Understanding irrigation impacts on low-level jets over the Great Plains

Abstract

Low-level jets (LLJs) over the Great Plains are well-known for their importance in transporting moisture from the Gulf of Mexico and forming precipitation in the Central US. However, the impact of irrigation practice on LLJs is rarely studied. To understand the irrigation impact on the Great Plains LLJs, the Weather Research and Forecasting (WRF) model is employed with and without an irrigation scheme. The simulated wind fields are evaluated against the observed wind from the Midlatitude Continental Convective Clouds Experiment (MC3E) as well as the North America Regional Reanalysis dataset. The results show that adding irrigation improves the simulated wind, especially at altitudes below 800 hPa. Irrigation reduces the LLJ frequency over the Great Plains, mainly by reducing the intense LLJ frequency. The reduced LLJ frequency is caused by the irrigation-induced weakening of meridional wind. The favorable conditions for LLJs have been identified by examining the mechanism associated with LLJs: a ridge-like pattern with weak and calm synoptic-scale conditions in the mid-levels and a strong west–east temperature gradient at lower levels that promote meridional winds. Irrigation-induced cooling leads to a baroclinic pattern with high pressure at lower altitudes over the irrigation region and low pressure in the mid- and upper levels in the downstream region, resulting in a northerly component that hinders the formation of LLJs. Low-level surface cooling also decreases the west–east temperature gradient and hence weakens the thermal wind forcing. The irrigation-induced mid-level low and weaker west–east temperature gradient are responsible for the reduced meridional winds and the reduced LLJ frequencies.