The dynamics of cyclonic systems over Iran using potential vorticity diagnostics: A case study for Nov-Dec 2003


Institute of Geophysics, University of Tehran


Potential vorticity (PV) is one of the dynamical parameters which is used in the study of weather system dynamics. It is conserved in the absence of friction and diabatic processes and therefore it can be used as a dynamical tracer in the formation and development of cyclonic systems. In this study we attempt to investigate the dynamic of cyclogenesis over the Middle-East and Iran for the period of Nov-Dec 2003, within the potential vorticity framework. It is aimed to study qualitatively and quantitatively the role of different mechanisms which have been involved in the cyclone development throughout the life cycle of the cyclone.
At first, using the NOAA analysis dataset, six hourly geopotential height maps for the period of Nov-Dec 2003 were examined. Only one noticeable cyclogenesis event with clear life cycle was found, beginning at 0000UTC 4 December and ending at 0600UTC 9 December 2003. The results suggest that the main reason that there was only one intense cyclone in the period studied here can be related to the occurrence of two significant blocking systems at the beginning and at the end of this period. As we expect, these blockings act to place stable weather conditions in the east and consequently to inhibit the cyclone development.
In the second stage, the Ertel-Rossby PV was calculated every six hours using the NOAA analysis dataset. The data used in the calculation of PV includes the horizontal wind components and potential temperature fields at 20 pressure levels with 50 hPa interval. Then the potential vorticity fields at pressure levels were interpolated to relevant isentropic surfaces, in order to study the contributions of the upper-level, mid-level, and surface PV anomalies in the cyclone development. To support the results, the vertical motions at 700 hPa were also used.
In general, three stages of cyclogenesis can be identified in the life cycle of the cyclone studied here. The results show that the upper-level PV anomaly plays the main role in the low-level disturbance development during the incipient stage, without remarkable contributions from the other PV anomalies. In the intensification stage, in addition to the upper-level PV anomaly both the thermal advection and the diabatically generated PV at low levels contribute significantly to the surface cyclogenesis. The results also suggest that there is interaction among the upper-level and low-level PV anomalies at this stage. Finally, the mature stage of cyclogenesis is encountered with the decay and breaking of the PV anomalies.