Joint Centre for Mesoscale Meteorology (JCMM)
Triple point development: two case studies
by Douglas Parker
Recent work has suggested that frontolytic strain may be a feature of a certain class of frontal wave instability. By considering one analysed case, it is suggested that this instability may manifest itself in the formation of triple point lows. Weak waves on the cold front of a frontal cyclone propagate on to the warm/occluded front at the triple point and develop significantly. When the warm front lies in frontogenetic strain, it is inevitable, from the geometry of the triple point, that the cold front will be in frontolytic strain. This process of triple point development seems to be relatively common over the Northeast Atlantic. Some of the implications of this model are discussed. It has been shown elsewhere that, at least in a barotropic instability model, frontolytic strain can yield very high growth rates of instabilities. Whether this is crucial to triple point development is not obvious, and it may be that the role of the cold front is simply to supply waves to the intense (frontogenetic) warm front.
In the second case considered here, the development at the triple point occurred through the same mechanism, and was associated with rapid propagation of the parent system. This process may be interpreted as being due to a transient Rossby wave behaviour.