Assessing the Errors in WRF Forecasts of Environmental Characteristics on WRF-Forecasted MCS Dissipation Time Errors

Mesoscale Convective Systems (MCS) are important atmospheric phenomena responsible for both a large percentage of precipitation and severe hazards over the central United States during the warm season. However, convection-allowing models can exhibit errors in their prediction of all stages in the life cycle of an MCS, particularly their dissipation. In a preliminary study, 40 nocturnal MCSs that occurred between June and September of 2018-2021 were simulated using the Weather Research and Forecasting model (WRF), and while no clear bias was found in the distribution of forecast dissipation time errors, the distribution had a standard deviation of 3.63 hours, indicating a large spread of WRF-forecasted dissipation time errors which requires further investigation. As it has been determined that Low-Level Jets (LLJ) play a significant role in maintaining nocturnal MCSs, the first focus of the present study is to evaluate the influence of WRF forecast errors related to the LLJ on errors in its forecasts of MCS dissipation time. A pair of main findings of this investigation are discussed. First, WRF forecast errors in the reduction of LLJ width from maturity to dissipation have some bearing on its errors in forecasting MCS dissipation time, with forecasted MCSs that dissipate too early associated with the WRF forecasting greater reduction in LLJ width compared to observations and forecasted MCSs that dissipate too late associated with the WRF forecasting a lesser reduction in LLJ width. Second, there is a tendency for the WRF to forecast more strongly veered winds and weaker storm-relative inflow in cases where it forecasts an MCS dissipation time that was too early. Meanwhile, the WRF tends to forecast winds that are less veered and stronger storm-relative inflow in cases where the dissipation time forecast is too late. The second focus of the present study is to investigate how errors in WRF forecasts of the downstream environment may affect its forecasts of MCS dissipation timing. When analyzing cases with a forecasted MCS dissipation time that is too late, the WRF tends to forecast higher values of moisture, equivalent potential temperature, and Most Unstable Convective Available Potential Energy than observed in the environment near the MCS. Further comparisons of the WRF-modeled and observed environments will be presented, with a particular emphasis on assessing forecast errors in the evolution of instability, moisture, and shear in the environments through which the studied MCSs propagated.