Sensitivity of Present and Future Seasonal Precipitation over Central United States to the Representation of Rocky Mountain Topography

Authors

Ranasinghe Disanayakalage Sherly Shelton Ranathunga, University of Nebraska-LincolnFollow

First Advisor

Ross D. Dixon

Committee Members

Clint Rowe, Michael Hayes

Date of this Version

8-2024

Document Type

Article

Citation

A thesis presented to the faculty of the Graduate College at the University of Nebraska in partial fulfillment of requirements for the degree of Master of Science

Major: Earth and Atmospheric Sciences

Under the supervision of Professor Ross D. Dixon

Lincoln, Nebraska, August 2024

Comments

Copyright 2024, Ranasinghe Disanayakalage Sherly Shelton Ranathunga. Used by permission

Abstract

Precipitation change across the Central United States (USA) is of great interest to the research community. Studies using global climate simulations suggest that the “100^th-Meridian”, which separates the “dry west” from the “moist east” will shift to the east as the climate responds to future emissions pathways. However, these simulations are run at relatively coarse resolutions, which do not accurately represent topography. Here, we perform regional simulations using the Weather Research and Forecasting (WRF) model to explore the sensitivity of present and future precipitation patterns across the central USA to the representation of the Rocky Mountains (RM). We first perform 20 km control simulations for both historical (1980-2009) and end-of-century (2070-2099) periods across the continental US. The end-of-the century runs use the SSP585 emissions scenario. We then run simulations where we decrease the RM height to 80% and 60% of the height in the control simulation. This emulates the “effective” height of the RM in coarser (1-2-degree resolution) global simulations. The findings show that decreasing RM heights change the annual and seasonal distribution of precipitation across the USA. The precipitation from December to February (DJF) over the central USA shows statistically significant positive change with reducing RM heights, while a contrasting pattern is seen from June to August (JJA). The precipitation minus evaporation (P–E) and soil moisture followed the same distribution pattern with changing RM heights. The moisture transport and moisture flux convergence are dominant over the central part of the USA after reducing the RM heights, while moisture divergence is concentrated in the RM region and eastern USA, leading to less precipitation than the control simulation, suggesting that RM heights modulate the low-level wind circulation and moisture transport.

Projected precipitation changes under SSP585 are not uniform across the entire country, particularly in the western and eastern parts of the USA, with increased precipitation due to changes in storm tracks and atmospheric moisture content. Meanwhile, regions in the southwestern USA experienced more negative changes during the 2080-2099 period. Interestingly, the DJF precipitation is increasing over the central, northern, and western USA, which is getting reversed for the JJA season. The result also shows that the RM height changes more strongly influence precipitation in the JJA season compared to the DJF season.

The results from these experiments are shown with emphasis on the seasonality of changes in precipitation patterns, the impact of moisture transport, and the importance of accurately representing RM topography in understanding projections of precipitation change across the central USA.

Advisor: Ross D. Dixon