Convective and Slantwise Trajectory Ascent in Convection-Permitting Simulations of Midlatitude Cyclones

Rasp, Stephan; Selz, Tobias; Craig, George C.

doi:10.1175/mwr-d-16-0112.1

Cited by 38 publications

(62 citation statements)

References 34 publications

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“…Since WCBs often feature higher vertical velocities (e.g. Madonna et al, 2014;Martínez-Alvarado et al, 2014;Rasp et al, 2016;Oertel et al, 2019a), we conclude that the large-scale ascent in mid-latitude wintertime WCBs is generally large enough to produce SLW.…”

Section: Synoptic Overview and Wcbmentioning

confidence: 77%

Microphysics and dynamics of snowfall associated to a warm conveyor belt over Korea

Gehring¹,

Oertel²,

Vignon³

et al. 2020

Preprint

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Abstract. On 28 February 2018, 57 mm of precipitation associated to a warm conveyor belt (WCB) fell within 21 h over South Korea. To investigate how the large-scale circulation influenced the microphysics of this intense precipitation event, we used radar measurements, snowflake photographs and radiosounding data from the International Collaborative Experiments for Pyeongchang 2018 Olympic and Paralympic winter games. The WCB was identified with trajectories computed with analysis wind fields from the Integrated Forecast System global atmospheric model. The WCB was collocated with a zone of enhanced wind speed of up to 45 m s−1 at 6500 m a.s.l., as measured by a radiosonde and a Doppler radar. Supercooled liquid water (SLW) with concentrations exceeding 0.2 g kg−1 was produced during the rapid ascent within the WCB. Vertical profiles of polarimetric radar variables show during the most intense precipitation period a peak and subsequent decrease in differential reflectivity as aggregation starts. Below the peak in differential reflectivity, the specific differential phase shift continues to increase, indicating early riming of oblate crystals and secondary ice generation. We hypothesise that the SLW produced in the WCB led to intense riming. Moreover, embedded convection in the WCB and turbulence at its lower boundary enhanced aggregation by increasing the probability of collisions between particles. This suggests that both aggregation and riming occurred prominently in this WCB. This case study shows how the large-scale atmospheric flow of a WCB provides ideal conditions for rapid precipitation growth involving SLW production, riming and aggregation. Future microphysical studies should also investigate the synoptic conditions to understand how observed processes in clouds are related to the large-scale circulation.

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Section: Synoptic Overview and Wcbmentioning

confidence: 77%

Microphysics and dynamics of snowfall associated to a warm conveyor belt over Korea

Gehring¹,

Oertel²,

Vignon³

et al. 2020

Preprint

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“…Despite this relatively strong local wind anomaly, its radius of impact is limited and the effect of the PV anomaly substantially decays beyond 40 km from the updraft. As hypothesized by Raymond and Jiang (1990), the relatively long-lived low-level positive PV anomalies interact with the background shear and thus could trigger new convective cells through the formation of local convergence lines on the downshear side. In this way, convective activity could be maintained.…”

Section: Flow Anomalies Induced By Pv Anomaliesmentioning

confidence: 89%

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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“…The ascent of this air mass to the upper troposphere results in a vertical reordering of air parcels originating at low latitudes which we interpret as corresponding to deep convection in the atmosphere. While the air mass originates at low latitudes, its ascent can occur in the midlatitudes, since extratropical cyclones advect air poleward before deep ascent occurs (22,23). The parcel rearrangement for winter (SI Appendix, Fig.…”

Section: Mapementioning

confidence: 99%

Changing available energy for extratropical cyclones and associated convection in Northern Hemisphere summer

Gertler

O’Gorman

2019

Proc. Natl. Acad. Sci. U.S.A.

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The circulation of the Northern Hemisphere extratropical troposphere has changed over recent decades, with marked decreases in extratropical cyclone activity and eddy kinetic energy (EKE) in summer and increases in the fraction of precipitation that is convective in all seasons. Decreasing EKE in summer is partly explained by a weakening meridional temperature gradient, but changes in vertical temperature gradients and increasing moisture also affect the mean available potential energy (MAPE), which is the energetic reservoir from which extratropical cyclones draw. Furthermore, the relation of changes in mean thermal structure and moisture to changes in convection associated with extratropical cyclones is poorly understood. Here we calculate trends in MAPE for the Northern extratropics in summer over the years 1979–2017, and we decompose MAPE into both convective and nonconvective components. Nonconvective MAPE decreased over this period, consistent with decreases in EKE and extratropical cyclone activity, but convective MAPE increased, implying an increase in the energy available to convection. Calculations with idealized atmospheres indicate that nonconvective and convective MAPE both increase with increasing mean surface temperature and decrease with decreasing meridional surface temperature gradient, but convective MAPE is relatively more sensitive to the increase in mean surface temperature. These results connect changes in the atmospheric mean state with changes in both large-scale and convective circulations, and they suggest that extratropical cyclones can weaken even as their associated convection becomes more energetic.

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Convective and Slantwise Trajectory Ascent in Convection-Permitting Simulations of Midlatitude Cyclones

Cited by 38 publications

References 34 publications

Microphysics and dynamics of snowfall associated to a warm conveyor belt over Korea

Microphysics and dynamics of snowfall associated to a warm conveyor belt over Korea

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

Changing available energy for extratropical cyclones and associated convection in Northern Hemisphere summer

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