Numerical simulation of a rapidly developing bow echo over northeastern Poland on 21 August 2007 using near-grid-scale stochastic convection initiation

Abstract. A rapidly developing fast propagating meso-β-scale severe bow echo that developed over northeastern Poland on 21 August 2007 caused significant property damage and resulted in 12 fatalities. The operational model of Consortium for Small Scale Modeling (COSMO) with a horizontal grid spacing of 2.2 km is used for its numerical simulation but encounters significant problems despite favorable environmental conditions. The implementation of a new stochastic convection initiation scheme in a 9-member ensemble enables the numerical reconstruction of the event as the cold-pool-driven convective system, with peak gusts closely matching the observed values. The scheme uses small-scale temperature perturbations arguably resembling near-grid-scale convective cells developing within the boundary layer. The perturbations influence not only CAPE and CIN, but also the lower-tropospheric vertical wind shear. Initial and boundary conditions for the experiment are based on ERA5 reanalysis. Additional data assimilation of local surface observations improves the reconstruction of atmospheric environmental conditions. A supplementary experiment tests the forecast sensitivity to an increase of low-to-mid tropospheric winds, and thus the vertical shear, and shows an increase of the maximum surface gusts within the ensemble when the convection initiation is implemented. The simulations' main drawbacks are about an hour delay with respect to observations in the development of maximum gusts and a tendency to produce isolated convective cells along the leading edge of the system's cold pool rather than a more coherent structure observed within the bow echo.