Joint Centre for Mesoscale Meteorology (JCMM)
JCMM INTERNAL REPORT NO.56
Secondary frontal waves
by Douglas Parker
Although interest in frontal waves, as instabilities of midlatitude frontal zones, goes back to the times of the Bergen school, with subsequent studies in later years, significant advances have recently been made, since the late 1980s. The idea of secondary frontal waves stems from a duality in the understanding of frontal instability, which again dates to the times of the Bergen school. In midlatitudes, cyclonic systems develop as baroclinic instabilities of the meridional temperature gradient, or 'polar front', as typified by the Charney and Eady models of baroclinic instability. Within these baroclinic waves, the action of the shearing of the basic state s to form locally intense fronts which will reach small lengthscales in finite time (as shown by Hoskins and Bretherton, 1972). Recently, it has been noted that the intense fronts associated with such developed baroclinic saves may themselves undergo instability, forming secondary cyclones, for example, on the trailing cold front. These secondary systems occur on the mesoscale, can have extremely large growth rates, developing over one or two days, and are notoriously hard to forecast. The recent studies of frontal waves focus on these secondary instabilities, although it could be argued that the difference between the secondary systems and their parent instabilities is only one of scale.
Detailed observations of explosive cyclogenesis have recently taken place in the ERICA project of 1988/1989, studying rapidly intensifying cyclones over the West Atlantic, close to the East coast of the USA. Although these systems develop rapidly on the baroclinicity between the Atlantic and the North American continent, with small horizontal scales, it may be argued that they represent 'primary' cyclogenesis, as they occur at the entrance to the Atlantic storm track. In any case, the planetary scale flow feeding these systems, with westerlies from the continent flowing over the warm Gulf Stream water, provides different conditions for cyclone growth than the mature fronts which approach Northern Europe. Other rapidly developing mesoscale cyclones include 'polar lows', which develop as cold polar air flows over high latitude oceans and are often associated with ice-boundaries. Again, these phenomena are here regarded to be different from the secondary frontal cyclones which commonly affect the UK and France.
The issues at stake are the energy source for the development (whether baroclinic, as the primary synoptic frontal cyclones are known to be, or barotropic as some studies have found, how intense or 'explosive' growth rates occur, the role of moist processes (in conditioning the primary front and in influencing the secondary system) and the structure of the mature frontal wave.