Study of Upper-level Frontogenesis from Potential Vorticity Perspective over the Middle-East Region and Iran

This paper investigates the evolution of the three middle- and upper-level fronts associated with the cyclones. The version 5 of the mesoscale model (MM5) is applied for the all cases, with 75 km * 75 km horizontal resolution. The boundary conditions are determined from the (United States) National Center for Environmental Prediction (NCEP), National Center for Atmospheric Research (NCAR) reanalysis. Using the model outputs, including PV, potential temperature, wind fields, the upper-level fronts have been studied. The analysis of the PV charts reveals that the upper-level frontogenesis is accompanied by the tropopause folding in all the cases. The penetration of dry and cold air with stratospheric PV values to midtropospheric levels increases PV gradient where the tropopause is folded and thereby enhances upper-level frontogenesis. The other noteworthy features are the existence of strong descending motions and high static stability near the regions of upper-level frontogenesis. The tropopause folding depth in the first case is larger than the other cases. The front in the first case is located on the upstream side of the thermal trough, whereas in the other two cases the fronts are seen on the base of the trough. It is suggested that the upper-level processes may have profound effects on the frontogenesis in the third case. On the other hand, the results show that both the middle- and upper-level processes have significant contributions to the frontogenesis in the other two cases.