Aerosol-Radiation-Climate Interactions over the Gangetic-Himalayan Region

Abstract

A growing body of evidence indicates that tropospheric aerosols, a major component of the global climate system, significantly influence the Earth’s radiation budget and climate forcings. Their multi-faceted impacts on climate with considerable lack of in-depth knowledge of their properties and spatio-temporal distribution makes aerosols one of the least understood components of the Earth’s climate. Over heavily polluted regions, the difficulty in quantifying aerosol effects is exacerbated. For example, the South Asian region of which about 1/7th of the world’s population lives in the Indo-Gangetic Plains (IGP) is one of the major hotspots of rising pollution due to rapid urbanization/industrialization and growing energy demands. Bounded by the high-altitude Himalayas in the north, the regional climate is largely governed by the summer (southwest) and winter (northeast) monsoons. This dissertation is an attempt to gain reliable insight into the complex interactions between aerosols, radiation and climate of the Gangetic-Himalayan region with a special emphasis on the regional hydrological cycle through the extensive use of multi-sensor satellite data together with ground radiometric measurements. Plausible couplings in the two contrasting seasons (winter and pre-monsoon/summer) namely, between the widespread winter haze and fog; and the connection between pre-monsoon dust transport and summer monsoon rainfall variability were investigated. The spatial and temporal distribution of fog occurrences is found to be influenced by the heavy pollution over the eastern IGP through its microphysical and radiative interactions. During the pre-monsoon period, dust plumes (mixed with local pollution) in the Gangetic-Himalayan (GH) region occurs at elevated altitudes (>5 km) and is found to be significantly absorbing in nature causing enhanced heating in the middle troposphere. There is an increasing trend in the atmospheric loading of absorbing aerosols during the pre-monsoon season and its likely response on the regional climate is observed in the possible amplification of the enhanced GH tropospheric warming as indicated by microwave satellite measurements in the past three decades. Tropospheric temperature data also indicate the strengthening of the land-sea thermal gradient, which is crucial to the onset and intensity of the Indian Summer Monsoon, followed by an increasing trend in the early summer monsoon rainfall. Finally, the relationship between tropospheric temperature trends and summer rainfall variability is examined and a possible future scenario of the regional hydrological cycle is proposed.