Effects of resolving convection in IFS simulations on the representation of warm conveyor belts

Abstract

Warm conveyor belts (WCBs) are coherent airstreams in extratropical cyclones that ascend from the planetary boundary layer in the cyclone’s warm sector to the upper troposphere within about two days. They are the primary cloud and precipitation-producing flow in extratropical cyclones and can trigger extreme precipitation events. The diabatic processes during the WCB ascent influence the dynamics of extratropical cyclones, and WCBs can affect the downstream Rossby wave pattern. Thus, the correct representation of WCBs is essential for the quality of numerical weather prediction. Convection embedded in WCBs frequently occurs and modifies the precipitation pattern. In this thesis, the influence of model resolution and explicitly sim- ulating deep convection on the representation of WCBs is explored in global high-resolution simulations. We calculate and examine WCB trajectories over a set of simulations using the hy- drostatic Integrated Forecasting System with parameterized and explicit convection at horizontal grid spacings of 1.4km, 4.5km, and 9km. We present a global comparison of the simulations, three WCB case studies, and a systematic intercomparison of WCB trajectories over 10 days. We find that the simulations with explicit convection suffer, especially in the tropics, from model spin-up and produce too much precipitation during the first hours to days. In the three case studies, differences in WCB representation between the simulations become apparent after only a few hours lead time. These comprise distinct differences in the location and ascent rates of WCB air parcels and the ascent structure at the cold front. However, the simulations show a large case-to-case variability in their differences. For the case study with fast ascending trajecto- ries and partly convective characteristics, the differences in the WCB trajectory characteristics are more pronounced than for the other two case studies with a more gradual ascent. The statistical analyses of the WCB trajectory characteristics performed over the 10 days confirm the differences found in the three case studies. Overall, a larger number of WCB trajectories ascend in the simulations with explicit convection than in the simulation with parameterized convection. Furthermore, WCB trajectories in the simulations with explicit convection ascend higher and faster and, thus, reach their minimum pressure after a shorter time. Among the simulations with explicit convection, increasing resolution decreases the number of WCB tra- jectories, the total ascent, and the ascent speed. Even though the characteristics of the WCB trajectories differ in the simulations, no apparent effect on the dynamical impact of the WCB and on the downstream development could be identified in the investigated WCB case studies. This highlights the missing knowledge under which synoptic conditions WCBs largely influence the downstream development.