Investigation of multi-scale and unsteady characteristics of deep Foehn events over Alborz Mountains

The Foehn characteristics and its temporal evolution events are investigated using a combination of observations, synoptic analysis and simulation data in the Alborz Mountains in northern Iran. Results show that the number and intensity of Foehns over these mountains, in the last ten years, have decreased and stronger ones often occurred in the west, in Gilan Province. Two mechanically-driven Foehns events occurred in Alborz Mountains during 4-5 December 2012 and 3-9 January 2013. These events caused an extensive and high-intensity fire in the Gilan and Mazandaran forests in the northern Iran. A strong pressure gradient in south-north direction across the Alborz Mountains is shown which was followed by a rapid increase in the intensity of the southerly warm wind in leeward side of the Alborz Mountains. On the synoptic scale, the Foehn event occurred due to the existence of a high pressure over the interior regions of Iran and lee cyclone over the southern Caspian Sea. Moreover, this study clarifies that in these cases, the synoptic scale circulation is largely responsible for the flow characteristics over the Alborz Mountains, and temperature variability during Foehn event depends on the large-scale advection and the source region of the flow which are often the dry central part of Iran. The results suggest that, the mountain waves excited over the northern slopes of the Alborz Mountains are the primary source of the localized southerly wind maximum around the lee side of Alborz Mountains.

Keywords: Foehn winds, Alboez Mountains, synoptic scale flows