Supercell thunderstorms

'We read with interest the account by L. A. Eyre (1992) oftwo violent thunderstorm events in Australia, and have some comments we would like to share with the readers. \tr7e believe that the events described might belong to a class of severe thunderstorms that has been relatively unrecognised heretofore, but which occasionally can be characterised by the sort of extreme wind and hail events associated with these two examples. Because the information about the storm environments available to us, either from Eyre's note or from other sources) is rather limited, our suggestions cannot be given a rigorous test. However, the descriptions of the events and our experience with similar storms in North America allow us tospeculate on the character of the storms producing these devastating events. The Sydney storm apparently occurred in an environment with strong vertical wind shear, according to Eyre. Eyre calls the storm 'multicellular' although he does not present any specific evidence in support of that description. It has been our experience that virtually all convective storms lasting more than 30 minutes have at least some multi-cellular aspects and so describing a storm in that way conveys little or no real information about the character of the stonn. It is not hard to imagine that the Sydney storm was, in fact, of the supercell variety, although it may not have had an intense mesocyclone at low levels. I7e have found that many people still cling to several of the old-fashioned descriptions of 'supercell characteristicsr' one of which is the unicellular character of the convective storm. Modern definitions of supercells focus on the presence of a deep, persistent mesocyclone (see, e.g., Doswell and Burgess 1993). Therefore, the multi-cellular character ofthe storm cited by Eyre is not really relevant in its classification; based on the events produced by the storm, we are fairly confident that it was a supercell. \7e also note with some dismay the comments by Eyre to the effect that evidence of 'twisting' in the damage can be used to imply something of the character of the high winds. The implication in Eyre's remarks is that some of the damage may have been tornadic, as evidenced by the twisted trees and electricity pylons. This implication apparently is based on the misconception that tornadic winds would show rotation on scales of a few metres. In virtually all tornadoes, the scale of rotation is at least one and perhaps two orders of magnitude larger than the scale oftrees and electric power pylons. Therefore, on the scale of the damaged obiects, the rotation will not be at all evident. For all practical purposes) the damaging winds in a tornado are 'straight' winds on a scale of a few metres. Twisted damage is a consequence of the way structures on that scale are affected by the wind. Based on the evidence in Eyre's note, it appears that the twisting is simply the result of what he calls 'local eddies' produced by the complex interaction of effectively straight winds and objects like trees and buildings. If the rotation is on the scale of the convective storm (several kilometres), as would be the case in a supercell without a tornado, the argument is even stronger that twisting damage on the scale of individual trees was not the direct result of that rotation. \ile think that the storm may have been a form of supercell with relatively strong stormscale rotation in mid-levels (say, higher than 3km above the surface), but with only weak storm-scale rotation near the surface. Such storms often take the form of what has been called a'high-precipitation' (or un) supercell by Moller et aI. (1990). Based on our observations of such storms in the USA and elsewhere, they can be accompanied by extensive swaths of damaging winds (and thus may fall into the class of events called 'derechos' by Johns and Hirt (1987)), as well as large hail and torrential rainfalls. Incidentally, although it has not been explained satisfactorily, the green colour noted by Eyre and the eyewitnesses has been associated with hailstorms for many years in the USA. I0flhile we do not have any suggestions for its origin, our experience has confumed repeatedly the association with hailstorms. The Arnhem Land event is also very inter-