Does terrain affect tornado genesis?

[Mike Birbeck]

Does geographical terrain/topology affect tornado genesis?

Two South African wind damage events (both classified as tornados by the SA weather authorities) occurred in the suburb of Mannenberg on the Cape flats just northwest of Cape Town (peninsular) in 1999 (deadly event) and again in 2002.

Mannenberg sits in the lee of Table Mountain.

These events are statistically interesting in the sense that tornados are extremely rare in the Cape Town area, although there is documented evidence of strong tornados (F3/F4) in the Cape Province (large land area covering multiple types of geography and affected by multiple weather systems with the potential for advection of strong coastal lows onto an escarpment and possible interaction with a desert dry line.

What strikes me as incredible is that the same suburb was struck twice within such a short space of time in an area where the statistical likelihood of a tornado is extremely low!

How likely is it that this phenomenon is related to local interaction with the mountain?

Does topology affect tornado genesis? What studies are out there relevant this question?

Event details:

http://news.bbc.co.uk/1/hi/world/africa/432942.stm

http://www.weathersa.co.za/Pressroom/2002/2002Oct3SevereStormCpt.jsp

 

[Mike Birbeck]

Topography.

I guess what I am asking is how important is topography in creating (or preventing) tornadoes?

 

[Nigel Bolton]

Here in the UK, there is increasing awareness that topography does play a part in the development of tornadoes.

As an example, our tiny Isle of Wight close to the south coast of England is quite effective at spawning tornadoes. The island acts very much like 'rock in a stream' and therefore generates wake vortices. Wake vortices are best seen from satpix depicting vortices shed into Sc from islands such and the Canaries and Jan Mayan.

During unstable weather, mainly during the winter when the sea is relatively warm, showers and or TS may pass over or close to the island. These storms can ingest a vortex, and stretch it to produce a tornado.

During the past 25 year, there have been numerous tornadoes close to this island, generally weak and short lived, but one on the 7th Jan 1998 attained EF2 strength and was 800 metres across. This tornado damaged 1000 properties in the town of Selsey, this town having captured 4 tornadoes in the last quarter century.

On 5th November 1999, the island spawned three tornadoes as a cold front swept east, causing damage in the Southampton area. On the 30th October 2000, another six tornadoes were spawned on another cold front, affecting mainly Portsmouth.

Other tornadoes more recently, after thorough investigation, have been deduced to have been caused by topographical factors.

N.

 

[Jody Radzik]

These storms can ingest a vortex, and stretch it to produce a tornado.

That would make these islandspouts, correct?

 

[Mike Birbeck]

Here in the UK, there is increasing awareness that topography does play a part in the development of tornadoes.

As an example, our tiny Isle of Wight close to the south coast of England is quite effective at spawning tornadoes. The island acts very much like 'rock in a stream' and therefore generates wake vortices. Wake vortices are best seen from satpix depicting vortices shed into Sc from islands such and the Canaries and Jan Mayan.

During unstable weather, mainly during the winter when the sea is relatively warm, showers and or TS may pass over or close to the island. These storms can ingest a vortex, and stretch it to produce a tornado.

During the past 25 year, there have been numerous tornadoes close to this island, generally weak and short lived, but one on the 7th Jan 1998 attained EF2 strength and was 800 metres across. This tornado damaged 1000 properties in the town of Selsey, this town having captured 4 tornadoes in the last quarter century.

On 5th November 1999, the island spawned three tornadoes as a cold front swept east, causing damage in the Southampton area. On the 30th October 2000, another six tornadoes were spawned on another cold front, affecting mainly Portsmouth.

Other tornadoes more recently, after thorough investigation, have been deduced to have been caused by topographical factors.

N.

Nigel

Your review of the tornadoes on the English South Coast is fascinating.

I have actually encountered a very minor example of wind interaction with the Isle of Wight when landing at Bembridge and encountering a strong rotor (in a prevailing breeze of no more than 12 knots).

It is easy to visual how this form of vorticity may be realigned and stretched under the right conditions. Indeed I believe I have also seen BBC footage of a small but violent looking tornado spinning up on the island in just the meteorological conditions you describe.

As I remember it, the Selsey tornado had my American boss (at the time) saying things like “I didn’t think you Brits had tornadoesâ€￾ . I am actually South African but am proud to be an honorary Brit and enjoy UK tornadoes. We may not have the big supercellular (or not that many) storms that those lucky guys across the pond enjoy (or not if they are victims of tornado destruction) but we certainly have interesting ones

 

[MJ Poore]

Does geographical terrain/topology affect tornado genesis?

Two South African wind damage events (both classified as tornados by the SA weather authorities) occurred in the suburb of Mannenberg on the Cape flats just northwest of Cape Town (peninsular) in 1999 (deadly event) and again in 2002.

Mannenberg sits in the lee of Table Mountain.

These events are statistically interesting in the sense that tornados are extremely rare in the Cape Town area, although there is documented evidence of strong tornados (F3/F4) in the Cape Province (large land area covering multiple types of geography and affected by multiple weather systems with the potential for advection of strong coastal lows onto an escarpment and possible interaction with a desert dry line.

What strikes me as incredible is that the same suburb was struck twice within such a short space of time in an area where the statistical likelihood of a tornado is extremely low!

How likely is it that this phenomenon is related to local interaction with the mountain?

Does topology affect tornado genesis? What studies are out there relevant this question?

Event details:

http://news.bbc.co.uk/1/hi/world/africa/432942.stm

http://www.weathersa.co.za/Pressroom/2002/2002Oct3SevereStormCpt.jsp

Yes topogrpahy plays a big role in S Africa.

In the case of the Mannenberg F3, terrain probably wasnt a factor with Table Mountain nearby so far as anyone knows. The smaller apparent tornado, or what ever it was in 2002, is thought to be a freak spout/funnel/downburst event of some sort. Cape Town is the least tornado prone part of the country, but occasionally in winter LP systems hit the Cape Peninsula head on (as in the 1999 F3 case) and this is probably the most likely cause of these two events.

On the east coast, from the Eastern Cape to KwaZulu-Natal, the most tornado prone part of S Africa, there is an escarpment rising from the warm Indian Ocean at 0m to the Drakensberg mountains at 3000 meters in a space of just 200km. Moisture comes straight off the Indian and blows upslope making a kind of dry line lift effect. Supercells initiate between the 600m and the 1000m level because the cap weakens here and the moisture is still strong. So most tornadoes happen in a corridor on the escarpment slopes here, like the 120km long Mount Ayliff F4 in 1999 for example. Small towns in the corridor area like Mount Ayliff, Mooi River and Impendle get hit regularly, and have recorded multiple F1/2/3 strikes over the past twenty years.

Up on the highveld plateau, the sub-continental interior, which varies between 1700m and 1000m in height, a more conventional US midwest type senario prevails with dry air from the Kalahari and moist air from equitorial Africa forming drylines on the grasslands etc. It has been noted that the 600km long Witwatersrand Ridge, on which Johannesburg sits, has an unsually high number of strikes and storms. This is because of its ramping effect.

 

[Nigel Bolton]

Thanks Mike.

UK tornadoes are interesting, and in general do not conform to the Amercan model, namely true supercell development within the UK is preciously rare, even though the term 'Supercell' was first coined here by Keith Browning, after a large hailer did a bit of damage across Wokingham, Berks. in 1959.

Most of our tornadoes here in the UK are cool weather phenomena and are most numerous either along an active cold front during the Autumn and Winter, when they can be fairly strong, more by dint of thier forward speed rather than rotation, or occur within the flabby curculation of a filling area of low pressure in the summer.

Our most powerful and long lasting tornadoes are normally 'Triple Point' tornadoes, at the intersection of three fronts or troughs. The Birmingham tornado (EF2; T5) was spawned as the northern end of a trough in warm air ran up the slope of a slow moving west-east aligned slow moving warm front. This instigated strong warm advection with strong southerly winds over-riding strong easterly winds beneath the front, causing the cell on the trough to spin, and generate a 'wedge' tornado.

One problem we do have in the UK now, is that the term 'Tornado' is very much in vogue with the media, and there are now numerous reports of tornado damage when straight-line wind was the culprit.

N.

 

[Nigel Bolton]

The last post bears no mention of 'Topography'. Duh!!!

As well as the Isle of Wight, other parts of the UK probably get more than their average fair share of tornadoes due to topography. These include the west Wales coast with vortices shed from the Lleyn Peninsula in and WNW'ly, downwind of the Bristol Channel, vis Northern Somerset and Wiltshire as storms succomb to the 'pontoon effect' from various headlands along the Bristol Channel, and Nottinghamshire and Lincolnshire by vorticity generated by the southern end of the Pennines.

N.

 

[Dan Robinson]

IMO there are two questions here: terrain's influence on the 1.) mesoscale (larger scale) and 2.) microscale (vortex scale).

On the mesoscale, mountains definately can create an ambient climate that is either conducive or detrimental to the formation of thunderstorms. Orographic lift is a well-known storm initiation mechanism. The Front Range of the Rockies would be a good example of an area where the terrain helps initiate thunderstorms and generate circulations (IE The Denver Cyclone) that can create a high-shear environment conducive for tornado-producing storms to develop. The Appalachians, on the other hand, are just the opposite, which tend to cut off low-level southerly flow, reducing moisture advection and limiting surface infow to storms. In that case, it is the large-scale environment that hinders supercells, not the terrain directly hindering storm rotation already in progress.

On the microscale, I'm not sure that terrain has as much of an effect. That is, once you have a storm-scale environment conducive to tornadogenesis, that the surface's role, if any, would be negligible in ehnacing or preventing tornadogenesis. Likewise, a non-rotating storm is likely not going to produce a tornado if a certain terrain feature is under it.

Many tornadic supercells-in-progress have passed over mountains with little or no change in strength directly related to the terrain at the surface. The Appalachians have a bad climate configuration for supercells - but when a synoptic setup does overcome those problems, supercells and tornadoes form regardless of what is at the surface (April 1974, November 2002, June 1998, etc).

 

[Martin North]

Of the two most powerful tornadoes we have had in the UK in the last twelve months...there are two distinct types.

1. The Bow Street EF2 tornado in Wales late last year was almost certainly formed as a result of topographical influences. The tornado was 'funnelled' up a fairly narrow valley with Fells (mountains) either side and then dissipated.

3. The Crumlin EF3 tornado in N. Ireland was most certainly the result of a Mesocyclone. It dropped from a Supercell which was the TEC (Tail End Charlie) of a squall line. A long tracked tornado this (26 kms).

 

[Martin North]

One problem we do have in the UK now, is that the term 'Tornado' is very much in vogue with the media, and there are now numerous reports of tornado damage when straight-line wind was the culprit.

N.

The other thing is...that even though tornado numbers are not necessarily up...every time there is one in the UK, it gets blamed on Global Warming. Even state run (BBC) forecasters and meteorologists run with these scare stories.

I can tell you that at the site investigation I was conducting of a previous tornado...a reporter (who was actually a tv weather presenter) had told a elderly householder whose house had been badly damaged that there would be more tornadoes like the one she'd had, because of Global Warming. What absolute nonsense....this tornado had nothing to do with that as it was spawned from the rear of a cell buried in a Rear Sloping Ana Front which had favourable shear/veer and some topographical influence.

 

[Chad Cowan]

This is an interesting topic and living in a big city in the midwest, is one that I have thought about quite a bit. I do believe that terrain (or in my case, buildings) can hinder tornadogenesis and tornadic circulation. Of course, tornadic processes at the surface are the least understood part, so trying to hypothesize what impact surface features have on a tornado is speculative. That said, I believe that the most important concept here is the amount of surface friction and its impact on developing circulation with a consistant speed.

 

[MJ Poore]

Thanks Mike.



Most of our tornadoes here in the UK are cool weather phenomena
N.

Quite. Please just say 'cold core funnel type tornado' the next time you say tornado. You're confusing people.

And it's no surprise that....

One problem we do have in the UK now, is that the term 'Tornado' is very much in vogue with the media, and there are now numerous reports of tornado damage when straight-line wind was the culprit..

Exactly what do you guys expect if you keep labelling every cold core funnel and gustnado a 'tornado'

That would make these islandspouts, correct?

Nope.Torronados

 

[tom hanlon]

Without a doubt terrain affects tornado formation.

As one poster mentioned, large scale effects such as orographic lifting. Also small scale effects.

The old mythology that hills prevent tornadoes is just not true. Meaning, table mountain is not going to protect Cape Town.

Cape Town is going to have so much else going on in addition to table Mountain. Sitting on the tip of Africa like that the Ocean temp and sea Breeze are going to come into play. Not to mention that surrounding landscape with a plateau not far to your north, plains briefly to the west but then the mountains around Stellenbosch.

As Tornado chasing outside of the US goes, I would think South Africa has great promise. The roads are good, the data is decent. Certainly the data is not like what we get in the USA but, it has to be better than what you would get in Argentina.

My brief time I spent in South Africa I did not "chase" but I happened across a number of impressive storms. As the other South African mentioned, I was in the more prolific terrain in Kwa-Zula Nataal . In and around the Drakensberg range, and then between there and J-Burg.

I had the feeling that unlike Great Britain where they may be "over reporting" Tornadoes, I felt like South Africa quite likely was under-reporting tornadoes. Everything was so remote. If a tornado strikes in the US and no-one sees it or contacts the NWS, then it is not a tornado. But since a supercell can hardly make a turn to the left or right without a horde of stormchasers on it's trail then a lot of our tor's get reported. In Kwa-Zulu Nataal I saw lightning rods on every building that could afford them. That is a good sign ;-) Bur when I headed out to get under a storm, no one else was sitting under the inflow with me.

--
Tom

 

[Tomas Pucik]

Quite. Please just say 'cold core funnel type tornado' the next time you say tornado. You're confusing people.

And it's no surprise that....



Exactly what do you guys expect if you keep labelling every cold core funnel and gustnado a 'tornado'



Nope.Torronados

Oooh and Mungo again does not believe that there are supercells and true tornadoes in Britain.