On the determining factors in cases of moderate to heavy precipitation in Tehran

Investigating the factors responsible for heavy precipitation events is important because of the financial and non-financial losses they often cause. Based on the ERA-Interim reanalysis and a numerical mesoscale forecasting model (WRF), the effects of four factors on significant precipitations in Tehran are studied. The four factors are upper-level potential vorticity (PV) anomalies, and mid-tropospheric PV anomalies as two meteorological factors; and the Alborz and Zagros mountains as two external factors. A factor separation analysis is applied to six days that are representative (most similar) of the six circulation types (CTs) previously identified for moderate to heavy precipitations in Tehran. The independent contribution of the upper-level PV anomaly to precipitation in each of the selected days of the second to the sixth CT is higher than 60%; hence, the upper-level tropospheric flow provided the main precipitation forcing factor, with much less contribution from the thermodynamic factor of the mid-level troposphere. Regarding the two orographic external factors, the higher positive impact on Tehran precipitation is found for the Alborz mountains. This is related to synoptic-scale weather systems that induce a northerly low-level flow towards the Alborz mountains. Moreover, in all the studied cases, except the fifth CT, the independent impact of the Zagros mountains on Tehran precipitation is positive. In all of the selected days except that of the first CT, the main factor responsible for precipitation is the intrusion of dry and cold stratospheric air associated with a large positive PV anomaly (streamer) at upper-tropospheric levels. For the first CT, further quest is necessary to find out the main factor.