Air­-land Interactions: Effects of Soil Moisture in the Past, Present, and Future

Abstract

The land surface and the atmosphere influence each other through processes that occur in the lowest layer of the atmosphere, where people live. This atmospheric layer is called the planetary boundary layer (PBL), and its dynamics are crucial for accurate weather prediction and climate projection by numerical models. In this thesis, I present three projects related to air-land interactions in the PBL. The projects span three different spatial scales (regional, synoptic, and global), and three different time periods (past, present, and future). On the regional scale, I propose an agricultural drought index that accounts for the mitigating effect of irrigation before the existence of satellites \citep{tang2023} by applying a recent theory that assumes strong air-land coupling. This drought index can be used to better understand past droughts. On the synoptic scale, we investigate how soil moisture gradients control the intensity of convective storms. These mechanisms are particularly relevant to understanding storms in the central United States, where synoptic scale east-west soil moisture gradients are a robust feature of the current climate. At global scales, I simulate an idealized land world in which surface moisture is fixed, and I use it to develop basic insight into how dry land surfaces impact continental climate. I derive a simple theory that parsimoniously explains the response of near-surface temperature and humidity to variations in soil moisture. The theory will provide insight into changes in rainfall and drought with future warming.