The Response of the Surface Wind Speed to the Synoptic Pressure Gradient in Central Japan

Abstract

The response of the surface mean wind vector to the synoptic pressure gradient was statistically calculated over the region of central Japan. The response was examined as related to the thermal effect, caused by daytime heating and nocturnal cooling in the surface layer. In the process of vector accumulation for a large number of stations, the wind components which originate from local circulations cancel each other. Consequently, in the analysis, it is made possible to extract the surface wind component induced by the synoptic pressure gradient from the actual wind data (this component is termed the SWC). The thermal effect was parameterized by the diurnal component of temperature change (DCTC), using a high-pass filter applied to the AMeDAS temperature data. As a result, a close relationship among the SWC, the synoptic pressure gradient, and the the thermal effect was found. Although the strength of the SWC is directly proportional to the strength of the pressure gradient, the response of the SWC to the pressure gradient can be modified by the DCTC. That is, the response is increased by daytime heating and decreased by nocturnal cooling. Moreover, the response undergoes large change at a critical pressure gradient (1.2mb/100km). The critical gradient is more obvious under cool conditions. It was concluded that the critical gradient separated surface airflow patterns into two categories; airflow patterns dominated by local circulations and airflow patterns mainly controlled by the SWC.