Role of Uncertainty in Microphysical Processes for Warm Conveyor Belt Ascent

Abstract:

The characteristic large-scale cloud band in extratropical cyclones is often formed by the so-called warm conveyor belt (WCB), a coherent and strongly ascending airstream in extratropical cyclones that typically ascends cross-isentropically from the boundary layer into the upper troposphere within two days. This transport of air into the upper troposphere can significantly influence the large-scale flow evolution and lead to ridge amplification downstream. The cross-isentropic ascent and the WCB outflow strength in the upper troposphere are strongly driven by latent heat release from the formation of liquid, mixed-phase and ice clouds. In this way, WCBs provide an environment where small-scale cloud microphysical processes are directly linked to the large-scale atmospheric circulation in extratropical cyclones. The need for parameterization of microphysical processes and convection in numerical weather prediction models introduces uncertainties in their representation which can feed back on the larger-scale flow. In particular, ice cloud formation and the phase partitioning are often poorly represented in numerical weather prediction models. Here, we quantify the role of uncertainty in microphysical processes in WCBs for (i) the detailed WCB ascent behavior and (ii) the large-scale flow evolution. Our diagnostic framework systematically explores parameter uncertainty in microphysical parameterizations in an ensemble of sensitivity experiments using the non-hydrostatic model ICON for a WCB case study in the North Atlantic that occurred during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX). The sensitivity experiments with systematically varying microphysical parameters, and including the effect of the convection parameterization scheme, are based on two-way nested simulations with two refined nested domains. Thereby, the inner-most convection-resolving nest focuses on the microphysically active WCB ascent region, and can feed back on the global domain allowing to quantify the upscale effects of microphysical processes within the WCB on the large-scale circulation.