Abstract
A novel method that quantitatively evaluates the development processes of extratropical cyclones is devised and applied to the explosive cyclones over the northwest Pacific in the cold season (October–April). By inverting the potential vorticity (PV) tendency equation, the contribution of dynamic and thermodynamic processes at different levels to explosive cyclone development is quantified. In terms of geostrophic vorticity tendency at 850 hPa, which is utilized to quantify cyclone development, the leading factors for the explosive cyclone intensification are upper-level PV advection by the mean zonal flow and the PV production from latent heating. However, explosive cyclones are also subject to hindrances from vertical and meridional PV advections. Quantitatively, the sum of thermodynamic contributions by the latent heating, vertical PV advection, and surface temperature tendency is about 1.6 times more important than the dynamical PV redistribution by horizontal advections on the explosive cyclone intensification. This result confirms the dominant role of thermodynamic processes in explosive cyclone development over the northwest Pacific. It turns out from further analysis that the interactions of lower-level anomalous flows are important for thermodynamic processes, whereas the advections by the upper-level mean flow are primary for dynamic processes.
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