A Lagrangian investigation of the (micro)physical processes controlling warm conveyor belt moisture transport and cloud properties

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, WCB air parcels experience various microphysical processes that produce mixed-phase clouds and large amounts of precipitation. They also transport water vapour and cloud condensate to the upper troposphere/lower stratosphere (UTLS), which is important for Earth’s radiative budget. 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. We conduct a Lagrangian investigation of the physical processes governing WCB moisture transport and cloud composition with a particular focus on (i) the microphysical processes controlling moisture loss from the WCB, and (ii) the cloud microphysical properties of the cirrus clouds in the WCB outflow.

To this end we conducted a case-study from the HALO-WISE campaign and ran a high-resolution doubly nested ICON simulation with a maximum (convection permitting) resolution of ~3km. 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 the behaviour of moisture transport to the UTLS for trajectories with different ascent timescales. Fast ascending trajectories ascend further south and to much lower pressures and temperatures than their slower counterparts. They also produce much more precipitation and have markedly different hydrometeor contents throughout the ascent. In the ice phase, slow ascending trajectories mainly produce ice and snow through depositional growth, whereas fast trajectories also produce graupel and hail by collision-coalescence. Warm rain processes dominate the moisture loss for all ascent timescales, but for fast ascending trajectories the conversion of moisture to precipitation by microphysical processes in the ice phase increases. These findings are important because widely used coarse-resolution simulations with convection parameterization run the risk of missing the physical processes we see for the fastest ascending trajectories.