Understanding daily precipitation over Monsoon and Southeast Asia in observations and regional climate models

Abstract

Monsoon Asia (MA) is the world’s most populous continent with high vulnerability to extreme weather, notably precipitation extremes. Due to sparse observations and limited modelling, past trends in extreme precipitation and future projections over many parts of the region are not well known. This thesis investigates regional precipitation (e.g., distribution, seasonality, variability, extremes, and past and future changes) over different sub-regions of MA using observations and climate models. The intercomparison of multiple observational precipitation products reveals the high temporal and spatial consistency in precipitation extremes in high-station density areas (e.g., Japan, India) and the large inter-product spread over limited-station regions (e.g., Southeast Asia - SEA). Products with high consistency in trends and variability for individual sub-regions of MA are selected to evaluate the performance of an ensemble of high-resolution regional climate models (RCMs) from the Coordinated Regional Downscaling Experiment (CORDEX). Rainfall patterns are investigated using various aspects of the precipitation distribution in CORDEX-SEA RCMs and compared with their forcing global climate models (GCMs). We find that RCMs are wetter and generally not as close to observations as their forcing GCMs. The more intense precipitation in RCMs is associated with 1) an increased supply of moisture from both local and large-scale sources and 2) a widespread increase in convective precipitation across the region. Our findings suggest that the RCM setup (e.g., parameterization scheme) is more important than the choice of GCM. Given the range of RCM performance, two sub-ensembles representing “better” and “worse” performing models are selected and their respective projections are compared to assess how past model performance can affect future projections. The thesis results highlight that careful model evaluation is needed and could lead to more well-informed future projections at the regional and seasonal scales relevant to the complex region of SEA. The framework and method developed in this thesis enable many avenues of research, such as understanding biases in regional and global models and how these could impact future projections. Ultimately, our understanding of regional rainfall patterns is improved, which in turn helps to better inform modelling strategies and the risks associated with future changes in precipitation under a warmer climate.