Interaction of microphysics and dynamics in a warm conveyor belt simulated with the ICON model

Abstract

Abstract. The representation of warm conveyor belts (WCBs) in numerical weather prediction (NWP) models is important, as they are responsible for the major precipitation in extratropical cyclones and modulate the large-scale flow evolution. Their cross-isentropic ascent into the upper troposphere is influenced by latent heat release mostly, but not exclusively, from cloud formation whose representation in NWP models is associated with large uncertainties. The diabatic heating additionally modifies the potential vorticity (PV) distribution which influences the circulation. We analyse diabatic heating and associated PV rates from all physics processes, including microphysics, turbulence, convection, and radiation, in a case study of a WCB that occurred during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) campaign using the Icosahedral Nonhydrostatic (ICON) modelling framework. In particular, we consider all individual microphysical process rates that are implemented in ICON's two-moment microphysics scheme, which sheds light on (i) which microphysical processes dominate the diabatic heating and PV structure in the WCB, and thus, potentially influence the large-scale flow, and (ii) which microphysical processes are most active during the ascent and influence cloud formation and characteristics. For this purpose, diabatic heating and PV rates are integrated along online WCB trajectories. Our convection-permitting simulation setup also takes the reduced aerosol concentrations over the North Atlantic into account. Complementary Lagrangian and Eulerian perspectives on diabatic heating and PV modification confirm that microphysical processes are the dominant diabatic heating contribution during ascent. Near cloud top longwave radiation cools WCB air parcels. Due to the longevity of the WCB cloud band, the diabatic heating contributions from radiation, and corresponding PV modification in the upper troposphere, are non-negligible. The turbulence scheme is active in the WCB ascent region, despite large gradient Richardson numbers, and process rates from turbulence and microphysics partially counteract each other. From all microphysical processes condensational growth of cloud droplets and vapor deposition on frozen hydrometeors most strongly influence diabatic heating and PV, while below-cloud evaporation strongly cools WCB air parcels prior to their ascent and increases their PV value. PV production is strongest near surface, and extends up to 4 km height with substantial contributions from condensation, melting, evaporation, and vapor deposition. In the upper troposphere, PV is reduced by diabatic heating from vapor deposition, condensation, and radiation. Activation of cloud droplets as well as homogeneous and heterogeneous freezing processes have a negligible diabatic heating contribution due to small overall mass conversion, but their detailed representation is likely important as the hydrometeor size distributions influence other microphysical processes. Generally, faster ascending WCB trajectories are heated markedly more than more slowly ascending WCB trajectories, which is linked to larger initial specific humidity content of fast WCB trajectories providing a thermodynamic constraint on total microphysical heating. Yet, the total diabatic heating contribution of convectively ascending trajectories is relatively small due to their small fraction in this case study.

Why Moist Dynamic Processes Matter for the Sub‐Seasonal Prediction of Atmospheric Blocking Over Europe

“…WCBs are challenging to predict due to the small‐scale cloud‐microphysical processes associated with their air stream (Oertel et al., 2020; Oertel, Miltenberger, et al., 2023) thereby facing both intrinsic limits of predictability and problems in an accurate representation in models. In a previous study, we could show that skillful predictions of WCBs in current numerical weather prediction (NWP) models are possible only until around 8–10 days (Wandel et al., 2021).…”