PNNL

Abstract

This study investigates the role of the land surface on the precipitation produced by an elevated mesoscale convective system in Iowa on 24-25 June 2015 during the Plains Elevated Convection at Night (PECAN) field campaign. Previous studies have shown a strong effect of low-level moisture on location of Mesoscale Convective System (MCS). A series of semi-idealized and realistic simulations with irrigation are conducted to understand the effect of low-level moisture perturbations on the MCS precipitation displacement. In general, numerical simulations place the MCS northeast of the observed location. Adding moisture directly in the low-level atmosphere in the idealized experiments reduces this displacement error. However, experiments with perturbed soil moisture result in drying over Iowa induced by moisture flux divergence from cooler low-level temperature and higher surface pressure, causing the MCS to move further to the east. The irrigation impact on low-level moisture is highly dependent on the length of simulation period. Shorter simulations on the order of days generate similar drying over Iowa but the opposite is found for the month-long simulations. Despite the lack of low-level moistening in soil moisture and irrigation experiments, the substantial sensitivity to low-level moisture is similar in all runs. More low-level moisture generates a more convectively unstable environment with less inhibition and a lower free convection level that leads to more rapid MCS development and a change in MCS location.