PNNL

The Science                                

The timing and location of precipitation associated with mesoscale convective systems (MCSs) are sensitive to low-level moisture in the atmosphere. Previous studies suggest that the displacement error in a modeled MCS over Iowa, from June 24 – 25, 2015, was caused by an underrepresentation of low-level moisture. Researchers conducted a series of simulations to understand the effect of moisture changes on the MCS precipitation. Adding moisture directly to the air close to the ground makes the storm form closer to its observed location, while changes to the moisture in the soil can make the storm move further away. The impact of irrigation on the moisture near the ground depends on the length of the simulation. Overall, this study finds that more moisture in the air close to the ground leads to a more unstable atmosphere that causes the storm to form more quickly, which better matches observations.

The Impact

MCSs are a collection of organized lines or clusters of moist convection that produce heavy and extreme warm-season rainfall, significantly affecting the hydrologic cycle. While progress has been made to improve physical understanding of the organization and maintenance of MCSs, the predictability of MCSs remains limited in terms of the timing, location, and rainfall intensity. Numerical models often have biases in low-level moisture, which may lead to substantial errors in precipitation. Therefore, understanding potential sources of error in low-level moisture and how they may affect MCS displacement is crucial for a better comprehending and predicting intense precipitating MCSs.

Summary

Researchers investigated how the land surface affects the precipitation produced by a cluster of thunderstorms, an elevated MCS, that occurred in Iowa between June 24 – 25, 2015. Researchers used semi-idealized simulations to study the effects of moisture on the location and amount of precipitation from the storm. In general, numerical simulations place the MCS east of the observed location. Adding moisture directly in the low-level atmosphere in the semi-idealized experiments reduces this displacement error. Adding soil moisture is not equivalent to adding atmospheric moisture because it cools temperature and induces low-level divergence, causing the MCS to move further to the east. Short-term irrigation simulations decrease atmospheric moisture and move the MCS to the east, but long-term irrigation does the opposite. Despite the lack of low-level moistening in the perturbed soil moisture and short-term irrigation experiments, the sensitivity of the MCS to low-level moisture is similar in all runs.

PNNL Contact

Jerome Fast, Pacific Northwest National Laboratory, jerome.fast@pnnl.gov

Funding

This research was supported by the Office of Science of the Department of Energy (DOE) as part of the Atmospheric System Research Program via Grant KP1701000/57131. The research used computational resources from National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science user facility operated under Contract DE-AC02-05CH11231.