PV comparison
A definition of the tropopause in terms of PV is more useful than a lapse rate definition (Hoskins et al. 1985).In this study a 2.0-PVU surface is defined as the dynamic tropopause, which approximates the boundary between the smaller PV magnitudes of the troposphere and the larger cyclonic PV reservoir of the stratosphere. A positive upper-level PV anomaly in the Northern Hemisphere typically corresponds to a cyclonic vorticity maximum, and a physically lower dynamic tropopause. A positive lower-tropospheric PV anomaly typically occurs as a result of an upward increase in latent heating. A positive potential temperature anomaly near the surface is a surrogate for a positive PV anomaly (MA et AL 2002).
The onset of reintensification is well correlated with a decrease in the 1000–500-hPa thicknesses, a drop in height of the dynamic tropopause, and an increasingly stable stratification of the troposphere above the cyclone. After the disintegration of the low-level PV anomaly, the cyclone weakens. We can use the PV fields to describe the entire process in terms of the magnitude of the PV anomalies and highlights the importance of their coexistence in the intensification. This storm evolution is consistent with the general behavior described by Hart and Evans (2002). In particular, the reintensification of Alex during the early part of the hurricane season illustrates the importance of the interaction of the baroclinic region and the sea surface temperatures (SSTs)(MA et AL 2002).
Alexs remnants appear as a single closed contour of 1 PVU . A strong upper level positive PV anomaly is evident to the north. On 6 August 2005 at 00 UTC the upper anomaly elongates and intensifies, and a family of low level PV anomalies, ahead of the trough, suggests convective instability in the region. McTaggart-Cowan et al. (2001) discuss the important role of latent heat release in fueling an increase in PV due to convective precipitation near the cyclone center during the reintensification.(MA et AL 2002)
The upper-level PV anomaly is positioned to the north of the low-level anomaly. The rapid elongation and southeasterly extension of the upper-level PV maximum are associated with the digging of the trough as it moves. The largest stratospheric intrusions during such a rapid upper-level development typically occur at the southeastern edge of the PV anomaly as seen from 6 August 2005 18 UTC. Further, the southeastward propagation of the elongating anomaly places Alex's remnant in an ideal location for cyclogenesis after 00 UTC 7 August 2005. Intensification of the near-surface circulation continues as the anomalies interact by wrapping around each other. When the storm reaches its greatest intensity (975 hPa at 00 UTC 8 August 2005), the anomalies are virtually collocated and thus can no longer positively interact.