Three-dimensional potential vorticity structures for extreme precipitation events on the convective scale

Müller, Annette; Niedrich, Benjamin; Névir, Peter

doi:10.1080/16000870.2020.1811535

Cited by 9 publications

(14 citation statements)

References 34 publications

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“…1, second term). The formation of such quasi-horizontal PV dipole structures have been found in idealized simulations of isolated cumulus-scale convection (Chagnon and Gray, 2009), in case studies of mesoscale convective systems (Davis and Weisman, 1994;Chagnon and Gray, 2009;Hitchman and Rowe, 2017;Clarke et al, 2019), in mid-latitude convective updrafts with varying large-scale flow conditions (Weijenborg et al, 2015(Weijenborg et al, , 2017Müller et al, 2020), and embedded in the baroclinic WCB ascent region (Harvey et al, 2020;Oertel et al, 2020). These PV anomalies can reach a magnitude of ±10 PVU and hence exceed the typical range of synopticscale PV.…”

Section: Introductionmentioning

confidence: 72%

“…The radar reflectivity structure of convective clouds is relatively heterogeneous compared to homogeneous stratiform clouds (e.g., Steiner et al, 1995) and often appears as confined towers (Hogan et al, 2002;Murphy et al, 2017). Here, we visually identify convective regions, where narrow plumes of increased values of Z rise above the 0 • C isotherm (see Hogan et al, 2002;Murphy et al, 2017;Oertel et al, 2019). Further applied indications for convective activity are locally enhanced radar reflectivity, a broken cloud top, spatially heterogeneous cloud top height and the absence of a pronounced radar bright band.…”

Section: Airborne Radar Observationsmentioning

confidence: 99%

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Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

Oertel

Sprenger²,

Joos³

et al. 2021

Weather Clim. Dynam.

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Abstract. Warm conveyor belts (WCBs) are dynamically important, strongly ascending and mostly stratiform cloud-forming airstreams in extratropical cyclones. Despite the predominantly stratiform character of the WCB's large-scale cloud band, convective clouds can be embedded in it. This embedded convection leads to a heterogeneously structured cloud band with locally enhanced hydrometeor content, intense surface precipitation and substantial amounts of graupel in the middle troposphere. Recent studies showed that embedded convection forms dynamically relevant quasi-horizontal potential vorticity (PV) dipoles in the upper troposphere. Thereby one pole can reach strongly negative PV values associated with inertial or symmetric instability near the upper-level PV waveguide, where it can interact with and modify the upper-level jet. This study analyzes the characteristics of embedded convection in the WCB of cyclone Sanchez based on WCB online trajectories from a convection-permitting simulation and airborne radar observations during the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX) field campaign (intense observation periods, IOPs, 10 and 11). In the first part, we present the radar reflectivity structure of the WCB and corroborate its heterogeneous cloud structure and the occurrence of embedded convection. Radar observations in three different sub-regions of the WCB cloud band reveal the differing intensity of its embedded convection, which is qualitatively confirmed by the ascent rates of the online WCB trajectories. The detailed ascent behavior of the WCB trajectories reveals that very intense convection with ascent rates of 600 hPa in 30–60 min occurs, in addition to comparatively moderate convection with slower ascent velocities as reported in previous case studies. In the second part of this study, a systematic Lagrangian composite analysis based on online trajectories for two sub-categories of WCB-embedded convection – moderate and intense convection – is performed. Composites of the cloud and precipitation structure confirm the large influence of embedded convection: intense convection produces very intense local surface precipitation with peak values exceeding 6 mm in 15 min and large amounts of graupel of up to 2.8 g kg−1 in the middle troposphere (compared to 3.9 mm and 1.0 g kg−1 for the moderate convective WCB sub-category). In the upper troposphere, both convective WCB trajectory sub-categories form a small-scale and weak PV dipole, with one pole reaching weakly negative PV values. However, for this WCB case study – in contrast to previous case studies reporting convective PV dipoles in the WCB ascent region with the negative PV pole near the upper-level jet – the negative PV pole is located east of the convective ascent region, i.e., away from the upper-level jet. Moreover, the PV dipole formed by the intense convective WCB trajectories is weaker and has a smaller horizontal and vertical extent compared to a previous NAWDEX case study of WCB-embedded convection, despite faster ascent rates in this case. The absence of a strong upper-level jet and the weak vertical shear of the ambient wind in cyclone Sanchez are accountable for the weak diabatic PV modification in the upper troposphere. This implies that the strength of embedded convection alone is not a reliable measure for the effect of embedded convection on upper-level PV modification and its impact on the upper-level jet. Instead, the profile of vertical wind shear and the alignment of embedded convection with a strong upper-level jet play a key role for the formation of coherent negative PV features near the jet. Finally, these results highlight the large case-to-case variability of embedded convection not only in terms of frequency and intensity of embedded convection in WCBs but also in terms of its dynamical implications.

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Section: Introductionmentioning

confidence: 72%

Section: Airborne Radar Observationsmentioning

confidence: 99%

Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

Oertel

Sprenger²,

Joos³

et al. 2021

Weather Clim. Dynam.

View full text Add to dashboard Cite

show abstract

“…The formation of such quasi-horizontal PV dipole structures have been found in idealized simulations of isolated cumulusscale convection (Chagnon and Gray, 2009), in case studies of mesoscale convective systems (Davis and Weisman, 1994;Chagnon and Gray, 2009;Hitchman and Rowe, 2017;Clarke et al, 2019), in mid-latitude convective updrafts with varying large-scale flow conditions (Weijenborg et al, 2015(Weijenborg et al, , 2017Müller et al, 2020), and embedded in the baroclinic WCB ascent region (Harvey et al, 2020;Oertel et al, 2020). These PV anomalies can reach a magnitude of ±10 PVU, and hence, exceed the typical range of synoptic-scale PV.…”

Section: Introductionmentioning

confidence: 72%

Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

Oertel¹,

Sprenger²,

Joos³

et al. 2020

Preprint

View full text Add to dashboard Cite

Abstract. Warm conveyor belts (WCBs) are dynamically important, strongly ascending and mostly stratiform cloud-forming airstreams in extratropical cyclones. Despite the predominantly stratiform character of the WCB's large-scale cloud band, convective clouds can be embedded in it. This embedded convection leads to a heterogeneously structured cloud band with locally enhanced hydrometeor content, intense surface precipitation and substantial amounts of graupel in the middle troposhere. Recent studies showed that embedded convection forms dynamically relevant quasi-horizontal potential vorticity (PV) dipoles in the upper troposphere. Thereby one pole can reach strongly negative PV values associated with inertial or symmetric instability near the upper-level PV waveguide, where it can interact with and modify the upper-level jet. This study analyses the characteristics of embedded convection in the WCB of cyclone Sanchez based on WCB online trajectories from a convection-permitting simulation and airborne radar observations during the North Atlantic Waveguide and Downstream Impact EXperiment (NAWDEX) field campaign (IOPs 10 and 11). In the first part, we present the radar reflectivity structure of the WCB and corroborate its heterogeneous cloud structure and the occurrence of embedded convection. Radar observations in three different sub-regions of the WCB cloud band reveal the differing intensity of its embedded convection, which is qualitatively confirmed by the ascent rates of the online WCB trajectories. The detailed ascent behaviour of the WCB trajectories reveals that very intense convection with ascent rates of 600 hPa in 30–60 min occurs, in addition to comparatively moderate convection with slower ascent velocities as reported in previous case studies. In the second part of this study, a systematic Lagrangian composite analysis based on online trajectories for two sub-categories of WCB-embedded convection – moderate and intense convection – is performed. Composites of the cloud and precipitation structure confirm the large influence of embedded convection: Intense convection produces locally very intense surface precipitation with peak values exceeding 6 mm in 15 minutes and large amounts of graupel of up to 2.8 g kg−1 in the middle troposphere (compared to 3.9 mm and 1.0 g kg−2 for the moderate convective WCB sub-category). In the upper troposphere, both convective WCB trajectory sub-categories form a small-scale and weak PV dipole, with one pole reaching weakly negative PV values. However, for this WCB case study – in contrast to previous case studies reporting convective PV dipoles in the WCB ascent region with the negative PV pole near the upper-level jet – the negative PV pole is located east of the convective ascent region, i.e., away from the upper-level jet. Moreover, the PV dipole formed by the intense convective WCB trajectories is weaker and has a smaller horizontal and vertical extent compared to a previous NAWDEX case study of WCB-embedded convection, despite faster ascent rates in this case. The absence of a strong upper-level jet and the weak vertical shear of the ambient wind in cyclone Sanchez are accountable for the weak diabatic PV modification in the upper troposphere. This implies that the strength of embedded convection alone is not a reliable measure for the effect of embedded convection on upper-level PV modification and its impact on the upper-level jet. Instead, the profile of vertical wind shear and the alignment of embedded convection with a strong upper-level jet play a key role for the formation of coherent negative PV features near the jet. Finally, these results highlight the large case-to-case variability of embedded convection not only in terms of frequency and intensity of embedded convection in WCBs but also in terms of its dynamical implications.

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“…Various energetic variable are applicable on large scale. A potential driver to control small scale models is the Dynamic State Index (DSI) (Müller et al, 2020;Müller and Névir, 2019), an "adiabaticity indicator". Other large scale variables driving the smaller scale stochastics are the available moisture or vertical wind shear.…”

Section: Discussionmentioning

confidence: 99%

“…The sample size of the reanalysis data set used in Sect. As vertical layer the 600 hPa surface is considered, because here the latent heat release takes place and the vertical velocity reaches its maximum, as shown in (Müller et al, 2020).…”

Section: Meteorological Datamentioning

confidence: 99%

Direct Bayesian model reduction of smaller scale convective activity conditioned on large scale dynamics

Polzin¹,

Müller

Rust

et al. 2021

Preprint

Self Cite

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Abstract. We pursue a simpliﬁed stochastic representation of smaller scale convective activity conditioned on large scale dynamics in the atmosphere. For identifying a Bayesian model describing the relation of different scales we use a probabilistic approach (Gerber and Horenko, 2017) called Direct Bayesian Model Reduction (DBMR). The convective available potential energy (CAPE) is applied as large scale ﬂow variable combined with a subgrid smaller scale time series for the vertical velocity. We found a probabilistic relation of CAPE and vertical up- and downdraft for day and night. The categorization is based on the conservation of total probability. This strategy is part of a development process for parametrizations in models of atmospheric dynamics representing the effective inﬂuence of unresolved vertical motion on the large scale ﬂows. The direct probabilistic approach provides a basis for further research of smaller scale convective activity conditioned on other possible large scale drivers.

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Three-dimensional potential vorticity structures for extreme precipitation events on the convective scale

Cited by 9 publications

References 34 publications

Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

Observations and simulation of intense convection embedded in a warm conveyor belt – how ambient vertical wind shear determines the dynamical impact

Direct Bayesian model reduction of smaller scale convective activity conditioned on large scale dynamics

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