Numerical Modeling of the Exceptional South Foehn Event over Alborz Mountains during December 2005 Extensive Forest Fire

Abstract

This study aims to document the features of an exceptional south Foehn event which occurred in December 2005 over Alborz Mountains and conducive to an extensive forest fires in northern Iran. The Foehn event is investigated by using a combination of observations and simulation data. A high resolution numerical model (WRF) is used to clarify the Foehn event at different scales. The event resulted an extensive, intense fire in the Gilan, Mazandaran and Golestan forests in northern Iran. <br><br>The results indicate that a mechanically-driven Foehn occurred in Alborz Mountains during 16-18 December 2005. The results indicate 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. A high resolution numerical simulation revealed that, strong meridional surface pressure differences, along with a southerly flow is blocked upstream of the Alborz Mountains, resulting in higher nonlinearity and created large amplitude, vertically- propagating mountain waves over the Alborz Mountains. The large, long-lived mountain waves in turn, created a down-slope windstorm (Foehn wind) in the lee of the Alborz Mountains. The study also indicated that the wave-breaking region on the lee side was responsible for reflecting the mountain wave energy back to the ground and for creation of severe down-slope wind in lee side of Alborz Mountains. The exceptional south Foehn event first has been appeared in early time on December 16 in western part of Alborz Mountains, when western Alborz has hit by a severe down-slope wind for the first time. The event extended to east by time, following the spatial and temporal variations of down-slope wind in lee side. The Foehn event persisted over the area until a mid-tropospheric trough destroyed the permanent southerly wind and changed it to westerly wind.