Importance of Equivalent Potential Vorticity in Determining the Trajectory of Sudan Lows

Potential vorticity (PV) which depends on relative vorticity and static stability in each latitude is a useful quantity in analyzing the synoptic systems at middle latitudes. Since PV does not depend on moisture field, it is not useful for determining the trajectory of moist synoptic systems such as those affecting south of Iran during the winter time. To diagnose the movement of such systems it is possible to use equivalent and we-bulb PV which depend on specific humidity through equivalent and wet-bulb potential temperature respectively. In this study the NCEP/NCAR reanalyzed datasets is used to calculate the equivalent and wet-bulb PV for two synoptic cases affecting south of Iran. Results show that small and negative amount of equivalent and wet-bulb PV aggress well with the location of low pressure systems at the surface. Also, it is seen that the pattern of equivalent and wet-bulb PV are similar except that the value of equivalent PV is larger than that of wet-bulb PV.