Title: Dynamical Origins of Warm-Season Precipitation Extremes over the Northern Extratropics: A Multiscale Diagnostic and Modeling Study

Date and Time: August 12, 2025, at 2:00 PM

Location: Ford ES&T 1229 (in-person)

Committee Members: Dr. Yi Deng (advisor), Dr. Jean Lynch-Stieglitz, Dr. Jie He, Dr. Ali Sarhadi, and Dr. Jingfeng Wang

Abstract:

Extreme precipitation events are increasing in frequency and intensity under climate change, leading to severe socioeconomic consequences. While many studies have focused on thermodynamic drivers, regional hydrological variability is often governed by multiscale interactions of atmospheric flows. The mechanisms range from planetary-scale forcing to regional meso- and micro-scale processes and remain poorly understood and inadequately represented in models. This study addresses these challenges by investigating the large-scale dynamical origins of warm-season precipitation extremes across the northern extratropics, with a particular focus on Mesoscale Convective Systems (MCSs) over the United States and precipitation extremes at the margins of the Asian summer monsoon. A hierarchical framework is developed through observational analysis, model diagnosis, and idealized modeling to disentangle the roles of large-scale forcing, synoptic variability, and upscale feedback in shaping the distribution and variability of regional extremes.

First, this study presents a diagnostic analysis of MCSs over the U.S. Great Plains during boreal spring. We identify five upper-tropospheric circulation patterns associated with MCS genesis. One cluster, linked to disturbances in the North Pacific storm track, is the main contributor to the increasing MCS trend and is tied to the Pacific Decadal Oscillation. Model evaluation shows that GFDL AM4 captures the large-scale forcing patterns but misplaces MCS activity due to biases in both the mean state and transient processes. Next, this study uses a moist two-layer quasi-geostrophic model to examine how MCS heating provides feedback to midlatitude synoptic variability. Latent heating from MCSs enhances downstream eddy activity and improves the structure of synoptic variability across the northern midlatitudes. Finally, this study investigates summertime extremes in Northeast China and Pakistan, integrating both multiscale diagnosis and dynamical analysis. Using hierarchical clustering and a barotropic vorticity model, we show that evolving seasonal mean flows are changing the frequency of regional precipitation extremes through their influence on the excitation and propagation of atmospheric disturbances. Together, these findings highlight key multiscale processes controlling regional extremes and offer insights for improving model simulations and climate projections.