DUSTIEAIM

The focus of this project is to characterize interactions between the land surface and the atmosphere and over a desert megalopolis (Phoenix) set in complex topography that results in steep environmental gradients. We will interrogate the interaction between storms and aerosol during the summer North American Monsoon (NAM) in the context of spatial contrasts at the urban interface with agricultural and natural lands in the Sonoran Desert. We will collect observations to understand the radiative properties, aerosol interactions, cloud formation and precipitation in Arizona in synergy with the Southwest Urban Integrated Field Laboratory (SW-IFL) funded by the Department of Energy Office of Science. Our science focus is to quantify the bimodal hydroclimate at the urban/rural/agricultural interface of the southwestern U.S. as regional air masses and meteorological patterns change throughout the year in response to the seasonal cycle, including winter Pacific storms and the NAM.

Phoenix is the 5th-largest metropolitan statistical area (MSA) in the U.S. with ~5 million residents in the greater metropolitan area. The spatial footprint is also very large, spanning approximately 50-80 km across depending on the direction. Phoenix was the fastest growing city in the U.S. last year (~2% growth rate). It is also considered to be America’s hottest major city with, for example, a record of 31 straight days with temperatures above 110 °F in summer 2023. The Phoenix metropolitan area depends on the Colorado River for 38% of its water, a source that has decreased due to climate change and rising demands. Phoenix is surrounded by high terrain to the north and east which receive winter snow and monsoonal rain, where precipitation is essential for replenishing the groundwater supply in and around Phoenix. Phoenix’s population is racially and socioeconomically diverse, with more than half the population comprised of traditionally underrepresented individuals, including Hispanic, Black and people of color, and Native American, as well as low-income families, based on the latest U.S. Census. We propose to do this with 3 science objectives and 4 testable hypotheses within each science objective:

• SO#1: Land-Atmosphere Interactions and Impacts: Determine how the urban-rural interface of the Phoenix metro affects atmospheric processes within the Sonoran Desert.

• SO#2: Aerosol Processes and Interactions with Clouds and Radiation: Identify which aerosol sources and processes dominate seasonally in Phoenix and evaluate how well Earth System Models capture their local and regional impacts on clouds and radiation.

• SO#3: Precipitation Processes: Evaluate how orographic, surface, and aerosol processes intersect with larger-scale meteorological variability to affect spatial and temporal precipitation patterns in and around the Phoenix metro.

For all these reasons, now is the time to collect atmospheric observations to fill current data gaps and improve the understanding of atmospheric interactions with the land surface and the water cycle in this underserved and under-resourced geography by filling current data gaps in the southwestern United States. The hot summer months coincide with the North American Monsoon, when the thermal low over the southwest U.S. draws in moisture from the Gulf of Mexico, eastern Pacific Ocean, and Gulf of California, which is released during late afternoon and evening thunderstorms. While most monsoonal precipitation falls over the mountains, the Phoenix basin often experiences convectively triggered dust storms (haboobs) and sometimes heavy rain, leading to flash floods. Lightning can spark intense fires in the higher-altitude forests where storm cold pool outflow creates erratic wind gusts and dangerous fire conditions. Smaller scale processes remain understudied and lack in situ observations: for example, how aerosols from different sources, both natural and anthropogenic, interact with the boundary layer and convection.

Timeline