Dynamical analysis of the effect of tropopause folding on the upper and lower-level frontogenesis

Tropopause folds are intimately linked to upper level frontogenesis and jet stream dynamics. They play an important role for stratosphere-troposphere exchange, the dynamical coupling of upper and lower tropospheric levels, and for generating severe weather events. In this study, the effect of upper-level positive potential vorticity (PV) anomaly on upper- and lower-level frontogenesis over the Middle East and Iran is investigated. In this regard, first three frontal systems associated with deep tropopause folding and strong fronts were selected by using the ECMWF data with the horizontal resolution of 0.75×0.75 degrees on the latitude and longitude. Then, PV anomaly was removed by replacing the zonal mean of PV in the study area and inverted to obtain the modified fields. To do this, a program package (PV inversion), comprising several different steps, is used which allows to isolate PV elements and then to study their impact on the atmospheric flow field as well as the temperature distribution. In the next step, the weather research and forecasting (WRF) model was applied by using the ECMWF data to perform two simulations with real (unchanged) and modified data, as initial conditions, in two domains with 9km resolution for the inner domain. Finally, the frontogenesis function terms, including deformation, tilting, diabatic heating and vertical frontogenesis were computed, using the WRF outputs. By comparing the results of the two simulations, we can determine the effect of tropopause folding on the frontogenesis function terms in the upper- and lower levels throughout the lifecycle of the fronts.
Results show that in the absence of tropopause folding, the horizontal and vertical temperature gradients, horizontal velocity, as well as negative vertical velocity are decreased significantly in the upper and lower levels. Also, positive vertical velocity is increased and its pattern is changed mainly in the lower levels. Generally, large positive values of deformation and vertical frontogenesis terms are collocated well with the gradient of potential temperature (frontal zone), and these terms are declined due to decreasing of horizontal velocity and temperature gradient when the fold of tropopause is removed. The tilting term does not have a fixed pattern in the upper and lower levels, and it follows the vertical velocity pattern. By removing the tropopause folding, vertical velocities are changed, thereby having variable effects on the tilting term and the gradient of potential temperature. The diabatic heating term produced by the release of latent heat intensifies ascending motions, and so affects the gradient of potential temperature (frontogenesis) in the lower levels. This term is also increased in the absence of tropopause folding in the lower levels. The other noticeable point is that the diabatic heating term does not significantly affect the frontogenesis in upper levels, because heating mostly takes place in the lower half of the troposphere far to the east of the upper level front. The total amount of frontogenesis function follows the vertical frontogenesis term in the upper and lower levels, and vertical gradient of potential temperature is generally very large. Although, the amount of the diabatic heating term is larger than the vertical frontogenesis term in the lower levels, but it is limited to a small area. Removal of the tropopause fold causes the total amount of frontogenesis function, similar to the vertical frontogenesis term, to decrease throughout the region of the upper-level front, especially in the downstream of the upper-level trough close to the center of tropopause folding omission. The results of the three cases studied here indicate that frontogenesis function terms are considerably more intense in the presence of tropopause folding. Therefore, it is concluded that tropopause folding has a remarkable positive effect on the formation and intensification of the upper-and lower-level fronts.