Comments on 'The Evolution and Dynamical Role of Reduced Upper-Tropospheric Potential Vorticity in Intensive Observing Period One of FASTEX'

摘要

Using trajectory-based PV diagnosis on a case of extratropical cyclogenesis, Pomroy and Thorpe (2000) attempted to demonstrate that diabatic reduction of PV through direct Lagrangian change produces negative PV anomalies at the tropopause level that are comparable in importance to the diabatically produced positive PV anomaly in the lower troposphere. However, because they did not consider the indirect effect of latent heating in enhancing advection of PV at the tropopause, which previous studies had shown to be dominant over the direct effect, their finding of dynamically significant upper-level negative PV anomalies remains unreconciled with the results of those previous studies. In addition, the extreme weakness of the diabatically produced positive PV anomaly in the lower troposphere and its induced wind fields in the case studied by Pomroy and Thorpe (compared to other recent studies) undermines the general applicability of their conclusion about the comparable importance of negative and positive PV anomalies produced by direct diabatic change in PV in extratropical cyclone evolution.