Numerical reconstruction 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 reconstruction but encounters significant problems despite favorable environmental conditions. Implementation of a new stochastic convection initiation scheme within a 9-member ensemble allows to reconstruct the event as the cold-pool-driven convective system with maximum gusts close to the observed ones. The scheme uses small-scale temperature perturbations arguably resembling grid-scale convective boundary layer thermals that influence not only MCAPE and MCIN, but also the lower-tropospheric vertical 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 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.