442 Simulation of the Structure of Foehn Wind over the Alborz Mountains using FNL Analysis data

Tuesday, 24 January 2017
4E (Washington State Convention Center )
Farahnaz Taghavi, University of Tehran, Tehran, Iran; and M. Salmani Rahimi

In this study, a southerly Foehn wind on lee side of the Alborz Mountains in northern Iran is investigated using a combination of observations, reanalysis and simulation data.  A high-resolution numerical model (WRF) with three nested domains 5, 15, 45 km spatial resolution with FNL analysis data is used to clarify the Foehn event. This event is caused an extensive and high-intensity fire in the Gilan forests in northern Iran.

The results indicate that a mechanically-driven Foehn occurred in Alborz Mountains during 7-9 January 2016. On the synoptic scale, the Foehn event occurred due to existence of a high pressure over the interior regions of Iran and lee cyclone over the southern Caspian Sea, with a strong pressure gradient in south-north direction across the Alborz Mountains, which was followed by a rapid increase in the intensity of the southerly wind in leeward side of the Alborz Mountains. Alborz Mountains are the primary source of the localized southerly wind maximum around the lee side of Alborz. The Foehn event persisted over the area until a mid-tropospheric trough destroyed the permanent southerly wind and changed it to northerly wind. The WRF simulation reveals that strong meridional surface pressure differences along with a southerly flow is blocked upstream of the Alborz Mountains, resulting in higher nonlinearity and creating large amplitude vertically-propagating mountain waves over the Alborz Mountains. The large long-lived mountain waves, in turn, create a down-slope windstorm (Foehn wind) in the lee of the Alborz Mountains. To quantify these effects the Froude number is calculated. A value of 1.2 is found at 1500 UTC 08 January 2016, indicating a supercritical flow on the lee side of Western Alborz. The study also indicates that the wave-breaking region on the lee side is responsible for reflecting the mountain wave energy back to the ground and for creation of severe down-slope wind in leeward side of Alborz Mountains.

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