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Abstract:
The characteristic and strongly precipitating cloud band in extratropical cyclones is associated with the so-called warm conveyor belt (WCB), which is a coherent airstream ascending cross-isentropically from the boundary layer into the upper troposphere within two days. The WCB ascent behaviour and associated diabatic heating are influenced by microphysical processes and environmental conditions in the WCB inflow region. The former relies on parametrisation schemes, the latter on initial and/or boundary conditions of thermodynamic variables. Altogether, this introduces uncertainty in numerical weather prediction and ultimately for the evolution of the large-scale mid-latitude flow. Based on a case study from the NAWDEX field campaign, we quantify the relative importance of perturbations to various microphysical processes and WCB inflow temperature and moisture (via modification of sea surface temperature). Thereby, we focus on uncertainty in WCB ascent behaviour, associated precipitation characteristics, as well as properties of the amplifying ridge downstream of the ascent region. To disentangle individual uncertainty contributions, we build a 70-member perturbed parameter ensemble which systematically combines the perturbations, and subsequently perform variance-based sensitivity analysis. Our results suggests that changes to WCB inflow properties most strongly influence WCB ascent behavior, surface precipitation sums, and the ridge amplitude. Yet, the microphysical perturbations locally modify vertical velocity in the WCB and determine the precipitation efficiency, which affects local to meso-scale precipitation characteristics and its spatial distribution. Moreover, the microphysical perturbations are distinctly imprinted in the large-scale flow pattern - albeit to a lesser extent than the perturbations applied to the WCB inflow characteristics.
