Diurnal Variations in Summertime Surface Wind upon Japanese Plains: Hodograph Rotation and Its Dynamics

Abstract

This study investigates diurnal variations in surface wind in Japan during June-August of 1992-2006 using data from the Automated Meteorological Data Acquisition System (AMeDAS) and the Sapporo City Multisensor (MULTI). Harmonic analysis and hodograph analysis are employed to investigate the rotation direction and rotation rate of the wind vector at about 1000 stations. An analysis of six major plains in Japan reveals distinct clockwise and anticlockwise hodograph regions within each plain. The rotation direction is attributed to two lagged pressure-gradient forces of contrasting orientations: one between the land and sea, and another between the plain and adjacent mountains. An analysis of the linearized equations of motion reveals that rotation of the wind vector is mainly controlled by the balance between the pressure gradient force and the frictional force, with a small but non-negligible contribution by the Coriolis force, particularly near the coast. The observed rotation rates of the hodographs show a brief stagnation during the mid-afternoon and a longer stagnation during the nighttime. This irregular rotation rate is well explained by taking into account the semidiurnal component of the wind. The linearized equations of motion indicate that this semidiurnal component results from the semidiurnal component of the pressure gradient force, which is generated by non-sinusoidal solar heating over the course of a day.