Influence of Irrigation on Diurnal Mesoscale Circulations: Results From GRAINEX

In order to understand the impact of irrigation on weather and climate, the 2018 Great Plains Irrigation Experiment collected comprehensive observations straddling irrigated and non‐irrigated regions in southeast Nebraska. Using these observations, we examine how irrigation affects diurnal terrain‐generated slope circulations, specifically the slope wind. We find that irrigation applied to upslope regions of gently sloping terrain reduces terrain‐induced baroclinicity and the associated pressure gradient force by up to two‐thirds. This leads to the reduction in the afternoon and evening upslope wind and is supported through comparisons to the High‐Resolution Rapid Refresh operational model, which does not explicitly account for irrigation. Additionally, the presence of irrigation decreases daytime sensible heat flux (Bowen ratio reduced 40% compared to non‐irrigated regions), weakening turbulent transport of momentum. Modifications to the terrain‐forced circulation by irrigation has the potential to affect moisture transport and thus cloud and precipitation formation over the Great Plains. Plain Language Summary Agricultural irrigation alters input of heat and moisture from the land surface to the atmosphere, which can affect weather and climate. Irrigation is expanding on all continents except Antarctica and is thus a major pathway through which humans impact the environment. However, the observations required to study the mechanisms though which irrigation affects weather and climate are lacking. The Great Plains Irrigation Experiment (GRAINEX) was conducted to collect such observations near the boundary between irrigated and non‐irrigated regions in southeast Nebraska. During the daytime, the slope of the Great Plains causes near‐surface upslope winds and downslope winds in the atmosphere above. At night this pattern reverses. This wind system influences storm formation by forcing upward motion and transporting moisture. Using the observations from GRAINEX and comparing to a weather model we find that irrigation weakens this wind system, potentially affecting cloud and rain formation in this region. Key Points First extensive observational study of modification of slope wind circulations by irrigation Presence of irrigation in upslope regions weakens terrain‐induced baroclinicity Irrigation‐reduced baroclinicity weakens the afternoon slope wind circulation