Search

This study presents a systematic investigation of the characteristics and meteorological impacts of warm conveyor belts (WCBs). For this purpose, we compile a new WCB climatology (1980–2022) of trajectories calculated with the most recent reanalysis dataset ERA5 from the European Centre for Medium-Range Weather Forecasts (ECMWF). Based on this new climatology, two-dimensional masks are defined that represent the inflow, ascent, and outflow locations of WCBs. These masks are then used to objectively quantify the key characteristics (intensity, ascent rate, and ascent curvature) and meteorological impacts (precipitation and potential vorticity (PV) anomalies) of WCBs in order to (i) attribute them to different stages in the life cycle of the associated cyclones and to (ii) evaluate differences in the outflow of the cyclonic and anticyclonic branches. The approach was applied globally, but this study focuses on the North Atlantic, one of the regions where WCBs ascend most frequently. The method is first tested and illustrated through three case studies of well-documented cyclones, revealing both the similarities and the case-to-case variability in the evolution of the WCB characteristics and impacts. We then extend the analysis to about 5000 cyclones that occurred in winter between 1980–2022 in the North Atlantic. The case studies and the climatological analysis both show that WCBs are typically most intense (in terms of air mass transported, ascent rate, precipitation rate, and volume) during the intensification period of the associated cyclone. The northward displacement along the storm track and diabatic PV production lead to an increase in low-level PV in the region of WCB ascent during the cyclone life cycle. The negative PV anomaly at upper levels, associated with the WCB outflow, remains relatively constant. The investigation of the WCB branches reveals an increasing intensity of the cyclonic WCB branch with time, linked to the increasing strength of the cyclonic wind field around the cyclone. Due to a lower altitude, the outflow of the cyclonic WCB branch is associated with a weaker negative PV anomaly than the anticyclonic one, which ascends to higher altitudes. In summary, this study highlights the distinct evolution of WCB characteristics and impacts during the cyclone life cycle and the marked differences between the cyclonic and anticyclonic branches. [ABSTRACT FROM AUTHOR]

Warm conveyor belts (WCBs) are strongly ascending, cloud and precipitation forming airstreams in extratropical cyclones. The intense cloud-diabatic processes produce low-level cyclonic potential vorticity (PV) along the ascending airstreams, which often contribute to the intensification of the associated cyclone. This study investigates how climate change affects the cyclones' WCB strength and the importance of WCB-related diabatic PV production for cyclone intensification, based on present-day (1990–1999) and future (2091–2100) climate simulations of the Community Earth System Model Large Ensemble (CESM-LE). In each period, a large number of cyclones and their associated WCB trajectories have been identified in both hemispheres during the winter season. Compared to ERA-Interim reanalyses, the present-day climate simulations are able to capture the cyclone structure and the associated WCBs remarkably well, which gives confidence in future projections with CESM-LE. The comparison of the simulations reveals an increase in the WCB strength and the cyclone intensification rate in the Southern Hemisphere (SH) in the future climate. The WCB strength also increases in the Northern Hemisphere (NH), but to a smaller degree, and the cyclone intensification rate is not projected to change considerably. Hence, in the two hemispheres cyclone intensification responds differently to an increase in WCB strength, which however is consistent with the opposite changes in near-surface baroclinicity expected with rising temperatures. Indeed, baroclinicity is expected to increase in the SH, which interacts positively with the direct effects of enhanced WCB-related diabatic heating to produce stronger cyclones, whereas it is expected to decrease in the NH, which counteracts the effects of the moist processes in the WCB ascent. Cyclone deepening correlates positively with the intensity of the associated WCB, with a Spearman correlation coefficient of 0.68 (0.66) in the NH in the present-day (future) simulations, and a coefficient of 0.51 (0.55) in the SH. The number of explosive cyclones with strong WCBs, referred to as C1 cyclones, is projected to increase in both hemispheres, while the number of explosive cyclones with weak WCBs (C3 cyclones) is projected to decrease. A composite analysis reveals that in the future climate C1 cyclones will be associated with even stronger WCBs, more WCB-related diabatic PV production, the formation of a more intense PV tower, and an increase in precipitation. They will become warmer, moister, and slightly more intense. The findings indicate that (i) cyclones will be more diabatic in a warmer climate, (ii) WCB-related PV production will be even more important for explosive cyclone intensification than in the present-day climate, and (iii) the interplay between dry and moist dynamics is crucial to understand how climate change affects cyclone intensification. [ABSTRACT FROM AUTHOR]

This study investigates how warm conveyor belts (WCB) will change in a future climate. WCBs are strongly ascending airstreams in extratropical cyclones which are responsible for most of their precipitation. In conjunction with the strong cloud formation, latent heat is released which has an impact on the potential vorticity distribution and therefore on the atmospheric circulation in the mid- and upper-troposphere. Because of these and other impacts of WCBs, it is of great importance to investigate their changes in a warmer climate. To this aim, future climate simulations (RCP8.5 scenario; 2091–2100) are performed with the Community Earth System Model version 1 (CESM1) and compared to a present-day climate (1991–2000). WCB trajectories are calculated based on the six-hourly 3D wind fields. WCBs are represented reasonably well in terms of location and occurrence frequency compared to WCBs in the ERA-Interim data set. In a future climate, WCB inflow regions in the North Pacific are systematically shifted northward in winter, which is in agreement with the northward shift of the storm track in this region. In the North Atlantic, increased frequencies are discernible in the southwest and a decrease to the south of Iceland. Finally, in the Southern Hemisphere, WCB frequencies increase in the South Atlantic, whereas they decrease near Madagascar. Part of these changes are consistent with corresponding changes in the occurrence frequencies of extratropical cyclones, i.e., the driving weather systems of WCBs. Changes are also found in the WCB characteristics, e.g., in specific humidity of the WCB inflow, the WCB-related precipitation, the cross-isentropic ascent and the isentropic level reached by the WCB outflow. This has implications for WCB impacts in a future climate. For instance, the strong increase in inflow moisture leads to: (i) a strong increase in WCB-related precipitation, especially in the upper percentiles, thus extreme precipitation related to WCBs might increase; (ii) a strong increase in diabatic heating in the mid-troposphere; and (iii) a higher outflow level which favours WCBs to more strongly interact with the upper-level Rossby waveguide. In summary, by investigating a distinct weather system, the WCB, and how it changes in its occurrence frequency and characteristics in a future climate, this study provides new insights into the dynamics and impacts of climate change in the mid-latitudes. [ABSTRACT FROM AUTHOR]