The formation of a baroclinic leaf

A baroclinic leaf begins with the undulation of an air mass boundary. This process can be best observed in water vapour images because they show as a tell-tale black stripe. During the development process, the leaf takes on a typical s-shaped moisture pattern in WV imagery, with a concave and a convex part (figure 1a). In the majority of cases, the convex part of the leaf structure is well defined in the beginning, but may become less distinct with further development. In WV images, a dark zone that narrows over time can often be seen at the poleward edge of the concave part. A dark spot corresponding to a dynamic tropopause anomaly can also be seen in this part. Even though its shape changes, this feature will remain prominent throughout the leaf structure's development.

The formation of a baroclinic leaf typically occurs together with significant PV anomalies on the 1.5 PVU surface. Moreover, the leaf's type is determined by its position relative to the jet stream.

Figure 1b: VIS image from 20 May 2012, 12:00 UTC – the development of the baroclinic leaf (red square) is not as pronounced as in the WV image.