Abstract
The bow echo, a mesoscale convective system (MCS) responsible for much hail and wind damage across the United States, is associated with poor skill in convection-allowing numerical model forecasts. Given the decrease in convection-allowing grid spacings within many operational forecasting systems, we investigate the effect of finer resolution on the character of bowing-MCS development in a real-data numerical simulation. Two ensembles were generated: one with a single domain of 3-km horizontal grid spacing, and another nesting a 1-km domain with two-way feedback. Ensemble members were generated from their control member with a stochastic kinetic-energy backscatter scheme, with identical initial and lateral-boundary conditions. Results suggest that resolution reduces hindcast skill of this MCS, as measured with an adaptation of the object-based Structure–Amplitude–Location method. The nested 1-km ensemble produces a faster system than in both the 3-km ensemble and observations. The nested 1-km simulation also produced stronger cold pools, which could be enhanced by the increased (fractal) cloud surface area with higher resolution, allowing more entrainment of dry air and hence increased evaporative cooling.
Highlights
Within the group of mesoscale convective systems (MCSs), systems that display bowing structures along the convective line are among the most poorly forecast [1]
Two ∆x = 3 km ensemble simulations of a bowing MCS, one with a nested 1-km domain, have addressed the hypothesis that a smaller ∆x increases the uncertainty within an ensemble
The spread of Total absolute SAL (taSAL) scores is larger in the single-nest ensemble
Summary
Within the group of mesoscale convective systems (MCSs), systems that display bowing structures along the convective line are among the most poorly forecast [1]. Results from a study of linear MCS evolution [21] at horizontal grid spacings of 250 m and 750 m suggest finer resolutions may limit or inhibit the descent of downdrafts within the system’s development This yielded faster cold-pool propogation at 750 m than at 250 m, but as the simulation progressed, the 250-m simulation generated a deeper cold pool. The two EPSs use a single set of ICs and LBCs that yield a bow echo in all simulations Both EPSs comprise ten likely perturbation members, created with a stochastic kinetic-energy backscatter (SKEB) scheme [23,24,25]. We chose the SKEB scheme to generate perturbations, and increase the simulation sample size, to allow use of fixed ICs and LBCs that yield a bow echo in each member. While our focus on a single case and model configuration precludes a more general conclusion about season-long performance as a function of resolution, it allows a deeper analysis of the physical reasons for systematic sensitivity to ∆x
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