The impact of various potential‐vorticity anomalies on multiple frontal cyclogenesis events

Abstract

SUMMARYThe in uences of various potential vorticity (PV) anomalies on multiple frontal cyclogenesis events are examined using 60 h simulations of a family of six frontal cyclones that developed over the western Atlantic between 0000 UTC 13 March and 1200 UTC 15 March 1992. The cyclogenesis events are characterized by pronounced low-level baroclinicity along a large-scale cold front moving over the warm ocean and by multiple propagating perturbations in the two PV rings on the cyclonic-shear side of an upper-level jet stream. It is found that the tracks of the frontal cyclones follow closely the distribution of the PV rings, whereas the cyclogenesis occurs in the cold-frontal zone, and in close proximity to the propagating PV perturbations in the central portion of an upper-level parent trough. A piecewise PV inversion reveals that the low-level thermal advection (or the bottom thermal anomaly) contributes the most to the depths of two major frontal cyclones, followed by latentheat release and the upper-level PV anomalies, whereas the opposite order is true for the remaining four weaker frontal cyclones.A series of sensitivity experiments is performed to test the impact of removing the upper-level PV anomalies individually, or collectively, on the development of a major frontal cyclone (MFC) as an initial-value problem. It is found that the MFC genesis and its nal intensity depend on how much its track departs from the upper-level PV perturbations. Removing a PV anomaly associated with a major short-wave trough delays the MFC genesis by 18 h and weakens its nal depth by 13 hPa. The largest departure occurs when two upstream PV anomalies are removed collectively, delaying the MFC genesis by 36 h and weakening its nal depth by 21 hPa. It appears that the MFC genesis in the rst 36 h depends more on the PV anomalies in an inner PV ring, whereas its subsequent development depends more on the PV reservoir in an outer PV ring.It is shown that most of the frontal cyclones can still develop in the absence of diabatic heating, indicating the important role of dry dynamics in controlling the multiple frontal cyclogenesis events. However, the 'dry' MFC fails to form a closed isobar when the trough-related PV anomaly is removed. The associated PV inversion and sensitivity simulation results suggest that even though the low-level thermal advection plays an important role in deepening the frontal cyclones, it is the upper-level PV anomalies that provide the necessary forcing for its ampli cation and the frontal cyclogenesis.It is concluded that the upper-level propagating multiple PV anomalies play a major role in triggering, but a secondary role in determining the nal depth of the multiple frontal cyclones. It is the subsequent growth of the bottom thermal anomaly that accounts for most of the nal depth of the frontal cyclones.

Frontal‐wave cyclogenesis in the North Atlantic –a climatological characterisation

“…tip vortices developing at the end of a PV band versus low-level instabilities of enhanced PV bands. Finally the importance of the upper-level PV anomaly is also highlighted by Zhang et al (2002) in their study of three frontal-wave events over the western NA, using a similar PV inversion strategy as Fehlmann and Davies (1999). The authors also underpin the importance of the relative position to the jet, with the anticyclonic-shear side of the jet being less favourable for frontal-wave development if the growth is not strongly dominated by diabatic mechanisms, e.g.…”

Is the isentropic surface always impermeable to the potential vorticity substance?

“…Potential vorticity (PV) is an important dynamical variable due to its conservative property in the absence of diabatic heating and friction and to its invertibility principle that would allow one to obtain the threedimensional (3D) structures of balanced flows (Hoskins et al, 1985;Davis and Emanuel, 1991;Huo et al, 1999;Zhang et al, 2002;Kieu and Zhang, 2010). Haynes and McIntyre (1987, hereafter referred to as HM87), extended Hoskins et al (1985)'s work and presented the most complete and diligent treatment of the PV concept, particularly from the point of view of PV substance (PVS), defined as ρQ, where ρ is air density and Q is PV.…”