The role of warm conveyor belts for the sub-seasonal prediction of blocked weather regimes

Variability of the large-scale atmospheric circulation in the midlatitudes is characterised by transient extratropical Rossby waves and stationary patterns, such as blocking anticyclones. On sub-seasonal time scales, the extratropical variability can be depicted by weather regimes which are quasi-stationary, persistent, and recurrent flow patterns. Accurate forecasts of the weather regimes are particularly relevant due to their longevity on sub-seasonal time scales and potential socio-economic impact. Research in recent years emphasized the modulation of blocked weather regimes by processes on synoptic scales, in particular by diabatic processes in extratropical cyclones. These diabatic processes are related to latent heat release in the so-called warm conveyor belt (WCB). The WCB is an ascending air stream in the warm sector of an extratropical cyclone and its “diabatic outflow” modulates the upper-level jet, resulting in the amplification of an upper-level ridge and eventually a block. This study systematically investigates the representation of WCBs around the onset of blocked weather regimes over the Atlantic-European region in the sub-seasonal reforecasts of the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS). In general, there is a strong link between the flow amplification around blocking onsets and high WCB outflow activity in the region of the incipient block. The analysis of the 11 ensemble members in the extended winter season from 1997 – 2017 shows that the ECMWF’s IFS reforecasts underestimate both the wave amplification and WCB frequencies for regime onsets in forecast week 2 (forecast day 8 – 14). Due to the persistence of the weather regimes, onsets on these time scales typically lead to active life cycles in week 3 and sometimes even week 4. The analysis of the temporal evolution prior to the onset reveals that the reforecasts establish large-scale flow anomalies via WCB outflow differently than observed with a more amplified large-scale flow and high WCB activity upstream of the incipient block. Our findings show that forecast biases in WCB activity might be partly responsible for the relatively poor skill of blocking forecasts in week 2 and 3 and highlight the potential for further improvement of sub-seasonal prediction by improving the representation of WCBs in NWP models.