UCLA

The earth system has been modified by human activities over the past few centuries, mainly through the greenhouse gases (GHGs) and aerosol emissions, land use and land cover changes (LULCCs), and fire regime changes induced by human. This PhD study seeks to understand and quantify the impact of different forcings and disturbance on regional and global climate, especially through land-atmosphere interactions, using climate model simulations. Chapter 2 examines the aerosol indirect effects on ice cloud particle size and consequent impacts on monsoon climate. Simulations show a negative radiative forcing (0.5–1.0 W m-2) at the top of the atmosphere, caused by both particle size decreases and cloud cover change. This radiative forcing is influenced directly by cloud microphysics and indirectly by large-scale circulation, which contributes to large uncertainties in aerosol modeling studies.In Chapter 3, the large-scale LULCC impact on global temperature and precipitation is investigated by implementing a yearly-updated vegetation map into a climate model. LULCCs influence the local climate by modifying surface biophysical properties (albedo and roughness). Chapter 3 shows that LULCC-induced non-radiative cooling can lead to an asymmetric atmospheric cooling and an anomalous heat transport across the equator, accomplished by changes in Hadley circulation and ITCZ. This, for the first time, connects the local and global effects of LULCC from the perspective of energy budget and energy transport within the climate system. Wildfire is one of the primary disturbances to the world's ecosystem yet the modeling study of fire effects is still in an early stage. A fire-coupled vegetation model is developed to investigate fire effects on terrestrial ecosystem and surface energy in Chapters 4 and 5. In the long term, fire reduced tree fraction by 30-50% in the tropical savanna, decreasing leaf area index and vegetation height by 0.52 m2 m-2 (12.5%) and 5.76 m (49.1%). While current fire models mainly work on the fire effect at annual and longer time scales, Chapter 5, for the first time, quantifies monthly to annual fire impact using a fire-vegetation model. The characteristics of simulated fire effects at intra-seasonal to seasonal scales are analyzed and are compared with observations.