The Role of Soil Moisture over Irrigated Land on Precipitation in the Northern Great Plains at Convective Scales

The sudden onset of irrigation in the spring and crop maintenance through the summer in the US Northern Great Plains produces changes to the water and energy budgets on both the local and regional spatial scales. For example, irrigation wets the surface leading to a decrease in albedo and a subsequent increase in net solar radiation at the surface. Subsequent evapotranspiration moistens and cools the boundary layer leading to soil moisture-precipitation feedbacks both over and downwind of irrigated lands. During irrigation onset, synoptic scale forcing is still prevalent while during the summer thermal forcing becomes increasingly more common. To investigate the role of changing soil moisture over irrigated lands a series of cloud resolving numerical simulations were conducted in which the soil moisture was systematically varied, after a 5-year offline adjustment phase, by as much as ±15% in regions where irrigation was the dominant land use. One set of simulations was conducted for a 10-day period in late May when large-scale forcing was dominant and irrigation induced modifications to the precipitation distribution was reduced. A second set of simulations was conducted in mid-July when large-scale forcing was largely absent. Initial results suggest that changes to surface thermal low development and perturbations to the 850 hPa geopotential due to irrigation modify the great plains low-level jet resulting in changes to the precipitation distribution.