Butterflies and Bow Echoes

Mesoscale bowing structures, either as line-echo wave patterns or progressive bow echoes, are often poorly predicted in deterministic numerical weather prediction forecasts. Poor forecasts may be related to lower inherent predictability of bowing structures, a result of heightened sensitivity to model configuration and initial-/boundary-condition error. Ensemble simulations better address this potential heightened sensitivity, and may determine low-predictability environments, but the spread of bowing structures in an ensemble is unknown. Further, how is the ensemble's output to be interpreted, when the ensemble mean of simulated radar reflectivity is not likely to yield a physically realistic structure?

In this study, we discuss the spread of modes in various ensemble configurations. We show how better initial conditions would improve positioning and timing, but that bowing is still sensitive to the smallest “butterfly-wing flaps”. We also present evidence that increasing grid resolution might yield a more diverse spread of bowing radii, and potentially a more difficult interpretation: it even suggests that a higher resolution deterministic forecast may be more likely to misrepresent a bowing feature. Finally, predictability of progressive bow echoes appears lower than with line-echo wave patterns, and potential reasons for this are discussed.