Toward a better understanding of elevated convection

The term “elevated convection” has become widely used in the operational and research communities in the past two decades to refer to convection that originates from a layer other than the atmospheric boundary layer. Elevated convection is of considerable forecast significance as it can produce severe hail, damaging surface wind and excessive rainfall in places well-removed from areas of strong surface-based instability. While it would seem easy enough to determine the layer of origin of a given convective tower with the aid of an appropriate proximity sounding, in practice, often this is not the case. For example, supercells in the vicinity of a warm front at first glance often appear to be elevated. Upon further examination, however, it is apparent that some such storms draw air into their circulation from the planetary boundary layer.

Cloud classification schemes, meanwhile, based primarily on appearance and employed by the International Cloud Atlas to facilitate the taking of surface-based meteorological observations, have long used the term “castellanus” to designate any patch of elevated layer-type cloud that assumes a turreted or cumuliform part on its upper surface. Physically-based definitions, such as that proffered by Richard Scorer, are more restrictive, using castellanus to refer to cumuliform clouds that owe their buoyancy to the occurrence of condensation rather than to the presence of pre-existing thermals. On some occasions, clouds that visually are castellanus by either definition may be quite deep and yield thunder, while on others, the clouds remain shallow or quickly dissipate.

Because it is not clear that a given convective cloud is necessarily entirely elevated or surface-based, and because there exist various interpretations of the terms “elevated convection” and “castellanus,” we believe that study should be directed to clarify these definitions and to better understand the physical processes associated with them. Using conventional meteorological and photographic data, this paper will present several examples of elevated convection to illustrate some of the ambiguities involved.