Enhancement of Forest Fires over Northeastern Japan due to Atypical Strong Dry Wind

Abstract

A large number of forest fires were enhanced by atypically strong dry winds over northeastern Japan on 27 April 1983. The strong surface winds were induced by vertical mixing of momentum in the unstable planetary boundary layer (PBL) with strong dry convection under fair weather and strong synoptic wind conditions. This type of strong wind was investigated by making use of observations and data analyses. Characteristics of this wind are summarized as follows. 1) The surface wind speed remains weak during nighttime hours, even when the upper-level synoptic wind is strong. This is due to the nocturnal stable layer (NSL), which prevents the intrusion of the upper-level synoptic wind into the lowest levels near the surface. Under such conditions, when the NSL is completely dissipated by daytime surface heating or synoptic upper-level cold-air advection, the surface wind speed exhibits a sudden strengthening within 1 h. 2) Fluctuations in the surface wind speed are induced by variations in the incoming solar radiation due to the passage of small fair-weather cumulus clouds. 3) The onset of the atypical strong winds is related to the strength of the NSL 4) Routine observations of the wind speed at a mountaintop having an altitude of 500?1000 m above sea level can be utilized to forecast the atypical strong wind, since the NSL does not form in such areas.