropogenic sources and contribute large uncertainties to estimates of the Earth’s changing energy budget. It is thus of great importance to understand the mechanism through which aerosols play a role on global climate. In this thesis, we investigate the direct and indirect effect of aerosols on global and regional climate variability (e.g. West Africa, South Asia and East Asia) using an atmospheric general circulation model, GFS (Global Forecast System) coupled with SSiB2 (the second version of Si mplified Simple Biosphere Model). The three-dimensional aerosol data from the Goddard Chemistry Aerosol Radiation and Transport (GOCART) model has been adopted in this study.

We first analyze the direct effect of aerosols, especially absorption aerosols, on global and regional energy budget, precipitation, and surface temperature and the mechanism involved. For instance, we find the dust aerosol in North Africa produces a heating in the atmosphere, which generates a cyclonic circulation in middle layer over Sahel region, which further brings about upward motion in the lower level and results in precipitation increase by 0.96 mm/day in June-July-August.

We also examine the impact of aerosols on ice clouds effective radius by applying an advanced ice cloud parameterization in the GCM. We find that increased aerosol loading reduces ice crystal size due to aerosol first indirect effect, with the maxima occurs in South Asia and North Indian Ocean. Ice clouds with smaller crystal sizes can absorb both shortwave and longwave radiation, thus resulting in less downward solar flux and less outgoing longwave on top of atmosphere (TOA). Global mean net radiation change on TOA is about 0.5W/m2 and its sign is largely dependent on the relative magnitude of shortwave and longwave change and precipitation changes primarily respond to cooling/warming of the atmosphere.

Lastly, we use sulfate data in both pre-industrial and present-day case to test the impact of aerosols on liquid cloud effective radius. We find aerosols can act as cloud condensation nuclei, and hence change the shortwave optical properties of liquid clouds. Radiative cooling occurs globally because smaller droplets size leads to increased cloud albedo. The mean value is about 2.5W/m2. Moreover, most radiative cooling occurs in North Hemisphere, where anthropogenic sulfate aerosols locate, such as East Asia and North America.

We also compare the radiative forcing of aerosol direct and indirect effect on both ice clouds and liquid clouds on global as well as the three monsoon regions. All aerosol effects result in radiative cooling on global scale. However, surface net radiation changes are different in West Africa, East and South Asia, which relates to local atmospheric conditions such as cloud cover and convection. Aerosol direct effect and aerosol indirect effect for liquid clouds have comparable impact on surface net radiation change (more than -3 W/m2), while aerosol indirect effects for ice cloud are smaller (~1 W/m2) because of the negative feedback from cloud cover. Decreased land surface temperature can be found over North Hemisphere continent in all three effects, especially over higher latitude, with varied magnitude. The precipitation changes are less predictable. Aerosol indirect effects on averaged global precipitations are close to zero because the precipitation changes are different or even opposite in different regions. The effect of aerosol on precipitations can be influenced by convection strength, topography, and even the relative location of aerosols and monsoon system.