Weather Divide Distribution during Winter along Several Valley Profiles in Northeastern Honshu

Abstract

According to the synoptic situations the author examined the concentration positions of weather divide during winter months along six east-west orientated valley profiles in Northeastern Honshu. Then the difference of positions and frequencies of weather divide occurring were explained by the influence of surrounding topographic features. Here the weather divide is defined as the easternmost limit of precipitation more than 1.0mm per day along each valley profile.
Two categories of synoptic situation were adopted: air flow type and wind velocity of 850mb surface at Akita. As the flow types relevant to this research NW, WNW, W and WSW flow types were selected, and concerning wind velocity four classes were set up.
In each profile the concentration position of weather divide appears in a different way according to air flow type and wind velocity.
The results are summarized as follows:
1) Along the profiles having a single watershed appear two or three concentration positions, in general, closely with each other on the leeward slope, and an exception, when the windward massive highland exerts an effect blocking the air flow in a definite flow direction, another concentration position appears between the highland and the watershed.
2) Along the profiles having main and secondary watersheds (the lower secondary one on the leeside of the main watershed), two or three concentration positions appear in case of NW, WNW and W flow types as follows:
Without the above blocking effect, they appear near the main watershed and on both slopes of the secondary one.
With the above blocking effect, they appear near the main watershed and on the wind-ward slope of the secondary one with a few exceptions.
With the strong blocking effect, they appear on both slopes of the main watershed.
In case of WSW flow type, the concentration positions appear as follows:
Without the above blocking effect, they appear only near the secondary watershed.
With the strong blocking effect, appear no obvious concentration positions.