Abstract
Abstract. Warm conveyor belts (WCBs) are important airstreams in extratropical cyclones. They can influence large-scale flow evolution by modifying the potential vorticity (PV) distribution during their cross-isentropic ascent. Although WCBs are typically described as slantwise-ascending and stratiform-cloud-producing airstreams, recent studies identified convective activity embedded within the large-scale WCB cloud band. However, the impacts of this WCB-embedded convection have not been investigated in detail. In this study, we systematically analyze the influence of embedded convection in an eastern North Atlantic WCB on the cloud and precipitation structure, on the PV distribution, and on larger-scale flow. For this reason, we apply online trajectories in a high-resolution convection-permitting simulation and perform a composite analysis to compare quasi-vertically ascending convective WCB trajectories with typical slantwise-ascending WCB trajectories. We find that the convective WCB ascent leads to substantially stronger surface precipitation and the formation of graupel in the middle to upper troposphere, which is absent for the slantwise WCB category, indicating the key role of WCB-embedded convection for precipitation extremes. Compared to the slantwise WCB trajectories, the initial equivalent potential temperature of the convective WCB trajectories is higher, and the convective WCB trajectories originate from a region of larger potential instability, which gives rise to more intense cloud diabatic heating and stronger cross-isentropic ascent. Moreover, the signature of embedded convection is distinctly imprinted in the PV structure. The diabatically generated low-level positive PV anomalies, associated with a cyclonic circulation anomaly, are substantially stronger for the convective WCB trajectories. The slantwise WCB trajectories lead to the formation of a widespread region of low-PV air (that still have weakly positive PV values) in the upper troposphere, in agreement with previous studies. In contrast, the convective WCB trajectories form mesoscale horizontal PV dipoles at upper levels, with one pole reaching negative PV values. On a larger scale, these individual mesoscale PV anomalies can aggregate to elongated PV dipole bands extending from the convective updraft region, which are associated with coherent larger-scale circulation anomalies. An illustrative example of such a convectively generated PV dipole band shows that within around 10 h the negative PV pole is advected closer to the upper-level waveguide, where it strengthens the isentropic PV gradient and contributes to the formation of a jet streak. This suggests that the mesoscale PV anomalies produced by embedded convection upstream organize and persist for several hours and therefore can influence the synoptic-scale circulation. They thus can be dynamically relevant, influence the jet stream and (potentially) the downstream flow evolution, which are highly relevant aspects for medium-range weather forecast. Finally, our results imply that a distinction between slantwise and convective WCB trajectories is meaningful because the convective WCB trajectories are characterized by distinct properties.
Highlights
1.1 Warm conveyor belts and embedded convectionMoist diabatic processes are known to play an important role in the evolution of extratropical cyclones and are frequently associated with rapid cyclogenesis (e.g., Anthes et al, 1983; Kuo et al, 1991; Stoelinga, 1996; Wernli et al, 2002) and increased forecast error growth
We analyzed the influence of embedded convection in the Warm conveyor belts (WCBs) of Cyclone Vladiana in the North Atlantic in September 2016 on precipitation, cloud structure, the local mesoscale dynamics, and larger-scale circulation features
Two categories of online WCB trajectories – very rapidly ascending “convective” WCB trajectories and more slowly ascending “slantwise” WCB trajectories – were identified in a convection-permitting COSMO simulation, and their impact was investigated in a composite analysis and, in more detail, for a representative example
Summary
Moist diabatic processes are known to play an important role in the evolution of extratropical cyclones and are frequently associated with rapid cyclogenesis (e.g., Anthes et al, 1983; Kuo et al, 1991; Stoelinga, 1996; Wernli et al, 2002) and increased forecast error growth Diabatic processes are important in warm conveyor belts (WCBs), which are coherent, typically poleward-ascending airstreams associated with extratropical cyclones (Harrold, 1973; Browning, 1986, 1999; Wernli and Davies, 1997) During their typically slantwise cross-isentropic ascent from the boundary layer ahead of the cold front to the upper troposphere, they form large-scale, mostly stratiform cloud bands and play a key role in the distribution of surface precipitation (e.g., Browning, 1986; Eckhardt et al, 2004; Madonna et al, 2014; Pfahl et al, 2014; Flaounas et al, 2018). The specific PV signatures of (i) large-scale WCB ascent and (ii) smaller-scale convective updrafts and their potential implications for the flow evolution differ substantially and are discussed in the following
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