We investigate coastal low-level wind jets flowing through the Tsugaru Strait from a case study on 7-9 June 2003 by using high-resolution satellite observations and numerical simulations. In June-August, the easterly wind often blows from the Pacific Ocean toward the northern Japan, associated with the high-pressure system over the Sea of Okhotsk. This easterly wind usually persists for several days and accompanies cool and wet air, low-level clouds and fogs due to a thin mixed layer and an upper stable layer. This condition is favorable for forming strong wind jets in the Tsugaru Strait with width of 20 km.

First, we look into wind distributions by using high-resolution wind field from RADARSAT and SeaWinds. The RADARSAT-derived wind field reveals strong winds in eastern inlet zone of the strait, in a neighboring bay connected to the strait and in the west of the Tsugaru Strait. Along the Japan Sea, several strong winds extend 100-200 km from the terrestrial gaps, and strong and weak wind regions are alternating. Wind measurements by two SeaWinds on QuikSCAT and ADEOS2 represent diurnal variations of the winds. In the eastern inlet zone of the strait, wind speed is minimum in the morning and reaches maximum in the evening. On the other hand, in the west exit of the strait, wind speed starts to increase from the afternoon and reaches a peak in the midnight. The time lag of wind variation between east and west of the strait is shown.

Then, using meteorological numerical model MM5, the simulated fields are analyzed. Observed surface wind distributions and their diurnal variation are consistent with the simulated wind fields. We also compare simulated wind variations with in situ observations and confirm the consistency between them. Wind structures and evolution similar to those of surface winds are seen up to 500 m high. Temperature fields reveal low-level cold air intrusion below 500 m high into the strait in the nighttime and cold air completely pass through the strait. They coincide with the wind variations.