عنوان مقاله [English]
This research investigates the evolution of middle-and upper-level fronts in three frontal cyclones. The first case occurred over the northern Mediterranean in February 2005 and the other cases were located over the Middle-East region and Iran during November and December 2004. The version 5 of the meso - scale model (MM5) was run for the all cases, with 75 km × 75 km horizontal resolution. The boundary conditions were determined from the (United States) National Center for Environmental Prediction (NCEP) – National Center for Atmospheric Research (NCAR) reanalysis. Using the model outputs, including wind and temperature fields, the vector frontogenesis was calculated for each grid point in the different levels. Then, the changes in the magnitude and direction of the horizontal potential temperature gradient along the three upper-level fronts were diagnosed. The vector frontogenesis has two rotational and scalar components. The scalar component comprises three terms related to divergence, deformation, and tilting, whereas the rotational component comprises three terms related to vorticity, deformation, and tilting.
Results of the calculations for scalar component of vector frontogenesis show that divergence term is smaller than deformation term in the all cases. Both terms are positive in the first and third cases, leading to frontogenesis, but they caused frontolysis in the second case. Besides, the tilting term leads to frontogenesis in the second and third cases, and is dominated along the front in the region of active frontogenesis. Against, the tilting term is negative in the first case, indicating that the total frontogenesis is primarily a result of the deformation term.
The analysis of the rotational component of vector frontogenesis reveals that the tilting term is negative in the all cases, leading to anticyclonic rotation of the isentropes. The vorticity term is positive and the dominant one in all the cases, coincident with the region of positive vorticity. It results in the cyclonic rotation of the isentropes relative to the mean flow, thereby initiating cold advection along the front. The deformation term is positive (negative) on the upstream (downstream) side of the front.