Impacts of initial condition perturbation blending in 10- and 40-member convection-allowing ensemble forecasts

Abstract

A series of convection-allowing 36-hour ensemble forecasts during the 2018 Spring season are used to better understand the impacts of ensemble configuration and blending different sources of initial condition (IC) perturbation. Ten- and 40-member ensemble configurations are initialized with the multi-scale IC perturbations generated as a product of convective-scale data assimilation (MULTI), and initialized with the MULTI IC perturbations blended with IC perturbations downscaled from coarser resolution ensembles (BLEND). The forecast performance of both precipitation and non-precipitation variables is consistently improved by the larger ensemble size. The benefit of the larger ensemble is largely, but not entirely, due to compensating for under-dispersion in the fixed-physics ensemble configuration. A consistent improvement in precipitation forecast skill results from blending in the 10-member ensemble configuration, corresponding to a reduction in the ensemble calibration error (i.e., reliability component of Brier Score). In the 40-member ensemble configuration, the advantage of blending is limited to the ∼18-22 hour lead times at all precipitation thresholds, and the ∼35-36 hour lead times at the lowest threshold, both corresponding to an improved resolution component of the Brier Score. The advantage of blending in the 40-member ensemble during the diurnal convection maximum of ∼18-22 hour lead times is primarily due to cases with relatively weak synoptic scale forcing while advantages at later lead times beyond ∼30 hours lead time are most prominent on cases with relatively strong synoptic scale forcing. The impacts of blending and ensemble configuration on forecasts of non-precipitation variables is generally consistent with the impacts on the precipitation forecasts.