Source: http://www.cimms.ou.edu/~doswell/

It is an honor to have with us today an internationally renowned meteorologist, Dr. Charles A. Doswell III, who throughout his working life has studied in depth the severe storms and tornadoes to become one of the foremost scientist of the subject.

He is currently on a short stay in Mallorca, collaborating with the group of Meteorology of the Department of Physics at the University of the Balearic Islands , but we know that he has already visited Mallorca several times.

1 .- How did your relationship with the island begin?

I met Prof. Climent Ramis, now head of the Physics Department, during an international meeting in Barcelona.  He invited me to visit Mallorca and I accepted his invitation.  During that visit, we decided that it would be good to collaborate on some research of mutual interest, which we began the next year.

2 .- After several times in the UIB. What is the most attractive aspect of Mediterranean meteorology for you?

What is interesting for me is to compare the meteorology of the Mediterranean to the meteorology of the United States.  I find there are both similarities and differences.  The differences mostly are associated with the complex topography of the Mediterranean area.

3 .- Speaking about severe weather in the Mediterranean, do you think that this area is a focus of severe weather?

Yes, the Mediterranean has severe thunderstorms very much similar to those in the United States, although not usually so strong or so frequent.

4 .- At the European level, do you think that severe storms are investigated sufficiently?

It is not possible for a scientist like me to think his favorite subject can ever be investigated enough!  I believe there is much, much more to learn about severe storms in Europe and in the Mediterranean.  Further, I believe the scientific understanding that research provides needs to be applied to the task of helping European (and Mediterranean) society reduce the impact of severe storms on lives and property.

5 .- You are currently collaborating with the study of severe phenomena here in the Mediterranean. Briefly, what are its features? Can we find differences with the American plains?

The most compelling feature of severe storms in the Mediterranean is their relationship to the topography.  The complex features of the land and the significant effect from the Mediterranean Sea can be critical in knowing when and where storms will develop and how severe they might be.  In the central United States, where severe thunderstorms are most frequent, the topography is much more flat – no major mountains and the oceans are many hundreds of kilometers away from the storms.

 6.-The development of big storms seems to be favored in large areas with no relief. In the U.S. there is the Great Plains, but here we have a quite complex orography. In this respect, do you believe that the sea acts in the same way as the Great Plains? What difference represents the fact that this is a sea and not flat land?

I definitely do not believe the sea interacts with the weather in the same way that the Great Plains do.  The surface of the ocean heats up much more slowly than the land, so in the spring, when major weather systems interact with the Mediterranean, the sea surface is relatively cold and storms become less likely.  During the autumn, however, the sea surface is quite warm, which makes severe storms more likely because moisture evaporated from the sea is a major source of the energy in a severe storm, so the main season for dangerous thunderstorms in the Mediterranean is not the spring, but rather is the autumn.  In the United States, it is exactly opposite – severe storms are more common in the spring than in the autumn.

7.-The severe storms in the American plains are sometimes associated with large hail, but instead in the Balearic Islands are much less frequent and large hail events are rare. What do you think this is due?

Unfortunately, the occurrence of large hail is not well-understood within the science of meteorology.  However, the temperature and moisture content of the air that provides the energy for severe thunderstorms is usually greater on the average in the United States than for storms in the Mediterranean region.  This is likely to be important in the occurrence of large hail.  But I cannot answer this question with a great deal of confidence.  The science of severe storms has much yet to learn.

8 .- Despite not having Doppler radar information, can you ensure that the storm of October 4, 2007 was or was not a supercell? If not, what process produced the tornadoes that affected the island during the storm?

Without clear evidence from Doppler radar, I cannot say for certain whether the storm that produced the tornado was or was not a supercell.  There are other types of storms besides supercells that can produce tornadoes, including what are called “bow echo” storms, which this storm may have been.  No one in the science of meteorology yet understands in detail how tornadoes develop, so anything I might suggest would only be speculation.  We believe that not all tornadoes develop in exactly the same ways, however.

9 .- Many people that day saw a greenish sky. We understand that is indicative of severity, isn’t it? Do you know why the green color is produced?

Unfortunately, although I have seen the so-called “green sky” many times, no one has yet produced a completely satisfactory scientific explanation.  I have no evidence to say that this green color only happens with severe storms, but in my experience, all the storms that I saw with this green color have been severe.

10 .- Tornadoes in the Mediterranean usually form without the presence of a supercell and therefore without mesocyclones.  Why? Does this determine the size of the tornado?, Does this suggest that tornadoes in the Mediterranean are weaker than in US because a mesocyclone is almost never present?

I have seen with my own eyes, on my visits to Mallorca, several supercell storms.  These have not been very strong storms, but I have seen visual evidence of mesocyclones in those storms.  Therefore, I do not have any compelling reason to believe that supercells are extremely uncommon in the Mediterranean region.  Some of the perception that supercells are rare here is the general lack of Doppler radar data.  In parts of the United States, people told me before they had Doppler radar that rotating storms were very rare.  But when they turned on their new Doppler radars, they began to see examples of supercells they’d not seen before!

If, as I believe, storms in the Mediterranean usually have less powerful instability in the air that feeds the storm than the storms in the United States, this means that the storms here are correspondingly less severe than those in the United States.  This may be a factor in how few really strong tornadoes have occurred here.  But a lot of work needs to be done to have a comprehensive understanding of Mediterranean severe storms.

11 .- Does the mesocyclone determines the strength of the tornado? Is there a direct relation between the structure of the mesocyclone or the supercell itself and the size and strength of the tornado?

No one knows exactly what controls the strength of a tornado.  It is true that supercell storms produce almost all of the really powerful tornadoes in the United States and that tornadoes from other types of storms almost never become very intense.  But the science of severe storms is not yet capable of giving a definitive answer to such questions.

12 .- Although the physics does not change, is the structure and behavior of a tornado formed on the sea differ from that formed over land? How does a waterspout change when it comes ashore? Does the same process occur when a tornado moves of over a lake, sea, ocean?

Tornadoes and waterspouts are difficult to observe in detail, because their powerful winds tend to destroy scientific instruments, and because it is uncommon for them to pass over (or even close to) any instruments.  Although scientific storm chasing has increased our knowledge of tornadoes on land, we have no such information regarding tornadoes over the water (waterspouts).  There are no scientific data-gathering projects that have attempted to chase waterspouts.  Therefore, I have no information I could use to answer such questions.  A tornado over a lake almost certainly would behave very much like a tornado over the ocean, at least for the time it was over the water.

13 .- Wind shear and unstable environment seems to be the key for supercell development. But could a very intense upward current without shear lead to a rotation in a convective structure?  And by extension, can rotating structures be found in an area of cold air in the upper layers if it is accompanied by marked divergence (thus in an environment with little shear but strongly unstable)?

If by shear you mean vertical wind shear, then it is possible to develop rotation in a convective storm, provided there is the appropriate horizontal wind shear.   This is what has been speculated to be happening in certain types of tornadoes that are not associated with supercells.  I assume the question about cold temperatures aloft actually implies a strong decrease of temperature with height, which is associated with the instability that leads to deep convective storms.  Strong divergence aloft is inevitably associated with strong convergence somewhere below it, and upward motion is inevitably present in between – strong divergence aloft simply cannot exist on its own, because of the laws of atmospheric physics.  If there is no vertical wind shear and no horizontal wind shear of the type that produces rotation, then no matter how strong the upward motion is within the storm, it will not begin to rotate unless it lasts long enough to begin to feel the effects of the Earth’s rotation.  This is in accordance with the well-known physical principle of the conservation of angular momentum.

14 .- Can Mesoscale Convective Systems (MCS) mask supercells?

I would prefer to say that MCSs can include supercells.  Yes, indeed supercells can be part of an MCS.

15 .- Why if the intensity of liquid precipitation (heavy rain) or the size of the solid precipitation (large hail) are considered severe weather, the great strengths of small size hail or very intense snow are not considered severe weather?

In the United States, storms producing only heavy precipitation are not considered to be “severe thunderstorms”, but I cannot explain why this is so.  This decision was made many years ago and although I don’t agree with this choice, it is not up to me to make it for the whole United States.  I am only one scientist.  All decisions like this are essentially arbitrary and usually have little or no physical basis.

16.-Dr Doswell, I would like to ask you about supercell storms which occur every year in Spain (and the Spanish maritime areas surrounding our country). It has been noticed that most of them seem to meet the current criteria fixed for them in the USA. In our country, supercells are usually smaller, weaker, short-lived, and even doesn’t show all the characteristics showed by supercells from other countries (they can reach a half point between other storm system, like a multicell storm, and an ideal supercell storm, whatever kind we consider, classic, HP or LP). Then, my question is: what do you think about establishing a less restrictive criteria for supercells in Spain, in the same way that the criteria for mesoscale convective systems (MCS) fixed by Maddox in 1980 were adapted for Europe? Of course, some of the supercells documented every year reach the organization level of an ideal supercell storm (classic, HP or LP). 17th August Cordoba HP supercell is a good example.

A small correction – Maddox (1980) established criteria for the so-called Mesoscale Convective Complex (MCC), not for the more general MCS.  And his criteria were not intended to become some sort of physically significant threshold.  Rather, it was his intent to study only a small subset of MCSs for the purposes of his specific research.
It might be that many of the supercells in the Mediterranean are not so powerful as their cousins in the United States.  I don’t know enough about these storms to say one way or the other.  I think the criteria for supercells that I have tried to use can be used here in the Mediterranean without any need for modification:  a supercell storm is one that contains a deep, persistent mesocyclone.  That is, the mesocyclone extends through a deep layer within and possibly beneath the cloud,  it persists for a least as long as it takes air to ascend from near the surface to the top of the storm, and the mesocyclone reaches some arbitrarily-defined level of intensity.

Thank you for your attention. We expect you enjoy your stay in Mallorca

Moltes gràcies! Any stay in Mallorca is always enjoyable for me!

Special thanks to Prof. Climent Ramis for his help in this interview