Sting .... Jet?

[Martin Rowley [West Moors/East Dorset]]

... In the latest issue of 'Weather', there is a short paper by Schultz and Browning entitled: " What is a sting jet? ".

 

As one of the authors is Keith Browning, who coined the term, it is as you might expect clear and succinct in answering the question. Essentially Schultz & Browning are emphasising that the name should only be used .... " specifically for the descending air stream, not more generally for the wind maximum near the tip of the cloud head ...".

 

I hesitate to suggest this - as Professor Browning is after all meteorological 'royalty' for us, but I wonder if the name needs rethinking? "Jets" as used in the meteorological community are essentially quasi-horizontal features, with change in altitude along the flow relatively small (relative that is to the speed dimension of the jet itself), whereas with the phenomenon discussed in this article the wind 'packet' descends sharply in altitude by some 200 mbar or more, over a discrete interval in space and time, within the overall cyclone life-cycle.

 

As it meets the Earth's surface it's forced into its potentially damaging near-horizontal phase, which presumably is why 'jet' was used.

 

To my mind, we need to 'take the eye' away from the 'jet family' and thus a more accurate idea of what's going on would be to characterise the wind energy generated as a 'burst', which conveys the short-lived nature of the event, much as 'microburst' does with some severe thunderstorms or 'cloudburst' more colloquially when describing intense, short-lived heavy convective rainfall.

 

How about . . . Sting Burst?

 

The reason I'm posting this here is to see if anyone knows if this naming of the phenomenon has been discussed elsewhere?

Martin.

[Jack Harrison]

Martin: How about . . . Sting Burst?

Far better.  Jet (for me) has connotations with strong winds at altitude.

Jack

[Smartie]

4:57 PM (2 hours ago)

>>Martin: How about . . . Sting Burst?

>>Far better.  Jet (for me) has connotations with strong winds at altitude.

>>Jack

How about ... something 'airstream'?

There are a number of jet-like flows in a rapidly deepening extratropical cyclone (though how you define a jet or airstream is open to question) eg

- upper jet 'jet stream''

-Low Level Jet  - in the warm air ahead if the cold front ~850 hPa

-Cold Conveyor Belt jet (CCB/CJ) wrapped around the Bent-Back Front (BBF) within the boundary layer

-jet associated with cold air advection (not generally described in the literature) eg ~700 hPa.

-jet associated with dry intrusion , located around base of dry air below ~500 hPa

-descending jets from cloud head eg sting jet (SJ)

[Smartie]

On Sunday, March 19, 2017 at 3:27:48 PM UTC, Martin Rowley [West Moors/East Dorset] wrote:

... In the latest issue of 'Weather', there is a short paper by Schultz and Browning entitled: " What is a sting jet? ".

 

................................................

 

How about . . . Sting Burst?

 

The reason I'm posting this here is to see if anyone knows if this naming of the phenomenon has been discussed elsewhere?

Martin.

In his original paper reanalysing the Oct'87 storm Browning, in a footnote, suggests a comparison with a convective 'heat burst'. The idea of a baroclinic or slant-wise downburst has been toyed with...

I don't know if Prof Browning would, in hindsight, choose to try to change his terminology- the historical raison d'etre  is laid out in the article. The thing about his original paper is that it identifies a new area for research and coining a name for the phenomenon is an important part of that.

The SJ phase of a storm persists for several hours and there may be more than one SJ in any storm, however because these storms tend to translate at high speeds and the duration of SJ winds at any given location may be short lived. In addition although the jet itself may possess quasi-steady state properties above the boundary the transport of these winds downward to the surface is usually associated with shallow convection and may take on downburst-like properties see eg-

Browning, K. A., Smart, D. J., Clark, M. R. and Illingworth, A. J. (2015), The role of evaporating showers in the transfer of sting-jet momentum to the surface. Q.J.R. Meteorol. Soc., 141: 2956–2971. doi:10.1002/qj.2581

Smart& Browning 

Smart, D. J. and Browning, K. A. (2014), Attribution of strong winds to a cold conveyor belt and sting jet. Q.J.R. Meteorol. Soc., 140: 595–610. doi:10.1002/qj.2162

find a sting jet associated with Windstorm 'Ulli' possessed rather unsteady properties in a simulation, however identifying steady, coherent airstreams may be dependant on the definition and methodology. Pulse-like properties have been attributed to SJs because satellite imagery of the cloud head hook cloud  filaments in animation typically show a stabbing forward motion possibly associated with individual sting jets or 'sting jet-lets' suggesting a pulse-like behaviours,see eg

HEWSON, Tim D.; NEU, Urs. Cyclones, windstorms and the IMILAST project. Tellus A, [S.l.], sep. 2015. ISSN 1600-0870. Available at: <http://coaction.lib.sfu.ca/index.php/tellusa/article/view/27128>. Date accessed: 19 Mar. 2017. doi:http://dx.doi.org/10.3402/tellusa.v%v.27128.

The possible role of evaporation  in a SJ suggests a comparison with convective downbursts, but there is inconsistent evidence for this as a sting jet mechanism. I can't say too much ATM, as the  paper is sub-judice at a journal, but we may have a solution to that puzzle- and it does involve identifying another descending jet or airstream and no we haven't coined a catchy name for it...yet.

PM if you'd (anybody would) like preprints.

[Martin Rowley [West Moors/East Dorset]]

... thanks for that: most interesting.

I'm musing on whether to write a Letter to 'Weather' asking the question I've posed here - if only to stimulate a debate. The term 'Sting Jet' is well established in the literature now so I wouldn't expect a change. The 'Sting' is fine & highly descriptive - with good provenance; it's the 'Jet' aspect, as Jack points out, that might need refining.

Martin.

[Smartie]

Yes- I think that  would be a perfectly legitimate question, and I think Keith Browning would be interested in answering it.

In fluid mechanics a jet is defined something like-

'A jet is a stream of fluid that is projected into a surrounding medium'

one way in the extratropical cyclone case is to identify the airstream with certain conservative properties eg a particular (small) range of theta w (WBPT), we do that in SJ cases with trajectory calculations. But see-
http://journals.ametsoc.org/doi/abs/10.1175/MWR-D-14-00382

Detection of Coherent Airstreams Using Cluster Analysis: Application to an Extratropical Cyclone

Neil C. G. Hart, Suzanne L. Gray, and Peter A. Clark

and references therein.

[Smartie]

I don't think my comments yesterday meet the high standard for clarity met by the Schultz and Browning article, so I'll have another go.

Firstly, this isn't just of academic interest.  Since the days of the Norwegian school it has been known that many atmospheric jets are associated with discontinuities in the wind and temperature fields (ie. 'fronts'') and thus have sensible weather impacts at the surface. For example the 'sting jet' (SJ) is associated with strong wind gusts, the cold conveyor belt jet (CJ) is associated with strong winds and heavy precipitation (including snowfall) and occasionally thunderstorms.

For our purposes I will define a 'jet' as a local maximum in the horizontal windspeed field. Major jets, such as the jet stream-upper-level jet- and the pre-frontal low level jet can be identified in time to space conversion of radiosonde profiles, see any textbook on meteorological analysis, but because a SJ is localised on the mesoscale, ie. its natural scale is  10-100s km in space and a few hours in duration this is much more difficult to do without high frequency/high density  RAOBS. However, wind profilers and, occasionally, research aircraft dropsonde curtains have been able to provide insights into the low level wind structure of rapidly deepening cyclones. We can use these observations and high-resolution satellite and radar imagery to validate our numerical simulations of these storms and analysis of the model fields offers further insight.

Looking at one such analysis

Smart, D. J. and Browning, K. A. (2013), Attribution of strong winds to a cold conveyor belt and sting jet. Q.J.R. Meteorol. Soc.. doi: 10.1002/qj.2162

available here-

https://www.ucl.ac.uk/hazardcentre/documents/Smart_Browning_2013

their figure 6 consists of model sections through the BBF of a sting jet storm defined in a system relative framework. Jets in the horizontal windspeed field are associated with closed isotachs which trajectory analysis (Figs 9-10) shows have some continuity in space and time and have properties , eg. theta w, which are quasi-conserved. It is possible to find windspeed maxima that do have closed isotachs that do not fulfil these properties and are therefore not classed as 'jets' for our purpose. This isentropic, system-relative (semi-Lagrangian) framework goes back to the work of JS Green and others from the Imperial College school in the 1960s. A number of SJs and a CJ are identified in the analysis, the upper level jet is in, or just out of, the upper-left part of the sections (eg. 6f). It can seen from Fig 9 that although the CJ and one of the SJs attain similar velocities they have markedly different properties. The CJ remains at low levels within the frontal zone and has low theta w, whereas the SJ descends from aloft with relatively high theta w.

Smart and Browning (p16) explicitly address the issue of 

"...the extent to which the SJ can be considered to be a semi-Lagrangian feature in which air parcels can be assumed to travel along the entire axis of the jet as observed at an instant in time" and conclude

"...it appears likely that the SJ typically dissipates or diminishes in intensity before a given parcel has time to travel along its entire length."

thus the existence of this SJ as a continuous airstream or jet is limited in time and appears to be in contrast to the more synoptic scale / mesoscale alpha upper level jet and warm sector LLJ where air parcels and airstreams can be tracked back  for many hours and even days. Circumstantial evidence from satellite imagery and other studies also suggest the limited spatial and temporal extent of individual SJs, however storms may produce more than one SJ over a long period of adding up to a prolonged damaging SJ phase. October 1987 appears to be an example of this type of storm.

[Martin Rowley [West Moors/East Dorset]]

... Many thanks for taking the time to lay out the background - informative. Given this (in your second post):

" For our purposes I will define a 'jet' as a local maximum in the horizontal windspeed field ... "

it is probably churlish of me to question the 'jet' aspect as I did; my only concern was that the use of the word 'smudged' this highly interesting phenomenon into the world of the 'horizontal' jets with no (apparent) link to the critical downward-penetration phase.

Perhaps we need a category (and distinct word) between "jet" and "burst"!

Martin.

[Smartie]

Martin,

I see your point, however I'm not personally unhappy about it. If the need for clarity is felt then one could refer to the `sting-jet airstream', implying reference to the descending airstream component only (as advised by S&B). There are many examples of jets or jet-like flows in meteorology eg the descending Rear Inflow Jet (RIJ) in Mesoscale Convective Systems (a slant-wise flow enhanced by moist processes a bit like a SJ) or the nocturnal LLJ (a form of inertial oscillation). Indeed the supercell simulations by Orf (pointed out by Richard) seem to have jet-like flows, the Streamwise Vorticity Current seems to be a jet in the fluid dynamics sense in that it is a current where vorticity is advected from an upstream boundary. Descending Rear Flank Downdraft (RFD)s in supercells have a strong slant-wise component and are enhanced by evaporation. So I would class a SJ as a species of the wider family of jet-like flows in meteorology.

I've searched for a formal definition of a jet in a meteorological context in vain-mostly the first call is to the upper 'jet stream'. In a cursory look at the fluid dynamics literature it's hard to find definitions as well, mostly textbooks are concerned with circular or slot nozzle jets. . Unsteady turbulent flow is a concern here and pulse-like jets and this may have some relevance to SJ where evaporation of cloud filaments may inject momentum in a pulse-like fashion. I think that you could term the SJ as an 'unsteady' jet, but it depends on what scale you're looking at things. At small scales the jet stream appears turbulent and unsteady, but at larger scales it is steadier, more laminar and 'meandering'. Scale is everything in turbulent flows.

If you like I can approach Profs S&B about the point and report back here, or you could email Dave Schultz as corresponding author or  indeed write a letter to the editor.

David

[Martin Rowley [West Moors/East Dorset]]

... Leave it with me: I'll contact the lead author with a shortened, slightly amended version of my original text to this group and see what happens. If anything 'substantive' arises, I'll post back here.

Thanks again for your help - fascinating research; I envy you!

Martin.

[Martin Rowley [West Moors/East Dorset]]

... contacted Prof. Schultz as outlined earlier; he also wondered if the 'jet' element might have been worth avoiding (or qualifying) in the early days, but the term is now buried in the literature and would be difficult to alter at this stage.

It reminded me that one of the last entries in the uk.sci.weather Glossary that I wrote (before I gave it up in 2007 - where have those 10 years gone?) was the entry ... "Sting Jet". As the term had been coined only a few years before, perhaps I should have spoken up sooner :-/

Martin.

[Richard Dixon]

On Thursday, 23 March 2017 11:38:37 UTC, Martin Rowley [West Moors/East Dorset] wrote:

... contacted Prof. Schultz as outlined earlier; he also wondered if the 'jet' element might have been worth avoiding (or qualifying) in the early days, but the term is now buried in the literature and would be difficult to alter at this stage.

It does make me wonder what another of Keith Browning's terminologies - "supercell" could have been named!

That one definitely has stuck. And we have Wokingham's weather to thank for it...!

Richard