The main factors responsible for heavy snowfall in Tehran: A case study

Heavy precipitation events often cause irreparable damage to human and economy. The ability to accurate prediction of the occurrence of such phenomena is very important especially for early warning systems run by operational centers. Identifying the main factors involved in heavy precipitation can result in more accurate predictions. The “factor separation method” (FSM) is used for this purpose. Based on the previous studies on factors affecting heavy precipitation especially in the Alpine region of Europe, and with regard to the geographical location of the Tehran Province, four factors are included in the application of the FSM. These factors are (1) the potential vorticity (PV) anomalies in the upper troposphere manifested in the form of PV streamers, (2) the PV anomalies in the middle troposphere, (3) the Alborz mountain range, and (4) the Zagros mountain range. The contributions of these factors were assessed in the heavy snowfall event of the 6^th of January 2008 in Tehran by applying the FSM using the ERA-Interim data and the WRF (Weather Research and Forecasting) model. To apply the FSM, 16 simulations, which cover all possible combinations of the four factors, were performed by the WRF model using 3 nested domains with horizontal resolutions of 45, 15, and 5 kilometers, respectively, and 60 vertical levels. These 16 simulations are:  the control run (1), the removal of PV anomalies of the upper (2) and the middle (3) levels of the troposphere, the reduction of the heights of the Alborz (4) and Zagros (5) mountain ranges, the removal of two-factor combinations (6 to 12), the removal of three-factor combinations (13 to15), and the removal of the four factors (16). To carry out the experiments involving the removal of PV anomalies, a PV inversion procedure was used to construct the initial states.
On the 6^th of January 2008, a case of heavy snowfall occurred in a significant part of the country from Tehran to the west in the south of the Alborz mountains. The case involved the intrusion of cold and dry air with large positive PV anomalies from the stratosphere to the upper troposphere in the form of a PV streamer. The PV streamer acts to substantially decrease the static stability in the lower to the middle troposphere. The resulting PV anomalies are associated with low-level winds, which can provide the low-level moisture supply for the precipitation event. The results show that the PV streamer is the factor with the highest impact. The independent contribution of the PV streamer is about 76.5% as obtained by the detailed analysis of the FSM using the outputs of 16 simulations. Another factor to consider is orography which can block air flows or lead to substantial ascent or descent. The magnitude of the impact due to orography depends on the characteristics of the flows over and around the mountains. The determining properties of the flows are the angle of hitting the mountains and the intensity of the flow. In those simulations that the altitude of the Alborz mountain range was reduced, the dominant air flow was from the windward to the leeward side of the mountain. As a result of the increase in the penetration of the cold air from the windward to the leeward of the Alborz mountain range, the pressure on the windward (leeward) of the mountains reduces (increases). In other words, the pressure gradient is reduced in the Tehran area, when the height of the mountains is reduced. So the warm front in the southern slopes of Alborz mountain ranges is weakened. The results of the corresponding simulations and the application of the FSM show that the contribution of the Alborz mountain range in this case is next to the upper-level PV anomalies. Quantitatively, the absolute contribution of the Alborz-mountain factor is about 48% in the case of the heavy snowfall that occurred in Tehran. Finally, it is worth mentioning that the Zagros mountain range and the PV anomalies of the middle troposphere play negligible role in this case.