The evolution and dynamical role of reduced upper-tropospheric potential vorticity in intensive observing period one of FASTEX

The existence and production of reduced upper-tropospheric potential vortic ity (RUPV) by heating is considered. An objective technique is used that id entifies anomalies of PV arising from a particular physical process (here l atent heat release). The evolution of two RUPV anomalies and a related diab atically increased lower-tropospheric PV (ILPV) anomaly occurring during In tensive Observing Period One of the cyclones from the Fronts and Atlantic S torm Track Experiment (FASTEX) is examined using model analyses, sounding d ata, and trajectory calculations. Three distinct airflows are identified em anating from the ILPV anomaly each with a different evolution. Results show that RUPV anomalies exist in the atmosphere and, in a weaker form, in nume rical models. The dynamical role of RUPV anomalies is examined using a nonlinear balance PV inversion and reruns of the U.K. Meteorological Office Limited Area Mode l. This shows that instantaneously the Row and temperature perturbations as sociated with RUPV anomalies are of at least comparable magnitude and exten t to those induced by a similar positive anomaly. Over time one RUPV anomal y is seen to have a significant effect upon the development of its parent l ow. This low is more compact and more rapidly developing in the absence of the anomaly. The effect of the positive anomaly is also significant, but re moving it has only a short-term effect as the anomaly quickly reforms. Thes e results show that it is important to consider the role of RUPV in the PV model of a midlatitude cyclone.