Predictability and dynamics of a squall line and bow echo event during BAMEX

An ensemble of 40 cloud-resolving WRF forecasts initialized with perturbations from an ensemble Kalman filter (EnKF) analysis is used to explore the predictability and dynamics of a bow echo event during BAMEX (Bow Echo and MCV Experiment). On 9-10 July 2003, damaging surface winds in excess of 60 mph were associated with two distinct bow echoes moving southeast across eastern Nebraska and southwestern Iowa. These bow echoes evolved from two distinct tornadic high-precipitation supercells eventually developing into a large (~600 km) line echo wave pattern.

Subjectively studying the individual ensemble members showed large variability between the member's mode and evolution, convective spatial and temporal characteristics, and larger grid scale flow and environmental properties. Given realistic uncertainties depicted by the EnKF analysis perturbations, certain members were found to have evident squall line and bow echo characteristics including strong rear inflow jets, notches, and bowing features while others developed discrete and widespread convective structures with no apparent organization. This variability in ensembles with similar synoptic characteristics can be attributed to the role moist convection plays on squall line and bow echo development and the dominating effect of ingested CAPE (convective available potential energy). Further correlations and ensemble energy analysis applied to study the ensemble member discrepancies illuminate the possible connection between CAPE, environmental shear, LFC (level of free convection), and convection initiation to the development of members with “good” versus “poor” squall line or bow echo characteristics. The findings are in strong agreement with previous model and observed conditions for bow echo and squall line evolution which affirm the viability of this type of predictability and dynamic study of initial condition variability in deducing dynamical features responsible for storm evolution.