Quantifying cloud microphysical uncertainties in an extratropical cyclone’s ascending airstream using Lagrangian diagnostics

The characteristic large-scale and strongly precipitating cloud band in extratropical cyclones is associated with the so-called warm conveyor belt (WCB), which is a coherent cyclone-relative airstream that ascends cross-isentropically from the boundary layer into the upper troposphere. Cloud microphysical processes along this ascending airstream determine the total diabatic heating, cloud structure, and associated surface precipitation characteristics.

We disentangle uncertainty related to the representation of cloud microphysical processes in the two-moment microphysics scheme of the ICOsahedral Nonhydrostatic (ICON) modeling framework in a convection-permitting simulation setup for an extratropical cyclone case study in the North Atlantic. To quantify uncertainty, we employ a perturbed parameter ensemble (PPE) approach, whereby five selected uncertain parameters in the cloud microphysics scheme and environmental conditions relevant for cloud formation are perturbed simultaneously and systematically. Specifically, cloud microphysical uncertainty is quantified along Lagrangian WCB trajectories which are calculated online during the ICON simulations from the resolved 3D wind fields at every model time step for each of the 70 PPE members. The Lagrangian perspective not only facilitates the characterisation of the airstream’s ascent behaviour but also provides detailed insight in cloud and precipitation formation along the ascent. The application of the Lagrangian diagnostics to all PPE members enables the quantification of dominant contributions of uncertainty from the perturbed parameters for WCB ascent characteristics, such as ascent timescales and tracks, as well as for precipitation formation along the ascent.

For example, we show that the precipitation efficiency along the ascending airstream is most strongly influenced by cloud condensation nuclei (CCN) concentrations modifying the cloud droplet to rain drop conversion. Moreover, a trajectory-based airstream-relative composite analysis shows that increased CCN concentrations result in a downstream shift of the surface precipitation relative to the eastward propagating airstream as the precipitation efficiency is reduced. In addition, the Lagrangian diagnostics can illustrate the feedback between diabatic heating from cloud microphysical processes in the mixed-phase and local vertical velocity. In this contribution we present our analysis framework and show how the perturbed parameters influence various Lagrangian diagnostics for WCB ascent and associated cloud and precipitation formation.