Convective activity in an extratropical cyclone and its warm conveyor belt – a case‐study combining observations and a convection‐permitting model simulation

Oertel, Annika; Boettcher, Maxi; Joos, Hanna; Sprenger, Michael; Konow, Heike; Hagen, Martin; Wernli, Heini

doi:10.1002/qj.3500

Cited by 68 publications

(227 citation statements)

References 76 publications

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“…WCB ascent has been shown to include sporadic periods of intense, convective‐like ascent (e.g. Browning, ; Oertel et al ., ) and may explain the strongest dependence of the WCB to the convection and cloud parametrizations found here. For this case‐study we have demonstrated that modifying parametrized diabatic processes changed the properties of the WCB, the diabatic heating of air parcels in the WCB ascent, and the upper‐level ridge amplification and block forecast.…”

Section: Discussionmentioning

confidence: 55%

Impact of model upgrades on diabatic processes in extratropical cyclones and downstream forecast evolution

Maddison

Gray

Martínez‐Alvarado

et al. 2020

Quart J Royal Meteoro Soc

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Models are continuously developed at numerical weather prediction (NWP) centres to improve forecast skill, with new operational model configurations adopted every few years. The parametrizations of diabatic processes are probably the most frequently updated part of NWP models as they are crucial for accurate weather predictions and contain uncertainties in their formulation. The impact of model developments is assessed here in forecasts from the Met Office's weather forecast model initialised throughout the North Atlantic Waveguide and Downstream Impact Experiment field campaign period in autumn 2016. Planned model parametrization developments are considered, together with an 'inexpensive coupled' forecast with daily updating of the sea surface temperature and sea-ice fraction. Forecasts produced from the coupled system have, on average, indistinguishable skill from the control forecasts, suggesting the benefits of coupled atmosphere-ocean NWP systems can be small. In contrast, a reduction in forecast error (∼4%) is identified in forecasts produced using an upgraded convection scheme. Periods of low forecast skill during the study period are shown to be associated with the onset and decay of blocking events and increased diabatic heating of air masses reaching the upper troposphere. In forecasts of a specific block development case that was not accurately predicted in any of the experiments or in the operational ensemble forecast from the Met Office, the representation of diabatic heating in the warm conveyor belt of an upstream cyclone is shown to moderate the subsequent block development; forecasts in which the heating is stronger generally have a more-amplified blocking ridge and amplified heating contributions from all parametrizations as diagnosed using diabatic tracers. Hence, we demonstrate that plausible changes to the representation of several different diabatic processes in models can impact forecast block development via changes within upstream cyclones. K E Y W O R D Satmospheric blocking, numerical weather prediction, parametrized diabatic processes, warm conveyor belts This article is published with the permission of the Controller of HMSO and the Queen's Printer for Scotland.

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Section: Discussionmentioning

confidence: 55%

Impact of model upgrades on diabatic processes in extratropical cyclones and downstream forecast evolution

Maddison

Gray

Martínez‐Alvarado

et al. 2020

Quart J Royal Meteoro Soc

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“…Another ridge exists around 20 • W-10 • E (label R3), which is characterized by ridge building between 2 and 3 days (Figure 2a,b). This ridge building was associated with the development of cyclone Vladiana (labelled V in Figure 2), which was another observational highlight during the NAWDEX period, due to the occurrence of pronounced warm-conveyor-belt ascent (Schäfler et al, 2018;Oertel et al, 2019). In the following, ridge R3 is characterized by a large amplitude and a similar nonlinear evolution to ridge R1, albeit its spatial extent is smaller than that of ridge R1 (Figure 2c,d).…”

Section: Synoptic Overview and Variance Evolutionmentioning

confidence: 86%

“…; Oertel et al . ). In the following, ridge R3 is characterized by a large amplitude and a similar nonlinear evolution to ridge R1, albeit its spatial extent is smaller than that of ridge R1 (Figure c,d).…”

Section: Quantitative View Of the Variance Amplificationmentioning

confidence: 99%

Processes governing the amplification of ensemble spread in a medium‐range forecast with large forecast uncertainty

Baumgart

Riemer

2019

Quart J Royal Meteoro Soc

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This study provides a process‐based perspective on the amplification of forecast uncertainty and forecast errors in ensemble forecasts. A case from the North Atlantic Waveguide and Downstream Impact Experiment that exhibits large forecast uncertainty is analysed. Two aspects of the ensemble behaviour are considered: (a) the mean divergence of the ensemble members, indicating the general amplification of forecast uncertainty, and (b) the divergence of the best and worst members, indicating extremes in possible error‐growth scenarios. To analyse the amplification of forecast uncertainty, a tendency equation for the ensemble variance of potential vorticity (PV) is derived and partitioned into the contributions from individual processes. The amplification of PV variance is, on average for the midlatitudes of the Northern Hemisphere, dominated by near‐tropopause dynamics. Locally, however, other processes can dominate the variance amplification, for example, in the region where tropical storm Karl interacts with the Rossby‐wave pattern during extratropical transition. In this region, the variance amplification is dominated by upper‐tropospheric divergence and tropospheric–deep interaction and is thereby mostly related to (moist baroclinic) cyclone development. The differences between the error growth in the best and worst ensemble members can, to a large part, be attributed to differences in the representation of cut‐off evolution around 3 days, which subsequently amplifies substantially in the highly nonlinear region of the Rossby‐wave pattern until 5 days. In terms of the processes, the differences in error growth are dominated by differences in the error growth by near‐tropopause dynamics. The approach presented provides flow‐dependent insight into the dynamics of forecast uncertainty and forecast errors and helps to understand better the different contributions of specific weather systems to the medium‐range amplification of ensemble spread.

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“…Diabatic processes are particularly important in warm conveyor belts (WCBs), which are coherent, typically poleward-ascending airstreams associated with extratropical cyclones (Harrold, 1973;Browning, 1986Browning, , 1999Wernli 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).…”

mentioning

confidence: 99%

“…Although WCBs are typically described as gradually ascending and mainly stratiform-cloud-producing airstreams (e.g., Browning, 1986;Madonna et al, 2014), the concept of rapid convective motion embedded in the frontal cloud band of the WCB had already been proposed in 1993 (Neiman et al, 1993). Recent studies have suggested that the WCB is, at least in some cases, not a homogeneously ascending airstream: in contrast, the detailed ascent behavior of the individual WCB trajectories associated with one extratropical cyclone can vary substantially (e.g., Rasp et al, 2016;Oertel et al, 2019), and convective activity can be frequently embedded in the largescale baroclinic region of the WCB. This has been identified, e.g., with various remote-sensing data (Binder, 2016;Crespo and Posselt, 2016;Flaounas et al, 2016Flaounas et al, , 2018, with online trajectories in convection-permitting simulations (Rasp et al, 2016;Oertel et al, 2019), and in coarser simulations with parameterized convection (Agustì-Panareda et al, 2005;.…”

mentioning

confidence: 99%

Potential vorticity structure of embedded convection in a warm conveyor belt and its relevance for large-scale dynamics

Oertel¹,

Boettcher²,

Joos³

et al. 2020

Weather Clim. Dynam.

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128

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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.

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Convective activity in an extratropical cyclone and its warm conveyor belt – a case‐study combining observations and a convection‐permitting model simulation

Cited by 68 publications

References 76 publications

Impact of model upgrades on diabatic processes in extratropical cyclones and downstream forecast evolution

Impact of model upgrades on diabatic processes in extratropical cyclones and downstream forecast evolution

Processes governing the amplification of ensemble spread in a medium‐range forecast with large forecast uncertainty

Potential vorticity structure of embedded convection in a warm conveyor belt and its relevance for large-scale dynamics

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