Irrigation impact on precipitation during a heatwave event using WRF-ARW: The summer 2015 Po Valley case

Irrigation is crucial in sustaining food production and it is found to have a cooling effect. Changes at the surface affect both the circulation and the precipitation. The magnitude depends on the model used, the irrigation description and the water amount, as well as the region. The study focuses on northern Italy (the Po Valley) due to its vulnerability to heatwaves and dependency on local water sources. This study is performed with the Weather Research and Forecasting (WRF) model and newly developed irrigation parameterizations defined by different evaporative processes. The model runs at a 3 km convection-permitting resolution, starting in May 2015 and analyzing July. An irrigation amount of 5.7 mm is applied daily at 5 UTC for 3 h, starting from May 15. A set of convection-parameterized sensitivity simulations is used to investigate the results' dependency on timing. Using a convection-parameterized set of experiments, it is assessed that irrigation increases the precipitation accumulated over the region. While irrigation modifies the air mass properties, convection-permitting runs show that afternoon events are inhibited due to an increase in convection inhibition and decrease in boundary layer height. This happens despite the increase in the convective available potential energy (CAPE) and a decrease in both the lifting condensation level and level of free convection. For the nighttime event, irrigation increases significantly both boundary layer moisture and CAPE, increasing the precipitation. All cases are compared against the national radar composite accumulated over two hours, finding that the irrigated runs perform better than the control.