Interaction of Potential Vorticity Anomalies in Extratropical Cyclogenesis. Part II: Sensitivity to Initial Perturbations

Abstract

In Part I of this series of papers, the static potential vorticity (PV) inversion diagnostics are performed to examine the lateral and vertical interactions between two upper-level (northern and southern) troughs and their possible influences on the development of the March 1993 superstorm. This study continues the investigation of the relative importance of two PV anomalies associated with the short-wave troughs and their interaction in the surface cyclogenesis by considering their individual roles as an initial-value problem. The piecewise PV inversion technique is first used to isolate the upper-level disturbances with a balanced vortex for each. The vortex is then either removed or its intensity doubled by subtracting the balanced vortex from or adding to the control initial conditions. With the modified initial conditions, a mesoscale model is integrated for 36 h to examine how each of the upper-level disturbances contributes to the surface development. It is found that the northern and southern troughs play different roles in the surface cyclogenesis. Without the northern trough, the eastward propagation of the southern trough slows down and the surface development is severely impeded. More rapid cyclogenesis occurs when the northern trough’s intensity is doubled. On the other hand, the deepening rate increases considerably when the southern trough is removed. Doubling the southern trough intensity slows the eastward progression of the northern trough and leads to a reduced deepening rate. In conclusion, the northern trough is crucial for the rapid development of the superstorm, whereas the southern trough is important only during the incipient stage of the cyclone. In particular, a much stronger surface cyclone could be spawned in the absence of the southern trough. This finding appears to contradict the previous work on the roles of trough mergers in extratropical cyclogenesis.