Coherent evolution of potential vorticity anomalies associated with deep moist convection

Weijenborg, Chris; Chagnon, Jeffrey M.; Friederichs, Petra; Gray, Suzanne L.; Hense, Andreas

doi:10.1002/qj.3000

Cited by 22 publications

(34 citation statements)

References 65 publications

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“…This quasi-horizontal PV dipole structure is a robust response of convective updrafts in the presence of vertical wind shear (Chagnon and Gray, 2009;Weijenborg et al, 2015Weijenborg et al, , 2017Hitchman and Rowe, 2017). Such convectively generated PV dipoles were previously identified in idealized simulations of isolated cumulus-scale convection (Chagnon and Gray, 2009), in case studies of mesoscale convective sys-tems (Davis and Weisman, 1994;Chagnon and Gray, 2009;Hitchman and Rowe, 2017;Clarke et al, 2019), and in midlatitude convective updrafts with varying large-scale flow conditions (Weijenborg et al, 2015(Weijenborg et al, , 2017. The amplitude of the horizontal PV dipoles can strongly exceed the typical amplitude of synoptic-scale PV.…”

Section: Pv Modification By Wcbs and Convectionmentioning

confidence: 67%

“…where ω h denotes the horizontal vorticity (ω h = ξ i + ηj ). Previous studies showed that the horizontal diabatic heating gradients play a major role in the PV modification in isolated convective updrafts (e.g., Chagnon and Gray, 2009;Weijenborg et al, 2015Weijenborg et al, , 2017 and in narrow smaller-scale heating regions embedded in a larger-scale flow (Harvey et al, 2020). The localized diabatic heating in a vertically sheared environment generates upper-level horizontal PV dipoles centered around the convective updraft ( Fig.…”

Section: Pv Modification By Wcbs and Convectionmentioning

confidence: 94%

“…PV is frequently considered for synoptic-scale dynamics (e.g., Hoskins et al, 1985;Stoelinga, 1996) but is also suited for the analysis of mesoscale convective systems (e.g., Conzemius and Montgomery, 2009;Shutts, 2017;Clarke et al, 2019) and has already been applied at the scale of individual convective cells (Chagnon and Gray, 2009;Weijenborg et al, 2015Weijenborg et al, , 2017. While for PV modification in synoptic-scale systems the horizontal gradient of diabatic heating is frequently neglected (as in Eq.…”

Section: Pv Modification By Wcbs and Convectionmentioning

confidence: 99%

See 2 more Smart Citations

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.

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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Section: Pv Modification By Wcbs and Convectionmentioning

confidence: 67%

Section: Pv Modification By Wcbs and Convectionmentioning

confidence: 94%

Section: Pv Modification By Wcbs and Convectionmentioning

confidence: 99%

See 1 more Smart Citation

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.

128

View full text Add to dashboard Cite

show abstract

“…Weijenborg et al . () demonstrated that horizontal PV dipoles with statistically significant flow anomalies are consistently associated with convection by compositing over several thousand convective cells. They also showed that these PV dipoles have a longer lifetime than their associated convection and so may go on to influence flow evolution.…”

Section: Introductionmentioning

confidence: 99%

Downstream influence of mesoscale convective systems. Part 1: influence on forecast evolution

Clarke

Gray

Roberts

2019

Quart J Royal Meteoro Soc

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Mesoscale convective systems (MCSs) are difficult to forecast due to their inherent‐60 unpredictability and development from scales that are subgrid in typical global models. Here the impacts of model representation of convection on MCS structure and downstream forecast evolution are examined using two configurations of the Met Office Unified Model: the convection‐permitting (4.4‐km grid spacing) limited‐area Euro4 and convection‐parametrizing (25‐km grid spacing) Global configurations. MCSs are associated with a characteristic potential vorticity (PV) structure: a positive PV anomaly in the mid‐troposphere and negative PV anomalies above and to the side of it. Convection‐permitting models produce larger‐amplitude MCS PV anomalies than convection‐parametrizing models. These differences are shown to persist after coarse graining the output from a Euro4 simulation to the 25 km grid spacing of the Global configuration for a case study from July 2012, and are largest in magnitude and extent in the upper troposphere. The effect of the poor representation of this PV structure by convection‐ parametrizing models on forecasts is investigated by adding “MCS perturbations”, calculated as differences between the coarse‐grained Euro4 and the Global outputs, to five‐day Global configuration forecasts. Upper‐level MCS perturbations lead to greater forecast differences than those at middle levels, though using perturbations at all levels yields the greatest impact. For the first 36 hr, differences grow on the convective scale related to the MCS and its influence on a developing UK cyclone, despite perturbation amplitudes initially reducing. Subsequently, differences grow rapidly onto the synoptic scale and by five days impact the entire Northern Hemisphere. MCS perturbations slow the eastward movement of Rossby waves due to ridge amplification. Thus, perturbing convection‐parametrizing models to include PV anomalies associated with MCSs produces synoptic‐scale forecast differences implying that the misrepresentation of the PV structures associated with MCSs are a potential source of forecast errors.

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“…The term that produces the PV dipole is marked with an underbrace. These theoretical results have been confirmed by idealized numerical simulations (Chagnon and Gray, ) and numerical weather prediction (NWP) models (Weijenborg et al ., ). Dipoles have been shown to have a magnitude about ten times higher than the well‐known vertical PV dipoles (generated by vortex stretching and vertical gradient of diabatic heating) once the wind shear exceeds values of the order of 1 m

\cdot

^{- 1} \cdot

^{- 1}

.…”

Section: Physical Basis Of the Algorithmmentioning

confidence: 99%

A physically based precipitation separation algorithm for convection‐permitting models over complex topography

Poujol

Sobolowski

Mooney

et al. 2019

Quart J Royal Meteoro Soc

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A new algorithm is described that can separate precipitation output from convection‐permitting models into three different types of precipitation: (a) convective, (b) stratiform, and (c) orographically enhanced precipitation. The algorithm is based on physical processes that underlie these types of precipitation and it is applicable over both ocean and land surfaces. It is particularly well suited for mountainous areas or other regions exhibiting complex terrain. The algorithm's performance is first demonstrated for a selection of well‐understood weather events and then for a 10‐year convection‐permitting climate simulation over Norway. The algorithm correctly separates convection embedded in frontal systems from stratiform precipitation and also properly identifies orographically enhanced precipitation when the frontal systems interact with local orography. The results suggest that this can be a powerful new tool for investigating characteristics of precipitation in convection‐permitting climate simulations, particularly in a climate change context and as researchers move towards models that explicitly resolve convection and its related processes.

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Coherent evolution of potential vorticity anomalies associated with deep moist convection

Cited by 22 publications

References 65 publications

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

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

Downstream influence of mesoscale convective systems. Part 1: influence on forecast evolution

A physically based precipitation separation algorithm for convection‐permitting models over complex topography

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