Sensitivities of warm conveyor belt ascent, associated precipitation characteristics and large‐scale flow pattern: Insights from a perturbed parameter ensemble

Abstract The characteristic and strongly precipitating cloud band near extratropical cyclones is associated with a coherent cross‐isentropically ascending airstream, the warm conveyor belt (WCB). The WCB ascent characteristics and associated diabatic heating are influenced by cloud microphysical processes and environmental conditions in the WCB inflow region. The former relies on the cloud microphysical parametrisation, and the latter on initial and/or boundary conditions of thermodynamic variables. Altogether, this introduces uncertainty in numerical weather prediction (NWP) forecasts of the WCB characteristics and ultimately the prediction of the large‐scale midlatitude flow. We quantify the relative influence of perturbations on various cloud microphysical processes and WCB inflow temperature and moisture (via modification of sea‐surface temperature) for a case study and thereby focus on uncertainty in WCB ascent behaviour, associated precipitation characteristics, and properties of the amplifying ridge downstream of the ascent region. To disentangle individual uncertainty contributions, we build a high‐resolution 70‐member perturbed parameter ensemble (PPE) with the Icosahedral Nonhydrostatic (ICON) modelling framework. The PPE systematically combines parameter perturbations and samples the phase space spanned by parameter uncertainty ranges. Based on the PPE, computationally cheap statistical surrogate models are developed that subsequently facilitate variance‐based sensitivity analysis for the target variables. Our results suggest that changes to environmental conditions controlling WCB inflow properties most strongly influence WCB ascent behaviour, total surface precipitation sums, and the ridge amplitude. Yet, the microphysical perturbations locally modify vertical velocity along WCB ascent and determine the precipitation efficiency, which affects meso‐scale precipitation characteristics as well as the spatial distribution and intensity of precipitation. Moreover, the microphysical perturbations distinctly influence the large‐scale flow pattern, albeit to a lesser extent than the perturbations applied to the WCB inflow characteristics.