Abstract
The Great Plains low-level jet (LLJ) is a contributing factor to the initiation and evolution of nocturnal Mesoscale Convective Systems (MCSs) in the central United States by supplying moisture, warm air advection, and a source of convergence. Thus, the ability of models to correctly depict thermodynamics in the LLJ likely influences how accurately they forecast MCSs. In this study, the Weather Research and Forecasting (WRF) model was used to examine the relationship between spatial displacement errors for initiating simulated MCSs, and errors in forecast thermodynamic variables up to three hours before downstream MCS initiation in 18 cases. Rapid Update Cycle (RUC) analyses in 3 layers below 1500 m above ground level were used to represent observations. Correlations between simulated MCS initiation spatial displacements and errors in the magnitude of forecast thermodynamic variables were examined in regions near and upstream of both observed and simulated MCSs, and were found to vary depending on the synoptic environment. In strongly-forced cases, large negative moisture errors resulted in simulated MCSs initiating further downstream with respect to the low-level flow from those observed. For weakly-forced cases, correlations were weaker, with a tendency for smaller negative moisture errors to be associated with larger displacement errors to the right of the inflow direction for initiating MCSs.
Highlights
The Great Plains receive much of their warm season precipitation from MesoscaleConvective Systems (MCSs) [1,2,3,4,5]
In the Type C cases for Weather Research and Forecasting (WRF) runs using the Yonsei University (YSU) scheme (Table 1), the Obs-inflow region showed statistically significant positive S values at most times and in all 3 vertical layers with respect to the total spatial displacement of Mesoscale Convective Systems (MCSs). This agrees with the hypothesis that errors in variables that would play a role in the level where saturation of an air parcel within ascending flow would occur will determine the displacement error of a simulated MCS
Since positive significant S values did exist at some times for some layers when using an inflow domain based on the observed MCS initiation, the general hypothesis still seems to be valid in these events, but none of the techniques tested in the present study was able to identify an inflow box that forecasters could use in advance of MCS initiation to predict displacement errors
Summary
Convective Systems (MCSs) [1,2,3,4,5]. Nocturnal MCSs typically ingest buoyant air parcels from layers of air above the convectively stable PBL [6,7,8,9]. Based on the findings of Peters et al [40] for one MCS event, the present study seeks to find whether or not a strong statistical correlation exists between errors in several thermodynamic variables in the upstream inflow regions of initiating MCSs in simulations, and the spatial displacement errors of the simulated MCSs. In addition, because Squitieri and Gallus [22] found differing behaviors in cases with strongly forced versus weakly forced LLJ cases, the present study explores sensitivity of the correlations to the amount of larger scale forcing. It is hypothesized that if negative (positive) simulated moisture errors exist in the inflow region for upscale growing convection, simulated MCSs will be displaced downstream (upstream) as more (less) lift would be required within the broad ascending airstream to bring parcels to their LFC, and that the greater the magnitude of the errors, the larger the displacements will be.
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