Diagnosis and Dynamics of Forecast Error Growth

Consideration is given to the diagnosis and dynamics of synoptic and subsynoptic forecast error from a potential vorticity (PV) perspective. A depiction of the extratropical forecast minus analysis PV pattern on a cross-tropopause isentropic surface serves to illustrate characteristic features of the PV-error field, and these features relate both to the instigation, development, and breaking of Rossby waves at the tropopause, and to surface cyclones and anticyclones. An outline is provided of a three-component diagnostic approach for studying PV forecast error. The approach exploits the quintessential PV concepts of quasi conservation, inversion, and attribution, and its essence is illustrated qualitatively by reference to one particular synoptic sequence over the North Atlantic. It also provides a framework for assessing the dynamics of possible mechanisms for generating realized PV-error features. The approach offers a conceptually attractive and diagnostically useful method of analyzing, assessing, and understanding the dynamics of forecast error growth.