Physical processes controlling warm conveyor belt moisture transport to the UTLS and dependence on model resolution

Warm conveyor belts (WCB) are regions of large-scale coherent airflow within extratropical cyclones that rapidly ascend from the boundary layer to the upper troposphere. During their ascent, WCBs transport water vapour and cloud condensate to the upper troposphere, and thereby significantly contribute to the moisture content of the extra-tropical upper troposphere-lower stratosphere (UTLS) as well as upper tropospheric cloudiness. UTLS moisture content and cloudiness are important for the radiative budget of the Earth and future changes thereof, but are often poorly represented in numerical models and reanalysis products. A detailed quantitative understanding of the processes governing water transport in WCBs provides vital clues to the origin of these biases and for evaluating predicted future changes in WCB moisture transport. Furthermore, recent studies have found that deep and embedded convection play an important role in WCBs. This points to the necessity of high-resolution simulations, that are well validated with observational data to provide a “benchmark” for coarser-resolution global (climate) models. Here we investigate the physical processes governing WCB moisture transport in simulations of a case-study from the WISE campaign with a particular focus on (i) the impact of grid spacing (including the use of convection parameterisations) on WCB moisture transport, (ii) the microphysical processes controlling moisture loss from the WCB, and (iii) the cloud microphysical properties of the cirrus clouds in the WCB outflow.

To this end we conducted two ICON simulations of an extratropical cyclone using (i) a global (~13km resolution), convection-parameterizing and (ii) a doubly nested (~13km, ~6km and ~3km resolution) convection permitting set up. In both set-ups online trajectories are calculated that capture convective ascent and allow for a Lagrangian analysis of WCB moisture transport and WCB cloud structure.

The Lagrangian metrics show large differences in ascent timescales and the efficiency with which water is transported from the boundary-layer to the UTLS. It is shown that this impacts the UTLS moisture content in the WCB outflow region. Local changes in UTLS moisture content induced by different representations of convection are shown to project onto larger-scale structures in the moisture and cloud fields over the 1-2 days after WCB ascent.