Cold Air Development

The mature stage of Cold Air Developments are represented by a distinct cloud spiral but without any Warm Front cloudiness, this is the main difference to the classical Occlusion types and is caused by a completely different development. While the classical Occlusion types are frontal developments, is the initial stage a non-frontal Comma cloud in the cold air (see Comma).

In connection with Commas the development of Polar Lows are a common phenomena especially in high latitudes. But the conceptual model Polar Low differs from the conceptual model Cold Air Development as in the Polar Low situation the low with a mesoscale Comma develops, while in the Cold Air Development situation frontal conditions develop leading to an increase of the scale of the cloud spiral.

Based on literature and the experience of case studies made at ZAMG, the following aspects to the physical background of Cold Air Development can be made:

PVA maxima are an important feature for Comma development. Commas may develop in front of upper level troughs (Fig. 1) with curvature vorticty dominating the process, as well as within the left exit region (Fig. 2) with shear vorticity dominating.

Fig. 1. Development of a CAD within an upper level trough.

In the case of a Cold Air Development both processes are relevant, but in any case the jet streak component of the physical background is very important.

Fig. 2. Development of a CAD as a result of a positive vorticity
maximum in the left exit region of a jet streak.

In literature very often the Commas are traced backward to the rear side of the upper level trough where they form in the left exit region of a following jet streak. The curvature vorticity maximum can be found more in the northern part of the Comma, in accordance with the spiral structure there. The shear vorticity maximum is more in the southern part of the Comma leading to the PVA maximum superimposed on the Comma tail. The jet axis crosses the Comma in the southern part, dividing regions of deeper convection in the north from those with more shallow convection in the south. The latter is connected with growing baroclinicity and will be discussed below. This situation stays nearly the same during all stages of the Cold Air Development. The intensive moving PVA maxima are regarded as the reason for cyclogenesis taking place within the cold air mass.

Two main processes may lead to the development of a Comma: CISK - which describes an interaction between large and small scale convection and/or a baroclinic state of the troposphere. Although both processes act together, one of the processes dominates the other in different developments. In the case of a Cold Air Development baroclinicity plays a crucial role.

In the initial stage mostly an organized southward continuation of the Comma tail in the low cloud area can be observed. This seems to represent the baroclinic zone which is confirmed by the existence of a weak surface trough with a front-like wind shift, a weak TFP and in some cases a zero line of temperature advection. The latter does not indicate strong temperature differences. In contrast to Cold Fronts these parameters do not mark a change to a different air mass, but only small drops in temperature and dew point.

During the Cold Air Development the baroclinic zone intensifies, which can be seen as increasing cloud as well as in a strengthening of all parameters mentioned before.

References