The effects of enhanced greenhouse gas concentrations on Earth’s climate are well understood. However, the impacts of anthropogenic aerosol particles, in particular due to the many aerosol-cloud indirect feedback mechanisms are not fully or even explicitly quantified as yet. This PhD seeks to contribute to improve our knowledge and understanding of aerosol – precipitation interactions over the Nepal Himalayas region and their consequences for precipitation patterns there. The research was carried out using the cloud-resolving Weather Research and Forecasting (WRF) model through a series of sensitivity studies and supported by literature reviews of satellite and field observations, although the latter are sparse. To complement the modelling studies, from March to December 2011, aerosols and surface meteorology were also continuously measured at Nagarkot (Lat: 27.7°N, Lon: 85.5° E, Alt: 1900m), Nepal, located in the eastern flank of a bowl shaped Kathmandu valley. The location was chosen to provide a representative vertical profile of aerosol and the impact on topographical flows. Our results showed a unique pattern of diurnal pollution circulation within the valley with a morning and evening peak. The evening peak, which is higher than the morning peak is attributed to the light wind blowing through the valley carrying locally generated fresh evening pollution, further enhanced by re-circulations of aged pollutants through suppression of the mixing layers as suggested by a previous study at a different location. The morning peak is caused by calm wind conditions followed by the transitional growth of the nocturnal boundary layer. It is found that the thermally driven mountain – valley wind circulations are responsible for ventilation of pollutants. The WRF simulations showed that a sophisticated double moment bulk microphysics parameterization scheme performed best, which did not show any statistically significant difference compared to the observed data at 80% confidence interval using a Chi-squared goodness of best fit test. A sensitivity analysis of aerosol and temperature perturbations on the monsoon precipitation was conducted. We found that the model represented the first indirect effect reasonably well however, rainfall was not particularly sensitive to the aerosol perturbations used, due to the poorly documented role of the ice phase processes which assume a greater importance in this region due to the influence of topography and diurnal heating cycle. Further model studies focusing on chemical properties of aerosol and sensitivity of Ice Nuclei (IN) to precipitation in this region are recommended. In contrast, the effects of temperature perturbation were found to be significant, more so than the currently modelled aerosol indirect effects, suggesting that reduced frequency but intense rain events are likely over the Himalayas as the climate warms.

The Himalaya plays a vital role in shaping the hydro-climate of South Asia and beyond, but their climate has not yet been monitored and modelled as well as some other regions. As the summer monsoon is the dominant climate system over South Asia, including the Himalaya, realistic simulation of the South Asian summer monsoon (SASM) should be a prerequisite for the satisfactory simulation of the Himalayan climate. The present research tests the assumption that higher resolution modelling will provide improved representation of the SASM, both regionally and over the Himalaya region. The first part of this research assesses the strength and stability of the temporal relationships between the monsoon rainfall indices (MRIs) and the large-scale monsoon circulation indices (MCIs), as a precursor to using such indices for model evaluation. The remainder of the thesis evaluates model performance in simulating various characteristics of SASM, mainly with regard to precipitation. In particular, the sensitivity of a regional climate model (RCM) simulation to domain size and added value of high resolution RCM simulation are evaluated. For this purpose, the Hadley Centre unified model - HadGEM is utilized in its regional and, in few instances, global configurations. The RCM simulations are performed at 0.44° and 0.11° horizontal resolutions and they are forced by the ERA interim dataset. Results show that i) the MRI-MCI relationship exhibits considerable low-frequency variability, ii) RCM simulation of SASM, particularly precipitation, shows sensitivity to domain size and simulation with a moderately sized domain that partially excludes bias prone equatorial Indian ocean outperform those with larger domains, iii) high resolution RCM simulation adds value in many aspects of SASM precipitation, including the seasonal mean, relative frequency distribution, extremes, and active and break monsoon composites, but the improvements are generally seen over the Indo-Gangetic plain rather than the Himalaya. The findings promote use of a high resolution RCM over a moderate sized domain (~ 25,000,000 sq. km) for the realistic simulation of SASM, but the study needs to be repeated with multiple realizations and different RCMs before arriving at a robust conclusion.