Domain-Independent Attribution. Part II: Its Value in the Verification of Dynamical Theories of Frontal Waves and Frontogenesis

Abstract

Theories of frontogenesis and frontal waves describe development in terms of the interaction of a basic state or environmental flow with a frontal flow. The basic-state flow may comprise a large-scale confluent?diffluent deformation field and/or an alongfront temperature gradient. The frontal flow is seen as evolving as a result of its interaction with the environmental flow. Such theories make specific predictions about the effect of the basic-state flow on the frontal flow. To test these predictions, counterparts of the basic-state flows and frontal flows used in theoretical models must be extracted from atmospheric data. Here the concept of attribution is used to identify such counterparts. In the present context, attribution refers to the process whereby a part of the wind field is attributed to a part of the vorticity or divergence field. It is mathematically equivalent to the process by which a part of a field of electric potential is associated with an element of total charge density in electrostatics. The counterpart of the frontal flow used in idealized models is identified as that part of the flow attributable to the vorticity and divergence anomalies within the frontal region. The counterpart of the basic-state flow is identified as that part of the flow attributable to vorticity and divergence anomalies outside the frontal region. Applications of the partitioning method are illustrated by diagnosing the flow associated with a North Atlantic front. The way in which the partitioning method may be used to test some theories concerning the effect of large-scale deformation on frontal wave formation is described. The partitioning method's ability to distinguish frontogenesis due to environmental flow from that due to frontal flow is also discussed. The analyzed front is found to lie at an angle to the dilatation axis of the environmental flow. It is argued that this feature must be common to all nonrotating finite length fronts.