A Numerical Study of a Dynamically Induced Foehn Observed in the Abashiri-Ohmu Area

Abstract

Observations are given for the foehn event which occurred in the Abashiri-Ohmu area (N44.3, E143.5), Hokkaido, northern Japan, on 12 May 1975. A numerical investigation is made of the foehn, using a 2-dimensional nonhydrostatic model.
The foehn is well simulated, even without any diabatic heating in the presence of an inversion layer below 900hPa. This foehn is confirmed to be dynamically induced, and is an unsteady, highly nonlinear phenomenon. It is accompanied by an atmospheric bore (a shock-like disturbance propagating upstream) and an internal hydraulic jump (a shock-like disturbance propagating downstream), but no wave breaking (wave induced critical layer). The foehn of a homogeneous fluid (with constant mean velocity U and constant Brunt-Vaisala frequency N) is also simulated. It is accompanied by wave breaking and hydraulic jump, but no bore.
A foehn index (a non-dimensionalized potential temperature rise on the lee slope) is introduced, and its dependence on the inverse Froude number (Nh/U) is examined for the 4-layered fluid of the Abashiri-Ohmu case and the homogeneous fluid. The critical mountain height for the occurrence of the foehn is found to be lower in the presence of an inversion layer below 900hPa than in the absence of an inversion layer.
In addition, the foehn event in the Abashiri-Ohmu area is compared with that of the European Alps (Hoinka, 1985).